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Thread: Choice of mounts

  1. #11
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    Quote Originally Posted by AustinPSD View Post

    Poor to mediocre choices include Newtonian reflectors - while a very few are designed to support imaging, most are not and can lead to difficulty, frustration, and poor results.
    I agree with what Austin has said apart from the above statement. There has been many good images taken with Newtonian reflectors. It takes a different way of thinking to use a reflector properly. And to really get into it, an SCT, an RC or a Mak are variations on the reflector telescope, using a primary mirror to collect and focus the incoming light rays into a cone. All the big professional scopes are an RC reflecting design, including Hubble.
    I will now go back to my corner.

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    In the interest of completeness of thought, and removal of bias, I'll expand a bit on Decepticon's legitimate comment/exception...

    First and foremost, there have been many, many excellent images acquired using Newtonian reflectors. And, as indicated, the catadioptric (folded optics) scopes like SCT's, Maks, and RC's are reflector variants and are probably among the most common designs for imaging use.


    Having said that, (and you'll hopefully find this my consistently stated opinion in other, related threads), my bias is against using a Newtonian reflector for imaging. The "why":

    - as to one specific type of Newt, the Dobsonian, aka. Dob: There are two issues here, one is the alt-az mount, which generally lacks tracking and guiding capability, and can't be polar aligned, so absent field de-rotatating capability the image field will suffer rotational effect. There are opto-mechamical, and optical considerations I'll address in the next point shared with all Newts. For completeness, it is possible to augment a Dob with tracking/guiding capability (i.e. ServoCat/ArgoNavis), as well as adapt it for polar alignment with an equatorial platform, or use a field de-rotator. One would still need to address the optic/opto-mechanical issues next described.

    - shared among all Newts are a small collection of optical, and opto-mechanical issues. The optics issue first, as it is simpler to address or avoid (sometimes): The faster the scope (more extreme primary mirror curvature) the more prone the scope is toward coma aberration. This shows up off-axis, in the edge field as "coma, or comet-shaped" stars. This can be avoided for the most part by choosing a Newt with a high-quality slow or moderate focal ratio, above f/4. Below f/4, a coma corrector (Paracorr) can be used to alleviate the coma defect.

    The opto-mechanical issues: most Newts, unless specifically designed for imaging, lack sufficient inward focuser travel, and total back focus distance for supporting arbitrary imaging/optical train configurations. The inward focuser travel issue comes about from the need to avoid interior OTA obstruction, and/or potential contact with the secondary mirror/mirror support, along with possible vignetting. The total back-focus distance issue relates to the focuser height, draw-tube travel, and distance from the secondary mirror and field stop to the imaging plane. The two issues taken together can sometimes prevent one from achieving fine focus with an imaging camera, or achieving focus at all. Most Newts have total back-focus distances of under 5". In contrast, an SCT, RC or Mak/Mak-Cas will have back-focus distance of at least 5", usually 6" for a 200mm aperture scope, and even longer for larger aperture instruments.

    In some cases this can be addressed via use of an aftermarket low-profile focuser. There are also more extreme modifications possible, for example moving the primary or secondary mirrors, etc. but these fundamentally alter the OTA and optical spec of the instrument.

    - lastly, mechanical/mount related issues... this collection is a result of the mechanical design of the Newt's OTA, and the characteristics of how it is mounted. First, the focuser assembly is located at the "wrong end" of the OTA, where it is usually elevated between the zenith and some number of degrees above the horizon. It is also located on the side of the OTA, off-axis from the tube centerline. When hanging an imager, related stuff like focal reducers, filter wheels, and of course cabling and related cabling drag, all this junk hanging off the side and wrong end of the tube leads to problems. The problems are increased mechanical moment that the mount drives must overcome during slewing, braking, and tracking, increased momentum that affect slewing and de-slew/braking, and tracking corrections. All things being equal, this imposes an additional workload, sources of error, and balance problems for the mount - in contrast to an SCT, RC, Mak, or refractor.

    As a final point of fact - there are fine imaging Newts available. Some are designed from the start for imaging, and address the aforementioned optical and opto-mechanical issues. They don't address the mount/mount workload mechanical issues, but none the less are fine instruments for imaging.

