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Aperture, Magnification & Exit Pupil

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46 replies to this topic

#26 cuzimthedad

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Posted 05 December 2020 - 02:10 PM

Welcome to Cloudy Nights!


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#27 MunichAtNight

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Posted 06 December 2020 - 04:04 AM

 

Hello and welcome.gif to Cloudy Night.  

 

Over the years I have been fortunate enough to acquire several Dobs and several are applicable to your situation, the 10 inch F/5 tube Dob, a 12.5 inch F/4.06 truss that was a tube Dob, the 13.1 inch F/5.5 truss Dob and the 16 inch F/4.4 that was a tube Dob and is now a strut/truss scope. For a 300mm aperture or greater, a truss style scope is the best choice.  It takes up so much less room in the car....

 

....Coma:  At F/5, coma can be seen and a coma corrector is nice to have but one can get by without one.  At F/4, I consider it a necessity. I sometimes view with the coma corrector in my F/5 scopes and even the F/4.4's but never the F/4.06.

....Each increase in aperture is worthwhile.  The step up from the 250mm F/5 to the 318mm F/4 makes a noticeable difference, the step up from the 318mm to the 406mm, it very apparent in terms of what i see.  

Hello Jon,

thank you for the information. I think it is a good idea to save some budget for a coma corrector but I will wait to buy until I have the F4 telescope, so I see personal the need for it! what kind of coma corrector do you use? I know one from ES but I have read that it need some additional adjustments when changing eyepieces - as far as I understand the best but the most expensive is from Televue built-in the feathertouch but this is OT here, I will start for this a new thread.

 

 

Yes, think about exit pupil

 

But Mel Bartels takes the position that what you want is 1 EP and multiple scopes--one scope for each desired FoV.

So if you are looking at all of M31, you want something like a 8" F/3 and a 21E, but if you are looking at Saturn nebula you want something like 40" F/3 for the light and for the (smaller) FoV and you can still use that same 21E !

Thanks Mitch! I didn't know about Mel Bartels - but very interesting homepage with lots of infos about lots of different telescopes! Gigantic! I have a lot to read! Yes you are right, each DSO needs it's specific instrument! The only, little problem to solve this is limited storing space, budget and time! smirk.gif As "German" native I have to ask, shame on me, 21E stand for 21 mm eyepiece?

 

New question! I got in my four month during experience that using eyepieces with an exit pupil below 1mm causes for me and my wife a viewing with much, much less contrast. A difference in matter of decreasing contrast is visible between 1.7 mm, 1.3 mm, 1.0 mm exit pupil. But when exit pupil becomes below 1mm for example to a exit pupil with 0.9 mm it is "dramatical" worse. Do others see / feel a similar exit pupil border, too?

 

CS - MunichAtNight - Ewald



#28 Asbytec

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Posted 06 December 2020 - 04:30 AM

 

New question! I got in my four month during experience that using eyepieces with an exit pupil below 1mm causes for me and my wife a viewing with much, much less contrast. A difference in matter of decreasing contrast is visible between 1.7 mm, 1.3 mm, 1.0 mm exit pupil. But when exit pupil becomes below 1mm for example to a exit pupil with 0.9 mm it is "dramatical" worse. Do others see / feel a similar exit pupil border, too?

 

CS - MunichAtNight - Ewald

Ewald, that objects are you viewing at that magnification. If dim objects, then they may become too dim for the eye to see as the image scale increases. I have not experienced any loss of contrast, so I don't know what to say about it. Actually contrast should remain unchanged in the telescopic image, but some degradation may occur on our eye as the object dims. Objects do become dimmer at smaller exit pupils. However, one should be able to hit 0.5mm exit pupil on bright objects like the moon and planets. The should be plenty bright to see at high magnification. Can you elaborate a bit more on what types of objects? 


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#29 Jon Isaacs

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Posted 06 December 2020 - 05:07 AM

Yes, think about exit pupil

 

But Mel Bartels takes the position that what you want is 1 EP and multiple scopes--one scope for each desired FoV.

So if you are looking at all of M31, you want something like a 8" F/3 and a 21E, but if you are looking at Saturn nebula you want something like 40" F/3 for the light and for the (smaller) FoV and you can still use that same 21E !

