Aperture and exit pupil: confusion and a neat calculator I made.

[pipnina]

In my quest to find out a theoretical limit to how much aperture a visual telescope could have before no more light can hit your eyes, two things happened:

1. I made a neat caluclator for different telescope parameters https://www.desmos.com/calculator/vhaqjro03u

          Key:

• f = focal ratio
• l = focal length (mm)
• a = aperture (mm)
• k = eyepiece focal length (ignore the fact it can be -10, that's the website being stubborn)
• P = eye exit pupil (mm)
• m = magnification

      2. I got confused... greatly:

In photography, lower f/ratio means more light hitting the fixed-size sensor... But for visual it's kinda made me confused as to why the exit pupil is the sole decider for brightness but more aperture can still lead to brighter views at the same exit pupil. If someone can explain this to me i'd appreciate it!

p.s. is my calculator accurate? I put my scopes pecs in (900mm focal length, 130mm aperture, 25mm EP) and the exit pupil was only 3.6! Does this mean I could get a longer focal length EP and get more FOV and brightness?

    ~pip

EDIT: forgot to add the link :3

[ronin]

Figures are right, 3.61mm exit pupil.

A longer focal length EP should give less magnification and so a brighter image but a smaller one also. There comes a time when the small disk becomes a dot and not much more. That is OK to locate something if you realise the dot is the object.

Really the questiion arises what is the "optimum" exit pupil and as long as all the light is utilised I am not sure what the answer is.

Have read that 2mm is it, but also that 1mm is it.

Ever had the feeling there is no "it" ?

With the proviso that you do not go insanely small.

[Stu]

In my quest to find out a theoretical limit to how much aperture a visual telescope could have before no more light can hit your eyes, two things happened:

• I made a neat caluclator for different telescope parameters https://www.desmos.com/calculator/vhaqjro03u

Key:

• f = focal ratio
• l = focal length (mm)
• a = aperture (mm)
• k = eyepiece focal length (ignore the fact it can be -10, that's the website being stubborn)
• P = eye exit pupil (mm)
• m = magnification

2. I got confused... greatly:

In photography, lower f/ratio means more light hitting the fixed-size sensor... But for visual it's kinda made me confused as to why the exit pupil is the sole decider for brightness but more aperture can still lead to brighter views at the same exit pupil. If someone can explain this to me i'd appreciate it!

p.s. is my calculator accurate? I put my scopes pecs in (900mm focal length, 130mm aperture, 25mm EP) and the exit pupil was only 3.6! Does this mean I could get a longer focal length EP and get more FOV and brightness?

~pip

EDIT: forgot to add the link :3

Larger aperture doesn't give brighter images, it just allows for the same brightness at a larger image scale. This means you can magnify smaller/fainter objects more and maintain brightness.

There is no single 'right' exit pupil as it varies depending upon what you are observing.

6 or 7mm is a max depending upon your pupil size for widefield or finder eyepiece use.

3 to 4mm is handy for larger objects such as the Veil, M31 (or when using an H beta filter for the horse-head)

Around 2mm is a good guide for smaller dso's as it gives optimum contrast.

For planetary, it is more a matter of a guideline minimum which would be around 1mm, although for small apo refractors or longer focal length newts, 0.5mm is useable.

[John]

I may have this wrong but in photography the aperture stop reduces the effective aperture of the lens being used and alters it's focal ratio. So the amount of light getting onto the sensor (or film in the old days) is reduced or increased as the stop is opened up or closed down.

With an astro scope the exit pupil that exits the eye lens of the eyepiece is a bundle of all the light that has been collected by the primary mirror or objective lens, assuming that no other aperture stop or mask has been added to the optical system. So a larger aperture mirror or lens, operating at the same focal ratio, produces a brighter bundle of light (exit pupil) because more light has been collected.

If I've confused things further or got this wrong I can only apologise !

[pipnina]

Larger aperture doesn't give brighter images, it just allows for the same brightness at a larger image scale. This means you can magnify smaller/fainter objects more and maintain brightness.

There is no single 'right' exit pupil as it varies depending upon what you are observing.

6 or 7mm is a max depending upon your pupil size for widefield or finder eyepiece use.

3 to 4mm is handy for larger objects such as the Veil, M31 (or when using an H beta filter for the horse-head)

Around 2mm is a good guide for smaller dso's as it gives optimum contrast.

For planetary, it is more a matter of a guideline minimum which would be around 1mm, although for small apo refractors or longer focal length newts, 0.5mm is useable.

