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Field of view limitations.


TheTalescopeMan
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Hi all,

I have just been looking at some figures relating to the field of view available with various 'scopes, and it strikes me that when one takes into account the need to consider exit pupil size, big, 'fast' Newts' that most say are ideal for viewing extended, deep-sky objects, actually have rather limited fields of view.

For example, take a 250mm, F4.8 Newt' with a focal length of 1200mm. A 25 mm Plossl eyepiece having a 52 degree apparent field of view will give a magnification of 48x, and give a field of view of 1.08 degrees with an exit pupil of 5.2mm. (That is, assuming that my maths is correct and I am not missing something).

Now for someone my age 5.2 mm is probably the maximum that is acceptable, and it seems that anything that might be done to get a wider field of view will in turn result in an even wider exit pupil size. For example, a 32mm Plossl would give an extended filed of view - up to a still not remarkable 1.39 degrees - but at a cost of increasing the exit pupil size to 6.7mm. Same problem seems to apply to the use of expensive wide-angled eyepieces and 2" focuser and eyepiece setups.

There doesn't seem to be a way out of this, other than use a scope with a much shorter focal length, and of course if you are looking for a big aperture 'scope, there is a limit to how 'fast' the scope can be, with most visual scopes being not much faster than f5.

Of course, a short focal length refractor (in turning also meaning one of limited aperture) can give a wider field of view. (A finder scope can perhaps be seen as being an extreme example of this). I guess that this is why they are popular for imaging, with the lack of aperture not being such an issue when you can collect the photons over an extended time period.

So, am I missing something? It seems that a newt' with a moderate aperture and correspondingly short focal length, say a 150mm f5 with a focal length of 750mm will give a reasonable 1.7 degree field of view with a 25 mm Plossl, whilst having a 5mm exit pupli, but this will have limited light-gathering ability. On the other hand, 'go big' and the exit pupil issue soon comes into play, allowing more light to be collected but with a more restricted field of view, which is hardly ideal for something like a M31 (at 2 1/2 or more degrees across), objects which are supposed to be the forte of 'fast', large aperture scopes.

Ok, so now tell me where my reasoning is going awry. :)

Edited by TheTalescopeMan
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You are quite correct. Large and fast reflectors do suffer from an over large

exit pupil if you use eyepieces that give very low powers.

It is why large apparent field of view eyepieces have become popular, the latest

having a 100 degree field, so that extended objects can be seen at a higher

magnification and smaller exit pupil.

Rsgards, Ed.

Edit : welcome to SGL !

Edited by NGC 1502
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The 31mm Tele Vue Nagler eyepiece was designed to address the issue you identify - it has a very large apparent field of view (82 degrees) but a focal length that keeps the exit pupil at a managable level even in fast scopes.

In my 10" F/4.8 newtonian the Nagler 31mm gives a true field of view of 2.12 degrees while the exit pupil is a reasonable 6.46mm.

The 28mm William Optics UWAN follows a similar approach but it's exit pupil, in the same scope, is 5.83mm, closer to your optimum.

On the downside, these are costly and very heavy eyepieces.

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Thanks for the replies, I guess that this is just another aspect of the 'fast scopes need expensive eyepieces' thing that I keep reading.

One other thing. I understand that if you are using a 1 1/4 eyepiece set up, once you go above about 32 mm, the restriction imposed by the width of the eyepiece tube means that wider fields of view cannot be obtained. So, a 40mm Plossl will give almost the same field of view as a either a 32 mm Plossl or even a wide angle 25mm eyepiece.

If a 25 mm eyepiece is the longest focal ratio that still gives a reasonable exit pupil, would there really be anything to gain, field of view wise, by going to a 2" setup? After all, if the focal ratio and apparent field of view of a 1 1/4 eyepiece and a 2" eyepiece were the same, wouldn't they, mathematically speaking give the same field of view?

Edited by TheTalescopeMan
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...If a 25 mm eyepiece is the longest focal ratio that still gives a reasonable exit pupil, would there really be anything to gain, field of view wise, by going to a 2" setup? After all, if the focal ratio and apparent field of view of a 1 1/4 eyepiece and a 2" eyepiece were the same, wouldn't they, mathematically speaking give the same field of view?