 

I was saying that 15 years ago.  But only as a joke.  Large exit pupils are desirable for some object but very few.  For most, a small exit pupil that is somewhere near the diffraction limit of the human eye is a better choice.  If you can't see the Airy disk as an Airy disk, then you're not operating at full resolution, your eye is limiting resolution, not the telescope.  

 

A 20 inch at 500x-1000x will show the Saturn nebula better than a 40 inch F/3 with the 21mm Ethos. That's only 170x. It's a poor choice for splitting doubles, for the planets for small objects like the Saturn nebulae. 

 

Eyepieces are about matching the object, the telescope and the eye.  Large exit pupils are rarely optimal.... 

 

Jon


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#30 MunichAtNight

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Posted 07 December 2020 - 07:23 PM

Ewald, that objects are you viewing at that magnification. If dim objects, then they may become too dim for the eye to see as the image scale increases. I have not experienced any loss of contrast, so I don't know what to say about it. Actually contrast should remain unchanged in the telescopic image, but some degradation may occur on our eye as the object dims. Objects do become dimmer at smaller exit pupils. However, one should be able to hit 0.5mm exit pupil on bright objects like the moon and planets. The should be plenty bright to see at high magnification. Can you elaborate a bit more on what types of objects? 

Hello Asbytec,

 

sorry for my might be faulty expression. I am not so used to the wording in English language in astronomical environment. What I wanted to express is exactly what you described with dimming the object. We used to see this effect specially with planets for example this autumn with Mars.So with the ES 8.5 or 6.7 mm eyepiece in our 10" Newton F5 Mars looks great with an idea of impression recognizing structure on the surface. With the ES 4.5 mm Mars looks like a "bumby bowl". This might be an effect of a bad seeing, too. But we had this impression every night, even we had very good condition in seeing with the "longer" (= 6.7 / 8.5 mm) eyepiece. I am sorry for only having experience for about 20 nights and me and my wife still have to learn and to experience a lot! It is hard to describe, specially in not a native language. Sorry for this! - MunichAtNight - Ewald


Edited by MunichAtNight, 07 December 2020 - 07:24 PM.


#31 Asbytec

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Posted 07 December 2020 - 08:39 PM

Ewald, I apologize. I should have been more sensitive to language differences. But your English is actually pretty good, so I didn't catch any nuances. :)

In any case, dimming of the image is normal. The 4.5mm will be dimmer, bit I hope not be too dim for both of you. Seeing conditions, thermal instability, and collimation can cause a reduction in contrast and resolution on bright objects like Mars.

Edited by Asbytec, 07 December 2020 - 08:51 PM.


#32 Starman1

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Posted 07 December 2020 - 08:56 PM

Hello Asbytec,

 

sorry for my might be faulty expression. I am not so used to the wording in English language in astronomical environment. What I wanted to express is exactly what you described with dimming the object. We used to see this effect specially with planets for example this autumn with Mars.So with the ES 8.5 or 6.7 mm eyepiece in our 10" Newton F5 Mars looks great with an idea of impression recognizing structure on the surface. With the ES 4.5 mm Mars looks like a "bumby bowl". This might be an effect of a bad seeing, too. But we had this impression every night, even we had very good condition in seeing with the "longer" (= 6.7 / 8.5 mm) eyepiece. I am sorry for only having experience for about 20 nights and me and my wife still have to learn and to experience a lot! It is hard to describe, specially in not a native language. Sorry for this! - MunichAtNight - Ewald

When the atmosphere is steady, look to other issues that cause "bad" seeing.

1) set up the scope on dirt or grass, not concrete or asphalt. The harder materials get hot during the day and lose heat all night, causing local turbulence in the air.

2) don't look at an object directly above a roof.  Roofs leak heat into the air almost all night and cause bad seeing directly above their surfaces

3) don't look at objects below 30° if you can.  The atmosphere is twice as thick at 30° as it is at the zenith above and the extra air causes more turbulence and loss of transparency.

The atmosphere at the horizon is 10-12x as thick as at the zenith, so the lower the object is, the worse the "seeing".

4) Atmospheric refraction will cause chromatic aberration in a target at low altitude.

5) at many sites, the atmosphere is quieter and the seeing better after midnight.  That doesn't work for the planets right now.  It's one of the reasons why 

the best time to look at planets is when they are near opposition--rising at sunset and at the N-S meridian near midnight.