Figures are right, 3.61mm exit pupil.

A longer focal length EP should give less magnification and so a brighter image but a smaller one also. There comes a time when the small disk becomes a dot and not much more. That is OK to locate something if you realise the dot is the object.

Really the questiion arises what is the "optimum" exit pupil and as long as all the light is utilised I am not sure what the answer is.

Have read that 2mm is it, but also that 1mm is it.

Ever had the feeling there is no "it" ?

With the proviso that you do not go insanely small.

So both of you are saying that 7mm exit pupil isn't just default and preferred for DSO? That's odd. I would have expected that, when looking for dark faint fuzzies the brighter image would have been preferable...

I may have this wrong but in photography the aperture stop reduces the effective aperture of the lens being used and alters it's focal ratio. So the amount of light getting onto the sensor (or film in the old days) is reduced or increased as the stop is opened up or closed down.

With an astro scope the exit pupil that exits the eye lens of the eyepiece is a bundle of all the light that has been collected by the primary mirror or objective lens, assuming that no other aperture stop or mask has been added to the optical system. So a larger aperture mirror or lens, operating at the same focal ratio, produces a brighter bundle of light (exit pupil) because more light has been collected.

If I've confused things further or got this wrong I can only apologise !

Eeeh. I think you may have confused things more.

I just don't get this, however much I'd like to...

[John]

I find it gets confusing trying and relate photographic concepts to astronomical ones, particularly visual astronomy. So I sort of don't do it now. Maybe it was a mistake to try above

[Stu]

So both of you are saying that 7mm exit pupil isn't just default and preferred for DSO? That's odd. I would have expected that, when looking for dark faint fuzzies the brighter image would have been preferable...

Eeeh. I think you may have confused things more.

I just don't get this, however much I'd like to...

Acey may be our best bet for a proper explanation to this, although I will try to find some links to info which is helpful.

I can confirm absolutely that a default figure of 7mm is NOT optimum. It is not about pure brightness, but more about contrast between the object and the sky background. Decreasing exit pupil size darkens the sky background, as well as increasing image scale (EDIT Decreasing exit pupil size doesn't necessarily increase image scale, but it does if exit pupil reduction is achieved by increasing mag) . There are optimum exit pupils where contrast is at its highest so object visibility is best.

[Laurie61]

For visual its worth remembering that we cannot increase the surface brightness of an object with a scope, beyond its real surface brightness, so the best we can do is match an unaided, fully dark adapted eye with a scope. The scope just adds magnification to the view.    

[Piero]

This is what I have understood about this topic.

Decreasing exit pupil darkens the sky background, but darkens the object brightness too. This means that the contrast between the object brightness and the sky background is actually the same as long as you are in the limit between roughly 7mm and 0.5mm exit pupil. 

However, when you move this contrast difference, let's say 'up' or 'down' in this scale, the human eye behaves differently. 

If you move above ~7mm, the object brightness and the sky background sort of fuse together and therefore we cannot distinguish them at all.

We can distinguish them if they have different colours but not by simple contrast. 

If you move below ~0.5, the sky background is black and the object brightness is so dim that eventually will be black too. Again, we cannot distinguish them.

Certain objects are more suitable at a certain exit pupils range, other objects at another exit pupil range. 

There is a distinction between diffuse-light objects and point-source objects. For the former (e.g. nebulae, galaxies) the object brightness decreases by decreasing the exit pupil. For the latter (e.g. stars) the object brightness increases by decreasing the exit pupil.

Therefore, for the first group, you have something like this: 

- For instance, for low surface brightness nebulae, you tend to use high exit pupil (~4-5mm). 

- For galaxies, 2-3mm; for globular 1-2mm.

For the second group, instead:

- For planets 1.5mm-0.5mm (0.5 in refractors).

- For double stars, you can use very low exit pupil, even below the theoretical resolution of the telescope because other factors belonging to this class of objects play a role. For instance the stars can have different colour or different brightness and our eyes can capture these differences to split them even though the two objects appear attached. Beside this aspect, I prefer to see them when they are properly split! 

Although I know very very little about this topic, to me the concept of exit pupil is important because it allows you to understand eyepieces in terms of brightness, rather than magnification. Therefore the exit pupil is independent of the telescope, or better hides the telescope features, whereas the magnification is still linked to it. 

Then, the ranges of exit pupil for viewing objects are indicative and there are other factors playing too (e.g. seeing, transparency, light pollution, age). Therefore, I consider the exit pupil thing as a good theoretical guide, which however does not replace (but complements) the experiments!

