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Reflector with quality like APO

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#1 AlbertoJ

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Posted 24 February 2013 - 07:25 PM

Hello,

Which 10" reflector gives a dark black background like a premium APO?
I´m interested in deep sky observation but black and no grey backgrounds.

I think about several options:
-Takahashi Mewlon 250.
-Intes-Micro Maksutov 10".
-Intes-Micro Maksutov-Newton 10".
-Newton Zambuto 10".

#2 Ed Wiley

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Posted 24 February 2013 - 07:50 PM

A Royce 10" Dall-Kirkham, at least if it is like my 8" version. At F20 those skies will be dark. 25% obstruction, but you have to up up with small FOVs. For general DSO viewing a good F5 Newtonian is hard to beat, I have a 12.5" I like a lot.

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#3 bremms

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Posted 24 February 2013 - 09:48 PM

F6-F8 10" Zambuto newt. NO contest. Ever use a super high quality Newt?
I had a 10" F6 with an Alika Herring custom mirror and 1/30 wave quartz diagonal. It handily bested an AP super planetary and C14 on Jupiter and Saturn. Detail was close on the C14 but contrast was not. AP had similar contrast but the 10" had much more detail. Haven't used the other scopes in that list. But I can't see how a compound scope with high order spherical or. Tak Mewlon that has a very large CO could compete.

#4 azure1961p

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Posted 24 February 2013 - 10:52 PM

So long as the baffling and such is at least adequate and the coatings are of equal age and reflectivity they will all be the same black - wether its cheap overseas optic or custom - they are all the same shade of black, or in truth - dark grey.

Surface roughness and such can affect planetary contrasts and the like but a sky background isn't going to illuminate because its not a custom optic. A lot of where this misconception arises is in improper unbalanced comparison. Small refractors are going to show a darker sky at 50x than a ten inch reflector at 50x - it'll look velvety and the stars like diamonds and it has more to do with everything BUT the fact that its a refractor - less atmospheric seeing resolved makes for steadier appearing stars and a smaller exit pupil makes for a darker sky. Frankly the refractor can even be junk and those two things will lend a darker appearing sky. I once had a friend who actually bemoaned a C14 for always having grey washed out background sky while his 4" Unitron boasted a "better" sky contrast. He didn't even begin to get it.

To answer your question though : opt for the reflector if you want darker appearing sky's as cassegrain systems can be prone to off axis light in the baffles throwing flare where reflectors would see less of the same thing simply by not having again, the baffle tube. Also some cassegrains have light leaking in around the CO in cases where they kept it on the small side to improve contrasts - a contradiction with purpose though.

So long as reflective coating are similar and tubes are well baffled wether its a Zambuto or Synta - the sky is the sky and any wishes for a 7" apo darkness through a 10" mirror is impossible and you wouldn't want it.

Pete

#5 GlennLeDrew

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Posted 24 February 2013 - 10:53 PM

First. Unless the exit pupil is down to near the 1mm range, the sky in any telescope is NOT 'black'.

Second. Other things being equal, an achro and an apo will deliver identical sky brightness.

Third. If a reflecting telescope is well baffled, the sky surface brightness will be as dark as in a well baffled refractor.

#6 azure1961p

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Posted 24 February 2013 - 10:56 PM

Lol you said that so succintly . It's a common misconception though.



Pete

#7 roadi

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Posted 25 February 2013 - 01:11 AM

Hello,

Which 10" reflector gives a dark black background like a premium APO?
I´m interested in deep sky observation but black and no grey backgrounds.

I think about several options:
-Takahashi Mewlon 250.
-Intes-Micro Maksutov 10".
-Intes-Micro Maksutov-Newton 10".
-Newton Zambuto 10".


