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Thread: A Brief Guide to Choosing Equipment for Astronomy. Telescopes, Mounts, Eyepieces.

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    Default A Brief Guide to Choosing Equipment for Astronomy. Telescopes, Mounts, Eyepieces.



    General Intent:

    There is a lot of confusion about how to make good choices regarding astronomical equipment. I hope to remove some of that confusion but I know that I cannot remove all of it.

    Part of the problem is that there is no single answer as to what you should own. Each of us is different and we generally live in different locations and have different styles, physical abilities, and observation priorities. To complicate things further, our priorities and observing conditions change every day which means that the system which would have been ideal yesterday may not be suitable today.

    So not only can I/we not determine what is the ideal system for you, even if I/we could find the ideal system for you today, it may no longer be ideal tomorrow (or maybe even later tonight).

    You are going to have to compromise. You will need to choose one of the following approaches or something in-between:
    1. Purchase a whole lot of systems so that you can have close to the right system available for each clear night and set of observing priorities. I've heard of one amateur astronomer who was setting up 9 telescopic systems most nights in order to meet his particular observing priority. Most of us are not going to own and utilize that many systems.
    2. Purchase one system which meets most of your priorities most of the time - and will hopefully be fairly good at almost all your most important requirements. Then make sure you consider adding a binocular to the mix - almost every amateur astronomer should have a binocular.


    It is not possible to cover all telescope system components or to give an exhaustive explanation of all aspects of your choices. The intent is to give you just a bit more of the basics so that you will not make a grossly unsuitable choice and will be better equipped to ask questions regarding equipment.

    This will not be a typical thread. The idea is that this will be a moderator-driven sticky. You should feel free to start or visit a companion thread to post comments and/or suggestions which may then be incorporated into the thread. Any moderator may edit this thread as they see fit. The intent is to have a somewhat compact reference which is updated over time. The reader should not have to hunt through pages of posts in an attempt to find the needed information.

    I ask that you start and/or post to companion thread rather than sending private messages about the information. PMs of this sort are likely to have less perspective than an open discussion and improvements to the thread will be less complete and precise.

    There are still significant formatting issues I have been unable to fully resolve. My apologies. I do hope to fix them over time.
    Last edited by OleCuss; 08-20-2016 at 04:39 AM.
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    Default Re: A Brief Guide to Choosing Equipment for Astronomy. Telescopes, Mounts, Eyepieces

    The Warning(s):
    Only you can choose the right equipment for you but I want to make some fairly general recommendations at the beginning:


    • Please don't buy cheap new equipment if you have reasonable choices. The very low-end new astronomical equipment is of fairly poor quality and will generally give you a rather poor experience – and that experience can be enough to drive you away from exploring your universe. Also remember to stay within your budget, the stars will still be here years from now if it takes you a while to save up for the system you actually need.


    • The best way to buy cheap is to find well-treated used equipment. Doing this well requires the patience to wait for the right equipment to become available and just enough sophistication to make sure the equipment really is in good shape. But seriously, I've gotten great used equipment for as little as Ľ the cost if I'd purchased new.


    • Understand that especially if you are buying new you are generally getting what you are paying for. If you buy something new for almost no money then your system will likely be nearly worthless. If you pay a whole lot of money for a system from a reputable source then you will be getting a superb instrument.


    • There are diminishing returns, sort of. If you are getting quality products, then doubling the amount you pay you probably won't double the quality of the view.


    • Relatively small increments in quality can make a huge qualitative difference. So sure, you can pay twice as much for a system or component which is maybe only about 5% better than is the cheaper item. The obvious conclusion would be that you are wasting your money, but it could be that you have actually spent your money wisely. That relatively small difference might turn a mediocre experience into an incredible one.
      • After all, if you can see some valued detail or context with the better equipment which you would not see at all with the cheaper equipment then the better equipment is effectively infinitely better.
        • Let's use an illustration? Let's say you have a vehicle which you must use to get to your destination. If you pay for enough fuel to get 95% of the way to your destination then you simply cannot get to your destination at all. If you pay for enough fuel to get you 100% of the way to your destination then you will have achieved success.

      • The point being that if you do not invest enough to meet your goal there is really no point in investing in the first place.



    Define what you need your system to do, how it must do it – then marshal the resources to get the system.

    And remember that it is your money and your goal. Just because someone else thinks you should buy something does not mean you should.

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    Default Why a Star Party is Important.

    Why a Star Party is important:
    If you are like most of us you generally don't want to buy a particular vehicle unless you have at least test-driven either that vehicle or one very much like it. The vehicle may have great reviews and you may have had it highly recommended, but it may not fit your particular body and/or needs. You may also just not like the vehicle.

    It is much like that with telescopes/systems. You may just not like them or you may find them inconvenient, uncomfortable, or otherwise just unsuitable.

    The way you test-drive telescopes and such is to go to star parties. You go to visit a collection of amateur astronomers and show up early so that you can see what it takes to prepare the equipment and then use it. You ask if you may look through the eyepiece and discover what the instrument and accessories can do for you so that you can develop confidence that you understand what your choices will mean to you.