    With all that, I most often simplify when giving an opinion regarding choice of optical tube for imaging - and cast the Newt into the "poor / mediocre" choice bucket.
    Last edited by AustinPSD; 01-07-2010 at 06:41 PM. Reason: typo
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  4. #13
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    Quote Originally Posted by ottUp View Post
    To put some numbers to the issue of the sensitivity of the eye vs a camera:

    Camera sensitivity is measured as QE (Quantum Efficiency), which is the percentage of arriving photons that are detected and converted to a useful signal. My CCD camera - a fairly typical one - has a QE of 65%. The human eye has been ... measured as having sensitivity equivalent to a QE of about 5%. So just taking sensitivity into account, the CCD is many times more sensitive - then you add the additional considerations of long exposure times, noise sensitivity, etc.

    "See saturn's rings for less than the cost of a car"? A lot less - more like the cost of your pub tab for 2 weeks. However, if your wife was satisfied with the "car price" threshold, perhaps not correcting her downward any more would be a good strategy....

    - Richard

    What I was referring to in terms of the eye being more sensitive than a camera is the contrast range, enabling immediate perception in low contrast (dark) situations. As I recall, most cameras will start to creak a bit beyond a range of five or six stops within a single FOV, whereas the eye can perceive perhaps 15 stops of contrast. This seems to me to mean the eye will appreciate more at a glance that a camera where light is very low, but of course the camera is more sensitive in absolute terms for a (single) given light level, and has the retention/persistence of long exposure on its side.
    I guess its another good reason for stacking many shots to produce and image - that its possible to manipulate a wide contrast FOV to reveal more detail that would be burned out or too faint if part of one shot.

    Regards

    SS

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    Quote Originally Posted by AustinPSD View Post
    With all that, I most often simplify when giving an opinion regarding choice of optical tube for imaging - and cast the Newt into the "poor / mediocre" choice bucket.
    I've been overwhelmed with folk's generosity of time and knowledge, but I am grateful too for a simple opinion, which your approach provides, so many thanks.

    In so many conversations on widely varying forums I've noticed people can be loathe to give opinions because it might be taken to be "advice" (ooooh!). As you can see from my ID info I'm in the UK, and Brits avoid advice seeing it as a dangerous thing, . I guess the thing with advice is, its dangerous to ask for as it might be offered, and dangerous to offer as it might be accepted.

    Have a great day (that's my advice..)

    SS

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    Quote Originally Posted by AustinPSD View Post
    In the interest of completeness of thought, and removal of bias, I'll expand a bit on Decepticon's legitimate comment/exception...

    First and foremost, there have been many, many excellent images acquired using Newtonian reflectors. And, as indicated, the catadioptric (folded optics) scopes like SCT's, Maks, and RC's are reflector variants and are probably among the most common designs for imaging use.


    Having said that, (and you'll hopefully find this my consistently stated opinion in other, related threads), my bias is against using a Newtonian reflector for imaging. The "why":

    - as to one specific type of Newt, the Dobsonian, aka. Dob: There are two issues here, one is the alt-az mount, which generally lacks tracking and guiding capability, and can't be polar aligned, so absent field de-rotatating capability the image field will suffer rotational effect. There are opto-mechamical, and optical considerations I'll address in the next point shared with all Newts. For completeness, it is possible to augment a Dob with tracking/guiding capability (i.e. ServoCat/ArgoNavis), as well as adapt it for polar alignment with an equatorial platform, or use a field de-rotator. One would still need to address the optic/opto-mechanical issues next described.

    This is a fault with the mounting of the optical tube and not the design of the optical tube itself.


    - shared among all Newts are a small collection of optical, and opto-mechanical issues. The optics issue first, as it is simpler to address or avoid (sometimes): The faster the scope (more extreme primary mirror curvature) the more prone the scope is toward coma aberration. This shows up off-axis, in the edge field as "coma, or comet-shaped" stars. This can be avoided for the most part by choosing a Newt with a high-quality slow or moderate focal ratio, above f/4. Below f/4, a coma corrector (Paracorr) can be used to alleviate the coma defect.