All good choices, in this respect still though my vote would go to a 10" Zambuto for its excellent optical quality, best overall performance, and simple design with space for fiddling for improvements! ;)

#8 Erik Bakker

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Posted 25 February 2013 - 02:28 AM

Hi Alberto,

Good question. All scope you listed are excellent. If you want to use them for deep-sky while keeping your FS128, a 10" may be too small a step up in. I would go for something bigger, a 12" or better yet a 16" unless these become to big/heavy for you. I have an FS102 and a 16" f/5 and am very happy with that combo. And yes, at the proper exit-pupil it has dark backgrounds. Contrast is always wonderful.

Depending on your current mount, the Mewlon 250 could be nice, but like the fully mounted Questar 7 I once had, it is for medium-high powers only.

The biggest shock for me as a long time equatorially mounted refractor user was the wonder of using a quality dob. There is nothing like it to explore and observe the (deep-)sky. My dob was build by Matthias Wirth / APM around ultra-high quality Russian mirrors. The first object I observed with it was Jupiter in my binoviewer. Stunningly beautiful and the best Jupiter view I had ever seen, including decades at the eyepiece of a 266mm f/15 Alvan Clark refractor. Incidently the views are also better than in a Mewlon 300. And the FOV much wider of course. This and the light weight make the dob a very comfortable scope to set-up and use. A Mak-Newt is also very nice, but much heavier.

Since you live in Europe, if you wish to go for the 10" size, I can recommend a 10" f/4.8 Normand Fullum currently available at APM. It is Markus' personal scope and a real compact beauty. He also has an 11" f/4.36 Zambuto mirror available from stock at the moment, around which he can have Matthias Wirth build you a custom made high-end dob. The last option would make a stunning scope for sure.

Hope you can find a suitable companion to your FS128 soon!

#9 siriusandthepup

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Posted 25 February 2013 - 09:21 AM

Alberto,

I agree with everyone's advice offered above.

Here is another way to think about this "dark sky background" issue.

I was testing two same focal length eyepieces with a friend on his scope. He lives in a 5 mag zenith location with a fair amount of light polution. Think of that light polution as a gray mat on the sky. He said that he preferred eyepiece "A" over eyepiece "B" because "A" showed a darker sky background. I told him he had chosen the wrong eyepiece. Eyepiece "B" showed the "gray mat" of light pollution and "A" did not because "A" had much inferior light through put. "A" darkened the sky background - AND everything else. You could put a neutral density filter on the "B" eyepiece and achieve the same effect. You do not want the eyepiece with the built in filter.

If you are thinking this is incorrect, it's easy to verify on a known star field or cluster. Just pick out a section and identify some of the faintest stars you can see. Compare the two eyepieces.

Larger scopes gather more light and that includes the light from the "gray mat". At the same magnification, the smaller scope will have a darker background just because it gathers less light.

Higher magnifications will darken the background because you are looking at a smaller piece of the gray mat and spreading it over a larger area visually.

Moving your viewing to a dark site will of course give you a darker sky background in all the equipment you bring there.

#10 Eddgie

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Posted 25 February 2013 - 09:47 AM

As others have mentioned, sky blackness has nothing to do with whether the telescope is a refector or refractor.

It is a funtion of exit pupil.

If you want a bigger telescope, just find the biggest you are comfrotable setting up and using and be done with it.

But remember, to get the sky to be as black, you will have to use a similar exit pupil, and often this means that you will be forced to use very high powers.

Here is what I mean... 128x in a 5" telescope gives an exit pupil of 1mm.


If I look at M13 in my 5" APO at 127xm sure the sky is very black and you see a bit of granularity, but the sky is very very black to be sure.

But if you look at M13 in a 10" scope at 127x, the exit pupil is 2mm. The sky will not appear nearly as dark, but you see more stars in M13. The sky does not appear as dark only becuase the exit pupil is twice as large as in the 5" scope. But the better limiting magnitude of the 10 inch apeture allows you to see more stars.

But what happens when you go to 250x in the 10 inch scope.. Well, at 250x, the sky will be just as dark as in the 5" scope, but OMG, will the cluster look incredible. You will resolve it to the core.