    I have a number of telescopes. I'm not sure which I could classify as my favorite, but I sure know which is my wife's favorite. . . It is a limited edition Orion XT8 which is a sort of maroon color and on which I've put a (only relatively) stylish red-dot finder. It is the only telescope my wife would not let me sell and I think the color is a large part of the reason she likes it.

    The point being that what matters to me may not matter to you – and what matters to you may not matter to me. The only way to be sure you know what matters to you is to go try stuff out.

    My” astronomy club involves a 1.5 hour drive. It's worth it. I learn something useful every time I go.

    O
    n multiple occasions I've driven 3 hours one way to observe with a club at a dark site – and 3 hours back. I've never yet regretted it.

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    Default Visual (eyepiece) Astronomy and Imaging (Astrophotography) are not the same.

    Visual astronomy and Imaging (Astrophotography) are not that similar:

    It is important to realize that there are very significant differences between using an eyepiece with a telescope and using a camera with a telescope. Some would argue that they aren't even the same hobby/pursuit - and I think they have a point.

    I am addressing this because all too often people end up getting a great system for visual use and expect it to also be great for imaging – and it may be nearly completely unsuitable for imaging purposes.

    As an example? I have mentioned that I own an Orion XT8. It is really a very good telescope and I do not regret purchasing it at all. It is nicely transportable, it has pretty good optics which are suitable for use with a good variety of accessories and on lots of targets. It is also almost entirely miserable for use as an imaging instrument. It is not designed to achieve prime focus with the typical camera, it doesn't track, and it uses an Alt-Az (Altitude-Azimuth) mount.

    If I had purchased the XT8 for conventional imaging it would have been an almost unimaginably bad choice.

    Can it be used for imaging nonetheless? Well, yes, it can. But using it for imaging requires some combination of additional equipment, careful selection of targets, relatively unconventional imaging methods, and telescope modifications. With the appropriate combination of the above I've seen some seriously great Lunar images made using an XT8.

    I'm sure that with a sufficient investment of money, time, and energy that I could be using my XT8 to do some pretty good imaging of galaxies, nebulae, clusters, etc. It is actually tempting to do so just to show it can be done, but I don't think it is worth what it would take.

    OK, OK, I've been going on about how a good visual system may not be a good imaging system. How about the opposite approach? Will a good imaging system make a good visual system? Maybe, sort of, usually, is the non-straightforward answer.

    Most of us who have good astrophotography systems could readily attach an eyepiece and be doing some fine visual astronomy. But this is not necessarily the case.

    I own several optics which could be classified as telescopes which are really good for certain types of astrophotography but which would require serious modifications to use visually and would almost certainly be unpleasing for visual use. The only telescope which I currently plan to eventually purchase requires a certain kind of adapter in order to be used with an eyepiece and unless you buy that particular adapter you cannot use it with an eyepiece at all.

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    Default Telescope Design Parameters of note

    Telescope (OTA – Optical Tube Assembly) Design Parameters of note:
    There are many factors which go into the choices made by someone planning to manufacture or build an astronomical system. In at least broad terms you really need to know who the user will be.

    That user might be you or it might be the market to which the manufacturer/builder wishes to sell their product. This can imply certain physical characteristics of the intended user as well as the level of sophistication and dedication. Budgetary concerns are also a big issue. Location can be surprisingly important.



      1. Location: If the user is going to be using the telescope on or near the equator you will likely want a system with an Altitude-Azimuth mount. A lot of consumer equatorial mounts simply do not work at a latitude near the equator – or at the North and South poles. Modifications or special techniques have been (and are) used in order to utilize equatorial mounts near the equator but beware that these may be cumbersome or expensive.
      2. If the user has a limited budget then building a system which has incredible tolerances and precision would result in a price too high for it to be affordable.
      3. If the user is going to have to transport the system then it must be transportable and for most of us that limits the size and weight of the system which we should purchase or build.
      4. The intended use also changes many of the choices which will be made.



    You really must be careful and knowledgeable. The companies which make these systems have to make money in order to stay in business so they make systems designed to appeal to certain markets whether or not the systems will actually suit the intended purpose or user characteristics. It really doesn't help to sell systems if you put into the advertisement that the instrument is wholly unsuitable to certain people or purposes.

    When you are choosing a system you will need to understand a number of parameters of the design and what they mean to you. Let's list a few of them and briefly address why they matter:


    1. Aperture. For our purposes this is generally listed as the diameter in millimeters (or in inches) of the objective lens or the mirror (Typically the biggest piece of glass, mirrored or otherwise). Why does it matter?
      1. The aperture roughly determines the amount of light which is gathered by the system. We amateur astronomers are generally very interested in gathering more light so this light-gathering is of great importance to us.
      2. To a significant degree the aperture determines the potential resolution of the system (how well you can see details and especially how well you can make out two pinpoint stars in the sky despite their appearing to be extremely close to each other visually). There are calculations which roughly estimate the potential resolution based on the aperture. Some will say that aperture is really the only determinant of potential resolution but I contend that in real life the story may be a little more complex.
      3. In good observing conditions the potential resolution is also very related to your highest useful magnification. This is very important to remember because it means that your usable magnification increases with the the aperture. The general Rule-Of-Thumb (ROT) is that most of the time under decent skies your maximum usable magnification is the same as your aperture measured in millimeters.
      4. So if you have a 60mm objective/aperture then most of the time you will have to use 60x or less but if you have a 300mm objective/aperture then 300x will usually be your maximum usable magnification.
      5. When you start to exceed an aperture of about 300mm (an objective with a diameter of more than 300mm) then it starts to get a bit messy. . . Some people contend that here on Earth the atmosphere limits you to only about 300-400x.
        1. In truth, some people are limited by their local conditions to only about 100x whereas some people are taking some really big telescopes up to high altitudes and seem to be able to usefully use as much as 1000x.
        2. Along with high altitudes getting you above quite a lot of the atmosphere, part of it is probably mental/visual trickery to get to those very high magnifications but some of it is also a matter of optical excellence.