    Refractors are prone to this problem as wel,l and as with Newt's, the faster the F ratio, the worse it can become. Apo's can be reasonably bad due to the grind of the lenses to bring all the colours to one focal point, reducing the major fault that all refractors suffer from-chromatic abberation. I find it easier to deal with coma alone than struggle with false colour and coma.

    The opto-mechanical issues: most Newts, unless specifically designed for imaging, lack sufficient inward focuser travel, and total back focus distance for supporting arbitrary imaging/optical train configurations. The inward focuser travel issue comes about from the need to avoid interior OTA obstruction, and/or potential contact with the secondary mirror/mirror support, along with possible vignetting. The total back-focus distance issue relates to the focuser height, draw-tube travel, and distance from the secondary mirror and field stop to the imaging plane. The two issues taken together can sometimes prevent one from achieving fine focus with an imaging camera, or achieving focus at all. Most Newts have total back-focus distances of under 5". In contrast, an SCT, RC or Mak/Mak-Cas will have back-focus distance of at least 5", usually 6" for a 200mm aperture scope, and even longer for larger aperture instruments.In some cases this can be addressed via use of an aftermarket low-profile focuser. There are also more extreme modifications possible, for example moving the primary or secondary mirrors, etc. but these fundamentally alter the OTA and optical spec of the instrument.

    I have never seen or experienced this problem with any Newt's Ive had, and in My Deep Space Imaging group of our Astro Society, nobody elese has had a problem either. Thi is consistant with DSLR's, DSI/DSI Pro's, or Astro dedicated CCD cameras. More prevalant to this is the need for most of our groups refractors(and SCT, Mak's and RC's) to require extension tubes to bring the camera out far enough as the focuser has not enough out travel.

    - lastly, mechanical/mount related issues... this collection is a result of the mechanical design of the Newt's OTA, and the characteristics of how it is mounted. First, the focuser assembly is located at the "wrong end" of the OTA, where it is usually elevated between the zenith and some number of degrees above the horizon. It is also located on the side of the OTA, off-axis from the tube centerline. When hanging an imager, related stuff like focal reducers, filter wheels, and of course cabling and related cabling drag, all this junk hanging off the side and wrong end of the tube leads to problems. The problems are increased mechanical moment that the mount drives must overcome during slewing, braking, and tracking, increased momentum that affect slewing and de-slew/braking, and tracking corrections. All things being equal, this imposes an additional workload, sources of error, and balance problems for the mount - in contrast to an SCT, RC, Mak, or refractor.

    You balance everything else when you set up a scope on the mount, why wouldn't you balance the tube to offset the weight of your imaging gear and then balance the mount? Makes sense to me!!

    As a final point of fact - there are fine imaging Newts available. Some are designed from the start for imaging, and address the aforementioned optical and opto-mechanical issues. They don't address the mount/mount workload mechanical issues, but none the less are fine instruments for imaging.

    With all that, I most often simplify when giving an opinion regarding choice of optical tube for imaging - and cast the Newt into the "poor / mediocre" choice bucket.

    As I stated previously, it takes a different way of thinking to image with a Newt. Please don't try to make people believe that refractors and catadioptric are the only way to image. There are just as many pros and cons for all the different types of optical systems, it is unfair to the greater majority of forum users to label a Newtonian reflector as a "poor/mediocre choice for imaging"



  7. #16
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    In the continuing theme of completeness of thought, and bias removal...

    I'll first reinforce that I think Decepticon raises some excellent, legitimate points. I am not at all out to convince folks that Newts are out for imaging, or the catadioptric or refractors are the only option. I so state unequivocally.

    Since this thread has drifted a bit from its original query (mount choice), I'll continue a bit in the drift and raise some issues re. other optics choices.

    Folded optical path scopes, aka. catadioptric instruments include Schmidt-Cassegrains (SCT), Maksutov and Maksutov-Cassegrain (Mak/Mak-Cas) variants, Ricthey-Chretien (RC), Dall-Kirkham and Corrected Dall-Kirkham (DK / CDK) in the mainstream, along with other variants.

