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Thread: Article: A Framework For Choosing Telescope Eyepieces

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    Default Article: A Framework For Choosing Telescope Eyepieces



    There is much controversy over eyepieces and their selection which will persist for eternity. The reason for this is that each of us has a different set of priorities, abilities, and comfort zones.


    For this reason I am going to attempt to lay out a framework to help people make reasonable eyepiece choices. Understand that the idea is not to look at custom or very oddball eyepieces but to look at the more readily available eyepieces.


    To all of the above and all that will follow there should be a big caveat: Your eyepiece choice is very personal and is ideally made after you have managed to try out that eyepiece in your telescope. The best way to do this is to go to a friendly star party with your telescope, ask lots of questions, and see if you can try out a variety of eyepieces in your telescope – and typically you'll get to do just that.


    Now a little perspective to which many of us have not paid much attention. Your eyepiece is likely more than the optics of your telescope! A few days ago I received a very nice eyepiece with 9 optical elements – my Newtonian reflector Optical Tube Assemblies (OTAs) have only two optical elements. Pay attention to your eyepieces, your telescope's performance will never be better than the performance of your worst optical component.


    We really need to get into some important math to help set the parameters for making wise eyepiece choices, but first we need to deal with some things which are a bit variable and controversial.



    1. First we need to touch on upper magnification limits. To be rather brief and ignore some controversies, you will seldom be able to use a magnification greater than the diameter of your objective mirror or lens as measured in millimeters. This means that if you have a 150mm objective mirror or lens you'll usually be using a magnification of less than 150x, but there may be occasions when you can push to twice that (300x) and if you have superb optics and conditions you may be able to go even higher than that (although there is some question of delusional states if you are exceeding magnifications more than 2x the objective diameter as measured in millimeters).
    2. We tend not to pay enough attention to lower magnification limits. This is a mistake. Many of us particularly treasure low power and the typically wide fields of view that accompany that low power. Unfortunately, we tend to forget that there is a lower limit to the useful magnification we can use. Mathematically, this is the diameter of the objective lens or mirror as measured in millimeters divided by the diameter of your pupil (usually assumed to be 7mm).
      Let me illustrate by using my own mistake: I bought an XX12g which has a 300mm objective and lists the lowest useful magnification as 42x. So I got a very nice eyepiece which will give me that magnification. Unfortunately, I'm an old cuss and that means that my pupillary diameter is probably more like 5 or 6mm – and that means that my lowest useful magnification is likely more like 300/5 or 60x. This means that my nice eyepiece is a mismatch for my nice telescope and my tired old eyes. Pay attention if you want your best performance and don't want to waste money.



    OK, now for a bit more math:



    1. The way to calculate magnification is to divide the focal length of your telescope by the focal length of your eyepiece. So if your eyepiece is 10mm and the focal length of your telescope is 1,000mm, your magnification will be 1,000/10=100x.
    2. The “Exit Pupil” is calculated by dividing the focal length of your eyepiece by the focal ratio of the objective mirror or lens of the telescope. This means that if I use my F/5 telescope with a 30mm eyepiece, the exit pupil of the eyepiece will be 30/5 – or 6mm. This is important because if the diameter of the light from the eyepiece (exit pupil) is greater than the diameter of my pupil, then when I look through the eyepiece the view will be degraded. Consider, then, my tired old eyes which should be assumed to have a useful pupil of 5mm – I should not be using an eyepiece with a focal length greater than 25mm with a telescope which has a focal ratio of 5.



    OK, got all that? Let's now touch on something which many (if not most) on this forum will disagree with me on – what constitutes a “fast” or “slow” telescope and what that means to eyepiece choice. Sorry, this is my little write-up so I'm going to do it my way.


    Generally speaking, a telescope with a focal ratio of 10 or above is a slow telescope. A telescope with a focal ratio of about 6-10 is medium speed. A telescope with a focal ratio of 4-6 is fast. A telescope with a focal ratio less than 4 is ridiculously fast.


    This is important because a fast telescope requires more highly corrected (read more expensive) eyepieces in order to provide a sharp image to the edge of the field of view. A medium or slow speed telescope will not be so demanding. When you get into the realm of the very fast telescopes you may need a special coma corrector (maybe a Paracorr 2?) in addition to a highly corrected (expensive) eyepiece in order to have a sharp image across the entire field of view.


