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Equal exit pupil brightness in larger/smaller refractors

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#1 JP-Astro

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Posted 26 June 2020 - 08:09 AM

Since in one of the latest forum threads my question wasn't answered I hope this direct question will be addressed by good people here on CN.

Post #41 by Greg:

https://www.cloudyni...2#entry10289140

...suggested what I formulated as my following question.

 

Moved it from the above thread here to preserve the contents as it originally appeared in the above thread:

 

--------------------------------------

If I understand Greg's last explanation correctly then the same exit pupil of a larger aperture (refractor in our case) is brighter than in a smaller scope.

To exaggerate a little for the sake of clarity I understand that in a:

3" frac the exit pupil of 0.5 will appear to the eye rather dim

6" frac the exit pupil of 0.5 will appear to the eye much brighter in comparison

Is that because of... what? Sorry if I didn't get it from the Greg's comment above. Consider me dumb if you like. So much to learn!

Up to date I had an understanding that the exit pupil was a normalized measure to bring all devices to a common base.

To me it sounds more like 1 inch measured with a 10 inch long ruler is physically smaller than the inch measured with a 30 inch ruler.

Please give me more science on that. I'm not afraid to ask.


Edited by JP-Astro, 26 June 2020 - 08:10 AM.


#2 Asbytec

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Posted 26 June 2020 - 08:42 AM

I do not believe that is correct. Both at 0.5mm exit pupil will have the same image surface brightness. The reason is one will be twice the magnification of the other (150x and 300x), which means twice the light gathered in the larger aperture is spread over 4x the surface area of the smaller aperture image. The result is the same in both, just different image scale. So, the net result is the same image surface brightness.

As Glenn LeDrew likes to explain, the exit pupil is the arbitor of image brightness. It's proportional to the surface area of the objective and the exit pupil where the exit pupil is the objective aperture divided by the magnification. It's similar to any two scopes with the same same focal ratio (relative aperture), two different apertures at f/4 produce the same image surface brightness. It's related to image scale.

All else equal...

Often, folks do seem to observe at exit pupils far too dim for my tastes. I find Jupiter maxes out around 0.5mm exit pupil. Much higher and the image becomes too dim and I begin to lose detail. Some folks swear by Jupiter at 0.3mm exit pupils (often in refractors) which is nearly 700x in an 8" scope. Something I cannot imagine being good. But, I think it boils down to throughput and individual physiology.

Edited by Asbytec, 26 June 2020 - 11:15 AM.

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#3 JP-Astro

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Posted 26 June 2020 - 08:55 AM

Ok, thanks - I now feel myself somewhat better.

By the way, in the thread I referenced above in the numerous posts it was suggested that a higher aperture (refractor) would tolerate more power per inch for a given object.

 

In an article by Starizona they suggest completely the opposite: "A smaller aperture telescope will tolerate more power per inch than a larger instrument."

Here is a link to the article:

https://starizona.co...serving-theory/

 

So, they suggest that a larger 8" telescope will be normally used at a 30x per inch = 240 for maximum quality of the resulting visual image.

The exception as they suggest are smaller aperture telescopes that would allow from 40x to 50x per inch powers in the same conditions.



#4 SeattleScott

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Posted 26 June 2020 - 09:42 AM

Correct. The atmosphere is great for allowing us to breathe but not so great for stargazing. Let’s say the atmosphere is limiting magnification to 150x on a given night. With a 5” scope that is 30x per inch. With a 3” scope it is 50x per inch.

There is also some thought that a smaller scope is looking through a smaller column of air and therefore less affected by atmosphere. So a smaller scope might be able to hit 200x on a night when a bigger scope is only hitting 150x.

Scott

#5 Alan French

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Posted 26 June 2020 - 09:46 AM

The exit pupil is really a tiny image of the illuminated objective. All the light that strikes the objective passes through the exit pupil. So, yes, a 0.5mm exit pupil in a 6-inch refractor contains 4 times as much light as a 0.5mm exit pupil in a 3-inch refractor. 

 

But as others have pointed out, the 6-inch will give twice the magnification, so the image will have four times the area and will have the same surface brightness in both. The advantage of the larger scope is larger image scale, which allows its increased resolution to be perceived by your eye.

 

The primary function of telescopes is to make objects larger, revealing smaller details.  

 

Clear skies, Alan


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#6 daquad

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Posted 26 June 2020 - 10:06 AM

In addition to what others have said regarding extended objects having the same surface brightness an equal exit pupils, note that point objects, (i.e., the stars) will appear 4X brighter in the 6" vs. the 3".  