    All of these in some regard are variants of a Newtonian reflector, using one or more mirrors in a "folded" optical path configuration.

    One of the significant benefits is the resultant more "compact" instrument, again similar to a basic Newtonian reflector (as compared to the physical length of an equivalent focal length refractor).

    The theme with optical design of most catadioptric instruments is to find appropriate corrections for basic optical aberration, including:

    - coma, which worsens with "fast" mirrors, typically below focal ratios of f/4
    - chromatic aberration
    - astigmatism
    - spherical aberration
    - field curvature and other off-axis aberration

    In fact, catadioptric is a derived term, from "kata", meaning against, and "dioptric", meaning refractive. These telescope designs use a combination of reflective and refractive elements to "cancel out" many aberrations.

    A basic SCT uses a front "Schmidt" corrector plate at the open/pointing end of the telescope. A secondary hyperbolic mirror is installed in the center of the corrector plate. The main parabolic mirror resides at the rear cell of the instrument, mounted on a system that allows the mirror to move linearly in the telescope tube to achieve focus. The main mirror is perforated, with a hole in its center to allow the main light cone to exit to a tube that usually contains the eyepiece, perhaps an imager, secondary focuser, or other instrument.

    An RC is similar, with the exception that it uses a hyperbolic main mirror is fixed in the rear cell. A draw-tube focuser, typically with long travel and extended back-focus distance is used to achieve both coarse and fine focus.

    The Mak uses a meniscus-shaped corrector plate and a parabolic main mirror, and is similar in optical configuration to a standard Newt. These are sometimes called "Mak-Newts.

    A Mak-Cass uses a movable main mirror, as in the SCT, along with a secondary mirror and meniscus-shaped corrector plate.

    The DK/CDK use an elliptical main mirror, and a spherical secondary mirror. The CDK uses a pair of lens elements after the secondary to correct and flatten the optical field.

    In general, these instruments represent a compromise between a pure Newtonian reflector in terms of their optical properties, chiefly image contrast, and cost/length/weight as compared to a refractor of equivalent focal length and aperture.

    Like a Newt, these instruments all share the central obstruction created by the secondary mirror. Some, like the RC may have central obstruction area ratios slightly higher than other reflectors/catadioptric scopes because the spherical mirror may be deliberately over-sized to reduce potential field edge vignetting.

    While a catadioptric instrument may be "fast" in terms of its focal ratio, most are in the f/10 range, and in some cases higher than f/15 (Dall-Kirkham). This results in reduced contrast for a visual observer, as compared to a basic Newt or Dob. Many are able to be equipped with focal reducers, typical standard reducers include f/6.3 and f/3.3, and in some specialized cases f/2 (Celestron Fastar, now discontinued, or the Starizona Hyperstar for both Celestron and Meade instruments). Some specialized variants are very fast, below f/2.

    In addition to improving visual contrast, a focal reducer (which may also integrate field flatteners / correctors) increases the effective field of view when used with an instrument. They can also help reduce exposure times in imaging applications.

    To recap some important points (drawbacks) at this juncture:

    - catadioptric instruments may exhibit reduced image contrast for visual observers
    - most catadioptric instruments need a corrector when used in imaging applications to correct spherical distortion and field curvature (a field flattener)
    - moving main mirror instruments (SCT's, Mak-Cass) can suffer from image shift due to mirror de-spacing, or mirror movement during use

    From an opto-mechanical perspective, most catadioptric scopes are back-heavy. The visual and imaging components in the optical chain mount on the telescope's rear cell, and may include, but are not limited to:

    - a "visual back"
    - a single or multi-speed manual or motorized focuser
    - a focal reducer
    - for visual use, an eyepiece and/or binoviewer
    - for imaging use, a DSLR, CCD, or video imager
    - for mono-chrome CCD cameras and/or narrow-band imaging, a filter wheel
    - various adapter tubes and spacers
    - an off-axis guider for in-path manual or auto-guiding
    - an instrument like a spectrograph, interferometric camera, etc.

    For imaging purposes, it is not uncommon to see a relatively high equipment load, as well as a long, linear chain of components hanging off the telescope rear cell. This sometimes necessitates heavy-duty/high-capacity and over-size focusers.