The 2" format can accommodate a much larger field of view - 52 degrees is the max for a 32mm 1.25" eyepiece wheras a 32mm 2" can be 80+ degrees.

But if a 2" version of an eyepiece gives the fame FoV as it's 1.25" equivalent then, yes, it would be pointless using it.

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Thanks for the further clarification. So the bottom line is that if, for example, you have a f5 scope and need a 5mm exit pupil, the maths mean that the longest focal length eyepiece you can use is 25mm. Then all you need to do is decide how much you want to spend on buying an eyepiece in order to get the maximum possible apparent field of view, from 50 Euro for a 52 degree Plossl, to 100 Eur for a 24 mm / 68 degree Hyperion, right up to a something costing 700 Euros plus, but having a field of view as wide as 100 degrees.

Looking again at what eyepieces are available, the 2" option also begins to make sense, as many ultra-wide angle eyepieces don't seem to be made with focal lengths much longer than about 17mm, I guess because the same sort of physical limitations as are imposed by a 1 1/4 eyepiece tube start to come into play again.

It guess that one take home massage is that you should never consider buying a scope without first considering the cost of the eyepieces that will do the scope justice.

Thanks again!

Edited by TheTalescopeMan
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...It guess that one take home massage is that you should never consider buying a scope without first considering the cost of the eyepieces that will do the scope justice....

Yep :D

I've read advice somewhere that you should budget at least 50% of the cost of your scope for eyepieces - that could be a bit on the low side to be honest :)

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If a 25 mm eyepiece is the longest focal ratio that still gives a reasonable exit pupil, would there really be anything to gain, field of view wise, by going to a 2" setup? After all, if the focal ratio and apparent field of view of a 1 1/4 eyepiece and a 2" eyepiece were the same, wouldn't they, mathematically speaking give the same field of view?

But the "if" never holds for long focal length eyepieces. If you want a large apparent field of view to get a large true field at the still efficient minimum magnification, the eyepiece will become a 2" eyepiece. A 24mm 70° AFOV eyepiece can still be a 1.25" eyepiece, but even an 18mm 82° AFOV eyepiece is already a 2" eyepiece.

The field stop size is directly related to the field of view (by field_of_view = eyepiece_field_stop / telescope_focal_length * 57.3, 57.3 being the conversion factor from radians to degrees). And the reason for 2" eyepieces is to allow for effective field stop sizes beyond 28mm (the barrel size minus the thickness of the barrel and retaining rings) and up to 46mm.

So the largest field eyepieces are always 2" eyepieces, by definition.

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sixela,

Thanks for your contribution. I think i was saying something similar when I wrote "Looking again at what eyepieces are available, the 2" option also begins to make sense, as many ultra-wide angle eyepieces don't seem to be made with focal lengths much longer than about 17mm, I guess because the same sort of physical limitations as are imposed by a 1 1/4 eyepiece tube start to come into play again."

Is not another issue the design of the 'scope used? For example, aren't 'fast' Newt's prone to serious coma, so that even if a wide-field view is theoretically possible, much of the periphery of this view will not particularly pleasing to look at. (I have read that with a f5 scope, only the central 0.3 degrees of the field of view will free of coma).

Alternatively aren't some scopes, such as a Maksutov Cassegrain, less prone to such problems, so that even if the calculated field of view available in such a 'scope is smaller, much more of what is available will give a sharp image?

Edited by TheTalescopeMan
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Is not another issue the design of the 'scope used?

Well, for seeing an aberration free field, yes, but even a field with some aberrations shows you a lot more than the black of a field stop :).

For example, aren't 'fast' Newt's prone to serious coma, so that even if a wide-field view is theoretically possible, much of the periphery of this view will not particularly pleasing to look at.
Define "pleasing to look at".

