Exit pupil

[Stardaze]

So who’s measured theirs? I guess the only way to really know what limit, on paper, you should keep within, is to check it, no?

Is there a general point where the central secondary begins to become noticeable in a reflector? 
 

Thinking of what the widest practical possibility would be for my dob (1270mm f5)? A couple of targets I’ve noted would benefit from a wider fov than my 24 (82) has to offer.

[John]

I have not measured mine but generally assume that at my age it's going to be around 6mm or so, max.

So with an F/5 scope I reckon 30-32 mm is the longest focal length that will be reasonably efficient.

I've used a 40mm eyepiece with my F/5.3 dob and don't recall seeing the secondary shadow but it would have been when viewing a dark sky so I might not have noticed it ?

I find 31mm or even better 21mm more effective in DSO observing with that scope under my skies.

 

 

[Stardaze]

11 minutes ago, John said:

I have not measured mine but generally assume that at my age it's going to be around 6mm or so, max.

So with an F/5 scope I reckon 30-32 mm is the longest focal length that will be reasonably efficient.

I've used a 40mm eyepiece with my F/5.3 dob and don't recall seeing the secondary shadow but it would have been when viewing a dark sky so I might not have noticed it ?

I find 31mm or even better 21mm more effective in DSO observing with that scope under my skies.

 

 

I’ve noted many stating that they’ve tried the trial and error approach. Have looked at the 28 nirvana but the apm 30 flat field looks a good option too. Just another consideration. I guess further down the line, a shorter FL scope could be a benefit too?

So yours John is the 1600mm FL then? 

Edited July 13, 2020 by Stardaze

[Alien 13]

I have wondered about exit pupil size many times and have several rifle scopes that go well above 20 mm and they are fine but everything is bright almost like having night vision. Have looked through some military spotting scopes that again push the exit pupil into double figures and again they worked brilliantly on the night sky, possible better than any commercial scope/eyepiece but you do need a dark site.

Alan 

[John]

1 hour ago, Stardaze said:

..So yours John is the 1600mm FL then? 

Yes, that's the one. The widest true field I can get is about 1.7 degrees. If I want to observe the whole of the Veil Nebula I have to move to my 102mm F/6.5 refractor where I can get 4 degrees with a 6mm exit pupil.

With the 12 inch I can fit the whole of the East or West segments of the Veil in the FoV.

 

[Stardaze]

8 hours ago, Alien 13 said:

I have wondered about exit pupil size many times and have several rifle scopes that go well above 20 mm and they are fine but everything is bright almost like having night vision. Have looked through some military spotting scopes that again push the exit pupil into double figures and again they worked brilliantly on the night sky, possible better than any commercial scope/eyepiece but you do need a dark site.

Alan 

Isn’t that to do with a rifle scope being a refractor? They don’t suffer from a central obstruction becoming visible with a high exit pupil? 

6 hours ago, John said:

Yes, that's the one. The widest true field I can get is about 1.7 degrees. If I want to observe the whole of the Veil Nebula I have to move to my 102mm F/6.5 refractor where I can get 4 degrees with a 6mm exit pupil.

With the 12 inch I can fit the whole of the East or West segments of the Veil in the FoV.

 

I just get one if the veils segment In the fov in mine. I bet both looks pretty spectacular. 

[AstroMuni]

Taken from Astronomy Hacks book by Robert Bruce Thompson; Barbara Fritchman Thompson.

Your eye doctor can measure your fully dark-adapted entrance pupil for you, but you can also determine it for yourself. To do so, you’ll need a set of metric Allen wrenches. Allow yourself to become fully dark adapted, which may take half an hour or more. Look directly at a bright star, and hold one of the smaller metric Allen wrenches along your cheek so that the long portion crosses your eye parallel to and near the eyeball.

Move the wrench up and down until it is centered on your pupil. You’ll see the star split into two stars, one on each side of the wrench. Substitute larger Allen wrenches until you reach a point where the star no longer splits, but is visible only as a single star on one side or the other of the Allen wrench. The size of that Allen wrench is the size of your fully dark-adapted entrance pupil.

If you observe frequently from a light-polluted site, repeat the experiment there. You may be surprised at the difference light pollution, particularly from nearby local sources, makes to your dark adaptation. For example, if your entrance pupil is a full 7mm at a truly dark site, it may be only 5mm at a brighter site. Your eyes operate on the same principles as any optical instrument. Light gathering ability varies with the square of the aperture. That means a 7mm entrance pupil admits nearly twice as much light (7² versus 5²) as a 5mm entrance pupil, which in turn means that you can see nearly one full magnitude deeper from the darker site.

 

Edited July 14, 2020 by AstroMuni

[Second Time Around]

I'd certainly recommend that you check the size of your dark-adapted pupils - they vary hugely from person to person and also tend to get smaller with age.  Each eye is likely to be different too.  So the oft-repeated 7mm is likely to be incorrect for most of us. 

It's easy to do so.  Just stay in darkness for a couple of minutes for your pupils to dilate; you don't need to get fully dark-adapted as that's a chemical process that occurs after your pupils have fully dilated.  Then just put a ruler above your eyes and take a flash photo - the flash is so fast your pupils won't react.  But do make sure that you have red-eye and pre-flash turned off if your camera has these settings.  As your pupil size will vary depending on the observing site it's better to measure them there.