 

All of these problems are worse at higher powers.

 

Collimation of the scope becomes more critical at high power.  A collimation that is just OK at 100x will not be good at 250x.

Cooling of the mirrors is critical at high power.  As glass mirrors cool, they give off heat that warms a boundary layer of air right in front of the mirror and that boundary layer will blur the image

at high power.  The important thing is for the mirror to be close to the ambient temperature and unless a fan is used behind the mirror, this can take many hours--the planets will have set by then.

Cooling is especially important with a mirror as large as 10" (254mm).

 

Low powers in your 10" (40-100x) will generally be clear and stable.

Medium powers (100-200x) will usually be clear and stable

High powers (200-300x) will occasionally be clear and stable, but the atmosphere will say if this is true or not.

Ultra high powers (300-500x) will only be clear and stable rarely.  It doesn't really pay to have eyepieces to create these powers--a Barlow lens will do fine.

 

But note that nearly all your viewing will be done at 250x and lower (1mm exit pupil at larger)


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#33 Glory Eye

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Posted 09 December 2020 - 09:27 PM

Very nice thread.

 

Since I observe with a smaller aperture, it dawns on me the other half of the equation is can one observe the 5 or 8" image at 200x? Trade precious surface brightness for a little larger image scale. I observe a lot of bright galaxies between 2mm and just less than 1mm exit pupil, but I do not recall M51 at anything smaller than 2mm (around 100x). Or larger than 2mm, for that matter. 

 

Off topic and intended only for further reading for those interested and have not seen Glenn LeDrew's work on DSO visibility here and an earlier version here

Asbytec,

It is important to note that the brightness scale in Post #1 is Relative, NOT absolute. This means that if an object is the brightness as shown for the 25", the comparative brightness in the 8" is almost imperceptible, but since its a relative scale it doesn't mean that all objects are invisible in an 8" at 200x in absolute terms. In other words, I'm sure there are bright objects that would show up well in an 8" at 200x. The point is to show how much brighter or dimmer an object is in comparison with increasing or decreasing aperture. Whether or not an object visible in a 25" is also visible in an 8", is not determined by aperture and magnification alone, but is also determined by the object's absolute brightness.


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#34 Jon Isaacs

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Posted 10 December 2020 - 06:25 AM

Asbytec,

It is important to note that the brightness scale in Post #1 is Relative, NOT absolute. This means that if an object is the brightness as shown for the 25", the comparative brightness in the 8" is almost imperceptible, but since its a relative scale it doesn't mean that all objects are invisible in an 8" at 200x in absolute terms. In other words, I'm sure there are bright objects that would show up well in an 8" at 200x. The point is to show how much brighter or dimmer an object is in comparison with increasing or decreasing aperture. Whether or not an object visible in a 25" is also visible in an 8", is not determined by aperture and magnification alone, but is also determined by the object's absolute brightness.

 

One issue that hasn't been discussed is the fact that dark adaptation can be a function of exit pupil.  So while the image of M51 will be brighter in the 25 inch, the eye may not be as fully dark adapted.  

 

My own experience is much like Don said.  Larger apertures allow higher magnifications as well as brighter images.  Mostly though, I find that I use nearly the same exit pupils in my 16 inch as a I do in my 22 inch and for most objects, the gain in what I see is due to the increased magnification and not the brighter image.  Both scopes are F/4.4, F/5.06 with the Paracorr, so the same eyepiece provides the same exit pupil.  For eeking out small faint galaxies, I use the same eyepieces in both scopes.  

 

For objects like galaxies and many DSOs, 200X is an unrealistic limit for a larger aperture scope.  In 2 arc-second seeing, I will use much more than 200x in most of my Dobs.  

 

The other side to more magnification with a larger scope is a dimmer image but a greater relative magnification and a more fully dark adapted eye with a smaler scope.  At 200x in a 25 inch, the exit pupil will be 3.1mm.  In Norme's 8 inch it would be 1mm but I would be probably using something like 125x which is a still reasonably bright 1.6mm exit pupil.  

 

The biggest factor though is the skill of the observer.  I wouldn't bet against Don nor Norme.  It's not their telescopes that perform well beyond their aperture, it's Don and Norme perform well beyond normal beings.  