    2. Focal length. This is the distance from the objective (mirror or lens) at which the light rays come to a focus (sort of). This is important because:
      1. Depending on the optical design of the telescope this can greatly affect the physical size of the telescope. Let's take a quick look at an example.
        1. For a Newtonian telescope your telescope tube or strut system/frame is likely going to be roughly similar to the focal length. Given the Newtonian design, if your telescope's focal length is 1 meter long it is going to be really difficult to come up with a way to fit it all into a telescope tube which is shorter than 1 meter long. Let's look at some related trickery:
          1. If you look at the focal lengths of some commercially made Dobsonians you will discover that across an aperture range of 6-10 inches all their telescopes have a focal length of 1200mm. This is clearly not done because it is an optically ideal focal length. Choosing a focal length of 1200mm means that the telescopes are ergonomically pretty good for most of us and they are also fairly transportable. These are important design factors for the end-user.
          2. Bird-Jones telescopes. These are often sold as Newtonian reflectors but they use a mirror which is relatively inexpensive to grind/figure and then a lens to correct the optical aberrations and to increase the focal length of the system. If you keep the aperture below about 6 inches the optics can be decent and you end up with an effective focal length which is relatively long despite having a relatively short telescope tube. These sell pretty well and many like the performance (but I am not a fan).


      2. The magnification of a system is calculated by dividing the effective focal length of the telescope by the focal length of the eyepiece. This means that your eyepiece choice will be affected by the focal length of the telescope.
      3. We'll likely work on this a little later in a little more detail, but in astrophotography (AP)
        1. The image scale is determined largely by the focal length of the telescope and the size of the sensor. (Magnification is not really a good term to use in astrophotography.)
        2. If you keep the focal length of the telescope below 700mm you will likely have many more nights of pretty decent imaging. Increase the focal length and atmospheric turbulence tends to rapidly become more of an issue. It is not quite as straightforward as that, but it is still a handy ROT (Rule-Of-Thumb).


    3. Focal ratio. This is calculated by dividing the effective focal length of the telescope by the aperture. This is also (roughly speaking) the photographic speed of your optics. We usually write this as something like F/10 where “F” means”Focal”, “/” means “Ratio”, and “10” means “10” (oddly enough!). So “F/10” reads as “Focal ratio of 10”.
      1. Visually, smaller/faster focal ratios generally mean you are likely to have a wider Field Of View (FOV) and less perceived contrast (more contrast is better). Your eyepiece choices are going to tend toward shorter focal lengths.
      2. Visually, higher/slower focal ratios generally mean you are likely to have a narrower FOV with greater perceived contrast.
      3. So far as almost all amateur astrophotography is concerned the sole optical determinant of the length your exposures will take is the focal ratio. At first this seems counter-intuitive but it is the fact. An illustration?
        1. I have a small (25mm objective lens) optical device/telescope with a focal ratio of 1.5 (F/1.5). The huge Keck telescope has an objective mirror with an aperture measured in meters and with a focal ratio of 1.8 (F/1.8). My tiny telescope can image something like M13 faster than could the Keck telescope if it were using the same camera. However, my tiny telescope will show M13 as a tiny bright smudge whereas the Keck telescope is going to show many thousands of individual stars and other useful details. The point is that for fast photography the optical key is a fast (low) focal ratio.


    4. Image Circle. This is of interest if you are going to do astrophotography. The net effect is that it tells you the diameter of the “focal plane”. If you have a large image circle and are using a small sensor then you are missing much of the light you have gathered (not necessarily a bad thing). If you have a small image circle and a large sensor you will have lots of vignetting (technically a bad thing, but not necessarily a very bad thing).


    OK, we've been looking at the most basic of the optical design parameters. Let's look at just a few of the other things to which you should pay attention while choosing a system. By no means will these be all the points, just some of the major ones to which you should be paying attention. Most are related to what came above in one way or another.


