44yr old - Max exit pupil?

[Bart]

Hello all,

Would I be pushing it if I went to 6.6?

I was looking at a 30mm wide angle for my current F5 (exit pupil 6), whilst keeping an eye on a possible future move to a F4.5 (pupil 6.6). Is 6.6 getting a bit close to losing light?

It just struck me that anyone with a fast 4.5 scope can only ever go for a 30mm EP and that means a smaller FOV(as mag increases for the longer focal length) for low power searching. Not much you can do about that, I suppose.

Any comments welcome. Thanks.

Bart

[Moonshane]

I had a 35mm eyepiece with an f5 scope and swapped this for a 26mm (with a slightly wider field) as I was getting my f4 dob (f4.5 with Paracorr). This made a big difference to the darkness of the sky and I never regretted it. I am 45. smaller true field is a price for more aperture with decent focal ratios unfortunately.

[Bart]

Thanks Moonshane, thats what I thought. Only beginning to understand the impact the fast F ratio is having on the EP selection.

Regards

Barry

[John]

I'm 52 and I try and stay with 5mm as my max exit pupil.

[Bart]

Has anyone had someone measure it, without having a ruler stuck into your eye!?! Can you go into a dark bathroom and look into a mirror with red light and do it yourself? Em, am I spending too long on my own?

Bart

[Probablynot]

I'm 75. Left eye has a cataract and I get double vision through it. But my right eye seems OK so far, and I can use a 6.5 plossl with it easily enough.

However, I'm wondering. If I have surgery for the cataract (I almost certainly will, soon), and they replace my organic eye-lens with a plastic one, will I then get decent views with the modded eye through my telescope?

[JamesF]

I assume you're asking (amongst other things) if the lens exit pupil is likely to be larger than your eye's maximum pupil dilation?

It'll obviously be different for each individual, but I'm 45 and at my (first ever!) eye test last week my pupil size was measured in moderate to low lighting as around 6mm. I was told that it would increase in full darkness, but not by very much.

I don't recommend visits to opticians, btw. For me they now come into that class of people who you shouldn't see because you'll come away with a list of things wrong with you that you were perfectly happy not knowing about. In my case, I'm apparently slightly astigmatic, my long distance vision is measurably (though not perceptibly) out of focus, my left eye has better vision than my right despite the fact that I'm significantly right eye dominant and I need to have further tests to ascertain whether I genuinely have increased intra-ocular pressure (a potential precursor to glaucoma) in one eye or just a slightly thicker than usual cornea.

And I only went in because I've noticed that I can't focus quite close enough to be able to easily do things like remove small splinters any more.

James

[JamesF]

Has anyone had someone measure it, without having a ruler stuck into your eye!?!

That's actually pretty much what the optician did, in my case. He got me to stare straight ahead whilst measuring the pupil diameter against a scale.

James

[Bart]

james,

You must have left there having felt like you'd gone a few rounds with Tyson. I know what you mean though, last year I got a viral eye infection, and came away with permanent scarred cornea, and a possibility of deteriorating sight in right eye . But apart from that, it went well .....

[umadog]

I have a different perspective on this: forget about this "losing light" business and measuring your pupil. Just buy the eyepiece focal length that gives you the FOV and mag you want and get observing! There is no practical consequence to "losing light". The main reason for this is because the image won't become dimmer if you over-fill your eye's pupil; it just will stop getting brighter. So you won't start seeing dimmer images at eyepiece focal lengths longer than the 30 mm which you mention.

Think about it another way: if you wanted wider fields of view without over-filling your pupil you would have to buy a telescope with a shorter focal length. In practice that means a smaller mirror since focal ratios shorter than your f/4.5 are rare in larger apertures unless you pay big money. Why bother buying the smaller telescope when you could just use your 10" with an over-sized exit pupil. It amounts to the same thing and it's cheaper and easier. The worst that will happen is that you may see the secondary obstruction during lunar observation.

For quite a while I used a 55 mm Plossl at f/4.9. It worked perfectly well.

[Bart]

thanks all.

[Jon Sales]

I have acute astigmatism in both eyes yet I can get focus with a 5mm eyepiece with my better eye no problem.

Your pupil under very dark conditions still only opens to around 8mm.

[Moonshane]

for me the most obvious difference when using longer focal length eyepieces is that the sky is so bright with light pollution I cannot see much. if you have little LP it probably makes less difference I suspect.

[Eastridge]

51 year old hubby measured his with ruler & camera in the dark & was 5mm.

Now having the struggle re wide EP's deciding whether to limit ourselves to a low 20'smm wide angle (which is what Exit Pupil dictates) or go 28 / 30 mm anyway as Ummadog ( & Steve at FLO) suggest.

Surprised there are differing views on something that should be 'scientific'

[Macavity]

Low powers are more about "framing things" though. Let nature take it's course? <G> The "maximum" amount of light will anyway enter the eye tho'. Studiously avoiding low powers, on "theoretical" grounds, may deny you an "interesting" view? Low powers can be problematical re. reflectors... the secondary shadow, and all that.

[acey]

I have a different perspective on this: forget about this "losing light" business and measuring your pupil. Just buy the eyepiece focal length that gives you the FOV and mag you want and get observing!

Totally agree. Nineteenth-century professional astronomers routinely used over-sized exit pupils, the eyepiece generally serving as low-power finder.

[dph1nm]

However, I'm wondering. If I have surgery for the cataract (I almost certainly will, soon), and they replace my organic eye-lens with a plastic one, will I then get decent views with the modded eye through my telescope?

I have had both eyes done (I had triple vision in mine!) and there is no problem using the plastic lenses through a telescope - it is just like having good eyes again. If you had significant astigmatism in the lens before the cataract your view might even improve, although you can have astigmatism caused by the cornea (as I have in one eye) which is not improved by cataract surgery.

NigelM