 

Jon


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#35 Asbytec

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Posted 10 December 2020 - 06:43 AM

Jon, thank you for saying so. As you know, learning to see is something I share often because of the difference we can make. To the point of this thread, observing includes factoring in the exit pupil.

Gone are the old days of 7mm exit pupils, bright images, and disappointing faint fuzzies. Galaxies have taken on a new life, even if they lack the grandeur of a 16" aperture. All credit goes to the expert information shared on CN over the years.

Edited by Asbytec, 10 December 2020 - 06:44 AM.

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#36 Jon Isaacs

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Posted 10 December 2020 - 06:57 AM

Jon, thank you for saying so. As you know, learning to see is something I share often because of the difference we can make. To the point of this thread, observing includes factoring in the exit pupil.

Gone are the old days of 7mm exit pupils, bright images, and disappointing faint fuzzies. Galaxies have taken on a new life, even if they lack the grandeur of a 16" aperture. All credit goes to the expert information shared on CN over the years.

 

Norme:

 

It's only necessary to look at your sketches... 

 

Myself, I don't claim to be anything more than an average observer who tries hard to be the best possible observer I can be.  I do spend a fair amount of time observing with smaller scopes, 60mm-100mm because I think it's easier to push the limits of a small scope.  Seeing isn't such an issue, finding the object is easier and high magnifications/small exit pupils are more easily tracked. 

 

My skills hopefully honed in a smaller scope can then translate into pushing a larger scope where just operating the scope is a bigger challenge and conditions are often an issue.  

 

Jon



#37 Asbytec

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Posted 10 December 2020 - 07:27 AM

Jon, I was remiss in crediting your contributions, Don's too, among too many to mention and be thorough naming everyone.

That's the thing, my acuity is not what it once was, but it's probably always been average. Our max iris gets smaller with age, and other problems with our eyes develop. But we all learn and gain experience. In other words, maybe it's safe to say most of us are average.

If we have any advantage, surely its the knowledge we've gained and share and the effort and love we put into it. I don't see more than others by standing on the shoulders of giants. It's more like I might see as far getting a piggy back ride. :lol:

I agree. Smaller apertures force us to look harder, develop techniques, and use knowledge just to see anything at all. I think it's so important to learn to trust what we think we see just shy of our imagination and how to recognize it when we do see it. That comes with practice and making sense of the visual image we peice together over time. If we think we saw something, often it's because we did and learned to tell the difference when we didn't.

I was interested in the OPs image of M51 through an 8" at 200x. It's large and dim, too much so for my liking. But, I wondered if someone could actually observe that image at the eyepeice. After all, it is there to be seen.

Edited by Asbytec, 10 December 2020 - 07:45 AM.

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#38 Jon Isaacs

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Posted 10 December 2020 - 08:46 AM

I was interested in the OPs image of M51 through an 8" at 200x. It's large and dim, too much so for my liking. But, I wondered if someone could actually observe that image at the eyepeice. After all, it is there to be seen.

 

 

Norme:

 

There are a number of things wrong with the simulation.  Regardless of the aperture, it won't be in color.  

 

But more importantly, the issue that has been continuously raised is that comparisons of extended objects need to be done at constant exit pupil and not constant magnification.  The reason the disappear is that the exit pupil is too small.  At 64x, M51 will be the same brightness as it at 200x in the 25 inch.  It doesn't disappear, it just gets smaller.

 

As far as seeing the image in the eyepiece.  The eye has very poor resolution at low light levels.  A camera on a 3 or 4 inch will show details similar to what one sees in a relatively large Dob.  

 

I think the best way to see just how bad the resolution of the eye is at low light levels is to view the moon through a 100,000:1 solar filter.  The sky is super dark, the full moon has a surface brightness through the filter of about 16mpsas, a quarter moon has a surface brightness of about 18 mpsas.  The detail is there to be seen, it's just 12.5 magnitudes dimmer than it is without the filter.  I could never see any detail. 

 

Jon


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#39 Asbytec

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Posted 10 December 2020 - 12:50 PM

Jon, another issue with simulated images, in my view, is they do not represent the visual impression. M51 simply does not look like the sim above in my 8". It's not a dim, well resolved view of the image through a larger aperture. But that's okay for representitive images to illustrate a point. We get the idea being expressed.
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#40 RockyMtnRR

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Posted 10 December 2020 - 02:29 PM

attachicon.gifM-51-Brightness-Graph.gif

 

I credit TOMDEY as he posted a graphic in times past similar to the ones I am posting here that helped me to understand this topic. His graphic also inspired the creation of these gifs. Thank you Tom.