    Attention must be paid to balance, and it is not unusual to see weights added to the front of the OTA to counterbalance the rear cell load.

    When used on a GEM mount, additional counterweights and/or adjustment of the counterweight position(s) on the counterbalance arm of the mount will be required to balance the weight of the OTA, the instrument load, and any tube counterweights.

    The total of all the counterweight on the telescope tube, and the mount counterbalance arm represent "dead load" that reduces the mount's overall capacity, and causes potential increase in the error rate as the working load becomes heavier.

    On an alt-az, or wedge-adapted alt-az mount, the balance issues can become problematic, as there is no counterbalance arm as on a GEM mount. Another mechanical consideration is interference with the mount base, wedge and tripod by the long component chain hanging from the rear cell. This can be especially problematic if the telescope is pointed near the zenith - care must be taken to set slewing limits in the mount such that the optical chain does not collide with the mount or tripod legs during tracking or GOTO operation.

    To be sure, catadioptric instruments have their plusses and minuses, both for imaging and for visual use. Whether they are less significant, or more when compared to a standard Newtonian reflector is in part subjective. In some cases, for example the RC and CDK instruments, they are primarily designed for imaging. While they can be used for visual observation, it isn't optimal. The SCT is in some sense a "jack of all trades", and represents a set of mostly reasonable compromises to afford large aperture, long focal length, a compact mechanical package, and somewhat flexible optical system for visual or imaging use. On a GEM mount, the SCT can be configured in a relatively straightforward manner to avoid most mount-related/opto-mechanical problems for either visual or imaging application. The Mak-Cass is often used when a compact, flexible design is required for imaging and visual applications (i.e. the Celestron NexStar SE series and some of the Meade LX instruments). These can be "table top" telescopes, equipped with embedded GOTO drives on simple fork alt-az mounts, embedded flip mirrors and "dual ports" for visual and imaging use, etc.
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    SS,

    Quite a thread you started. Its developed almost into a book on telescopes. I might as well add another chapter.

    As I understand your first posting, you would like to get a telescope with a large aperture for visual viewing that has a mount that would allow some basic photography later on but do not care to spend the family fortune on it.

    One that comes to mind that sort of fits the bill is the SkyWatcher Explorer 200P 8 inch telescope on a EQ-5 equatorial mount that costs around £400. The mount is manually operated but can be upgraded ... either motorized for tracking or with Skywatcher SynScan GOTO system. The Newtonian will have all the issues already mentioned which limit but do not eliminate its photography capabilities and the mount is very marginal which will add complications mentioned earlier to your photographic efforts; however, you will be able to do some work. It will also be heavy and bulky in comparison to an 8 inch Dob.

    Your concern about having to manually track. I did that for years and really is not that difficult or that big of a distraction especially if you take the time to polar align your mount. Dob telescopes are very popular and essentially all of these scopes are manually tracked. However, many people do not care for manual tracking .... its a personal sort of thing.
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    For closure on the optics thoughts in this thread, I'll pick on refractors for a bit...

    These days, most refractors fall into three categories:

    - two-element achromatic
    - two-element semi-apochromatic (aka "ED")
    - three-element apochromatic

    The refractor types are enumerated in increasing order of cost/expense. For a given aperture, an achromat should be the lowest cost, an ED/semi-apo the middle ground, and an apochromatic instrument the most expensive. This will generally hold true for a given brand and series of instrument at the same aperture. "Your milage may vary" across brands/instrument series, as it is possible that one manufacturer's ED scope might cost more than another's apochromat at the same aperture...

    The two-element achromatic refractor these days uses an objective lens cell containing two lens elements. Once is crown glass, the second is typically flint glass. All achromatic refractors will suffer from an optical aberration known as "chromatic aberration", sometimes called chroma, or chromatic defect. This is the result of light's behavior as it passes through material with differing indices of refraction - light at different wavelengths will bend slightly differently, and "slow down", which causes the different wavelengths of light that make up the visible spectrum to arrive at the telescope's focal point at different times, and at slightly different distances.

