A Guide to choosing eyepieces

Discuss telescope eyepieces.
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OleCuss
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A Guide to choosing eyepieces

#1

Post by OleCuss »


There is much controversy over eyepieces and their selection and this 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 ½ the optics of your telescope! I do have a nice eyepiece with 9 optical elements - my Newtonian reflector Optical Tube Assemblies (OTAs) have only two optical elements. Pay attention to your eyepieces, your system'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.

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).

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 my pupillary diameter is actually around 6mm and that means that my lowest useful magnification is likely more like 300/6 or 50x. 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:

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.

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 (going just by age) should be assumed to have a useful dark-adapted 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.

To put it another way? If you are under light-polluted skies your eyes won't be fully dark-adapted anyway and you should not assume that your pupillary diameter is greater than 5mm. Unless you are relatively young and typically observe under nice, dark skies, keep the exit pupil of your system under 5mm.

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 a "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.

For the purposes of this post in an amateur astronomy setting, 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.

Now to trash the above paragraph? In the professional world it is not uncommon for an F/2 telescope to be considered "slow".

This distinction between "fast" and "slow" is important because a fast telescope typically requires more highly corrected (read more expensive) eyepieces in order to provide a good 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:

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. . .

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 some 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.

I have found the Baader zoom eyepiece particularly beneficial in public outreach. If I provide an eyepiece with a huge FOV they get lost and cannot find the selected target. With the zoom eyepiece I can zoom right to the target and there is just nothing else for them to see.

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 quite a bit of money for that bit of optic trickery.

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 may not 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 am 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 very nice 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 5mm for the pupillary diameter because even though I've been measured at 6mm in one eye and 6.5mm in the other, I'm almost always in light pollution and my dark adaptation will thus be limited.

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 because almost all of the time 200x will be the maximum.

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. Now parenthetically, you'll notice that the shortest focal length always turns out to be the same as your focal ratio. . .

This suggests that for the XT8 I should be getting a few eyepieces (3 or 4) with focal lengths between 6mm 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 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.

It is important, however, to remember that the typical Barlow (also called a Tele-Extender) will not only decrease the amount of light you see, the typical Barlow will also degrade the quality a little. A Powermate from Tele Vue or a good ED-Barlow will tend to minimize or eliminate that degradation.

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 of telescopes.

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 perform 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 at any given moment. 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.

It is important to realize one other thing. . . Your eyes just aren't all that good. If you choose a system with an exit pupil less than 0.5mm you are just going to have too many problems with the way your eye behaves to have a good look. Keep that exit pupil at more than 1mm for consistently good viewing. And if you get an exit pupil under 0.5mm and think you are getting good views, I may be able to get you a good Psychiatrist!

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. This is also often done at high altitude where atmospheric turbulence is less of a problem.

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.
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Re: A Guide to choosing eyepieces

#2

Post by DeanD »


Thanks for this: a good, common-sense introduction without getting into all the technical controversies that some people love (and which confuse newcomers!).

Only one comment for clarification is that you can go for a lower magnification than provided by (say) a 7mm exit pupil: it is just that you aren't getting all the light from the scope into your eye and so the view is not as bright as it would be if your exit pupil was matched. This happens for day-time views anyway: I might be using 10x50 binoculars for instance, with a 5mm exit pupil, but my eyes may only be dilated to (say) 2mm (because it is daytime!). I can still have a great view though.

Keep up the good work!

All the best,

Dean
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Re: A Guide to choosing eyepieces

#3

Post by GCoyote »


From one old cuss to another, thanks!
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Re: A Guide to choosing eyepieces

#4

Post by Michael131313 »


Thanks OC. I bookmarked it.
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Re: A Guide to choosing eyepieces

#5

Post by johnbear »


This should be part of every instruction manual for all consumer/beginner level telescopes.
Excellent information. I will refer this to all new telescope users that I meet.
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Re: A Guide to choosing eyepieces

#6

Post by MistrBadgr »


Excellent article! I enjoyed reading it.
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Many eyepieces, just not really expensive ones.
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Re: A Guide to choosing eyepieces

#7

Post by coopman »


Very good advice. Thanks.
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Re: A Guide to choosing eyepieces

#8

Post by Ylem »


Awesome write-up 👍

Thanks, said the guy with a box of cheap old Plossls.
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Re: A Guide to choosing eyepieces

#9

Post by Bigzmey »


Ylem wrote: Sat Nov 14, 2020 2:03 am Awesome write-up 👍

Thanks, said the guy with a box of cheap old Plossls.
Never underestimate the power of old Plossls. :D They new how to make them right then.
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Re: A Guide to choosing eyepieces

#10

Post by notFritzArgelander »


Ylem wrote: Sat Nov 14, 2020 2:03 am Awesome write-up 👍

Thanks, said the guy with a box of cheap old Plossls.
I have a bag of cheapish new Plossls. The Vixen NPLs perform very well and a re a good value to boot! :)
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Re: A Guide to choosing eyepieces

#11

Post by 25585 »


Wearing eyeglasses needs *effective* eye relief starting at 20mm, only 2 cms is not that much. Effective eye relief is the distance you get between the lowest your eye or glasses lens can go to the top of an eyepiece's barrel.

Manufacturers' figures are usually from the top of an eye lens, so if an eyepiece lens is recessed down a barrel, its *effective* eye relief will be less than in advertised specification.

Another caveat aspect is eye placement & exit pupil behaviour. Certain ranges &/or focal lengths are harder or easier viewing because of how a user finds getting his or her eye lined up to see the whole, or maximum view through an eyepiece, and ease in retaining or re-acquiring that view with head & eye movement. Only trial really lets a person find out what is acceptable. Even binoculars can have different eyepiece issues & aspects, same for spotting scopes with fixed eyepieces.
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Re: A Guide to choosing eyepieces

#12

Post by andy brown »


Thank you OleCuss for that entertaining and light hearted article.