This gif is amazing... however I feel as though I need to stop looking at it or it may become expensive.



#41 ButterFly

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Posted 10 December 2020 - 02:31 PM

Jon, another issue with simulated images, in my view, is they do not represent the visual impression. M51 simply does not look like the sim above in my 8". It's not a dim, well resolved view of the image through a larger aperture. But that's okay for representitive images to illustrate a point. We get the idea being expressed.

The eye's perceived brightness as a change in flux is rather complicated, but generally logarithmic.

 

I recently had an eye exam where I was going to be dilated anyway.  I asked the doctor for an eye exam, the dilated exams, then another eye exam.  The results were very interesting.  The refractive error was large, but that's fairly okay for astro use because we can refocus as long as the difference is not so large that it throws off the eyepiece optimizations.  The astigmatism shift is what was optimized for astro glasses.

 

On a side note, I mentioned how much better my my contacts were during the day compared to at night.  The doctor was not surprised.  "Your pupils are huge.  If the window of the contacts is only 6mm, the outer portions won't be corrected."  Eyes get terrible at big exit pupils and evolution never selected otherwise because everything is dim anyway.  Seeing the motion of a prowling puma was enough.



#42 Asbytec

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Posted 10 December 2020 - 02:49 PM

Sometimes I wish we evolved those reflective cat's eyes. :lol:
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#43 ButterFly

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Posted 10 December 2020 - 03:17 PM

Sometimes I wish we evolved those reflective cat's eyes. lol.gif

That would make eyepiece exit pupil design very complicated!


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#44 Glory Eye

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Posted 16 December 2020 - 11:32 PM

Norme:

 

There are a number of things wrong with the simulation.  Regardless of the aperture, it won't be in color.  

 

But more importantly, the issue that has been continuously raised is that comparisons of extended objects need to be done at constant exit pupil and not constant magnification.  The reason the disappear is that the exit pupil is too small.  At 64x, M51 will be the same brightness as it at 200x in the 25 inch.  It doesn't disappear, it just gets smaller.

 

As far as seeing the image in the eyepiece.  The eye has very poor resolution at low light levels.  A camera on a 3 or 4 inch will show details similar to what one sees in a relatively large Dob.  

 

I think the best way to see just how bad the resolution of the eye is at low light levels is to view the moon through a 100,000:1 solar filter.  The sky is super dark, the full moon has a surface brightness through the filter of about 16mpsas, a quarter moon has a surface brightness of about 18 mpsas.  The detail is there to be seen, it's just 12.5 magnitudes dimmer than it is without the filter.  I could never see any detail. 

 

Jon

Addressing the points that are mentioned in the above post:

1. I used a color picture because there are umpteen images to be found online that don't infringe on anyone's copyright, and I don't have an adequate sketch of my own to use. At any rate, you could plug any image or sketch into the gif that you want, the point is to illustrate the principles rather than what you may see at the eyepiece in absolute terms.

2. Holding magnification constant allows us to see what we can expect in terms of relative image brightness as you reduce aperture. The comparison that needs to be done is the one that demonstrates the principle in question. To wit, if I am limited to 200x magnification due to my local seeing, how much brighter can I expect M51 to be If I upgrade from 16" to 20"?

3. I say this with respect to you and all who have much more experience than I, but I have to disagree that the image will be the same brightness at 200x as it would be at 64x. Mathematically speaking, Exit pupil for an Obsession 25" at 200x is 635/200 = 3.175, and 635/64 = 9.92 for 64x. The larger the exit pupil, the brighter the image, and the smaller the exit pupil, the dimmer the image. At some point, depending on the object, there is a minimum exit pupil where the image is too dim to see. I looked at M51 just last night at 72x to find it, and 200x to observe it. It was noticeably brighter at 72x but too small to see detail well. Again, this entire exercise is to demonstrate the principles that A) Larger aperture telescopes will give you brighter images for a given magnification, and B) Larger aperture yields larger images for a given exit pupil.


Edited by Glory Eye, 16 December 2020 - 11:33 PM.