    1. Weight. The best telescope is the one which you will use. If the telescope is too heavy for you to easily move then it won't be used and it is a bad telescope for you. If your telescope is too heavy to be easily be handled by your mount, it is a bad telescope for you.
    2. Length. The length of the telescope matters. If it is too long it will be more difficult to transport and it will require a better (more expensive) mount than will a shorter one. But going too short can sometimes be a problem as well, but I won't go into all of that at the moment.
    3. Type of construction. You can get scopes with almost no opportunity for air circulation and that may (or may not be) a problem depending on the type of telescope. You may get a scope which has a solid tube and that might mean more problems with thermal equilibrium but better control of stray or unwanted light. The material used can matter – one of the intermittent debates I see is whether or not carbon fiber tubes are generally superior (for most purposes I don't think so).
    4. Any provision for a cooling fan may or may not be important depending on the design and size of the telescope.
    5. Is the OTA designed for astrophotography? Obviously, if you don't plan to do AP you will not be too concerned about this, but if nothing else being able to do AP is an asset if you ever choose to sell it. It is very common for a Newtonian telescope to be purchased and the new owner discovers that they cannot achieve prime focus – if the OTA is not advertised as being an “astrograph” you should assume that you will have to buy stuff or modify that Newtonian scope in order to do astrophotography.
    6. Ergonomics. Do think about how you will be looking through the eyepiece (or whatever else you want to do with the system) when it is pointed at various parts of the sky. You actually see better if you are sitting. Long telescopes of whatever design can literally be a pain for some of us oldsters (and a fair number of you youngsters) if aimed at some areas of the sky.


    OK, those are sort of the basic areas of interest.

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    Default Common Telescope Designs

    Some common telescope designs:

    Consider this section to be pretty much an apology. I am not even going to attempt to touch on the vast majority of the designs which are available. I want to just touch somewhat briefly on the most important characteristics of some relatively common designs and why you might want (or not want) that type of telescope.

    I plan to touch on:


    1. Refractors – still the stereotypical telescope.
      1. Objective lens at the front and an eyepiece at the rear which one looks through.
      2. This design is so dominant in general society that it is not uncommon to have people purchase a Newtonian telescope and try to duplicate the orientation of a refractor with utter failure – and then sell that telescope as “barely used” or using some similar phrase because they can't see a thing with it.


    2. Newtonian Reflectors.
      1. Light enters the telescope tube (or frame) and goes down to the base of the telescope where it hits the primary mirror and the light is reflected up to a smaller diagonal mirror which reflects/deflects the light into a focuser on the side of the telescope tube where the eyepiece resides.
      2. These are very popular since you generally get great value.


    3. Schmidt Cassegrain Telescopes (SCTs).
      1. The light goes through a corrector plate into the telescope tube. The light then encounters a spherically ground primary mirror with a hole in the middle. The light is reflected up to the secondary mirror which is held in place by the corrector plate – and then goes back to the center of the primary mirror and through the hole in the primary mirror to the eyepiece.
      2. These put a lot of capability into a relatively small package.
      3. In some sense, this is a design which does almost anything fairly well but is the best at nothing.
      4. This design started as an attempt to make a sort of cheap Maksutov-Cassegrain telescope. Despite its origin as an attempt to build a cheaper Maksutov-Cassegrain that is not all that it is since it has better cooling characteristics and relatively recent optical enhancements (Meade ACF and Celestron EdgeHD) mean the optics can be surprisingly good.


    4. Maksutov-Cassegrain Telescopes (MCTs).
      1. These are frequently referred to as a Mak or as a Mak-Cas.
      2. The light enters the tube through a glass meniscus and then goes back to the primary mirror which sends it up to the secondary mirror (attached to the meniscus) which sends the light back toward the primary mirror and through a hole in the primary mirror to the eyepiece.
      3. The optics are often surprisingly good but achieving thermal equilibrium is relatively difficult in large part because the meniscus is quite thick and acts as a fairly effective insulator.


    5. A whole lot of other designs will be largely ignored. Maybe I should cover the Ritchey-Chretien but people interested in one will likely have already done a lot of research. I'm going to ignore things like the CDKs, Schiefspiegler, Chiefspiegler, Schmidt-Newtonians, etc. not because they are not good or great but because most of us aren't considering getting one.


    I should also note that there is no design which is free of optical aberration(s). Some of the things you must look out for are field curvature, chromatic aberration, spherical aberration, astigmatism, coma, etc.

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    Default Refractor Factors

    Refractor factors:I'm going to touch on refractors first because they were developed first. These puppies have a long history! I'm going to just touch on a few general design variants and what they can do for you – and what you want to look for:

    1. Singlets. You aren't going to buy one of these but I think it is worth touching on these because this is where refractors started and the problems they have either persist or dictate some design features in the most modern of refractors.
      1. Old design.
      2. Advantages:
        1. Inexpensive because it requires only a single piece of glass to make the objective.
        2. Can be relatively lightweight because it requires only a single piece of glass to make the objective. In reality the tube will be relatively long and this will add weight

      3. The problem is that different wavelengths of light refract somewhat differently and this means that different wavelengths of light will focus at different distances from the objective lens. The result is that if you focus one wavelength of light then all the rest of the light you have captured is not focused and you get fringes of color visible around bright objects due to the chromatic aberration – and less detail/contrast.
      4. A partial fix for the chromatic aberration? Make the focal ratio very high (slow optics). When you do this the nature of the optics is to bring the wavelengths to a focus at something closer to the same point/plane and thus minimize the color fringing. This also tends to diminish the issues with field curvature. Unfortunately, this also means a very long telescope tube and eyepieces which have a relatively long focal length (not so easy to find/afford).