My own view? A % of astronomers are seduced by the high end products as a means to an end, when in fact, it may very well not be necessary.
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Re: A Guide to choosing eyepieces

#13

Post by mikemarotta »


Based on this article I made a chart of exit pupils to put into my log book.
exit pupils (1).jpg
They are useful guides to expectations.
---------------------------------------
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Re: A Guide to choosing eyepieces

#14

Post by OleCuss »


As long as it works for you, I'm happy!
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Re: A Guide to choosing eyepieces

#15

Post by Mike Q »


This is a great write up on how to pick eyepieces. Simple and easy to understand. This exactly what beginners need to know
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Re: A Guide to choosing eyepieces

#16

Post by messier 111 »


this report is very well written and explains the essence very well,
of what is the use of eyepeices.
thx for the fine report .
ps: too bad I only saw it now.
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Don Pensack United States of America
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Re: A Guide to choosing eyepieces

#17

Post by Don Pensack »


There are several ways to pick eyepieces:
1) Even % jumps. Like Pentax XW or Morpheus progressions, this is usually a 40% jump up to the next magnification (or focal length of eyepiece = 40% shorter).
Advantage: enough spread to create a noticeable difference.
Disadvantage: Low powers are too close together, high powers are too far apart.
2) Even magnification jumps. This is where the magnification jump up from low power is exactly the same, i.e. 50x/100x/150x/200x/250x/etc.
This is like the 1x/2x/3x method of choosing I mention elsewhere on this site. You can always add one in between at an oft-used range (say, the 100-200x range).
Advantage: low powers are far enough apart, higher powers get closer together, %-wise.
Disadvantage: low power jumps represent large changes in brightness, while high powers represent small changes in brightness.
3) uniform % loss of true field with increased magnification. This works best if all the eyepieces have the same apparent fields.
A 41.4% increase in magnification for each higher power eyepiece then has exactly 1/2 the area of the next lower power.
Advantage: True field areas are spaced well and it is easy to decide which eyepiece should be used to frame an object.
Disadvantage: see even % jumps above.
4) Exit pupil jumps to yield equal drops in brightness with each increase in magnification, like 6mm exit pupil..4.3mm..3.1mm..2.2mm..1.6mm..1.1mm..0.8mm..0.57mm
Advantage: each jump represents exactly the same % dimmer than the next lower power.
Disadvantage: see even % jumps above.
5) the "U" shaped selection. This is where you have a most-used, favorite magnification that just works with the scope. Since many objects need a little more magnification, and many more a little bit less,
you make the next magnification up and down from that favorite magnification a fairly small jump. Then, the eyepiece below those 3 and above the 3 have a larger jump, and each jump up and down from there gets progressively larger.
Example: 150x is a favorite magnification. You make the next lower and higher 120x and 180x, then to 80x and 220x, then 270x, 330x,400x,480x, etc., however your set gets filled in.
I like this one myself, but I make my jumps at the ends a bit larger.
Advantage: You always seem to find just the right magnification for that scope on each object.
Disadvantage: you need to have a larger set of eyepieces to cover the entire range of magnifications for the scope.

One thing to realize is that the vast majority of your observing will be done with exit pupils between 0.8mm and 3mm. Can you go higher and lower? Sure. But you just won't use them as much.
So pay attention to the eyepieces you select that yield exit pupils in that range. They'll get a lot of use, so that is probably the range in which to splurge.
Astronomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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Caldwell 14 Great Britain
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Re: A Guide to choosing eyepieces

#18

Post by Caldwell 14 »


An excellent common sense article. Math is not my strong point, but even I could follow your analysis. It is very easy to go down rabbit holes after reading posts in other spaces regarding magnification. Your piece explains perfectly why I use my 3 most commonly used eyepieces as much as I do. My scope has a fl of 8.6. My most commonly used oculars are a 8 mm Radian giving me x110 with an exit pupil of 0.9, a 13 mm Delite at x67.7 with an exit pupil of 1.5 and a 32 mm Plossl at x27 with an exit pupil of 3.7. The 5 mm is used for lunar, Mars and Saturn as a rule with a little double splitting when the mood takes me.

I have considered a TV zoom (6-3), how would the exit pupil change? I start developing floaters above my 8mm x110 magnification
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Re: A Guide to choosing eyepieces

#19

Post by Caldwell 14 »


I use my 11mm and 20 mm plossl exclusively for white light solar btw.
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Re: A Guide to choosing eyepieces

#20

Post by Don Pensack »


Caldwell 14 wrote: Fri Dec 08, 2023 10:49 am An excellent common sense article. Math is not my strong point, but even I could follow your analysis. It is very easy to go down rabbit holes after reading posts in other spaces regarding magnification. Your piece explains perfectly why I use my 3 most commonly used eyepieces as much as I do. My scope has a fl of 8.6. My most commonly used oculars are a 8 mm Radian giving me x110 with an exit pupil of 0.9, a 13 mm Delite at x67.7 with an exit pupil of 1.5 and a 32 mm Plossl at x27 with an exit pupil of 3.7. The 5 mm is used for lunar, Mars and Saturn as a rule with a little double splitting when the mood takes me.

I have considered a TV zoom (6-3), how would the exit pupil change? I start developing floaters above my 8mm x110 magnification
Exit pupil is eyepiece focal length / telescope f-ratio or telescope diameter in mm / magnification.
So if you see floaters with 8mm, a 3-6mm zoom would be pretty much unusable except, maybe, for double stars.
Astronomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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