On the other side of the coin though, at 250x, you might not see it framed as nicely in the 10" scope because well, the true field will only be half as large (eyepeiece types being the same). But M13 will be far better resolved than in the 5" scope.

So, what do you want. If you want big frames around objects with black skies, use a small telescope with a small exit pupil.

If you want to resolve more detail or stars in the target, use a big scope with a big exit pupil, and you get the framing, but you loose some sky blackness.

Or you can use the big scope with a small exit pupil to see the most detail from the target and keep the sky black, but loose true field.

Figure out what you want most.

Maybe the right step is to move to a 6" APO.

But if you want to see a lot more detail in more and more DSOs, you need more aperture.

You will trade off between sky brightness and image scale though. If you use similar powers that you use today, the sky will be brighter, but you will get the same fraiming.

If you want the sky as black, you will have to use similar exit pupils to what you are using today, but you will loose true field.

Only you can decide what the right compromise is.

I personally always recommend getting the biggest scope with the best optics that you can manage and afford.

Today, the number one recommendation I have been making is a 12" Go-To dob. If the mirrors are not great, you can spend another $650 and make them very high Strehl.

This will be my own next telscope I think.

#11 mmalik

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Posted 25 February 2013 - 12:37 PM

Here is a comparison of sorts...(Note: NOT recommending any..., just some numbers to look at)

Attached Files



#12 GlennLeDrew

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Posted 25 February 2013 - 12:50 PM

Whenever I see it written that magnification darkens the sky (and any extended object) because the light is "spread out", my impulse is to challenge this oversimplification which borders on myth.

That common explanation is a convenient fiction, because in a limited regime it just happens to describe what's observed. But it fails to address exactly what's really going on. This regime is found where the exit pupil is no larger than the iris.

It's not the increased *magnification* which causes the darkening. Rather, it's the reduced *exit pupil*. In the regime where the exit pupil is no larger than the iris, one can say that image surface brightness scales inversely with magnification. But once the exit pupil gets as large as the observer's iris, further reductions in magnification no longer results in image brightening; a ceiling is reached, even though the image is made to compress.

The common explanation for this image brightness ceiling at exit pupils equaling or exceeding the iris is that the objective is being stopped down. Yes, that happens to be true. But again, it misses an important aspect.

To understand what follows, I strongly urge you to do this. Aim your scope at any evenly illuminated source; it could be the daytime sky or a wall in your den. Swap out a variety of eyepieces so as to get a wide range of exit pupils. Look at each exit pupil from a distance of a foot or two. Note how, no matter the exit pupil diameter, the surface brightness of the exit pupil remains constant.

Here's the important part.

When you look into the exit pupil correctly, as when observing, or from a distance as when doing this test, the surface brightness of the sky never changes with exit pupil changes. Yes, you read that correctly!

When your eye is positioned correctly at the eye point, the exit pupil is located in the plane of your iris. If the exit pupil is the smaller of the two, it stops down your iris and thereby dims the view. An exit pupil smaller yet stops down your pupil further and hence further dims the image.

But when you examine the exit pupil from a distance, your entire iris always operates because the exit pupil is well removed from the plane of the iris. Through the exit pupil you see a small portion of the evenly illuminated target, whose surface brightness is equal to the direct, naked-eye view irrespective of exit pupil diameter.

You could prove this with a camera, be it a DSLR or point 'n shoot, as long as it permits manual exposure control. Take a metered shot of the target, lock the exposure and other settings, including focus, then shoot a series of a range of exit pupil sizes. In software, measure the brightness of all, to see essentially constant values.

(NOTE! to get sharp-edged images of the exit pupils for accurate measurements, focus *first on an exit pupil* from a foot or two, then lock the focus. Maintain this for all subsequent shots, including the target directly, even if it be the distant sky (it need not be 'in focus', although any evenly lit source cannot ever be out of focus anyway.) This is so that the effective f/ratio does not change due to the the lens--and its iris--moving along the optical axis.)