    2. Doublet achromats. These are made by using two piece of different kinds of glass to improve the control of different wavelengths of light. You may very reasonably choose to get one of these.
      1. Fairly old design.
      2. Advantages:
        1. Fairly inexpensive since the glass types are pretty commonly available and inexpensive. Since there are only two lenses the manufacturing process is not overly expensive.
        2. Despite the inexpensive nature you can really have pretty good optics.
          1. As with the singlets you generally have better focusing of the various wavelengths if you use a higher focal ratio. If you make an achromat long enough it will compete with much more expensive telescopes.
          2. On the relatively dim DSOs you may not notice any color fringing at all, but even though you may not be bothered – the image you are seeing is still not as sharp as it should be because some of the light you have gathered just isn't going exactly where you want it to go.

        3. Regarding use for astrophotography?
          1. If you are using a One-Shot-Color camera (like a DSLR) you will have a whole lot of problems with this design – but with extra work you may be satisfied with your images.
          2. If you are using a monochrome camera you can still do some superb imaging by using narrowband filters and then combining the images. You just have to re-focus with each filter change and you will be competing with (and maybe beating) the performance of far more expensive telescopes.





    3. ED-Doublets. These are effectively achromats with one of the glass types being an Extra-low Dispersion glass.
      1. These are frequently call “apochromats” by those who sell them, but IMHO almost none of them should be considered apochomatic (the possible exception being an ED-doublet which was made with Fluorite and Lanthanum). An apochromat should have essentially no detectable false color or fringing both visually and in images. An ED-Doublet just doesn't quite get there. I think, however, that the term “near apochromat” or the term “semi-apochromat” can apply as some of them are very, very good.
      2. Seriously, these are very good or superb for visual use and pretty good for imaging as well. If I knew I would not do any astrophotography with the scope and had the choice between a good ED-doublet and a triplet apochromat I would take the ED-doublet because it will be lighter, achieving thermal equilibrium will likely be faster, and visually I probably would not be able to tell difference between the optical quality on visual targets.
      3. Still struggle with color just a little. . .

    4. Triplet apochromats. These use three lenses to make up the three-element objective group and at least one of the lenses will be made of ED glass.
      1. You will have superb color control with a good triplet apochromats although a poorly designed one will be worse than the best of the ED-Doublets.
      2. Cool-down (achieving thermal equilibrium) is more of a problem due to having three lenses to insulate the system, but it really isn't a problem for most of us.
      3. Why do you want one?
        1. The modern triplet apochromat mostly exists because of the interest in AP.
          1. Remember how earlier I mentioned that the faster the optics (smaller focal ratio) the shorter your image acquisition times will be? Well, those faster optics also mean that color is harder to control so our optical experts figured out how to have relatively fast optics and still have super color control – and that took three (or more) lenses with carefully chosen glass types.
          2. Since the optics are relatively fast the telescope tube is also relatively short and that makes for a surprisingly compact and portable instrument. That compact instrument also means relatively little demand on the mount for better/reliable tracking and better imaging.
          3. Your typical triplet apochromat will be manufactured to better tolerances and is likely to have a more robust focuser. This is very important in order to maintain proper alignment from objective to camera.

        2. The contrast is wonderful and all the light is going exactly where it needs to go for a wonderful visual experience. You can also push the magnification higher than you can with other designs.

      4. Why you don't want one?. . .
        1. Again, cool-down is a little worse than if you were getting a doublet design.
        2. All that glass and design work means greater expense. It just may not be worth it to you.


    5. Other designs. There are quadruplets and such out there with special design features. They are typically superb instruments but typically take longer to reach thermal equilibrium and I'm going to leave it at that for now.


    Some more general thoughts which may be relevant:


    1. If you are doing imaging with a refractor you should consider using a field flattener to take care of that field curvature problem.
      1. Field curvature is a common issue and you will want the field flattener with a triplet apochromat, an ED-Doublet, etc.
      2. If you are interested only in the center of your image the importance of field curvature diminishes. However, I'd note that centering a target in your image is often the worst way to do the compose/frame your image.
      3. Some manufacturers are making telescopes with a built-in field flattener.
      4. The field curvature in a refractor is generally a function of the focal length. This means that your field flattener needs to be designed for a telescope with roughly the focal length of your telescope.

    2. A refractor has no central obstruction.
      1. The lack of a central obstruction means you do not have the diffraction which a central obstruction will cause.
      2. The lack of a central obstruction means you are not blocking any of the light which should be entering your telescope. This means that a good apochromat will have a brighter image than will a telescope of another common design with the same aperture. There are uncommon designs where this is not necessarily the case.

    3. A refractor with modern coatings has essentially no reflective loss and relatively little scatter.
      1. Most of the modern telescopes which use a mirror will be reflecting only about 90% to maybe 96% of the light which hit it. This means significant light loss with each mirror and consequent dimming of the image. Dielectric coatings have minimal reflective loss and may become more available and popular – I currently see this only on smaller mirrors.
      2. Loss due to light scattering by the mirror can be up to 20%. You also need to realize that this scattered light is not only lost to the eyepiece, it is also doing a little bouncing around in the telescope tube and reducing the contrast of the image. Flocking will reduce the contrast loss. Dielectric coatings may also reduce the scattering but this is, again, not yet commonly used.