    Chromatic aberration increases with aperture. It manifests itself as a "fringe" at one or more wavelengths at the image field edges. The fringe is also more pronounced on very bright objects like the moon and planets.

    Contemporary achromatic refractors use advances in materials (glass and coatings) to control the chromatic defect, and there are now relatively large achromats up to about 150mm that produce quite good visual results, reducing chromatic fringing on all but the brightest objects like the Moon.

    Semi-apochromatic, or "ED" refractors use Extra-low Dispersion (ED) glass, usually one fluorite element along with a traditional crown glass element to form a two-element objective lens cell. The fluorite element has a lower index of refraction than crown glass, so the combined shape of the fluorite element along with lower IR "correct" to a large extent the chromatic aberration seen in a two-element objective cell like the achromat. The materials properties of fluorite glass make it difficult and expensive to form large diameter lens elements. It is rare to find an ED instrument larger than 120mm, although there are a few 130mm and 150mm ED instruments manufactured. These of course are fairly expensive.

    The three-element apochromatic refractor uses an objective lens cell composed of three lens elements, usually of crown, flint, and extra-hard flint or another exotic glass element. This approach uses both the refractive index differences between the lens element glasses, as well as the lens element shape to bend the light down the optical path such that all wavelengths focus at approximately the same time/place, hence eliminating the chromatic aberration.

    True apochromatic refractors are typically the most expensive per unit of aperture diameter of all telescope designs. True apo's are manufactured in aperture diameters from about 60mm to 200mm and larger. The cost of instruments larger than 120mm is very high - a 200mm apochromatic objective lens cell can exceed $55,000 USD.

    Shared among all refractors is a fact of simple physics. As the aperture diameter increases, the focal length must also increase. As the aperture diameter and focal length increase, the telescope gets physically longer, heavier, and requires a consequently heavier, higher capacity, and more expensive mount to support it. If the telescope mount has a drive system, the motors must be larger and more powerful to carry the OTA weight, as well as any dead-load from counterweights.

    Manufacturing difficulty also increases with OTA diameter and length. The OTA interior of a quality instrument must be baffled to contain spurious reflections in the tube's interior. Manufacturing baffles, interior support structure, tube anti-reflection coating, etc. all increase in expense with diameter an length.

    At some point, usually around 1500mm focal length, the OTA's physical length becomes impractical. This requires either a sectioned OTA, which increases costs, creates the need for high-strength precision joints, or creates the need to leave the instrument in a fixed physical location, meaning it is no longer transportable. At this focal length, typical apertures begin to exceed 200mm, which dramatically increase the telescope's total cost, due to the significant cost of the objective lens cell.

    From an opto-mechanical perspective, refractors are generally simpler instruments than reflectors or catadioptric instruments. The objective lens cell "fixes" the lens elements in place at one end of the tube, and there are no mirrors, intermediate corrector elements, or other components with the exception of the focuser and eyepiece.

    Many refractors cannot be end-user collimated. This is not to say that refractors don't require collimation. It is rare, but they can require collimation, especially if transported frequently or roughly handled. Higher end refractors have objective lens cells that can be end-user collimated. In the case of a fixed cell, if the instrument requires collimation it must be returned to the factory optician for service, which can be costly and time-consuming.

    It is relatively rare, but refractors can be used with specialized optical components including focal reducers and field-flatteners.

    In imaging applications, the simplest approach is the prime-focus, or direct objective imaging configuration. As with Newts, SCT's, RC's, etc., an imager is installed directly in the telescope focuser's draw-tube, and the telescope acts like the imaging camera's prime lens. Imagers include DSLR's, CCD's, video imagers, and film cameras. The optical chain includes the camera, a bayonet or camera-specific barrel adapter, and a T-ring, which is throated into the focuser draw-tube. In most cases this is all that's required.

    In some applications, for example narrow-band imaging or use of a monochrome CCD, a filter wheel is inserted into the optical train. In applications that use manual or on-path auto-guiding, an off-axis guider may be present.

    In some circumstances, an extension tube may be required to allow the imaging camera to come into focus. This is a function of the telescope's focal length, the camera, the size of the telescope's image circle, and the distance from the focuser stop to the image field-stop.