 

Dom Q.


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#7 Asbytec

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Posted 26 June 2020 - 10:58 AM

In an article by Starizona they suggest completely the opposite: "A smaller aperture telescope will tolerate more power per inch than a larger instrument."
Here is a link to the article:
https://starizona.co...serving-theory/

So, they suggest that a larger 8" telescope will be normally used at a 30x per inch = 240 for maximum quality of the resulting visual image.

There is a lot going on here. Like everything else related to observing, there are few hard and fast rules because there are many variables. Seeing, object type, observer acuity to name a few. Optical quality always finds its way into the conversation, too.

Image breakdown seems to happen at two or three points on the magnification scale. Once where seeing becomes objectionable to our preference for a steady image. Next when aberration or obstruction effects become visible, and lastly when the (object type) becomes too dim to see well at small exit pupils.

In my 8", I'm at 400x easily enough on Jove as the standard extended object. Non point source, extended bright low contrast detail is about max for me.

From the article. "This works out such that the image through a telescope can never be brighter than the image as seen with the unaided eye."

An interesting thought experiment with exit pupil is imagine you have a 6" scope operating at 1mm exit pupil and 150x. Point your scope at a small faint galaxy you can see.

Now, drop in an eyepeice that gives 1x and a 150mm exit pupil. Imagine further your iris can open to an impossible 6" to accept the light from the exit pupil. Now look up at that galaxy with your naked eye 150mm iris. The view through the telescope and your eye are the same. You don't even need a 6" telescope with a 6" iris opening. (Actually the telescope image is a little dimmer due to transmission loss).

But, if you looked at that galaxy through the telescope at 1x with a 7mm iris, you would not see it in the same way you can't see it naked eye. Not because its not bright enough, but because its not big enough like it was at 150x and 1mm exit pupil. You need a telescope to magnify an object because its not any brighter than it is to the naked eye. Of course, you need light grasp to magnify the galaxy so it retains the same surface brightness at larger image scales. That's why we select exit pupils to pass the light grasp of the scope into our more normal iris apertures.

Telescopes do not make images brighter than they really are, telescopes magnify objects. The result of magnification is the exit pupil. Or, said differently, magnification is the ratio of entrance pupil (aperture) to the exit pupil.

(Had to rush my reply...)

Edited by Asbytec, 26 June 2020 - 11:01 AM.


#8 JAC51

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Posted 26 June 2020 - 12:17 PM

In terms of magnification per inch Televue on their website recommend a "350x maximum regardless of aperture" which would correspond to 50x per inch for 7" telescope but 85.7x for a 4" scope but only 25x per inch for a 14" scope. Televue do though also recommend a maximum of 60x per inch regardless of aperture. Their recommendations would be in the same sort of vein as those of the starizona link in the post above.

 

As for "There is also some thought that a smaller scope is looking through a smaller column of air and therefore less affected by atmosphere. So a smaller scope might be able to hit 200x on a night when a bigger scope is only hitting 150x" I have experienced this with my little Tak FS60 compared to my TMB 130mm.

 

Jupiter very low in the sky, altitude 15 degrees or so in the 130mm 50x just possible, 10x per inch, FS60 70x, about 28x per inch, not great but main bands still visible. I know this might be an extreme example but at least it agrees with the principle.

 

The column of air idea has me thinking about the limit of atmospheric seeing on resolution I was taught in the early1970's when I was about 8 or 9. This theory was I was told "the cell theory of the atmosphere" and essentially it's the columns of air mentioned above.

 

The idea was to imagine the air as being made of narrow columns/cells of stable air. In the UK the typical cell might be 4" across and occasionally larger to a maximum of about 6", the best in the world might be 12" across. Therefore in the UK there would be no real advantage for high resolution for work in going beyond a 6" telescope and that even the mighty 200" Palomar reflector could not take a higher resolution image than and amateur 12' reflector.

 

In fact the man, who was the local insurance man who came round the doors, then demonstrated the principle to me. He put one hand palm up facing me and then moved it backward and forward to represent one 4" wide cell which a 4" telescope could look through quite happily.

 

He then put his other hand palm up beside the first one then began each one moving backwards and forwards independently to represent two  4" cells. So as he said a 4" telescope can look through each cell separately, which is fine but an 8" telescope is trying to look through two different cells at the same time and so it's resolution is in effect lower most of the time in the UK.

