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Eyepiece Exit Pupil Help


Nick P

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I am revising my EP collection and am getting bogged down with optimal exit pupil size! :)

can anyone give me some advise please?

I have seen suggestions that exit pupil size should be in the 2 - 4mm range for ideal observing, however the low power EP I am thinking of (25mm plossl in a F4.5 scope) gives an exit pupil size of 5.5 (my eyes dilate to about 6 mm)

Would moving to a wider field EP of 19mm - giving an exit pupil of 4.2 - give a significantly improved view?

Thanks for any help

Nick

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I tend to work on the 7mm rule and don't have any problems apart from very occasionally seeing the secondary shadow when looking at the moon whilst the sky is still blue. I am 43. My 35mm Panoptic in my large dob gives an exit pupil of 6.6mm and this works completely fine.

Like all general rules they can be broken to some extent. You will have no problems with a 25mm EP and I would buy on the basis of the field of view you want and other characters such as eye relief etc than exit pupil size in that range.

In my view you could use a 30mm effectively too although the sky will be lightened due to light pollution assuming like most of us you have it locally.

Not that I'd recommend it to you but I know of people using 41mm Panoptics and 55mm Plossls in scopes which technically they shouldn't but they seemed happy!

Good luck with your choice.

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I'm 50 and I don't know my dilated exit pupil size but I use a Nagler 31mm in my 10" F/4.8 newtonian which gives an exit pupil of 6.46mm, 39x magnification and shows just over 2 degrees of sky, which I feel is just fine. That's what Nagler 31mm's were designed for :)

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Thanks for the input Tom and Shane, I seem to be worrying unnecessarily!

The view through the 32mm I used in this scope in the past was very dim on the central area because of the secondary mirror shadow - not really surprising as the exit pupil was (32mm/f4.5) 7mm! so am trying to avoid any similar problems now.

Would I be right in thinking that if my pupil size is say 6mm, any exit pupil below that wou;d not show any secondary obstruction or light loss?

It appears that the EP's I have shortlisted will be fine with exit pupils from 5.5mm - 3.6mm (all with the same TFOV)

Would the EP with a 3.6mm exit pupil have a significantly dimmer view than one with 5.5mm? Am thinking of the effect of light pollution on the view, contract increases with magnification but the view becomes dimmer.

Best

Nick

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I'm 50 and I don't know my dilated exit pupil size but I use a Nagler 31mm in my 10" F/4.8 newtonian which gives an exit pupil of 6.46mm, 39x magnification and shows just over 2 degrees of sky, which I feel is just fine. That's what Nagler 31mm's were designed for :)

Hi John,

Do you not see any hint of the secondary mirror at all?

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Thanks for the input Tom and Shane, I seem to be worrying unnecessarily!

The view through the 32mm I used in this scope in the past was very dim on the central area because of the secondary mirror shadow - not really surprising as the exit pupil was (32mm/f4.5) 7mm! so am trying to avoid any similar problems now.

Would I be right in thinking that if my pupil size is say 6mm, any exit pupil below that wou;d not show any secondary obstruction or light loss?

It appears that the EP's I have shortlisted will be fine with exit pupils from 5.5mm - 3.6mm (all with the same TFOV)

Would the EP with a 3.6mm exit pupil have a significantly dimmer view than one with 5.5mm? Am thinking of the effect of light pollution on the view, contract increases with magnification but the view becomes dimmer.

Best

Nick

No worries Nick.

I'm no expert on the technicalities but if I am using my 35mm Panoptic (6.6mm EP) then the view is nice and wide with no real problems but due to LP is a little grey at home. With my 22mm Panoptic (4.15mm EP) the sky background is a little darker but is in no way 'dim'. The same applies also to my 13mm Ethos (2.45m EP) but with a still slightly darker background.

My philosophy is as a general rule don't buy an eyepiece with an exit pupil of less than 0.5mm or more than 7mm. This way you will always have a view which is bright enough for the objects you are looking at. This works naturally as the things you can use the smaller exit pupil on are generally brighter than those where you need a wider exit pupil. Eg my best eyepiece for the Veil is the 35mm Panoptic and my best eyepiece for the moon is my 6-3mm Nagler zoom (1.3-0.56mm exit pupil range).

Other than the above exit pupil rule, my criteria are 1) cost 2) sharpness and contrast/optical quality 3) field of view 4) eye relief

This explains the reasons for my choices in the signature which I feel cover every single target quite well given the scopes I have.

Of course this means I will never need another eyepiece again (ahem.......honestly......). :)

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5.5mm is never going to be any kind of problem.

Depends on the scope ... with a frac an oversize exit pupil can be comfortable; with a reflector the central obstruction shadowing does restrict the low power that is comfortable. I find (6mm eye pupil) that anything less than 4mm exit pupil is uncomfortable. If I need a wider field I change down to a smaller instrument!