    Now for a few general usage issues:



    1. The Apparent Field Of View (AFOV) choice is of considerable interest and is largely a personal choice. Some people are very happy with an eyepiece with an AFOV of 68 degrees or less while others really want 70-80 degrees or more. The eyepieces with an AFOV of 80 degrees or more are frequently thought of as being immersive (you are looking around within the FOV rather than seeing the whole FOV at once). Some people love immersive views and others find them distracting at best – it is a personal thing. It is important to realize that you might really hate that wonderfully designed eyepiece that you paid a small fortune to purchase. . .
    2. Fumbling in the dark. You find a wonderful site with dark skies and have everything set up and ready to go. You decide to change the eyepiece and discover it is very difficult to choose the eyepiece when you can't see the eyepieces. . . And when you've fumbled around and found another eyepiece you discover that it took so long that the Jupiter is no longer in the FOV and you also have to re-focus in order to see anything. These kinds of experiences have led to many of us really preferring to use a good zoom eyepiece like the Baader Hyperion Zoom since you can change magnification rapidly (and without fumbling) and there is little re-focusing required. You do, however, end up with a minor decrease in optical performance with a somewhat restricted FOV. Many are willing to make the compromises inherent in the use of a good zoom eyepiece since their observing time is increased and they don't lose the object they are trying to observe while fumbling for a different eyepiece and trying to achieve focus.
    3. Eye relief. This is the distance from the eyepiece to what I'm going to call the best spot for you to put your eye. With eyepieces like Plossls it is generally going to be the case that a longer focal length means a longer eye relief and more comfortable viewing. If you get a 4mm Plossl eyepiece you will practically have to put your eyeball on the eyepiece in order to see anything and that really isn't a very pleasant experience. A 50mm eyepiece will have your eyeball at a pleasing distance. But with some designs you get good eye relief no matter what your eyepiece's focal length – you generally pay for that bit of optic trickery.
    4. Eyeglasses. This is sort of a continuation of #3. If you wear eyeglasses you are going to want at least 15mm of eye relief to account for the distance from your glasses and your eyeball. Most really want 18-20mm. The other thing to consider is that if you are merely near-sighted or far-sighted, then focusing will take care of that issue and you needn't wear glasses to use your telescope. But if you have astigmatism you will need to wear glasses (even contacts designed to take care of the astigmatism won't work very well for you) unless you get some very high-end eyepiece such as an Ethos which allows use of the Dioptrix system (some argue Dioptrix may be better than glasses for this purpose).



    So with all the above complexity, how do you choose? I dunno – that's up to you. But let's examine a few scenarios which might illustrate the choices and prove helpful.


    Let's assume I'm a rank amateur with a very limited budget and purchased an Orion FunScope.


    The FunScope has 76mm objective with a focal length of 300mm for a focal ratio of 3.9. It also has a spherical mirror which means it has built-in spherical aberration. Seriously, even with the absolute best of eyepieces you'd still have significant aberration and an eyepiece which might do a decent job would likely cost you 6 times what the telescope cost – and wouldn't fit into the focuser or would be too heavy anyway. Enjoy the telescope as you got it and don't buy more eyepieces for it.


    Let's assume a somewhat larger budget and I bought an Orion SkyQuest XT8. This telescope has a 200mm objective with a focal length of 1200mm for a focal ratio of 5.9. It has a parabolic mirror which means that it really will give excellent views.


    The assumed budget is not all that big so getting really expensive eyepieces probably is not going to be an option although with somewhat fast optics it could really benefit from them. But nevertheless I'm going to assume the use of the mostly adequate Plossl eyepiece. Now let's do the relevant math:


    For my minimum useful magnification I'm going to use my old eyes number of 5mm for the pupillary diameter as I suspect that is what many of us should be using. For one thing, what if you are at a star party and I show up with my old eyes and you want to show me your spectacular view designed for a 7mm pupil? It's going to look lousy to me and I'm going to think you're nuts. So old man's prerogative here, we're going with 5mm.


    So the minimum useful magnification is going to be 200/5 or 40x. The maximum useful magnification is going to be 200-400x depending on the viewing conditions, but for the sake of this discussion I'm going to sort of temporarily adopt a maximum useful magnification of 200x.


    Since we have a focal length of 1200mm the calculation of the range of useful eyepieces is going to use 1200mm divided by the lower and upper magnification limits. So. . . 1200mm/40 gives the longest focal length eyepiece of 30mm. 1200mm/200 gives me the shortest focal length eyepiece of 6mm.