 

The demonstration still sticks my memory after all this time though not saying that this theory is even correct.

 

I can not even recall anyone else using the "cell size of the atmosphere" since that day.


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#9 csphere.d

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Posted 26 June 2020 - 12:37 PM

I think the point to keep in mind is this:  For a given magnification, the larger aperture will always provide a larger exit pupil- and thus a brighter image.


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#10 russell23

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Posted 26 June 2020 - 02:51 PM

I've found using refractors from 60mm to 120mm that the exit pupil is an excellent guide to the level of image brightness an object can handle before it is too dim.   So if object doesn't seem to dim until a 0.7mm exit pupil in one scope, it is pretty close to the same in a different aperture scope.

 

However, one variable is that the resolution increases in the large scope because the scope gathers more light and the given exit pupil is at a higher magnification.  So it might seem that a larger scope could handle a smaller exit pupil but if you reset your analysis of the image on the brightness and not the resolution then you realize that the increased resolution can trick you into missing the fact that the image is getting dimmer.

 

But maybe with the larger scope the increased resolution allows a person to tolerate a dimmer image. 

 

Sorry if that adds more confusion than clarity.



#11 Asbytec

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Posted 26 June 2020 - 05:45 PM

I think the point to keep in mind is this:  For a given magnification, the larger aperture will always provide a larger exit pupil- and thus a brighter image.

In practice, folks tend to use larger exit pupils with larger apertures because the magnification is already pretty high. The eye likes a bright image, but it also like a large image. In smaller apertures, the eye is starved for image scale so we tend to push magnification a little higher, i.e., working at smaller exit pupils. 


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#12 Jon Isaacs

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Posted 26 June 2020 - 07:02 PM

In terms of magnification per inch Televue on their website recommend a "350x maximum regardless of aperture" which would correspond to 50x per inch for 7" telescope but 85.7x for a 4" scope but only 25x per inch for a 14" scope. Televue do though also recommend a maximum of 60x per inch regardless of aperture. Their recommendations would be in the same sort of vein as those of the starizona link in the post above.

 

 

Do you find those limits are reasonable in your experience?

 

It certainly doesn't work that way for me. On doubles, I use up to 80x/inch in scopes up to 10 inch, (820X)  I use higher magnifications with larger scopes.

 

On the planets, 400x is common, planetary Nebulae, in the bigger scopes, 800x-1000x. Maybe more.

 

The seeing is part of it but image brightness is very important. In a 4 inch, a 1.6 mm exit pupil is about 64x, in my big scope, it's 350x, I'm starhopping in the looking for faint galaxies. 

 

Jon


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#13 Jon Isaacs

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Posted 26 June 2020 - 07:06 PM

In practice, folks tend to use larger exit pupils with larger apertures because the magnification is already pretty high. The eye likes a bright image, but it also like a large image. In smaller apertures, the eye is starved for image scale so we tend to push magnification a little higher, i.e., working at smaller exit pupils. 

 

I think Norme nailed it. With a large scope, you have magnification to burn, you can back off some and buy a brighter image.

 

Jon


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#14 Bomber Bob

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Posted 26 June 2020 - 08:24 PM

Often, folks do seem to observe at exit pupils far too dim for my tastes. I find Jupiter maxes out around 0.5mm exit pupil. Much higher and the image becomes too dim and I begin to lose detail. Some folks swear by Jupiter at 0.3mm exit pupils (often in refractors) which is nearly 700x in an 8" scope. Something I cannot imagine being good. But, I think it boils down to throughput and individual physiology.

 

I'm in the "That's Too Bright!" category.  I have to magnify Mars / Jupiter / Saturn, or the finest details get washed-out in the glare.  I was born light-sensitive, and I spent decades & 1000s of hours picking out planetary details with an 80mm F15 refractor.  The first NVGs (Night Vision Goggles) I got in the USAF were TOO DANG BRIGHT at their lowest setting, and at times made me a dangerous flyer (that was 30+ years ago, the tech has greatly improved).

 

And, I can't stand glare in a scope.  As it relates to the topic, and the final image a scope presents, I'm a big-time advocate for Glare Control / Suppression / Elimination.  Most of my experience is with fracs, but I'm slowly learning the tricks with reflectors.  For my weird (probably aerial camera lens) ATM 5" F5 non-ED triplet, I made a cone baffle for about the last 3rd of the light cone to the focuser -- along with my usual flocking & blackening.  Huge improvement in contrast, and in view quality at the eyepiece.