Why is it important?  Well, if the exit pupil of your scope is greater than your pupil size you'll be wasting some of the aperture (although the field of view is likely to be bigger).  So it's a good idea to have an eyepiece that matches exit pupil and pupil size. 

You also need to take into account that the magnification and therefore the exit pupil will change if you use a coma corrector.  For instance, a Paracorr reduces the exit pupil by 15%, an Explore Scientific coma corrector by 6%.

To give an example I measured my dark-adapted pupil at 4.5mm.  I have a 10inch f/4.8 Dob fitted with an Explore Scientific coma corrector.  So the calculation for this scope is 4.5 x 4.8 x 1.06 = 22.9mm eyepiece.  As there are few 23mm eyepieces available, plus my pupil size will almost certainly get even smaller with age I rounded this down to 22mm.

 

Edited July 14, 2020 by Second Time Around

[Louis D]

17 hours ago, Stardaze said:

So who’s measured theirs?

Did you mean entrance pupil of the eye?

[Stardaze]

2 hours ago, Second Time Around said:

I'd certainly recommend that you check the size of your dark-adapted pupils - they vary hugely from person to person and also tend to get smaller with age.  Each eye is likely to be different too.  So the opt-repeated 7mm is likely to be incorrect for most of us. 

It's easy to do so.  Just stay in darkness for a couple of minutes for your pupils to dilate; you don't need to get fully dark-adapted as that's a chemical process that occurs after your pupils have fully dilated.  Then just put a ruler above your eyes and take a flash photo - the flash is so fast your pupils won't react.  But do make sure that you have red-eye and pre-flash turned off if your camera has these settings.  As your pupil size will vary depending on the observing site it's better to measure them there.

Why is it important?  Well, if the exit pupil of your scope is greater than your pupil size you'll be wasting some of the aperture (although the field of view is likely to be bigger).  So it's a good idea to have an eyepiece that matches exit pupil and pupil size. 

You also need to take into account that the magnification and therefore the exit pupil will change if you use a coma corrector.  For instance, a Paracorr reduces the exit pupil by 15%, an Explore Scientific coma corrector by 6%.

To give an example I measured my dark-adapted pupil at 4.5mm.  I have a 10inch f/4.8 Dob fitted with an Explore Scientific coma corrector.  So the calculation for this scope is 4.5 x 4.8 x 1.06 = 22.9mm eyepiece.  As there are few 23mm eyepieces available, plus my pupil size will almost certainly get even smaller with age I rounded this down to 22mm.

 

Thta's a much better and easier test, thanks!

3 minutes ago, Louis D said:

Did you mean entrance pupil of the eye?

Indeed, yes. Hadn't thought about the terminology per se, but you'd be right Louis with that. 

[bomberbaz]

I recently measured mine with the pinprick card test and got mine to around 6.3.

So hard to measure when you are using homemade devices with only a ruler to measure and magnifying glass the distances though.  

 

[tony210]

Hi - interesting topic - over 50 your looking at less than 5mm in most cases- so in my case although I always wanted 7x50 binoculars - 10x50 or8x42 will best fit my pupils now- such is life....

[Pixies]

29 minutes ago, tony210 said:

Hi - interesting topic - over 50 your looking at less than 5mm in most cases- so in my case although I always wanted 7x50 binoculars - 10x50 or8x42 will best fit my pupils now- such is life....

Yep- but binoculars again don't have the secondary obstruction. So my understanding is that the larger exit pupil is wasting light - but won't actually make the viewing worse. Whereas with a reflector, as the exit pupil gets larger than the actual pupil, the shadow of the secondary gets large enough to cause the view to dim.

When you look at the exit pupil hovering above the EP lens for a bright view, this makes sense. You can imagine placing your eye over it and just getting the secondary mirror shadow, rather than the light around it. I'll see if I can find a good pic

[tony210]

Yes -I think that is an important distinction and certainly is the impression with reflectors and low power eyepieces that give a large exit pupil.

[Pixies]

OK - here are some pics. Sorry for preaching to those who obviously understand this, but this is more for confirming I've got this right.

This is my 203 Bresser Dob (fl=1213mm). I don't have a wide-field eyepiece yet, so the 25mm plossl that came with it will have to do.

mag = 1213/25 = x 48.52,   exit pupil = 203/48.52 = 4.18mm

You can see this in the first pic. It's obviously bright (daylight) so you can see internal reflections, etc. and you don't get that 'hovering over the glass' appearance of the exit pupil. The secondary mirror is 60mm diameter - the 'shadow' of this will appear as (60/48.52) = 1.24mm

 

Now if I splashed out on a 41mm TV Panoptic eyepiece, I would get a 203/(1213/41)= 6.9mm exit pupil and a 60/(1213/41) = 2mm diameter secondary shadow. If my eyes were young that'd be fine, but my (young) middle-aged eyes are probably nearer a 5mm max pupil size, so that 2mm diameter obstruction would be significant.