[pipnina]

This is what I have understood about this topic.

Decreasing exit pupil darkens the sky background, but darkens the object brightness too. This means that the contrast between the object brightness and the sky background is actually the same as long as you are in the limit between roughly 7mm and 0.5mm exit pupil. 

However, when you move this contrast difference, let's say 'up' or 'down' in this scale, the human eye behaves differently. 

If you move above ~7mm, the object brightness and the sky background sort of fuse together and therefore we cannot distinguish them at all.

We can distinguish them if they have different colours but not by simple contrast. 

If you move below ~0.5, the sky background is black and the object brightness is so dim that eventually will be black too. Again, we cannot distinguish them.

Certain objects are more suitable at a certain exit pupils range, other objects at another exit pupil range. 

There is a distinction between diffuse-light objects and point-source objects. For the former (e.g. nebulae, galaxies) the object brightness decreases by decreasing the exit pupil. For the latter (e.g. stars) the object brightness increases by decreasing the exit pupil.

Therefore, for the first group, you have something like this: 

- For instance, for low surface brightness nebulae, you tend to use high exit pupil (~4-5mm). 

- For galaxies, 2-3mm; for globular 1-2mm.

For the second group, instead:

- For planets 1.5mm-0.5mm (0.5 in refractors).

- For double stars, you can use very low exit pupil, even below the theoretical resolution of the telescope because other factors belonging to this class of objects play a role. For instance the stars can have different colour or different brightness and our eyes can capture these differences to split them even though the two objects appear attached. Beside this aspect, I prefer to see them when they are properly split! 

Although I know very very little about this topic, to me the concept of exit pupil is important because it allows you to understand eyepieces in terms of brightness, rather than magnification. Therefore the exit pupil is independent of the telescope, or better hides the telescope features, whereas the magnification is still linked to it. 

Then, the ranges of exit pupil for viewing objects are indicative and there are other factors playing too (e.g. seeing, transparency, light pollution, age). Therefore, I consider the exit pupil thing as a good theoretical guide, which however does not replace (but complements) the experiments!

I guess I'm slightly disappointed about not being able to make any of these objects brighter. But at least now I have some new knowledge-weapons to help me see things that I wouldn't have before. Thanks for the info!

[Piero]

I guess I'm slightly disappointed about not being able to make any of these objects brighter. But at least now I have some new knowledge-weapons to help me see things that I wouldn't have before. Thanks for the info!

Pipnina, you will not see Hubble images with your Newton, but without your Newton you won't even see those objects!

Every telescope has pros and cons. I would suggest you to recognise the limitations of your telescope, but also to recognise the power.

For galaxies or globular clusters your telescope is limited because of the aperture. This does not mean you cannot see those objects. It just means that you can see most of them as very faint or fuzzy blobs. 

However, there are so many other objects to see in the sky! 

There are lovely open clusters where your telescope is fully capable of revealing nice details. 

There are a few double stars too, plus planets and moon which can take you busy for a long while if observed patiently. 

I really suggest you a good star atlas (e.g. the sky & telescope pocket star atlas) and a decent red dot finder or 8x50 standard finder. Have fun! 

This hobby is about fun and patience. When you see an object, take some note of it and then when at home search it on wikipedia or wherever you fancy. Read about its details. It is amazing how distant or how big or how old that object is! 

And you were there, looking at it, touching with your eye that light which travelled for so long time! And above you, there is this immense, extremely sparse but also powerful world! The same window that all our relatives and ancestors used to look at! That huge view apparently so immutable is actually moving and changing fast!

Try to think about these things next time you observe and free your emotions. Your little telescope is your gate! 

[pipnina]

Pipnina, you will not see Hubble images with your Newton, but without your Newton you won't even see those objects!

Every telescope has pros and cons. I would suggest you to recognise the limitations of your telescope, but also to recognise the power.

For galaxies or globular clusters your telescope is limited because of the aperture. This does not mean you cannot see those objects. It just means that you can see most of them as very faint or fuzzy blobs. 

However, there are so many other objects to see in the sky! 

There are lovely open clusters where your telescope is fully capable of revealing nice details. 

There are a few double stars too, plus planets and moon which can take you busy for a long while if observed patiently. 

I really suggest you a good star atlas (e.g. the sky & telescope pocket star atlas) and a decent red dot finder or 8x50 standard finder. Have fun! 