    This suggests that for the XT8 I should be getting a few eyepieces (3 or 4) with focal lengths between 6 and 30mm. I can cover that range pretty nicely by getting Plossls from a place like Orion (I like Orion so I'm going to use their Plossls for the illustration) in 6.3mm, 10mm, 17mm, and 25mm eyepieces (I could probably get by with the 32mm although it might be a tad long for my old eyes). These eyepiece choices would get me from a magnification of 48x to one of about 190x at a fairly low cost.


    Now we're going to cheat just a little. We're going to get a 2x Barlow. Yes, tossing in a good Barlow will slightly decrease the amount of light which reaches your eye, but it will double your magnification and do it while maintaining the eye relief of the eyepiece. So when you use the Barlow with the 10mm eyepiece you will get the magnification of a 5mm eyepiece but still have the far more tolerable eye relief of the 10mm eyepiece. What is more, you effectively change that 4-piece set of eyepieces into the equivalent of an 8-piece set for just the price of the Barlow! We're talking win-win. What is more, you'll notice that our maximum magnification just went to 380x which means we're starting to push the available magnification to 2x the diameter of the objective (in millimeters) for those rare occasions where some of us think you can use that kind of magnification.


    If you care to increase your budget a bit you can choose to get Super Plossl eyepieces. They are optically somewhat superior to the Plossl and are, IMHO, a surprisingly good deal. When you are looking for Super Plossls you should be aware that the current Meade Super Plossls appear to be simply Plossls. I've found that Agena Astro usually has the best deals on Super Plossls so when I've wanted those, that's where I've gone (Agena Astro calls them GSO SuperView). I think they do pretty well in my XT8.


    Now let's up the ante a bit. I've got a Meade LX200. This has a 250mm objective mirror with a focal length of 2,500mm to yield a focal ratio of 10 (F/10). This puts it into the at least marginally slow category.


    Now if you buy a telescope that costs as much as this one, you really should make the relatively small investment in order to get some pretty decent eyepieces. I think Super Plossls should be the minimum for a telescope of this caliber – although Plossls will preform quite nicely with optics this slow.


    Let's run the numbers again just to get in the practice: Minimum magnification is 250/5 to yield 50x. Maximum magnification will be assigned as 250x (but acknowledging the possibility of very rare conditions allowing 500x).


    2500mm/250=10mm and 2500mm/50=50mm. So my eyepiece range is 10mm-50mm. And yes, I can get a decent Barlow and expand that range. (Sort of off-topic, but of note, is that choosing a rather slow telescope has meant that I can use relatively long eyepieces with relatively good eye relief. This is one reason why so many people really like the optics of the Schmidt-Cassegrain and Maksutov-Cassegrain telescopes.)


    Now lets go to the XX12g. We've had plenty of practice at the math, so I'm just going to toss in my opinion that if you are going to get a telescope this fast (F/4.9) you should budget for a set of really good eyepieces. IMHO, Plossls give lousy views and Super Plossls aren't all that much better. Something like the Tele Vue Delos line or even the Panoptic line will do very nicely if you are not into the “immersive” view. If you want that immersive view then you should look into something like the 100 degree Explore Scientific line or the Tele Vue Ethos line. Interestingly, if you are willing to make the compromise, the Baader Hyperion Zoom works pretty well.


    Now to deal with some outliers.


    Some people are getting some very expensive telescopes with truly superb optics. They are occasionally reporting the ability to use magnifications significantly greater than what our calculations suggest could be useful. Opinions vary regarding this. Some people just think they are nuts and others are in awe. Some think the observer's mind and eye are sort of integrating glimpses of the target and sort of gradually filling in details of what is being seen to produce a view in the mind which is greater than what is seen in the eyepiece. Whether nuts or simply superior, they are getting their monies' worth and we should simply be happy for them. The point being that if you are one of these people you may want to invest in excellent eyepieces which go as high as a magnification of 3-4 times the diameter of the objective. I don't have the telescope or the expert observational skills to invest in those eyepieces.


    Another extreme is sheer high magnification. Yes, there are very skilled and knowledgeable people who say that ground-based observers are pretty much limited to 350-400x no matter the size or quality of the optics. It's pretty much a matter of the limitations of the observing conditions. This would suggest that even if I got a telescope with a 600mm objective mirror that I still would get a maximum magnification of 400x.