Coma in an f/5 scope is such that only the central 2.2mm is "coma free" in the sense that diffraction of the aperture is still dominant in determining the diffraction pattern. For a 200mm f/5, that's a field of 0.12° (that's the field within which you'd be hard pressed to detect coma in an in-focus image).

But of course, at low magnification you can't resolve the diffraction pattern or the coma in a field that is a lot larger than that.

In general, whether coma is objectionable in a Newt with given f/ratio depends mainly on the eyepiece AFOV, as coma is linear in the field. If you increase magnification you'll be looking at a field with less coma but magnifying it more, so it's all a wash.

In general, for f/5 Newts, most people roughly feel they don't need a coma corrector with 70° AFOV eyepieces. With 82° AFOV eyepieces, more will want a coma corrector. With 100° AFOV eyepieces, eve more people will want one. Of course, your tolerance for aberrations depends on what you're used to; most people who do have a Paracorr tend to use it even in f/6 scopes with Plössls.

Actually, designs different from a Newt are just as prone to off-axis aberrations, only different ones. SCTs have a lot of coma and field curvature relative to their effective f/ratio (in other words, an f/10 SCT has a lot more coma than an f/8 Newt). Maks and have a lot, too, though less than SCTs. Schmidt-Newts have a half the coma of a Newt with identical f/ratio, but they're usually a lot faster than Newts. APO Refractors have wide field without too many aberrations but field curvature unless you use a flattener.

And finally, at low power (which is what the thread is all about), at large exit pupils your own eye's aberrations may start to dominate the aberrations, and in fact do so all across the field! Your eye lens is a lot better stopped down than used over its entire surface...

Alternatively aren't some scopes, such as a Maksutov Cassegrain, less prone to such problems, so that even if the calculated field of view available in such a 'scope is smaller, much more of what is available will give a sharp image?
Not really.

As I said above, take a Newt with identical aperture and same focal length as the Mak (so that it shows the same field) and it will show less coma (It'll also be spectacularly less practical, but I digress). If you want wide field scopes that require no coma corrector, you're looking at APO refractors or very long achromats, but they're usually either very expensive or terribly impractical (or both) for large apertures.

If you want to see more field, there's no alternative than to shorten the focal length; that's because the maximum field stop of 2" eyepieces is 46mm, so that in the formula I posted above, maximum field of view is inversely proportional to the scope focal length.

And if you want the same aperture, you have to pick a scope with a shorter f/ratio. So it's either more off-axis aberrations (if you want to keep the exit pupil reasonable you will have to use a large AFOV eyepiece), less aperture (many people do have a small refractor to complement their larger scopes for large objects -- I also do), a Paracorr, a very very long achromatic refractor (think observatory only) or an extremely expensive and heavy APO refractor. Pick your poison.

Edited by sixela
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.... If you want wide field scopes that require no coma corrector, you're looking at APO refractors or very long achromats, but they're usually either very expensive or terribly impractical (or both) for large apertures....

I believe that the maksutov-newtonian design has significantly lower levels of coma than a conventional newtonian.

I have an Intes 6" F/5.9 mak-newtonian that seems to be that rare thing - excellent for high power viewing of planets due to it's small central obstruction AND pretty good for low power, wide angle views because of it's relatively fast focal ratio :)

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Wow, this is good stuff!

I have also just read that the 'coma' issue really only is noticeable with point sources, i.e. stars, so for viewing disperse objects, it seems it does not matter so much.

sixela, you say "at low power (which is what the thread is all about), at large exit pupils your own eye's aberrations may start to dominate the aberrations, and in fact do so all across the field! Your eye lens is a lot better stopped down than used over its entire surface..."

This is something else I was trying to get my head around. That is, what gives the best image (subjectively the brightest, having the best contrast etc), an exit pupil of say 5mm, which 'fills' all the available aperture, or one of say 3 mm where the same amount of light is focused on a smaller area of the retina. From what you say it is better not to fill all the available retinal aperture, if possible. Is that right?

