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Seeing Galaxies

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#1 AlvinPL

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Posted 06 April 2021 - 06:30 PM

So, ive got a question regarding seeing big objects, like galaxies or nebulas, to see them with a telescope do i need to be able to notice them with my eyes? (I.E. I can see orions nebula and the pleiades very well, but i can see little smudges on the sky with my eyes.) So, for example, given that Andromeda's Galaxy is supposedly 3 times the size of the moon on the sky, and with dark enough nights its visible to the naked eye; to be able to see with a telescope (whatever aperture it has) do i need to be able to notice anything on that spot on the sky with my eyes?  Or i can aim at that "blank" spot in the sky and with enough aperture will i be able to see it?

 

I can see a lots of bright stars on the sky every other day, i use 15x70mm binoculars; looking into one bright star and i can see hundreds of stars; so i dont think the light pollution is that bad in my area (unless it actually is and since its all ive known, i know no better).

 

So, question is, do telescopes allow me to see better what im already seeing? (i need to be able to "see" the nebula through "light polution" to see it on the telescope)

or do telescope allow me to see things my eyes cant catch? (a 130mm Aperture Telescope will show me more objects and details than a 70mm Aperture one)


Thanks in advance! :D


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#2 infamousnation

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Posted 06 April 2021 - 06:36 PM

Telescopes capture more light than your eye is able to. They then concentrate this light into your eye. This allows you to see things like galaxies that you would not otherwise be able to see. If fact finding a galaxy can be challenging without a go to mount because you can’t see it with your eyes to point the telescope at it. A good finder scope can help.



#3 Bean614

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Posted 06 April 2021 - 06:38 PM

Besides Andromeda,  there are very, very, very, very few Galaxies to be seen with the naked eye. The Triangulum (M33) is possible occasionally,  and M81/M82 is a long stretch.  


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#4 MellonLake

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Posted 06 April 2021 - 06:57 PM

 You can see a lot more with a telescope

 

I have a 10" Dobsonian telescope and in 2 hours form dark skies where Andromeda is visible by eye, I found 67 galaxies in the constellation Virgo alone.  I have seen well more than 200 galaxies in my telescope.  Nebulae are not as prolific in the sky but I have seem many many nebulae as well. Some of them are even as large (or nearly) as Andromeda (North America Nebula, and Veil).  

 

The larger the aperture (opening) of the telescope the more you will be able to see.  8" and 10" telescope are where faint DSOs really start to be easy to find and view.  In some galaxies like the Whirlpool, Bodes, Sunflower, Andromeda, Triangulum and a few others I can even visually see the spiral arms of the galaxy.  The veil nebula looks like filaments of plasma (which it is).  Wisps can be seen in the Orion nebula.   

 

So again. A telescope will show you a lot more.  

 

Rob  


Edited by MellonLake, 06 April 2021 - 06:58 PM.


#5 AlvinPL

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Posted 06 April 2021 - 07:21 PM

oooooh nice nice, thanks for the fast reply!


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#6 ButterFly

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Posted 06 April 2021 - 08:07 PM

For extended objects like galaxies, a telescope can only make an image appear bigger, never brighter than naked eye.  Regardless of how big an aperture is, if paired with an eyepiece that yields an exit pupil larger than your eye, just enough aperture is "missed" by your pupil to make sure it's never brighter than naked eye.  It's the making bigger that makes telescopes work for extended objects.

 

When we "see" an object, we are distinguishing it from its background.  Background is everywhere, so we are really comparing background, to object plus background.  The size of the feature against that background matters a lot.  Despite decreased brightness from magnifying, the size against the background increases.

 

Andromeda is whatever size and brightness it is.  When I change my background by adding light pollution, at some point, I can no longer make it out naked eye.  The cores of all spiral galaxies are about the same brightness, but the sizes differ wildly.  So even in the darkest skies, I can't make out more than two spirals naked eye.

 

I keep trying for M81.  It's by improving my contrast sensitivity that I will see that galaxy against the darkest background I can find, if ever.  The only way to do that is by practice.  No filter will help and I can't get the sky darker than natural light.  What you can see will improve over time, not because your eyes are getting better, but because you are training your eyes and brain to work together where they never have before.


