Eyepieces and exit pupil

[spaceboy]

I popped out last night for a quick observing session with my 80mm f/6. My low mag, wide field eyepiece for this scope is a 26mm 70° EP giving me 4.4mm on exit and 3.9° TFOV which fits most large clusters in comfortably but just for cracks I wanted to see how much free space I could get around the coat hanger cluster so dropped my 42mm 65° which in this scope gives 7.3mm on exit and 5.9° TFOV. I was actually impressed with the views even being able to pick out some of the fainter stars around the cluster despite the somewhat washed out background.

Now I understand the principle behind exit pupil and the larger cone of light basically being wasted by using such a large exit pupil but I enjoyed the view all the same. I guess I'm kind of getting at why not have focal lengths that have exit pupils beyond that of what your eye might manage. I mean I may well have been getting a pupil dilation of 7.3mm but I doubt it given the St lights around my garden but I enjoyed the views it offered so isn't that what matters at the end of the day ????

As it stands my 42mm EP is for one of my other scopes offering a more suitable exit pupil but have any other member bought eyepieces knowing the exit pupil was going to be too large but just wanted an eyepiece that offered up a huge FOV ??? My guess is that even under dark skies such a large exit pupil isn't going to wash out the background as most of the light is wasted anyway but your still going to get a floating through space experience ??

Have I got this all wrong ? Are there other disadvantages ??

[Moonshane]

I don't know much other than what you say about the technicalities but have some pertinent recent experience with a 40mm TV widefield eyepiece that I bought on a whim to try it - the price made it a no brainer really.

My scopes are fast newts (f4 or f4.5 with the paracorr) so this eyepiece gives a larger than usual exit pupil of nearly 9mm. I was also a little surprised by the views despite the very light skies last night. In comparison with the 26mm Nagler the view was a little washed out but the field was noticably different (1.94 degrees in the 40mm and 1.6 degrees in the 26mm) so I think it's worth it. Based on this I feel that in the same way that a 31mm Nagler provides a supplementary experience to a 21mm Ethos, this 40mm widefield will work for me at dark sites where a widest field is desireable.

I suppose this is another suck it and see situation where people should try things and decide for themselves what works. There's no doubt that the 26mm Nagler is a better eyepiece but at <1/3rd the price of a used one the 40mm is a good buy for me.

[michael.h.f.wilkinson]

A 42 mm (LVW? that's 72 deg, btw), gives an exit pupil of 7mm on an F/6 scope, not 7.3. 7mm is OK for most youngsters, but even at my age, the average maximum dilated pupil size is 6.2mm or so. I have attempted to measure mine, and come up with 7mm or so. The bottom line is, if it works for you, you are fine. I do prefer the 31T5 in my 80mm F/6. Bit smaller FOV, but better dark background

[Adrian]

From the little I've read, it would seem that the oversize exit pupil is more of a problem for reflectors as the size of the secondary becomes disproportionately larger. For refractors this isn't a problem - you just lose light (and as we know you can reduce the light quite a lot before the eye-brain complex notices the difference).

The Televue view: http://www.televue.com/engine/TV3b_page.asp?id=86#.U6lvcZRdVKA

Michael: The Revelation (& others) Superview is 42mm 65degree afov. Not as good as the LVW, but a tenth of the price! This could be the type of eyepiece in question.

[michael.h.f.wilkinson]

From the little I've read, it would seem that the oversize exit pupil is more of a problem for reflectors as the size of the secondary becomes disproportionately larger. For refractors this isn't a problem - you just lose light (and as we know you can reduce the light quite a lot before the eye-brain complex notices the difference).

The Televue view: http://www.televue.com/engine/TV3b_page.asp?id=86#.U6lvcZRdVKA

Michael: The Revelation (& others) Superview is 42mm 65degree afov. Not as good as the LVW, but a tenth of the price! This could be the type of eyepiece in question.

Didn't know the revelation, but there is a lot of confusion about the LVW FOV. Many places list 65 deg, the barrel lists 72 deg

[John]

I don't think having an over-large exit pupil is a problem if you enjoy the views. In practice, because I have some light pollution to contend with, I find that the background sky starts to look grey rather than black as the power reduces and the exit pupil size climbs so faint DSO's get a little harder to see and thats a practical reason why I don't use eyepieces with focal lengths longer than 32mm with my scopes. If I observed regularly under very dark skies I might be tempted to get a 40mm / 70 degree eyepiece. I have owned such eyepieces and even 50mm / 50 degree ones but the results were not very satisfying at home.

[Moonshane]

I agree John and sold my 35mm panoptic in favour of my 26mm nagler for these reasons. But even if i only use it a few times a year i am glad i bought the 40mm. Time will tell if i retain this view lol

[spaceboy]

From the little I've read, it would seem that the oversize exit pupil is more of a problem for reflectors as the size of the secondary becomes disproportionately larger. For refractors this isn't a problem - you just lose light (and as we know you can reduce the light quite a lot before the eye-brain complex notices the difference).

The Televue view: http://www.televue.com/engine/TV3b_page.asp?id=86#.U6lvcZRdVKA

Michael: The Revelation (& others) Superview is 42mm 65degree afov. Not as good as the LVW, but a tenth of the price! This could be the type of eyepiece in question.

Sorry Michael I should have mentioned what EP's I was using but as Adrian has figured out I was using the revelation superview http://www.telescopehouse.com/acatalog/Online_Catalogue_Revelation_Eyepieces_34.html I use this in my slower f/9.4 refractor and for the money they aren't bad but while the views are still acceptable they are not best suited to faster refractors and I'm sure not at all advisable for a fast Newtonians. I didn't see much point in pushing the boat out on the 42mm given I have only one scope the longer focal length is suited to but in hindsight it may well get to see some use in my 80mm f/6 next time I'm under darker skies.

[russ.will]

I bought the Maxvision 40mm SWA, simply because I figured that the chances of getting such a set of optics for £120 again, was going to be nil. They're still pumping them out though....

I thought it might find use in the 127L with a 4.25mm exit pupil and 30x magnification and it does. The background is a little washed out in in the F6 Apo at 12x, but it's rather like using a dirty great monocular and the Milky Way looks spectacular from darker sites. I dropped it in the F5 Dob and I didn't really even notice the secondary shadow. By gum, the outer FOV was full of coma and light grey skies were the order of the day, but it wasn't the crashing disappointment that I expected. It won't get much use in the Dob, but on those nights when I can't seem to find the proverbial barn door, it may yet still earn it's keep as a remedial finder EP!

Russell

[great_bear]

From the little I've read, it would seem that the oversize exit pupil is more of a problem for reflectors as the size of the secondary becomes disproportionately larger.

Even this is overblown too. In practice you just move your eye slightly to one side of the obstruction.