 

ATM 5x5T - Restore S21 (Cone Attached).jpg ATM 5x5T - Restore S22 (Cone to Eye View).jpg


Edited by Bomber Bob, 27 June 2020 - 09:43 AM.


#15 gnowellsct

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Posted 27 June 2020 - 12:26 AM

The exit pupil is really a tiny image of the illuminated objective. All the light that strikes the objective passes through the exit pupil. So, yes, a 0.5mm exit pupil in a 6-inch refractor contains 4 times as much light as a 0.5mm exit pupil in a 3-inch refractor. 

 

But as others have pointed out, the 6-inch will give twice the magnification, so the image will have four times the area and will have the same surface brightness in both. The advantage of the larger scope is larger image scale, which allows its increased resolution to be perceived by your eye.

 

The primary function of telescopes is to make objects larger, revealing smaller details.  

 

Clear skies, Alan

...not just resolution.  Color saturation too.  A dark room is shadowy-gray.  Turn on a light you see colors.  Let the sun in you see more intense colors.


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#16 gnowellsct

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Posted 27 June 2020 - 12:40 AM

Since in one of the latest forum threads my question wasn't answered I hope this direct question will be addressed by good people here on CN.

Post #41 by Greg:

https://www.cloudyni...2#entry10289140

...suggested what I formulated as my following question.

 

Moved it from the above thread here to preserve the contents as it originally appeared in the above thread:

 

--------------------------------------

If I understand Greg's last explanation correctly then the same exit pupil of a larger aperture (refractor in our case) is brighter than in a smaller scope.

To exaggerate a little for the sake of clarity I understand that in a:

3" frac the exit pupil of 0.5 will appear to the eye rather dim

6" frac the exit pupil of 0.5 will appear to the eye much brighter in comparison

Is that because of... what? Sorry if I didn't get it from the Greg's comment above. Consider me dumb if you like. So much to learn!

Up to date I had an understanding that the exit pupil was a normalized measure to bring all devices to a common base.

To me it sounds more like 1 inch measured with a 10 inch long ruler is physically smaller than the inch measured with a 30 inch ruler.

Please give me more science on that. I'm not afraid to ask.

   

Really all you have to do is push an 80mm to 160x and push a 130 mm to 260x, there is a difference.  The scale improves  but the colors improve too.  

 

Actually my latest experiments were on Wednesday, it was a 92mm CFF vs an AP 130 GT on Jupiter.  You don't just see more detail.  You see more color IME.

Perhaps I should say more intense color.

 

If you go the other way, controlling magnification: some of the brightest Jovian colors I've seen were in the C14 at 100x.  One does not typically use a C14 for planet viewing at this power, at 100x it is loitering (one uses a 40mm ocular).  The 80mm I had at that power showed some much dimmer colors and less distinct details.  At 100x it was working hard.     Jupiter at 100x in the C14 is blinding.  I should have deployed a filter of some kind.

 

GN



#17 JAC51

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Posted 27 June 2020 - 02:05 AM

" In terms of magnification per inch Televue on their website recommend a "350x maximum regardless of aperture" "

 

I'm not necessarily agreeing with Televue numbers as such. The highest magnification I have used with my 5" refractor so far on the moon is x300. As I'm just getting interested in double stars I'm already thinking of how to push my magnification beyond this to say try X400 x500 on for example Zeta Herculis which would give 80x/100x per inch (seeing permitting). 


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#18 John Huntley

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Posted 27 June 2020 - 04:08 AM

" In terms of magnification per inch Televue on their website recommend a "350x maximum regardless of aperture" "

 

I'm not necessarily agreeing with Televue numbers as such. The highest magnification I have used with my 5" refractor so far on the moon is x300. As I'm just getting interested in double stars I'm already thinking of how to push my magnification beyond this to say try X400 x500 on for example Zeta Herculis which would give 80x/100x per inch (seeing permitting). 

My ED120 splits Zeta Herc from around 200x and up. The optimum with that scope for that binary seems to be about 260x - 280x.


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#19 Tamiji Homma

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Posted 27 June 2020 - 07:39 AM

Since in one of the latest forum threads my question wasn't answered I hope this direct question will be addressed by good people here on CN.

Post #41 by Greg:

https://www.cloudyni...2#entry10289140

...suggested what I formulated as my following question.

 

Moved it from the above thread here to preserve the contents as it originally appeared in the above thread:

 

--------------------------------------

If I understand Greg's last explanation correctly then the same exit pupil of a larger aperture (refractor in our case) is brighter than in a smaller scope.