Edited by TheTalescopeMan
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P.s. sixela, you also say "Actually, designs different from a Newt are just as prone to off-axis aberrations, only different ones. SCTs have a lot of coma and field curvature relative to their effective f/ratio (in other words, an f/10 SCT has a lot more coma than an f/8 Newt). Maks and have a lot, too, though less than SCTs. Schmidt-Newts have a half the coma of a Newt with identical f/ratio, but they're usually a lot faster than Newts. APO Refractors have wide field without too many aberrations but field curvature unless you use a flattener."

Can you give any views on the newer version of the Tal 200K? This now has a focal ratio of 8.5, which is much faster than most Maks. (Most of which also have either a smaller aperture than the 200K or cost a lot more). It also seems to lack the Coma problem of a SCT. (See the test data below for the older F10 version).

TAL 200K OTA Review Nenad Filipovi?'s Homepage

I am on the brink of ordering one of these, and so far the main downside seems to be the somewhat restricted field of view. It now has a 1 1/14 Crayford focuser but can't use a 2" system because of the limitations set by the central baffle tube. I have calculated that with a 24 mm, 68 degree Baader Hyperion, the field of view would be 0.96 degrees with an exit pupil of 2.9mm mm. Even more economically a 32 mm Plossi would give a field of view of 0.98 degrees with an exit pupil of 3.8 mm. Which of these would give the 'best' view I don't know.

Another alternative is a 250mm f4.8 dob. With this a 24 mm Baader Hyperion would give a field of view of 1.36 degrees and an exit pupil 4.8 mm. I guess I could get a slightly wider field of view (1.52 degrees) with a 22mm, 82 degree Nagler but then I would be spending as much as getting the 200K.

OK, so the Dob would gather more light, but is the difference in the width of the field of view between the two something that is really worth worrying about, especially given the other advantages of the 200K? (For example, I was looking to mount the 200K on a HEQ5 and so have both tracking ability and reasonable portability).

P.s. My main priority is to get decent views of as many deep-sky objects as is possible, and would also like a 'scope that in the future can be adapted to do some basic imaging.

Edited by TheTalescopeMan
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Wow, this is good stuff!

I have also just read that the 'coma' issue really only is noticeable with point sources, i.e. stars, so for viewing disperse objects, it seems it does not matter so much.

It's not as obtrusive on extended objects, but just like other aberrations, it does kill contrast features stealthily on things like the moon and planets (of course it's only important on the moon, at least if you collimate properly and keep a planet centred in the view).

It's not important for any object that you see with night vision, because your night vision is really blurry. Coma's the least of your worries there.

This is something else I was trying to get my head around. That is, what gives the best image (subjectively the brightest, having the best contrast etc), an exit pupil of say 5mm, which 'fills' all the available aperture, or one of say 3 mm where the same amount of light is focused on a smaller area of the retina.

Actually, a 3mm exit pupil corresponds to a higher magnification than a 5mm exit pupil so it spreads the light of extended objects over a larger area of the retina. You seem to have an erroneous mental image of things.

The exit pupil is just a circle through which an infinity of light bundles (parallel with the eye focused at infinity) go through, but on your retina, it's the angle at which the different bundles pass through the exit pupil that dictates how light is spread, i.e. the magnification.

From what you say it is better not to fill all the available retinal aperture, if possible. Is that right?

I can't parse that.

For detection of extended objects you should (I'm cutting corners) make sure that in every dimension they appear at least 2° large, and beyond that, that they're as bright as possible. So for large objects detection is actually maximised at large exit pupils.

At least from a dark site; if the sky background is brighter than 21 mag/arcsec² then you do need to use more magnification to darken it and then observe for a couple of minutes to let your eyes adapt to the dark background in the eyepiece (otherwise the sky outside of the scope prevents full dark adaptation).

If e.g. your sky has a background brightness of 20 mag/arcsec², then as a rule of thumb it might be better to use an exit pupil that's roughly 1.5 times less (i.e. an exit pupil of roughly 4.5mm instead of 7mm, to give just one example). But it's a rule of thumb; if your pupil opens less then your eyes might well adapt to darkness quite well despite a slightly light polluted sky, or if your rhodopsin works differently you might have different experiences.