This hobby is about fun and patience. When you see an object, take some note of it and then when at home search it on wikipedia or wherever you fancy. Read about its details. It is amazing how distant or how big or how old that object is! 

And you were there, looking at it, touching with your eye that light which travelled for so long time! And above you, there is this immense, extremely sparse but also powerful world! The same window that all our relatives and ancestors used to look at! That huge view apparently so immutable is actually moving and changing fast!

Try to think about these things next time you observe and free your emotions. Your little telescope is your gate! 

I've already started aquiring some of those items. I got my 9x50 RACI about 2 months back and the red dot finder that came with the scope is awaiting duct tape (to be attached to the OTA). Although my charts need to be put in a new binder... The one they're currently in likes to close on me.

Jupiter was a good see (while it was closer to opposition, less so now, but back next year!) venus was good during the conjunction (although the atmospheric turbulence kept getting in the way, venus is all the way over the city :/) the orion nebula is remarkably difficult where I am, though, even from my "Dark site" (it's a beach car park about 10 minutes out of the city, by no means perfectly dark, but darker than my back garden, certainly- the milky way is actually visible there (from my back garden, a 3:30 exposure didn't bring out more than hints!) but this december another opportunity presents itself!

Maybe I'll give Uranus and Nepune a go when I figure out where they are. I know they won't look like much to me but I bet they'll still be distinctly blue!

Although, I never found open clusters particularly interesting. Globs are the ones that I like to imagine being in the middle of looking out.

[ghostdance]

I echo Piero's post absolutely...great advice that sums up the spirit I try to have in pointing my two small apertures skyward.

Uranus when I found it was *tiny* - but the loveliest blue/green dot I'd ever seen!

In contrast, I love open clusters. They're just so beautiful and can be so delicate...and not even Cluster status - I get so much pleasure from seeing patterns between stars.

My oddity I guess That, and finding little red stars like X Cancri

BTW, you are so lucky to have a view of the Milky Way just a few minutes from home! I must get (considerably) out more...

[pipnina]

I echo Piero's post absolutely...great advice that sums up the spirit I try to have in pointing my two small apertures skyward.

Uranus when I found it was *tiny* - but the loveliest blue/green dot I'd ever seen!

In contrast, I love open clusters. They're just so beautiful and can be so delicate...and not even Cluster status - I get so much pleasure from seeing patterns between stars.

My oddity I guess That, and finding little red stars like X Cancri

BTW, you are so lucky to have a view of the Milky Way just a few minutes from home! I must get (considerably) out more...

Well, I say that I have a view of it. But I haven't actually seen it myself yet. I'm waiting until septemberish time when It's dark enough early enough for me to get my dad to drive me there.

I'm basing the idea that I can see it off people at the astronomical society saying it's visible. That and a 30 second image taken by a member that shows it remarkably well.

But yeah. I guess people like you who live in capital cities have the ultimate "You think YOUR sky is bad"  response

EDIT: I guess I'll be looking for the other two blue planets when they reach opposition.

[ghostdance]

City observing has its rewards, even so. So much to see and wonder at - and that little me can stand in my tiny backyard with a metal tube and bits of glass and see across billions of years....phew...

To paraphrase Stephen Stills:

"If you can't be with the skies you love

Love the skies you're with....."

Good luck with Neptune and Uranus. And I hope you one day catch the magic of an open cluster or two

[Piero]

Maybe I'll give Uranus and Nepune a go when I figure out where they are. I know they won't look like much to me but I bet they'll still be distinctly blue!

Although, I never found open clusters particularly interesting. Globs are the ones that I like to imagine being in the middle of looking out.

I have not seen Neptune yet, but I did see Uranus a few times. I love it, but I have to say I love the fact that I can see a distant planet. You will not see more than a very little green / blue disk, I am afraid. I hope this will not disappoint you just because you cannot see details. Rather, try to think that those planets are not as big as Jupiter or Saturn and they are really distant. It is amazing that they are detectable even with small telescopes such as ours! 

Regarding open clusters.. have you looked at the Double Cluster? it is just a touch below Cassiopeia. There is M39 which is beautiful too. If you can see the Milky Way, you can simply lose yourself in Cygnus! The wide Duck M11 is another amazing target. NGC6633 in Ophiuchus is also pretty and of decent size. If the sky is clear for you in the South, just point your telescope over there and scan the sky between North North-West Sagittarius, South-West Scutum and South-East Ophiuchus. There are plenty of nice open clusters there.

This is just a hint. No need to wait until September.

Piero