To sum up. Neither magnification nor its kin, the exit pupil, alters intrinsic image surface brightness. Observed image brightness is controlled purely by the diameter of the pupil in the plane of the eye's iris.

It's incorrect to say that increased magnification dims the view because of the spreading out of the light, for this implies an *intrinsic* change which does not occur. Simply state--correctly--that image diminution occurs due to the reduced exit pupil, which is an *extrinsic* cause.

#13 laconicsax

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Posted 25 February 2013 - 12:57 PM

Also relevant is light pollution. If your skies are already glowing from light pollution, the scope you're using isn't the biggest relevant factor.

#14 azure1961p

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Posted 25 February 2013 - 11:52 PM

Glenn,

Yeah but-

I'm getting the smaller exit pupil and its affect on perceived contrast but in the same token - the light is being spread out. A 1000x Jupiter is the same light volume as the 50x Jupiter but spread out across a larger area and as a result it is dimmed. I don't see why this logic doesn't explain what is infact happening but in the same token Im getting the stopping down of smaller exit pupils . Through it all though the same light of the large exit pupil is now grossly spread out with the finest exit pupils - it can be measured with apparent angle.

Thanks in advance.

Pete

#15 azure1961p

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Posted 25 February 2013 - 11:53 PM

My post is a question, not a statement !

Pete

#16 Jon Isaacs

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Posted 26 February 2013 - 07:49 AM

Whenever I see it written that magnification darkens the sky (and any extended object) because the light is "spread out", my impulse is to challenge this oversimplification which borders on myth.

That common explanation is a convenient fiction, because in a limited regime it just happens to describe what's observed. But it fails to address exactly what's really going on. This regime is found where the exit pupil is no larger than the iris.



I have to disagree that the light spreading out borders on myth or an over simplification. One need not invoke the exit pupil to explain why the image is dimmer. The image exists at the focal plane, the eyepiece magnifies that image. With a given number of photons, the greater the magnification, the fewer there are per unit area to illuminate the retina. The image is dimmer.

It is worth understanding that the surface brightness of the exit pupil is, excepting for various transmission losses, constant so that the surface brightness of any extended object at the retina only depends on the area if the exit pupil and is independent of the telescope.

But I think the light spreading explanation is also valid because indeed, it describes exactly what happens at the retina and it is easily understood. When someone is not clear that increasing the magnification results in a dimmer image, looking at what happens at the retina is easily understood. Including the exit pupil in that discussion may be counterproductive because it often requires it's own discussion.

A couple more points:

- I do think it's normal to assume in this particular discussion that all the light enters the eye.

- Also, part of the refractor-like view has to do brightness of the stars and how it relates to seeing. In a larger scope, the stars are brighter than in a small scope up until one resolves the Airy disk. Last night the seeing was so-so and the moon was full. Looking at Polaris, about 33 degrees elevation, it was messy in the 10 inch but the 9th magnitude companion was a nice pinpoint. In the 80mm, the main star appeared less disturbed but the companion required averted vision. Rigel was similar, the companion was a pinpoint in the 10 inch, the primary was not.

I do think that the size of the central obstruction does have an effect on the appearance of the stars. When one is dealing with 35% COs, the energy spread into the diffraction rings is significant. Thermal issues are also important as is field correction.

Some of what is associated with a "refractor-like" view is due to the properties of a high quality, optic with fewer thermal issues. A fair amount though is simply due to the limited aperture of the refractors we have access to.

The most refractor-like views I have experienced have been with my old 12.5 inch F/6 Newtonian. With three fans to cool the mirror and the large diameter phenolic tube, it's thermally stable, not affected by my body heat and does provide some awesome views of objects like open clusters that are often thought of as best suited for a refractor.