    4. As the aperture of a good refractor increases the cost starts to skyrocket. Optically the refractor is superior up to an objective size of about 1 meter but most of us find a triplet apochromat with an objective size of more than about 4-5 inches to be impractical.
    5. As the aperture of the refractor increases the length of the telescope tube tends to also increase. This makes it more difficult to handle and to mount. Even if you get a big refractor onto a mount at the site where you want use it – you could have it hitting stuff in certain circumstances. This may or may not be an issue for you.
    6. Thermal equilibrium is a minimal issue for most of us with smaller refractors. Don't believe me? Remember that your binocular is effectively a modified refractor. A spotting scope is also a type of refractor. We don't generally do cool-down before using either a binocular or a spotting scope.
    7. Dew shields are frequently built into the design of a refractor. This both reduces the problems with dew formation and the problems with ambient light.
    8. If you are looking for a small refractor with some of the best optics for a relatively low price – you need to look at spotting scopes. Spotting scopes are made for a much larger market of bird-watchers and other terrestrial users and can use economies of scale to reduce the cost of great optics in a robust package. They also have lots of competition and this results in surprisingly good optics for a relatively low cost. This is value!
    9. One other note which has caused much frustration for many? The focuser of the typical refractor is configured so that if you are using an eyepiece you must use a diagonal and if you are using a typical camera you must NOT use a diagonal in order achieve focus. Remember this and you will likely have no problem.
    Last edited by OleCuss; 06-25-2017 at 01:15 PM.

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    Default Newtonian Reflectors

    Newtonian Reflectors:

    The Newtonian reflector has been around for a long time having been developed by Sir Isaac Newton.

    A Newtonian telescope should typically have light enter the telescope tube and travel down to the primary mirror (the objective), reflect back up to the diagonal mirror and out to the eyepiece (which is ensconced in the focuser).

    It is generally preferable that the primary mirror be figured to a parabolic curve in order to avoid spherical aberration. However, with small mirrors which have relatively high focal ratios the spherical aberration may not be a significant issue.


    1. Newtonian reflector advantages:
      1. Great value. You can generally get more light-gathering for less money with this design since the design is relatively simple and robust. Did anyone mention that Sir Isaac Newton was a genius?
      2. Pretty good optics. Especially if you keep the optics slow (higher focal ratio) you can have very good optical quality for the price. At higher focal ratios you can get some really good contrast and the diagonal mirror can be smaller to minimize the central obstruction.


    2. Disadvantages:
      1. Need for thermal equilibrium. This is often referred to as “cool-down” because most of us are taking a scope from a warmer area to a cooler area – but taking a scope from a cool area to a warmer area causes very similar issues. Until the mirror and air inside the telescope tube have become nearly the same as the ambient temperature you will have tube currents playing havoc with the view/image. There are some things which can be done to minimize this:
        1. Design choices:
          1. Use a truss design rather than a solid tube. This means less air is trapped and cool-down is more rapid. There is a drawback, however, as you will likely to be bothered more by ambient light pollution. The ambient light issue diminishes if you use a shroud, but then your cool-down is not as good.
          2. Choosing a telescope tube which allows at least ˝-inch of space between the mirror and the telescope tube will likely help with cooling.
          3. Use a cooling fan. Computer fans are often used to speed the achievement of thermal equilibrium.
          4. Choose a mirror cell which is well-ventilated. This would seem obvious but it hasn't been for some. . .
            • I once purchased a 13.1-inch Coulter Odyssey telescope. I bought it only because I wanted the mirror as a cheap mirror blank. The design was absolutely horrible with virtually no provision for mirror cooling – and beyond the cooling problems merely collimating it would cause distortion of the mirror. I got it cheap and only wanted the mirror so I got exactly what I wanted, but if I'd wanted the entire scope it would have been a pretty bad choice.



        2. Material choices:
          1. Pyrex mirrors are less affected by temperature changes than are the more typical glass choices. I'm not sure, however, that this makes that much of a difference. The bigger problem for most of us is likely turbulence within the telescope tube and this means the mirror is a heat source and using a cooling system to blow away the heat emitted by the mirror is probably more important then getting a Pyrex mirror.
          2. A mirror with materials which are designed to dissipate heat rapidly will more rapidly result in a mirror which is at ambient temperature and is no longer heating the air inside the tube and causing tube currents. Hubble Optics is using their “sandwich mirror” to help with this as is Optic Wave Laboratories with their cellular blanks: Optic Wave Laboratories - Products (not sure how available they are at this time).


        3. Keep your telescope at ambient temperature all the time. Storing in a garage or outbuilding may keep your telescope at nearly the ambient temperature. Some folk will air condition their observatory to the temperature they expect to have when they start observing.
        4. Choose a small scope. Smaller mirrors tend to achieve ambient temperature more quickly.


      2. Central obstruction. (That diagonal mirror is blocking the center of the path which the light is using to enter your telescope.)
        1. Having a central obstruction causes diffraction. This degrades/blurs the view.
        2. The central obstruction decreases the amount of light which gets to your eye.