But detection isn't everything. If you select objects with such a high surface brightness that they're quite plainly visible, you usually apply more magnification to make the view more pleasing (darkening the sky background) and to see details within the object, rather than just detect the object as a whole (M51 is a lot nicer with clearly defined spiral arms than as an easier to detect featureless blob). That's why a 2mm exit pupil is such a workhorse exit pupil for most DSO work, and why for objects that are small but hav every high surface brightness (like some planetary nebulae) you magnify even more.

And then you have filters. They make the objects themselves dimmer but enhance the contrast between them and the background, and you need less magnification if you use them.

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It also seems to lack the Coma problem of a SCT.

I'll suprise a lot of people by saying this myself, but if you start seeing coma as a "problem" then you're probably well down the path of overanalysis. The most important thing to do with off-axis aberrations when you observe is to ignore them. If they bother you, you can always slew the scope so that what you observe is in the middle. Yes, it may be a bit of a hassle (detecting galaxies in the Perseus A cluster around NGC 1275 is a bit of a chore if everything at the edge looks like one) but it's not fatal. And if it bothers you too much there are remedies (like a Paracorr), or you can stick with a somewhat smaller AFOV for your medium field eyepieces.

With anything like an f/5 Newt or a typical SCT or Mak, I wouldn't describe coma as a "problem". Once you get to f/4.5, f/4, f/3.66 and f/3.3 scopes, things are a lot different, of course.

I have calculated that with a 24 mm, 68 degree Baader Hyperion, the field of view would be 0.96 degrees with an exit pupil of 2.9mm mm. Even more economically a 32 mm Plossi would give a field of view of 0.98 degrees with an exit pupil of 3.8 mm. Which of these would give the 'best' view I don't know.
Depends on the object. For detection of really faint objects at the edge of detection or when you want to use aggressive filters, the Plössl. For seeing details in the less hard DSOs the Hyperion (or better, the Meade SWA while they're cheap) will be better.
Another alternative is a 250mm f4.8 dob. With this a 24 mm Baader Hyperion would give a field of view of 1.36 degrees and an exit pupil 4.8 mm.
Don't get fixated too much on your maximal efficient exit pupil (ad simply assume you pupil only opens up to 5mm). There's also such a thing as framing large objects properly. Such a scope screams for a 34mm Mead SWA or equivalent, even if you pupil opens up to only 5mm.

P.s. My main priority is to get decent views of as many deep-sky objects as is possible, and would also like a 'scope that in the future can be adapted to do some basic imaging.

Well, to be honest, if you want to image DSOs you want something that'll take 2" gear and you'll want to do yourself a favour and start with a short focal length if this is your first astrophotograhpy platform. Might even be another scope than your visual one; sometimes trying to do too much with one scope gives you a jack of all trades and master of none and will cost as much as two separate scopes.

Unless you're thinking of lunar and planetary photography, in which case the Tal 200K is perfect (possibly after you get another focuser).

Edited by sixela
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Thanks sixela for your very informative answers.

Your discussion of detection vs. viewing detail and contrast in relation to exit pupil size was particularly interesting. I was thinking that, as the minimum magnification for the 'new' 200k is about 53x without vignetting becoming an issue due to the baffle tube intruding, then it might not be particularly good for deep-sky work. However, from what you say, whilst being able to use a lower magnification than this (and, correspondingly, having an exit pupil wider than 3.8 mm) would aid in the detection of really faint objects, when it comes to viewing detail in objects that can be detected, more magnification along with an even narrower exit pupil (say 2mm) would be no bad thing, enhancing contrast by darkening the back ground sky and so forth.

On reflection, I have already observed something very similar with my current (and first) scope, a Tal 1, where I have found that things such as the Ring and Crab Nebula look more defined if I use a higher magnification. This is something that surprised me given I had read that as higher magnifications spread out the available light more thay can make very faint object fade out. Your comments also make me suspect that the ease I have had in finding objects such as the Ring Nebula, even with my very modest scope, probably reflects the relatively dark French countryside where my observation site is located.