To exaggerate a little for the sake of clarity I understand that in a:

3" frac the exit pupil of 0.5 will appear to the eye rather dim

6" frac the exit pupil of 0.5 will appear to the eye much brighter in comparison

Is that because of... what? Sorry if I didn't get it from the Greg's comment above. Consider me dumb if you like. So much to learn!

Up to date I had an understanding that the exit pupil was a normalized measure to bring all devices to a common base.

To me it sounds more like 1 inch measured with a 10 inch long ruler is physically smaller than the inch measured with a 30 inch ruler.

Please give me more science on that. I'm not afraid to ask.

 

It is different topic but there are many interesting discussions regarding surface brightness in this thread.

When you get free time, you might enjoy it :)

 

https://www.cloudyni...vs-focal-ratio/

 

Tammy


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#20 Asbytec

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Posted 27 June 2020 - 07:51 AM

Here's another.

https://www.cloudyni...ection-of-dsos/

#21 Alan French

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Posted 27 June 2020 - 09:48 AM

...not just resolution.  Color saturation too.  A dark room is shadowy-gray.  Turn on a light you see colors.  Let the sun in you see more intense colors.

A dark room is shadowy gray because there's not enough light to engage your color vision. When you turn on the light the cones come into play and you can see color. 

 

If you use a 3-inch and a 6-inch at magnifications yielding the same exit pupil the surface brightness will be the same, but the image in the 6-inch will appear twice as large. No difference in color saturation. 

 

In reality larger telescopes are more often seeing limited and rarely reach the magnifications need to fully meet their capabilities. I remember having Sue's 14.5-inch f/6 Newtonian out several years ago when Jupiter was well placed. I was playing wth a 6.5-inch off axis mask and was using about 200x, So the exit pupil with the mask was ~0.8mm and without the mask it was ~1.8mm. In this case, since the magnification was the same, the color saturation with full aperture was higher, and colors were more obvious. 

 

I also found the larger exit pupil more agreeable, and there were times when full aperture showed more detail. 

 

Clear skies, Alan 


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#22 Bomber Bob

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Posted 27 June 2020 - 09:54 AM

...not just resolution.  Color saturation too.  A dark room is shadowy-gray.  Turn on a light you see colors.  Let the sun in you see more intense colors.

When I rack-up the power on a refractor, I'm looking for fine details, so I'll sacrifice colors.  On my 1950s Edmund 4" F15 @ 300x (75x / inch), that meant whites, grays, & blacks... but, I could not only see a festoon on Jupiter, I could trace its "tail" in a zone until it faded away...

 

The first time I saw really intense colors on Jupiter, I was testing my Tinsley 6" F20 Cass after Majestic recoated its mirrors.  No Lie:  At just 60x with a spectros KE 50mm, I was stunned.  So many shades of brown, orange, & gray -- and the GRS looked like a bloody wound.  More amazing to me, the intensity didn't fade much going up to 100x & 200x in that scope.  (Yeah, it's a Keeper!)

 

Here's a sample from a Shoot-Out between the Edmund 4" F15 and a 6" F23 Mak-Cass:

 

ESC4_MM6 - Jupiter Sketches 20170430 S02.jpg


Edited by Bomber Bob, 27 June 2020 - 10:34 AM.


#23 Alan French

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Posted 27 June 2020 - 10:10 AM

We know that size plays a role in the perception of faint objects. A dim DSO may only become visible when it is enlarged and covers a larger area of the retina. 

 

The perception of colors can also depend on size.

https://research.tab...Matters-CGA.pdf

 

Anyone know of any articles on image scale and color perception in planetary viewing?

 

Clear skies, Alan



#24 Galicapernistein

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Posted 27 June 2020 - 10:14 AM

When I rack-up the power on a refractor, I'm looking for fine details, so I'll sacrifice colors.  On my 1950s Edmund 4" F15 @ 300x (75x / inch), that meant whites, grays, & blacks... but, I could not only see a festoon on Jupiter, I could trace its "tail" in a zone until it faded away...

Good point. Image brightness and high contrast don’t always go together.



#25 havasman

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Posted 27 June 2020 - 10:31 AM

" In terms of magnification per inch Televue on their website recommend a "350x maximum regardless of aperture" "

Televue makes small aperture refractors and for such scopes that may be sound advice. But in the modern world of high quality large aperture reflectors adhering to such a rule would deprive the observer of advantages such equipment brings. That statement has been roundly discredited in these forums.




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