      3. Diffraction due to the “spider”. Something has to hold the diagonal mirror in place and this device usually takes the form of a 4-vane “spider”. This causes diffraction spikes and degrades the image/view although some of us rather like the diffraction spikes in some images.
        1. Some fixes?:
          1. The prominence of the diffraction spikes can be diminished by making and using a wire spider. This approach has the advantage of actually decreasing the overall amount of diffraction.
          2. You can get or make a curved spider to diminish the prominence of the diffraction spikes. You may or may not get a decrease in the overall amount of diffraction with this approach. If you have no actual decrease in the amount of diffraction then you change the diffraction from spikes to an overall blurring of the view/image.
          3. You can make an optical window and use it to hold the diagonal mirror. This will have the effect of eliminating the spider and the diffraction due to that. There are some issues with doing this:
            • I don't think such optical windows are readily available so you will have to buy the right glass, grind and polish the glass, have the glass coated, and bore a hole in the glass in the right spot for the diagonal mirror.
            • This approach can be expected to slow your cool-down.
            • Some loss of light. I think this would be negligible.
            • Since your diagonal mirror is likely to be right at the very end of the telescope tube any ambient light sources are likely to be a big problem. Attaching a light shield to extend the tube will reduce or negate this issue.



      4. Collimation.
        1. Newtonians should be assumed to need collimation every time they are used. This means making all of your optical components align properly. While you should assume you will have to do it every time, with gentle handling you may not have to collimate often – and after a few times you won't find it to be much of a bother anyway.


      5. Size. As you get bigger telescopes the instruments can become very difficult to manage. Well, this is true with almost any telescope design but I toss it in as a disadvantage both as a warning that if you get a big Newtonian you may not be able to use it – and as an opportunity to discuss how to minimize the issue.
        1. Get a truss-tube design. Seriously, they make some bizarre-looking ultra-lightweight designs where if you can lift the mirror you are highly likely to be able to set up and take down that telescope.
          1. The lightest designs have been more expensive. This may be changing.
          2. You may want to use a light-shroud to decrease the effect of ambient light. In a very dark site this may not be enough of an issue to matter.
          3. It can take a lot longer to set up a truss-tube OTA than it may take you to set up a solid-tube Newtonian OTA. This really may not be an issue, however, since your time setting up the OTA may also be cool-down time for the mirror.
          4. It has been over one year since I last collimated my solid-tube Newtonian – I treat it gently and it star-tests beautifully each time I take it out. My truss-tube OTA has to be collimated every time I take it out. The collimation is not a big deal but it must be done and that adds a couple of minutes to the set-up time.


        2. Just leave it set up for use.
          1. If your area is secure you may be able to just put a shroud over your scope and leave it right where you want it.
          2. You can make or use some sort of observatory.


        3. Have someone else set it up for you. . .


      6. Ergonomic issues. When they get big they can be difficult to just look through. Ladders, chairs, platforms, mount choice, etc. can help.
      7. Coma. An optical aberration which means that nearer the edges of your FOV you get something a little like a cometary tail on your stars. I find this intolerable with standard eyepiece at F/5 or faster, bothersome at F/6, and no bother at all at F/8.
        1. Go with slower optics and coma is hardly an issue. This tends to also narrow your typical FOV with the scope – this is a problem on some larger targets and no problem at all on smaller targets.
        2. Use premium eyepieces and you can minimize the issue. This will cost you money. . .
        3. Get a good coma corrector. The best cost quite a bit of money but yield a great result.
        4. Do nothing. Some people just aren't bothered by coma.



    Watch out for design choices which you might not understand. If you look at the focal lengths of the typical commercially made Dobsonian telescopes with 6-inch, 8-inch, and 10-inch apertures you will find that they all have a focal length of 1200mm. This appears to be chosen for ergonomic accessibility, transport ease, and to keep the size from being intimidating. It is not done for optical excellence. Make sure the design choices match your own priorities fairly well.

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    Default Maksutov-Cassegrain Telescopes (MCTs)

    Maksutov-Cassegrain Telescopes (MCTs)


    1. Why you might want an MCT:
      1. MCTs tend to fit surprisingly great views with a long focal length into a relatively small package. Focal ratios in the range of F/13 are not uncommon.
        1. That relatively long focal length means that relatively inexpensive eyepieces work quite nicely and the really good eyepieces give amazing views.
        2. The compact size helps with the weight and transportability.

      2. No collimation is necessary. Well ordinarily you don't have to collimate them but sometimes strange things happen and people have either collimated their MCT or sent it to the manufacturer to be collimated. I'd guess, however, that the vast majority of owners will never need to collimate their MCT.
      3. Quite good for Astrophotography of the brighter planets. That relatively long focal length is an advantage. Hook up a video camera for "lucky imaging" and you can do some good stuff.


    2. Why you might not want an MCT:
      1. That relatively long focal length means relatively narrow Fields of View. If you are wanting to view small targets that is no problem at all. If your target is large you may only be able to see a small portion of the target.
      2. Cool-down is disproportionately long for the size of the instrument. With the larger MCTs this can be bad enough that you don't really reach thermal equilibrium until the environment starts to warm up again in the morning!
        1. Again, you can store the instrument at the temperature you expect to encounter when you will begin observing and make this a relatively minor problem.
        2. You can get a "CAT cooler" from Lymax. This device will circulate the cooler ambient air through the scope and give you much faster cooling.
        3. Stick with a smaller MCT - they cool faster.
        4. Be incredibly patient!

      3. Generally a bad choice for Deep-Sky imaging. Deep Sky Objects (DSOs) tend to be dim and as a result we usually want fast optics and long exposures. MCTs have slow optics and that can mean your exposure time may be unacceptably long.