So, it seems that if I go for the 200K I might not be able to get super wide-field views and some objects may be just beyond the range of detection at the minimum possible magnification, but for those objects that can be detected, I should be able to pick out reasonable detail, naturally within the limitations set by the aperture.

Perhaps a good, relatively inexpensive eyepiece for deep-sky work with this scope would be something like a 21 mm Hyperion, giving a magnification of 81 times with a field of view of 0.84 degrees and an exit pupil of 2.1 mm?

I also see what you say about the dangers of buying a scope that does not really excel in any area, and I have seen the 200K being described as an all-round ‘scope. However, the 200K does seem to tick all the boxes that are most important to me, apart from something like a Vixen VMC 200 L, which is more expensive. (A F6 focal reducer for the Vixen also costs almost twice as much as the Tal one does, should I look to doing some basic imaging later on).

Edited by TheTalescopeMan
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On reflection, I have already observed something very similar with my current (and first) scope, a Tal 1, where I have found that things such as the Ring and Crab Nebula look more defined if I use a higher magnification. This is something that surprised me given I had read that as higher magnifications spread out the available light more thay can make very faint object fade out.

The thing is that what we detect is contrast between the background and the foreground. Sure, if both are brighter it's easier to detect, but it's not a linear relationship at all. And in fact, contrast features are also detected more easily if they're larger, so you sometimes have to make details fainter to make them large enough to be seen.

See the graph in attachment. These are contrast ratio curves: for a given angular size of object for the eye (it's a log scale, in arcminutes) it depicts how bright the background (and thus the object) has to be for a given contrast ratio to be detected (from experiments done in 1946 by Blackwell).

Telescopes never change that contrast ratio at all, and not even changing telescopes does. What a given telescope does is allow you to view the object at maximum surface brightness but make it larger, and changing eyepieces makes you trade surface brightness of object and background versus size (the slanted lines are for one telescope and different exit pupils). As you can see from the graph, for many contrast ratios, the telescope line and the barely detectable contrast ratio curve often have similar slopes.

What you can also see is (black line) that making features larger than more or less 2° (120 arcminutes) doesn't help detection, but making the background (whether it's the sky background or the part of an object over which you want to see some bright part of the object) darker than 25 mag/arcsec² also doesn't.

Your comments also make me suspect that the ease I have had in finding objects such as the Ring Nebula, even with my very modest scope, probably reflects the relatively dark French countryside where my observation site is located.
The ring nebula has very high surface brightness, so it takes a lot of magnification.

So, it seems that if I go for the 200K I might not be able to get super wide-field views and some objects may be just beyond the range of detection at the minimum possible magnification, but for those objects that can be detected, I should be able to pick out reasonable detail, naturally within the limitations set by the aperture.

Yes. It's just that looking at M31, M42, M45, M33, the Rosetta Nebula, etc. you won't be able to frame the object very well for a panoramic view. And DSO imaging is going to be tough if you use a DSLR sensor and a reducer, because the edge will essentially be black. Not to mention that even with a reducer, the focal length means you'll be imaging with the big boys from day one, which makes the learning curve VERY steep (I see a small 80mm ED doublet in your future :) ).
However, the 200K does seem to tick all the boxes that are most important to me and, apart from something like a Vixen VMC 200 L, which is more expensive.
Optically, they're very good scopes. An observing colleague frequently brings one. I don't like the focuser too much, but that's a detail and it's not that hard to focus the scope (because of the fairly long f/ratio and the fact the focuser is at the back, not moving the fast primary).

post-22037-133877515941_thumb.jpg

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Ôîðóì îïòè÷åñêèõ ïðèöåëîâ, îïòèêè, îðóæèÿ è òåëåñêîïîâ

Noticed a lot of very good DSO pics taken with 200K's and Tal reducer recently in the Tal factory forum(link above). I'm presuming it's the newer F8.5 Klev with crayford(since they've been out in Russia for 4 years or so).

I'm no imaging expert, but they sure look nice(edit: shows what is possible with these scopes, I guess).

Andy.

Edited by AndyH
spelling and an addition
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