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#7 sevenofnine

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Posted 06 April 2021 - 11:48 PM

If you like learning from the discussion so far, Terence Dickinson's excellent astronomy book "NightWatch" covers this and much more. It has seasonal star charts plotting all the deep space objects (DSO's) and the equipment necessary to view them. All in a book that is very easy to read and a pleasure to have handy near your telescope. waytogo.gif


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#8 Dave Mitsky

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Posted 07 April 2021 - 12:43 AM

You may find some of the information on astronomy, amateur astronomy, and observing presented in my post (#22) at https://www.cloudyni...mers/?p=5184287 useful, AlvinPL.  There are sections on various books, observing guides, stellar atlases, planetarium programs, and astronomy apps, deep-sky observing, binocular astronomy, and urban astronomy.

 

I include lists of DSOs and binary stars each month in the Celestial Calendar.

 

https://www.cloudyni.../#entry11000657


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#9 rob1986

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Posted 07 April 2021 - 12:53 AM

For extended objects like galaxies, a telescope can only make an image appear bigger, never brighter than naked eye. Regardless of how big an aperture is, if paired with an eyepiece that yields an exit pupil larger than your eye, just enough aperture is "missed" by your pupil to make sure it's never brighter than naked eye. It's the making bigger that makes telescopes work for extended objects.

When we "see" an object, we are distinguishing it from its background. Background is everywhere, so we are really comparing background, to object plus background. The size of the feature against that background matters a lot. Despite decreased brightness from magnifying, the size against the background increases.

Andromeda is whatever size and brightness it is. When I change my background by adding light pollution, at some point, I can no longer make it out naked eye. The cores of all spiral galaxies are about the same brightness, but the sizes differ wildly. So even in the darkest skies, I can't make out more than two spirals naked eye.

I keep trying for M81. It's by improving my contrast sensitivity that I will see that galaxy against the darkest background I can find, if ever. The only way to do that is by practice. No filter will help and I can't get the sky darker than natural light. What you can see will improve over time, not because your eyes are getting better, but because you are training your eyes and brain to work together where they never have before.


your first statement is not true. the brightness ratio depends on how much area the collected light is focused on. if the increase in light collection is more than the magnification then it will brighter.
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#10 sctbrd

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Posted 07 April 2021 - 02:47 AM

This article covers the main tricks for seeing fainter objects. A focal reducer and averted vision really help. Also, the longer you look at a particular object, the more detail you will see.

 

https://skyandtelesc...-sky-observing/


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#11 Dave Mitsky

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Posted 07 April 2021 - 01:18 PM

your first statement is not true. the brightness ratio depends on how much area the collected light is focused on. if the increase in light collection is more than the magnification then it will brighter.

A telescope can't make an extended object look brighter than it appears to the unaided eye.  The surface brightness of an extended object is fixed.  A telescope can, however, make the object look larger, which makes it easier for the eye to detect.

In fact, because no optical system perfectly reflects or transmits light, the brightest possible view of extended objects through a telescope is actually a bit dimmer than the naked eye.

https://medium.com/@...ew-24507147d7fc

 

A telescope can simply never present a view that is brighter than what the eye can see when fully dilated to 7mm.
 

https://medium.com/@...pe-cfa6e2cbdd7f

 

This does not apply to stars, which are essentially point sources.

Again, it’s important to note that this effect only applies to anything that can be magnified. Stars can’t because they are too small and too far away, but nebulae, solar system objects, and galaxies can be.
 

http://umich.edu/~lo...dscobel.13.html

 

True or False?  - More aperture makes extended objects look brighter.

 

Well, this is kind of true, but a more accurate statement is that more aperture lets you magnify faint, extended objects more, making them easier to see.  But with more aperture, objects are not necessarily always brighter to your eye.  The reason is similar to what I explained in the previous discussion.  Suppose you are looking at M81 (a galaxy in Ursa Major) in the 8” f/5 scope with a 25 mm eyepiece.  You are looking at the galaxy at 40x.  Now switch to the 16” f/5 scope with the same eyepiece.  You’re now looking at it at 80x.  You’ve got four times the light, but the galaxy is now covering four times the area in your eyepiece’s field of view.  So, to your eye, its brightness per unit area is the same in both scopes.  Moreover, the sky background is the same brightness too, so contrast of the galaxy against the background is the same.  So why does it look so much better?  It’s because the galaxy appears twice as large in the 16”, covering four times the area, and you see much, much more detail.
 

https://skyandtelesc... you can see it.

A classic 1946 paper by H.R.Blackwell, based on wartime research, revealed that in very dim light the eye has better contrast sensitivity if an object has a large angular size (greater than 3° in some cases). Roger N. Clark explores this topic in his book Visual Astronomy of the Deep Sky (1990).