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    Default Re: A Brief Guide to Choosing Equipment for Astronomy. Telescopes, Mounts, Eyepieces

    Schmidt-Cassegrain Telescopes (SCTs)


    1. Why you might want an SCT?
      1. A good SCT can do a lot of things pretty well.
        • Visual observing – SCTs tend to have slow optics meaning that they have a relatively large focal ratio (focal length is relatively long compared to the aperture of the instrument). These slow optics mean that relatively inexpensive eyepieces work nicely and the perceived contrast is usually pretty good.





        • Astrophotography - There have been a lot of very good astrophotographs made with SCTs – and there will be in the future as well.
          • Very good for planetary astrophotography. Doing “lucky imaging” with the relatively long focal length of the SCTs can give you a pleasing image scale and a lot of detail. The optical aberrations of the SCTs tend to be closer to the periphery which means that they do not interfere with imaging of those tiny planets. Some of the best planetary imaging is being done by folk like Christopher Go using an SCT: http://astro.christone.net/
          • Pretty good at imaging DSOs (Deep Sky Objects). There are a lot of targets which can be imaged very nicely with an SCT. Given that a lot of DSOs are relatively large and dim it is common to do this type of astrophotography using a combination focal reducer/coma corrector.
            • There have been a number of SCT variants which should probably mentioned in this regard.
              • For a time Meade made an F/6.3 variant. This was a very nice idea for astrophotography since the faster optics meant images could be acquired twice as fast.
                • Some F/6.3 scopes were reportedly pretty good – and others were reportedly miserable. If you consider buying one make sure you look through it first to assure yourself that it is one of the examples with good optics.
                • Most of us don't want the F/6.3 version because the central obstruction is relatively large and causes more diffraction. We'd rather get the typical F/10 SCT and use a focal reducer for optics which are effectively just as fast and with less diffraction.

              • Both Meade and Celestron have been making ACF and EdgeHD variants which are designed to correct much of the optical aberration(s) found in the typical SCT. It is my understanding that the optics are very good.
              • Meade is now making F/8 SCTs with ACF optics. These are obviously intended for the
              • astrophotography market and I've seen some images made with them which were very good, indeed.

            • Celestron SCTs with “Fastar” capability. It is important in this regard to note that the primary mirror of the SCT is spherically figured and typically has a focal ratio of about F/2. For a while Celestron made the Fastar which replaced your secondary mirror and allowed you to then attach your camera and do imaging at about F/2 for very fast and relatively wide-field astrophotography. Celestron no longer makes the Fastar but you can get the Hyperstar from Starizona to do that job for you. You need to be careful about camera selection and pay attention to the image circle size, but there are a lot of folk who are delighted at what they can do with a Hyperstar! I believe some Hyperstars were made for Meade SCTs but are no longer available.





      1. Inexpensive. The SCT is effectively an attempt to make a relatively inexpensive Maksutov-Cassegrain telescope. It took some work to figure out how to make the corrector plate, but they got it figured out and these have become some of the more popular telescopes in amateur astronomy.
      2. Portable. Because of the folded light path, the SCT is usually a relatively compact and light-weight instrument considering its aperture.
      3. Accessory availability. The SCTs are very popular and that means lots of accessories are available.
      4. Cool-down is not quite as immediately necessary as with some other designs. The corrector plate makes this an enclosed system and serves to both reduce the initial need for thermal equilibrium with the environment – and slows the achievement of good thermal equilibrium (the CAT/SCT cooler can help).

    1. Why you might not want to get a SCT.
      1. Not the best at anything. Seriously, you can argue that the SCT is not the best for any particular task in amateur astronomy. You can nearly always find something better for an individual task/use. The SCT can do almost anything, however, and do it pretty darned well and that is why so many of us think it is the best telescope design for our purposes.
      2. You can get a Dobsonian-style Newtonian with all the same visual potential for less money. If you really want inexpensive there is probably no design which gives you so much for so little money as does the Dobsonian. Do remember, however, that you can sometimes get an SCT on the used market which is in great shape and surprisingly inexpensive.
      3. Problems with dewing. The SCT's corrector plate is right out there in the breeze and is famous for dewing over and thus ruining your view. Dew shields help and you can also get or make devices to do a little heating of the corrector plate to minimize this. A dew shield also blocks a lot of extraneous light and is a good idea for use with most telescopes.
      4. Again, it has issues with optical aberrations and the older ones have more of an issue with this. If you are not doing astrophotography the aberrations may not be particularly bothersome to you. EdgeHD and ACF optics will largely correct these issues.
      5. Cool-down. An SCT can take quite a while to come into thermal equilibrium with the surroundings. Most folk can set the scope up and let it achieve thermal equilibrium as part of their routine and just not find this to be a problem. And, as with the Maksutov-Cassegrain scopes you can get an SCT cooler and speed up the process.
      6. Collimation. I have an SCT which is about 11 years old and has not yet required re-collimation. I have another SCT which was badly out of collimation when I purchased it (maybe why I got it inexpensively?) but after I collimated the thing it has required no further work. Net effect is that you may need to collimate an SCT on an occasional basis, but it is doable and will likely not be something you must do often.


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