    Now, I'd really like to have a 600mm telescope and really wouldn't like to be limited to 400x when I've got the optics to handle at least 600x. Well, I can take a little comfort in the fact that some very skilled and reliable observers are reporting the use of 1,000x under very good conditions and with large aperture telescopes. Again, a portion of this may be the mind and the eye integrating what they are seeing through atmospheric turbulence to create an image in the mind which is better than the image actually seen in the eyepiece. But the point is that with big optics and good conditions you may be able to usefully use much more than 400x.


    The flip side of the above is that there are people who have good size optics and really should be able to use 200x or more but find that their typical conditions typically limit them to 100x. The point of this is that just because your optics are good enough it doesn't mean you can use the higher magnification. If your local conditions are not that good, you can concentrate your eyepieces to lower magnifications and get more for your money.


    Again, the best thing to do is to join your local astronomy club and go to a few star parties and try things out to see what works for you in your location. My hope is that the above has given you an idea of what to look for and how to look for it.


    FWIW



    If this interests people, I need to apologize in advance. I do not have moderator powers so I will not be able to update this or add to it in an orderly manner. So I'd like to take the opportunity to ask the moderators to modify the above as they please in order to keep things updated or to properly clarify or edit this. Please make it clear that it has been edited by people other than myself and I'll be happy as a clam (assuming clams are happy).
    Last edited by OleCuss; 12-25-2011 at 11:32 PM.
    Orion XX12G and XT8; 10 inch LX200GPS; Celestron NexStar 8SE; Tele Vue NP-101is (Petzval apochromat); Meade LS-8; WO GT-71; AVX and Losmandy G-11 mounts.

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    Excellent Thread!! Very detailed and well written...Great Job!! This would make excellent Sticky!

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    Wonderful, well written, I could actually follow it although a lot of it is over my head but I think a few times read through and I'll have it. Thanks...this has to be a sticky
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    If you could let us know what you perceive as over your head, the moderators might be willing to edit the text as to make it a bit more clear.

    I can no longer edit it so it will have to be up to them to fix it.
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    Great lessions. Every beginner should read it.

    And you didn't mention that you can use more magnification on stars and clusters (point sources) than on galaxies or nebula (extended sources). Maybe that is in your next lesson.

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    That is a good point.

    I didn't think of it as a lesson, however. Mostly as a guide that people can use to get started in making eyepiece choices.

    I do have at least a start on something similar for choosing a telescope, but I know that it will need a lot of editing/formatting and I'm quite certain I could not get it put together before my editing privileges for the post would go away.

    I'm on a forum or two where you can go back and edit your post years afterward. It makes it possible to clear up errors for people who might want to use it for reference in the future. No chance of that at this time so I need to think long and hard before I attempt the telescope choice post. I may need to figure out how to do it without the formatting I want.
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    Excelent! Great read. Thank You!

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    I made this thread a sticky for now. I don't know how we would go about keeping it updated. Maybe you can post the updates via reply? It is a VERY nice article btw. Thanks

    Jim
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    Thanks OleCuss!
    If I could just add a little comment from personal experience:
    The weight of EPs on small telescopes and mounts.

    Some EPs are the size of hand grenades and can easily overpower small setups. You touched on the weight issue briefly, but since a lot of amateur astronomers have these small mounts, a word or two about EP weight might be useful.
    (And since edits are time-sensitive, I guess this message really goes out to the moderators as well).
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    Yeah, the answer to the edits would be either for the moderators to simply edit the original post as they see fit (I would be very happy for this to become a collaborative effort - most all the moderators know more about this stuff than I do anyway), or to change the editing policy on the forum to allow edits in perpetuity.

    I don't see the editing policy changing any time soon (I'm sure there is good reason for the current policy).

    Since I really don't like having extended sticky threads on subjects of this nature I really don't want to try to do what would be sort of long-distance edits. This is especially true on the Beginners Forum where it would be especially valuable to have the relevant information consolidated - it is much more newbie friendly to have the development of the language of the original post go on in the background and just keep modifying the #1 post.

    And seriously, what I would be happiest with would be if the moderators would continually modify the OP as they see fit and I'd even be happy if they took my username off it entirely. Pride of ownership is not an issue here.

    The alternative is that I can try to occasionally start a whole new thread with updates and the moderators can either choose to let the new thread die or they can zap this thread and put the new thread in as a sticky. But that gets kind of odd as well.

    It is already clear to me that some edits would be a good idea - and this does not surprise me at all. There are plenty of others on the forum who know this stuff better than I do - and that is not false modesty on my part but a statement of fact.
    mick286 likes this.
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