    Other imaging applications include eyepiece projection, where the eyepiece is installed in the focuser, and a T-ring/projection adapter are used with a camera body - this is usually the method where high magnification is desired, for example in planetary imaging.

    The afocal method may also be used, where a bracket is used to hold a DSLR, digital point-and-shoot, or web camera some distance from the telescope's eyepiece. In this case, the camera bracket supports the weight of the imager by using the telescope's focuser assembly to "hang" it from the OTA.

    Similar to the balance and counterweight issues that apply to Newts and SCT/RC/Maks, the same issues arise here. The OTA outboard end (objective lens side) may need counterweights to balance the OTA, and the mount (GEM) may need additional counterweights and/or spacing adjustment on the counterbalance arm. It is extremely rare to see a refractor on a wedge-adapted alt-az mount, as even relatively short focal length OTA's become ungainly and unstable on this style mount.

    The high-points at this juncture:

    - refractors are the most expensive in terms of aperture per unit cost of all telescope designs
    - it is rare to find an amateur class, "affordable" refractor larger than 150mm
    - the least expensive refractors, achromats suffer from some degree of chromatic aberration, increasing with aperture
    - physical properties make refractors above a certain aperture/focal length large, heavy and impact things like the mount cost
    - physical properties tend to make refractors unusable for imaging on wedge-adapted alt-az mounts

    Expense not withstanding, the appeal of a large aperture refractor, especially a high-quality apochromatic instrument are nearly unparalleled image contrast and brightness.

    All reflectors, including Newts and catadioptric instruments use mirrors, or a combination of mirrors and lenses in the light path. They may also have central obstructions from mirror or corrector supports. No mirror is perfect. The highest quality mirrors are 99% reflective, which commodity mirrors are usually 94% - 96% reflective. Slight collimation errors, reflection loss, and absorption/refraction loss in these optical designs reduce by some amount, however slight, the light that reaches the observer's eye.

    These issues are not present to any significant extent in a refractor. The only losses are absorption/refraction loss in the objective cell and eyepiece, generally there are no collimation or off-axis losses. This results in a brighter, higher contrast image than an equivalent diameter reflector.

    The significance of this is left to the subjective opinion of the visual observer... although measurable and quantifiable with instruments, there are human observers who will swear by no other than a big refractor.

    For imaging, particularly in the digital realm, we have lots of controls over many of the variables in the imaging equation. We can make up for contrast loss, process out certain kinds of optical aberrations, correct tracking errors within limits, and increase (or decrease) exposure times based on aperture, focal ratio, etc. Because of that flexibility, its possible to obtain high-quality images from a variety of telescope and telescope/mount combinations, imagers, etc.

    It all comes down to cost, efficiency with time, how much frustration is involved, what skills are necessary, and so on. In turn this leads to opinion based in part on fact, in part on experience, and in part on subjective factors about what kind of scope and/or mount are "best" for any given application.

    I freely admit my bias - for imaging, I prefer a DSLR or one-shot color CCD as the choice of imaging camera. I prefer an RC or SCT in the 8" (200mm) or larger range. I prefer a heavy-duty GEM mount with a total working load of 80lb. or more. I prefer a short-tube guide-scope and standalone auto-guider.

    For visual observing, I prefer a Dob - the biggest one I can afford, move, and transport (about 12").

    For casual observing, I like big binoculars - my 25x100 binocular telescope is very, very sweet... it also gives me something to do while my telescope(s) are busy imaging.

    My preferences and opinions are based on roughly a decade of imaging experience, including my own work, and helping others at all levels, with many, many different mount/scope/camera combinations.