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#12 havasman

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Posted 07 April 2021 - 01:32 PM

your first statement is not true. the brightness ratio depends on how much area the collected light is focused on. if the increase in light collection is more than the magnification then it will brighter.

Respectfully, your 1st statement is certainly not true. Telescopes cannot create brightness. All photons available to the retina are present at the telescope objective aperture and in fact brightness is lost to the optics in a scope. Hopefully only a bit, but some for sure.

 

This topic deals peripherally with the phenomenon while discussing equally counterintuitive observing tenets and also prompts me to say how much I miss Glenn's contributions - https://www.cloudyni...e/#entry8661327


Edited by havasman, 07 April 2021 - 01:40 PM.


#13 rob1986

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Posted 07 April 2021 - 02:32 PM

Respectfully, your 1st statement is certainly not true. Telescopes cannot create brightness. All photons available to the retina are present at the telescope objective aperture and in fact brightness is lost to the optics in a scope. Hopefully only a bit, but some for sure.

This topic deals peripherally with the phenomenon while discussing equally counterintuitive observing tenets and also prompts me to say how much I miss Glenn's contributions - https://www.cloudyni...e/#entry8661327

glenns argument is the reason why some of our contributers here are getting amazing images of very faint objects with very small apertures. however in visual astronomy he is argument amounts to a tautology. the very fact that he uses exit pupil, which is a derived quantity, means that he has twisted himself into a pretzel.

jon isaacs put it very well there.

the other issue is that our rods and cones are not well behaved linear detectors. they are optimized for certain brightness ranges, and not outside of them. if you lower the backround brightness beliw that range, you have increased contrast for the same reason that vernier scales work.

you mentioning that the same number of photon are present at the aperture only reinforces the contention that you do not understand the discussion.

the orion nebula will patently prove you wrong.

Edited by rob1986, 07 April 2021 - 02:34 PM.

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#14 sg6

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Posted 07 April 2021 - 02:35 PM

Seeing galaxies with the naked eye is useful - you know where to point the scope.

The arguement about never being brighter in a scope then by eye comes up often. I have never bothered to look at why. But very few galaxies are naked eye objects, but they are telescope objects - consider the Leo triplet. So the 2 arguements seem not to match up.

 

The opposing sides seems to be: None via a scope are brighter then naked eye, but to see them you need a scope so they are brighter.

 

Your scope will allow you to see more, otherwise why, even how, do we view most DSO's ?

 

You wouldn't have to learn the sky if all were naked eye visible, you would look around and say there is M98, M99, M100. All of which seem to need a scope and you need to estimate pretty well where they are. Cos you aren't going to see them naked eye. They and quite a few others are part of the numerious Virgo cluster, but you need a scope, even binoculars show several, eyes don't.

 

And Andromeda is 6x the full moon, not 3x.

 

Just hope someone locks this before the arguement gets "nasty". It already is going downhill.


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#15 MellonLake

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Posted 07 April 2021 - 02:55 PM

Other than M31 and M33, no other galaxies can really be seen without a telescope (the Magellanic clouds are dwarf galaxies that can be seen as well in the southern hemisphere but are substantially closer than any other galaxies and are captured by the Milky Way so I kinda don't count them) .  Even M81/M82 which relatively speaking are bright, will look like dim stars to the unaided eye (if they are even visible, I have never picked them out by eye) so I don't count this as being visible galaxies.   It really takes averted vision and good dark adaptation to see galaxies (whether in the telescope or by unaided eye), Even M31 is not directly apparent and requires averted vision (the core is directly visible in a telescope but the disc requires averted vision).   While I agree with the theory that nothing appears brighter in telescope than with the unaided eye, this is not really a functional comparison.  I found 67 galaxies in Virgo in one night but none of them are visible by the naked eye.  So does a telescope make them brighter than the unaided eye, theoretically no, but practically speaking the answer is yes, without the telescope the vast majority are simply not visible.  The telescope collects more light and magnifies the images so we can actually see these objects (that are otherwise non-detectible to our eyes).  


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#16 ButterFly

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Posted 07 April 2021 - 03:09 PM

I found 67 galaxies in Virgo in one night but none of them are visible by the naked eye.

It really is the size of the object against the background.  Try the same object in different skies.