Jon

#17 Eddgie

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Posted 26 February 2013 - 08:53 AM

Whenever I see it written that magnification darkens the sky (and any extended object) because the light is "spread out", my impulse is to challenge this oversimplification which borders on myth.



You've addressed this to me as if this is something I said in my post. I didn't say anyting at all about the light being "spead out" in my post.

I said that exit pupil darkens the sky.

Perhaps you ment to address it to someone else.

#18 GlennLeDrew

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Posted 26 February 2013 - 01:13 PM

Eddgie,
I've never gotten into using the 'Reply' button heading each post, instead jumping immediately to the 'Quick Reply' field at the page bottom. hence the confusion as to whom I'm addressing (if I don't name the respondent.)

All,
I was trying to differentiate between what the instrument does from what our eye does.

All the retina cares about as regards image surface brightness is the solid angle of the admitting pupil. In other words, the f/ratio of the eye's own lens.

Imagine the following. Your scope is working at an exit pupil of 5mm, and your iris is 5mm. While gazing into the eyepiece, for some reason your iris constricts to, say, 4mm. The image will get visibly a bit dimmer. Nothing in the scope has changed.

We can consider telescopic image surface brightness as being relative to the maximum possible, the baseline set by the unaided eye at maximum pupil dilation. This differs from person to person (not to mention the dimensions of the eye itself, which affects its resultant f/ratio.)

Of course, because the eye and telescope do form an intricately linked *system*, for practical purposes it's not necessary to decouple them and separately consider their contributions. The net effect is that the eye's own lens is in a sense replaced by one of larger aperture and focal length, the f/ratio of which determines image surface brightness.

#19 siriusandthepup

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Posted 26 February 2013 - 09:39 PM

Glenn,

I understand and agree with most of your explanations regarding the eye, pupil size, etc.

Respectfully, I disagree with this quote:

Whenever I see it written that magnification darkens the sky (and any extended object) because the light is "spread out", my impulse is to challenge this oversimplification which borders on myth.


Let's reframe the discussion a bit in hopes of clarification.

Light pollution = no one gets a 100% dark sky back ground in their scope.

No light pollution = everyone gets a dark sky background regardless of magnification, exit pupil, telescope size. Don't we all wish!!

Now, let's go ahead and eliminate non terrestrial and near earth effects for simplification. No aurora, Gegenschein, Zodiacal light, etc.

The light pollution we most contend with are related to dust particles, pollen, pollutants (such as NOx), and water vapor. When these are in our observing air they cause light from cities, Moon, and other astro sources to be scattered and some of that scattered light ends up in our telescopes.

Now for the sake of argument lets say that a 1 degree circle on the sky that we are observing has 100 (1000, million, doesn't matter) particles scattering light. These contribute to the "graying" of our sky background. If we increase our magnification to the point that we are only looking at 10% of that 1 degree area, then only 10 light scattering particles are in our field of view (our eye pupil size still remains the same size as before) and of course assuming that our high power eyepiece has the same apparent field of view as our low power eyepiece. Having only 10% of the previous amount of light pollution in the same apparent field makes our sky background noticeably darker. This is the effect of "taking a patch of the gray mat of light pollution and stretching it over the field of view using magnification". I totally get that you might not be comfortable with saying it that way - everyone has a preferred way of expressing ideas.

This is my explanation of how increasing magnification darkens the background in your eyepiece. This is how I view and explain the effects of light pollution. I welcome your comments.

#20 Jon Isaacs

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Posted 27 February 2013 - 09:15 AM

Of course, because the eye and telescope do form an intricately linked *system*, for practical purposes it's not necessary to decouple them and separately consider their contributions. The net effect is that the eye's own lens is in a sense replaced by one of larger aperture and focal length, the f/ratio of which determines image surface brightness.



:waytogo:

I think both explanations, that of the exit pupil and that of the light on the retina are both useful. There are many ways to look at any given situation. To my mind, looking at the light on the retina is the simplest, most intuitively obvious, so it is a good place to begin.