 

So that's my excuse for not getting a 41mm Panoptic.

 

[AstroMuni]

10 minutes ago, Pixies said:

So that's my excuse for not getting a 41mm Panoptic.

Nice one  🤣

[Pixies]

All this exit pupil chat reminded me that the family, including myself, need an eye test. So I called up my opticians to see what the arrangements are. Currently in Scotland, opticians are still only doing emergency work.

But I asked about getting my maximum dark pupil dilation measured when things were back to normal. She was very interested and said they will have a go - but would have to use a dim red light. Apparently there is equipment that can do it without using visible light, but not on the NHS!

One interesting thing she mentioned is that when you are trying to measure it yourself, if you eye is trying to focus on something close, like a mirror, this will cause your pupils to contract. Worth bearing in mind, depending on how you are doing it.

[John]

I occasionally use a 40mm 70 degrees eyepiece with my F/5.3 12 inch dob. I don't recall seeing the shadow of the secondary but the skies have been dark when I've done this so I might not have noticed it.

It is not the most effective eyepiece for DSO's in that scope though. The 31mm Nagler and 21mm Ethos are much preferred usually.

And this is my excuse for not getting a 41mm Panoptic

[andrew s]

I was trying to use some Schmidt camera optics F2 with a large diagonal to make it easier to use. Trying an eyepiece with it I had to look round the hole in the image the secondary made 

Regards Andrew 

Edited July 16, 2020 by andrew s

[Captain Scarlet]

On 14/07/2020 at 11:17, AstroMuni said:

Taken from Astronomy Hacks book by Robert Bruce Thompson; Barbara Fritchman Thompson.

Your eye doctor can measure your fully dark-adapted entrance pupil for you, but you can also determine it for yourself. To do so, you’ll need a set of metric Allen wrenches. Allow yourself to become fully dark adapted, which may take half an hour or more. Look directly at a bright star, and hold one of the smaller metric Allen wrenches along your cheek so that the long portion crosses your eye parallel to and near the eyeball.

Move the wrench up and down until it is centered on your pupil. You’ll see the star split into two stars, one on each side of the wrench. Substitute larger Allen wrenches until you reach a point where the star no longer splits, but is visible only as a single star on one side or the other of the Allen wrench. The size of that Allen wrench is the size of your fully dark-adapted entrance pupil.

If you observe frequently from a light-polluted site, repeat the experiment there. You may be surprised at the difference light pollution, particularly from nearby local sources, makes to your dark adaptation. For example, if your entrance pupil is a full 7mm at a truly dark site, it may be only 5mm at a brighter site. Your eyes operate on the same principles as any optical instrument. Light gathering ability varies with the square of the aperture. That means a 7mm entrance pupil admits nearly twice as much light (7² versus 5²) as a 5mm entrance pupil, which in turn means that you can see nearly one full magnitude deeper from the darker site.

Unfortunately that method only works in Europe, and in the UK for now but for only a limited amount of time. In the US and shortly in the UK it'll only work using Imperial allen keys  

M

Edited July 16, 2020 by Captain Magenta

[Captain Scarlet]

1 hour ago, Pixies said:

... One interesting thing she mentioned is that when you are trying to measure it yourself, if you eye is trying to focus on something close, like a mirror, this will cause your pupils to contract. Worth bearing in mind, depending on how you are doing it.

seriously though, because the light from a star is parallel, I would think the allen key method should also work with the allen key held a distance in front of your face, avoiding forcing you to squint?

I'm definitely going to try it

[Stardaze]

2 hours ago, John said:

I occasionally use a 40mm 70 degrees eyepiece with my F/5.3 12 inch dob. I don't recall seeing the shadow of the secondary but the skies have been dark when I've done this so I might not have noticed it.

It is not the most effective eyepiece for DSO's in that scope though. The 31mm Nagler and 21mm Ethos are much preferred usually.

And this is my excuse for not getting a 41mm Panoptic

How much do you favour/use the 31 John? Is it your 'finder' EP mostly?

[miguel87]

If I use my 32mm eyepiece to look at the full moon through my 8inch, 200p, I can just see the central shadow. 

I'm 33 years old and the above set up creates a 6.5mm exit pupil.

The effect is impossible to notice on any planets or deep sky objects.

Might be useful.

[John]

1 hour ago, Stardaze said:

How much do you favour/use the 31 John? Is it your 'finder' EP mostly?

I used the 31 more before I got the Ethos 21 which shows almost as much sky but at a higher magnification.

The Nagler 31 is still a favourite for extended DSO's such as the Veil Nebula, N A Nebula etc. With my Vixen 102 F/6.5 ED refractor the big Nagler will show a 3.8 true field of view and the whole of the Veil Nebula complex

 

[Stardaze]

2 hours ago, John said:

I used the 31 more before I got the Ethos 21 which shows almost as much sky but at a higher magnification.

The Nagler 31 is still a favourite for extended DSO's such as the Veil Nebula, N A Nebula etc. With my Vixen 102 F/6.5 ED refractor the big Nagler will show a 3.8 true field of view and the whole of the Veil Nebula complex

 

Definitely fancy trying the APM 30 UFF but need the 20 and 9 first.