 

M51 really shows this effect for me.  In my Bortle 5 backyard, the crests of the arms become direct eye visible at 21 mm.  The 13 washes them out quite a bit.  Holding the bridge and the arms at the same time is very difficult for me with any eyepiece.  In Bortle 3 skies, I can find a zoom level where I can hold both the arms and the bridge direct eye at the same time.  It's typically somewhere in the 10-20mm range of zooms, depending on transparency mostly.  At my Bortle 2 site, at 8mm, I can hold both at the same time with very little effort.

 

The size and brightness of the various features is the same in all those sites.  The eyepieces make them bigger in exactly the same way at any site.  The background is the only difference there.  So why should that matter?  Try it out.

 

At a dark site, at least 30 of the Messier objects are easy naked eye objects.  From Lincoln Center, I had to look for the Pleiades.  It's the background.


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#17 AlvinPL

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Posted 07 April 2021 - 03:10 PM

Well; i  think already got my answer, we are technically "seeing" those galaxies with our naked eyes; but due to light pollution we cant see the details and they get mixed with the night sky and thats why we dont see it, until we zoom with a telescope; which will increase the surface i will be seeing; and indirectly increase the constrast when compared to the background sky; (because its more "object" vs less background).

 

Did i get it right? or did i miss half the points grin.gif

 

My question came because i was wondering if you could still look for galaxies and nebulas in the sky with moderate light pollution with good enough aperture; or if my skies are not dark enough for it to be seen; doesnt matter how much aperture ill have; ill never find it in the sky.

 

Based on what i have read im thinking i might miss major details due to light pollution, but with enough aperture i should be able to see at least a little blur of some of the galaxies.

 

 

Thanks


Edited by AlvinPL, 07 April 2021 - 03:13 PM.


#18 rob1986

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Posted 07 April 2021 - 03:25 PM

the other crux in this issue it the way our brains process light, essentialy a double ended exponent/log curve.

the change in light concentration uses a square ratio. this means that shifting the relative portions of the scale changes perceived contrast, and may shift the object into a range at which we more readily percive the difference in contrast.
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#19 rob1986

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Posted 07 April 2021 - 03:34 PM

It really is the size of the object against the background. Try the same object in different skies.

M51 really shows this effect for me. In my Bortle 5 backyard, the crests of the arms become direct eye visible at 21 mm. The 13 washes them out quite a bit. Holding the bridge and the arms at the same time is very difficult for me with any eyepiece. In Bortle 3 skies, I can find a zoom level where I can hold both the arms and the bridge direct eye at the same time. It's typically somewhere in the 10-20mm range of zooms, depending on transparency mostly. At my Bortle 2 site, at 8mm, I can hold both at the same time with very little effort.

The size and brightness of the various features is the same in all those sites. The eyepieces make them bigger in exactly the same way at any site. The background is the only difference there. So why should that matter? Try it out.

At a dark site, at least 30 of the Messier objects are easy naked eye objects. From Lincoln Center, I had to look for the Pleiades. It's the background.


its the concentration of light in given angular area. what you are trying to do is "zero" the noise level, by placing the backround brightness below threshold sensitivity. or else place the object in a contrast sensitive value range.

#20 rob1986

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Posted 07 April 2021 - 03:57 PM

Other than M31 and M33, no other galaxies can really be seen without a telescope (the Magellanic clouds are dwarf galaxies that can be seen as well in the southern hemisphere but are substantially closer than any other galaxies and are captured by the Milky Way so I kinda don't count them) . Even M81/M82 which relatively speaking are bright, will look like dim stars to the unaided eye (if they are even visible, I have never picked them out by eye) so I don't count this as being visible galaxies. It really takes averted vision and good dark adaptation to see galaxies (whether in the telescope or by unaided eye), Even M31 is not directly apparent and requires averted vision (the core is directly visible in a telescope but the disc requires averted vision). While I agree with the theory that nothing appears brighter in telescope than with the unaided eye, this is not really a functional comparison. I found 67 galaxies in Virgo in one night but none of them are visible by the naked eye. So does a telescope make them brighter than the unaided eye, theoretically no, but practically speaking the answer is yes, without the telescope the vast majority are simply not visible. The telescope collects more light and magnifies the images so we can actually see these objects (that are otherwise non-detectible to our eyes).


then why does the orion nebula look so much brighter in the telescope than to my unaided eye?

#21 ButterFly

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Posted 07 April 2021 - 04:32 PM

then why does the orion nebula look so much brighter in the telescope than to my unaided eye?

"Look so much" is the key.  It's better distinguished against the background because our eye/brain system is non-linear.