#45 Jon Isaacs

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Posted 17 December 2020 - 04:12 AM

Addressing the points that are mentioned in the above post:

1. I used a color picture because there are umpteen images to be found online that don't infringe on anyone's copyright, and I don't have an adequate sketch of my own to use. At any rate, you could plug any image or sketch into the gif that you want, the point is to illustrate the principles rather than what you may see at the eyepiece in absolute terms.

2. Holding magnification constant allows us to see what we can expect in terms of relative image brightness as you reduce aperture. The comparison that needs to be done is the one that demonstrates the principle in question. To wit, if I am limited to 200x magnification due to my local seeing, how much brighter can I expect M51 to be If I upgrade from 16" to 20"?

3. I say this with respect to you and all who have much more experience than I, but I have to disagree that the image will be the same brightness at 200x as it would be at 64x. Mathematically speaking, Exit pupil for an Obsession 25" at 200x is 635/200 = 3.175, and 635/64 = 9.92 for 64x. The larger the exit pupil, the brighter the image, and the smaller the exit pupil, the dimmer the image. At some point, depending on the object, there is a minimum exit pupil where the image is too dim to see. I looked at M51 just last night at 72x to find it, and 200x to observe it. It was noticeably brighter at 72x but too small to see detail well. Again, this entire exercise is to demonstrate the principles that A) Larger aperture telescopes will give you brighter images for a given magnification, and B) Larger aperture yields larger images for a given exit pupil.

This is how I see it:

 

- 1 and 2:  I am not sure how many hours you have spent looking at M51 in larger aperture telescopes.  I have a fair amount in 16 inch, 22 inch and 25 inch scopes.  Even in poor seeing, I will use more than 200x.  Objects like M51 are not affected as much as by seeing as the planets and double stars.  

 

My main point is that as Norme suggested, 200x limits the 8 inch because it is too much magnification.  Comparisons are ideally done at magnifications that are optimized for each scope.  

 

- 3. I can see I should have been more explicit. That would have avoided the confusion, I was comparing the 8 inch at 64 x to the 25 inch at 200x.  I was responding to Norme's statement:

 

"I was interested in the OPs image of M51 through an 8" at 200x. It's large and dim, too much so for my liking. But, I wondered if someone could actually observe that image at the eyepeice. After all, it is there to be seen.:

 

I wrote:

 

"But more importantly, the issue that has been continuously raised is that comparisons of extended objects need to be done at constant exit pupil and not constant magnification.  The reason the disappear is that the exit pupil is too small.  At 64x, M51 will be the same brightness as it at 200x in the 25 inch.  It doesn't disappear, it just gets smaller."

 

The context was the 8 inch image and a constant exit pupil.  What I meant to imply was 

At 64x in the 8 inch, it will be the same brightness as it is at 200x in the 25 inch.  

 

As Don said, larger apertures generally mean both a larger exit pupil and more magnification. To simulate M51 across a range of apertures, an optimal exit pupil/magnification should also be employed.  

 

Jon


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#46 Glory Eye

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Posted 17 December 2020 - 06:11 PM

Jon,

Sad to say I've never looked through anything bigger than my 16" and it doesn't seem likely that I will, so I enjoy your narrative about your experience with the larger scopes.

 

"Objects like M51 are not affected as much as by seeing as the planets and double stars."

 

Earlier this week, I had a very fine observing session which included a 600x view of the Eskimo Nebula where I could see the face fairly well. It wasn't completely sharp due to the seeing which would not yield a good view of Mars. My observing experience affirms your above statement.

 

Since you have looked through larger apertures would you mind describing the viewing experience of M51 in a 25" as compared to a 16"?

 

Blessings,

David.



#47 Jon Isaacs

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Posted 17 December 2020 - 07:15 PM

David:

 

The Eskimo is small and bright and definitely benefits for stable seeing and takes magnification.

 

M51, how to describe the difference.. :scratchhead:

 

M51 is about 12' across.  With the 16 mm Nagler, it was about 230x, 45 degrees across with a 2.8 mm exit pupil, with the 12 mm Nagler, about 60 degrees across with a 2.1 mm exit pupil.

 

In my 16 inch, the spiral structure is very apparent, plenty to see. In the 25 inch, its much like the photos. It's one of the most impressive objects because it's such a good fit for your eye, big but not too big.  

 

In your face.

 

Jon




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