    I'm going to close with this summary of the territory covered in this thread's many segments:

    Newtonian Reflector:

    - Pro's: optically straight-forward, relatively low cost, high visual contrast, relatively lightweight and compact
    - Con's: can exhibit coma below f/4, must pay careful attention to weight and balance, possible limitations for imaging due to inward focuser travel limit, total back-focus

    Dobsonian reflector (Newt on alt-az ground-board mount):

    - Pro's: absolute lowest cost per unit of aperture
    - Con's: can't easily be polar aligned, generally lack tracking/guiding capability

    Catadioptric (SCT/RC/Mak/Mak-Cass/DK/CDK):

    - Pro's: relatively compact, no back-focus limit, well-corrected, flexible optical configuration
    - Con's: reduced visual contrast, can be heavy, must pay attention to weight and balance issues

    Refractors:

    - Pro's: optically simple, virtually no need for collimation, high image contrast for visual use
    - Con's: most expensive per unit of aperture, chromatic aberration can be high (achromats), weight/length increase with aperture/focal length

    Mounts:

    - alt-az:
    - Pro's: generally lower cost, somewhat more straightforward for beginners
    - Con's: consumer-grade/commodity alt-az mounts tend to have high tracking error, single arm mounts are capacity limited

    - equatorial wedge-adapted alt-az:
    - Pro's: generally lower total cost than GEM, re-purposes/preserves existing alt-az mount investment
    - Con's: wedge can add mechanical instability, increase tracking error, mounting difficulty, can be more difficult to polar align

    - german equatorial mount (GEM):

    - Pro's: quality mounts can have high tracking accuracy, relatively easy to polar align accurately, less prone to loading problems, highly adaptable
    - Con's: can be complicated for beginners to use, low-quality, low-capacity mounts problematic for AP use, can be costly
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    Quote Originally Posted by sxinias View Post
    SS,

    Quite a thread you started..
    I know - I was beginning to feel like a trouble maker!

    Its good though - so much good objective (pun definitely intended) information.

    Of course we are all tempted to champion our own choices so when I eventually get my telescope (which I feel sure will be a dob or an equitorially mounted newt) I will suddenly become an expert on why they are the only possible options. I'm amazed that replies have been so unbiased actually.

    Oddly though I am beginning to think of other issues now, and getting maps out to look for dark places.

    I remember coming down from a mountain in the english Lake District last year. I'd got to the summit of a hill called Great Gable just before sunset and had about 1200 feet to descend in the dark (with headtorch etc) to my car/ When I got to the mountain pass where the car was I turned my head torch off and happened to look up - I'd never seen so many stars in my life. Thast was probably what made me start thinking about telescopes again. Its clear that even the best telescopes cannot compensate for simple things like light pollution and rotten atmospherics.

    Had a look at the scope you suggested. They look quite tempting and can be bought very competitively.

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    Quote Originally Posted by Steelystan View Post
    Oddly though I am beginning to think of other issues now, and getting maps out to look for dark places.

    I remember coming down from a mountain in the english Lake District last year. I'd got to the summit of a hill called Great Gable just before sunset and had about 1200 feet to descend in the dark (with headtorch etc) to my car/ When I got to the mountain pass where the car was I turned my head torch off and happened to look up - I'd never seen so many stars in my life. Thast was probably what made me start thinking about telescopes again. Its clear that even the best telescopes cannot compensate for simple things like light pollution and rotten atmospherics.
    I'll pipe in with my limited and admittedly amateur experience on this topic. One other advantage of refractor design telescopes is coping with light pollution. Larger reflector designs, because of the larger aperture and that they reflect light, tend to reflect high levels of LP as well. This amplifies the skyglow, yielding poor washed out images and a loss of object detail, or an inability to see through the LP reflection to see the object.

    Since refractors are generally smaller aperture and do not pull in as much of that ambient light, and also do not reflect that light, they tend to fair much better at seeing through light pollution. This while still grasping the light from the target object and bringing it to your eye with sharp contrast. This often makes them preferred instruments for areas with high degrees of light pollution.

    I am of course decidedly biased since I own one, but I can attest to it's abilities at dealing with LP. I live in a valley with 750,000 people and substantial light pollution, and my refractor does an excellent job of grabbing targets I can't even see naked eye due to the LP. The contrast is always excellent and the sky appears inky black, with razor sharp object clarity, despite the surrounding light pollution.

    That said I will now respectfully bow out. I feel like a high school grad that just interrupted a doctorate seminar on Astronomy at Harvard after the quality, knowledgeable and in depth posts by Austin. He's an astronomical hero around here!!
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