 

Take a look at the figure in the Constrast Sensitivity section.  The curve that you see is a mapping of the non-linearity of your eye brain system.  It can change from hour to hour.  I can see a lot more details in hour four compared to hour two because of how my own curve changes as afterimages clear.

 

How would I possibly predict that at the eyepiece?  It's easier just to swap the eyepiece to see which gives you a better view of the feature you are after, or turn the knob on a zoom eyepiece.



#22 rob1986

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Posted 07 April 2021 - 04:58 PM

"Look so much" is the key.  It's better distinguished against the background because our eye/brain system is non-linear.

 

Take a look at the figure in the Constrast Sensitivity section.  The curve that you see is a mapping of the non-linearity of your eye brain system.  It can change from hour to hour.  I can see a lot more details in hour four compared to hour two because of how my own curve changes as afterimages clear.

 

How would I possibly predict that at the eyepiece?  It's easier just to swap the eyepiece to see which gives you a better view of the feature you are after, or turn the knob on a zoom eyepiece.

its better than that, the refresh rate on our retina is adjustable too with practice. which should change dark sensitivity.

 

But thats my point.

 

on the other hand, I just realized why dad never saw any dsos with his 8800. his 45 mm celestron was giving him a 10mm exit pupil at 20x. and an 8 inch telescope produces identity for brightness at 40x, (an 80mm frak does this at 16x - makes me wonder what the percentage of people running this argument are frak lovers) this means he was halfing the surface brightness.


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#23 jjbag

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Posted 07 April 2021 - 05:02 PM

oooooh nice nice, thanks for the fast reply!

Just realize you will not see Hubble type images. More like bright or dim greys out there with definitions (though globular clusters (yum M13 season) are a different story).


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#24 Asbytec

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Posted 07 April 2021 - 05:26 PM

your first statement is not true. the brightness ratio depends on how much area the collected light is focused on. if the increase in light collection is more than the magnification then it will brighter.

Try a thought experiment. Let's say you are looking at a galaxy through a 150mm aperture at 150x. That's a 1mm exit pupil and the galaxy is visible at that magnification. Now, the impossible part, let's drop the magnification all the way down to 1x with a 150mm exit pupil (with a HUGE eyepiece). Say our iris can also increase to an impossibly large 150mm exit pupil as we are looking through the scope so all the light passing through the exit pupil enters our eye. The galaxy, if you can see it at all, has a certain image brightness being very small at 1x.

Now, turn your 150mm eye from the eyepiece at 1x magnification to the sky also at 1x. You may be surprised our naked eye view of the galaxy, then, is actually the same(sic) as the view through the telescope. The naked eye galaxy has that same "certain image brightness" as above. Both the eye and the telescope are operating at 150mm aperture and 1x magnification. It really is not any brighter through the 150mm aperture than our 150mm naked eye. In fact, you do not even need a 150mm telescope at all except to magnify the galaxy (reducing the exit pupil).

The same holds true for viewing the galaxy through the 150mm telescope at 150x and 1mm exit pupil. Since the exit pupil is the final "aperture" to the eye, our eye is actually stopped down to 1mm exit pupil, too, regardless of our dark adapted iris. So, the galaxy is "no brighter" (surface brightness per unit area) through the telescope with a 1mm exit pupil than the naked eye with a 1mm iris. The only difference is the galaxy is larger in the telescope because it is magnified more than 1x.

The only reason we need a telescope at all is to increase magnification. The larger aperture collects light, forms an image, then squeezes it through the exit pupil into our eye. The result is magnification proportional to the exit pupil (ratio of Aperture/Magnification). Then apply the inverse square law, the magnified galaxy is not any brighter *per unit area* in the telescope than the small naked eye image with our dark adapted iris "aperture" and 1x. (Our eye is a telescope, too. Its a 1x refractor. :) The telescope we use just extends our capabilities using the same principles).

Edited by Asbytec, 07 April 2021 - 06:29 PM.

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#25 rob1986

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Posted 08 April 2021 - 04:36 AM

just found this

http://adsabs.harvar...JBAA..114...73L

and want to point out, the spread on the graph indicates that the exit pupil formula is only an approximation. and from my calculations the formula itself uses unity as the means for approximating the exit pupil for a given eyepeice.

the actual exit pupil is a complicated computation involving the image size, apparent feild of view, eye relief etc to calculate the actual result. this means that in reality, the number given (16x for an 80mm frak at f11 and a 56 mm ep for instance) is variable and dependent on ep type.


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