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Thanks everyone a lot of really useful info there,

Shane unfortunately I live on the East Sussex coast so too far to pop over but thankyou for the offer!

However I now have a plan which should work well which will give a an exit pupil of 5.4 which should work perfectly, gives maximum FOV for 1.25' focuser and fits perfectly into the magnification range.

Many thanks

Nick

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It is, and so it should be for what it costs :)

The 28mm Nirvana / UWAN delivers similar benefits for a lot less £'s though.

I've been reading some really good reports of these, the Meade 5000 SWA and UWA, and Celestron Axioms, over on 'Cloudy Nights' and looking at the price points these are hitting now in the UK with some heavy discounting going on, they have become quite irresistible!

I haven't read a bad word about the 5000 SWA's which is pretty good for an eyepiece that's been around for several years.

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I am revising my EP collection and am getting bogged down with optimal exit pupil size! :)

There is not "an" optimal exit pupil. The range for you indeed starts at 6-7mm if you have skies dark enough not to hinder dark adaptation too much (i.e. roughly Mag 5.7 skies or better), and goes all the way to very small pupils.

The larger pupil is the best for detection of fairly large but very faint objects (things like the Witch Head nebula, the California nebula with an H-beta filter, etc.), the 2-4mm range is usually the workhorse range for seeing detail in (rather than just detecting) somewhat smaller but brighter objects (the range is shifted towards larger exit pupils when you work with filters) and the low exit pupil end is for very small objects with large surface brightness (like many small planetary nebulae) and point objects.

Even on a single object you'll see different things at different magnifications. Taking M51 as an example, with 4mm exit pupils you'll likely see (from a dark site) the faint extension of the arm that goes away from the companion better, and with a 2mm exit pupil you'll see the spiral structure much more clearly and perhaps even the bright knots in the arms. If you have a light polluted site, you're likely to want more magnification for a given detail.

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In my 10" F4.7 Dob I use a Nirvana 28mm ep it gives me an exit pupil of 5.9mm, I never see the secondary and it is a really great low power/finder ep. I also use a 17, 12, 8 and occasionally 5mm ep's which give me a good spread of exit pupil sizes.

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The larger pupil is the best for detection of fairly large but very faint objects (things like the Witch Head nebula, the California nebula with an H-beta filter, etc.), the 2-4mm range is usually the workhorse range for seeing detail in (rather than just detecting) somewhat smaller but brighter objects (the range is shifted towards larger exit pupils when you work with filters) and the low exit pupil end is for very small objects with large surface brightness (like many small planetary nebulae) and point objects.

I think this is where my conundrum lies, understanding the application of various exit pupil sizes and how they interact with light pollution. I have a fair amount of LP with a naked eye limiting magnitude of around 4.5, now my concern is twofold; 1) with too large an exit pupil light is lost or the secondary shadow comes into view - this problem has been answered by keeping the exit pupil below the size of my eye pupil. 2) Due to the level of light pollution what exit pupil size would give the best contrast to detect feint fuzzies in the low power EP?

I am looking at EP's giving exit pupils of 5.4mm, 2.5mm, 1.8mm, and 1.1mm. The higher power values work very nicely under my skies for picking out different levels of detail, but is there too big a jump from 5.6 to 2.5 in the above series?

In my 10" F4.7 Dob I use a Nirvana 28mm ep it gives me an exit pupil of 5.9mm, I never see the secondary and it is a really great low power/finder ep. I also use a 17, 12, 8 and occasionally 5mm ep's which give me a good spread of exit pupil sizes.

Kris do you see a big jump in the quality of view when you move from the 28 to the 12 in your set up - and what is your usual limiting magnitude?

Thanks to everyone for your responses and help, this is proving very useful for me - and sorry for so many daft questions!

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Personally, I'd prefer an exit pupil of 4mm under these skies. With a 5.4mm exit pupil the sky will basically be as bright in the eyepiece as with the naked eye, and if it's bright enough to look coloured it's certain to prevent good dark adaptation.

With a 4mm exit pupil if you keep looking for more than 2 minutes your eyes will adapt to the darker background in the eyepiece, especially if you e.g. use a hood to isolate your eyes from stray light.

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Personally, I'd prefer an exit pupil of 4mm under these skies. With a 5.4mm exit pupil the sky will basically be as bright in the eyepiece as with the naked eye, and if it's bright enough to look coloured it's certain to prevent good dark adaptation.

With a 4mm exit pupil if you keep looking for more than 2 minutes your eyes will adapt to the darker background in the eyepiece, especially if you e.g. use a hood to isolate your eyes from stray light.

This is really useful stuff thanks, looks like I am homing in on an 18 / 20mm eyepiece rather than the 24 I was thinking of.

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Kris do you see a big jump in the quality of view when you move from the 28 to the 12 in your set up - and what is your usual limiting magnitude?

Yes certainly the 12mm is a Nagler 12T4 so with the 82 degree FOV it is excellent for close up views of DSO's and the images are nice and bright.

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