Understanding the exit pupil can be counter-intuitive and often requires considerably more explanation, though in someways it is more fundamental. We both know that stating something as simple as "the surface brightness of an extended object through a telescope cannot be brighter than it is naked eye" can lead to endless discussions that never quite make it all clear.

Sometimes I think it is best to provide both the simple explanation and the more complicated one.

Jon

#21 GlennLeDrew

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Posted 27 February 2013 - 11:04 AM

Ed,
Jon has highlighted the paragraph from my previous post which might go some way toward addressing your concerns.

In my zeal to differentiate the physics of the telescope from the part the eye plays, I had obviously decoupled the two to an inappropriate degree.

As outlined earlier, it's instructive to examine exit pupil surface brightness, either by eye alone from a distance of a foot or two, or with a magnifier focused on the exit pupil. Aim the scope at the sky in daytime or at a well illuminated wall indoirs. Then swap eyepieces and/or stop down the objective. With any combination of these methods you'll see that the exit pupil surface brightness doesn't change. This illustrates the optical principle of etendue.

Another illustration of this is afforded by a variety of lenses having different forms and curves. While looking through them toward any bit of scenery, with lens near to the eye or far away, the surface brightness is always the same as that seen by eye alone. In spite of any enlargement, de-magnification or de-focusing done by the lenses.

#22 GlennLeDrew

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Posted 27 February 2013 - 11:41 AM

Hmmmm... I'm seeing this thread in the Cats and Casses Forum; wasn't it originally in the Reflector Forum?

#23 Jon Isaacs

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Posted 27 February 2013 - 01:50 PM

Hmmmm... I'm seeing this thread in the Cats and Casses Forum; wasn't it originally in the Reflector Forum?


Glenn:

As far as I know it has always been right here in the Cat's and Casses forum. It should be in the reflector forum because that is where scopes that are capable of providing apo like views are discussed. :)

Probably the scope that comes the closest to providing the sorts of views possible with an apochromatic refractor would be either a Maksutov-Newtonian or a standard Newtonian with a coma corrector. Both these are able to provide the well corrected widefield views possible with an apo and can be used with very small central obstructions so the energy is concentrated in the Disk. My little 130mm F/5 fitted with a coma corrector and the 31mm Nagler came surprisingly close to the sorts of views possible with my NP-101..

Jon

#24 siriusandthepup

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Posted 27 February 2013 - 02:21 PM

Thanks Glenn,

I have enjoyed the thought provoking discussion. Made me think about a few more interrelations.

Now I'd like to give my suggestion on the original question that started this thread.

Which 10" reflector gives a dark black background like a premium APO?
I´m interested in deep sky observation but black and no grey backgrounds.

I think about several options:
-Takahashi Mewlon 250.
-Intes-Micro Maksutov 10".
-Intes-Micro Maksutov-Newton 10".
-Newton Zambuto 10".



Several posters have said this already and I agree with them completely. A 10" or 12.5" f/5 or f/6 Newt with Zambuto, Royce or Orion UK premium mirror is very satisfying under a dark sky. Any of the scopes on your list are very excellent, but for deep sky I reccommend the f/5 or f/6 Newt. (It's much cheaper too!) Go for the 12.5" if you can fit it in your vehicle. Then drive to the darkest site you have available. A grey background is more related to the amount light polution you have than the much less important contrast differences between premium scopes. Premium scopes won't cure light pollution and neither will light pollution filters. Otherwise, we amateurs wouldn't put up with all those road trips. ;)

#25 Jon Isaacs

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Posted 27 February 2013 - 03:27 PM

A grey background is more related to the amount light polution you have than the much less important contrast differences between premium scopes.



I agree, a Gray background depends on the sky brightness and the exit pupil. In terms of the background sky brightness, the focal ratio does not matter, only the exit pupil.

Jon






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