6mm+ exit pupil a good idea for nebulae?

[Size9Hex]

Looking for advice on a possible eyepiece purchase for nebula hunting please.

My current kit consists a 10" f4.7 dob and a 4" f4.9 refractor, with a 24mm ES82 eyepiece to give a 5.1mm and 4.9mm exit pupil in the two scopes. It's a great combination (especially with nebula filters), but I'm finding nebulae to be a massive draw for me and wondered whether I could get even better views with a longer eyepiece.

Do you think a longer eyepiece (e.g. I was considering a 30mm length for a 6.4mm and 6.1mm exit pupil) would be worthwhile? Is 30mm pushing it far enough for it to be noticeably different/better on the nebulae than the 24mm eyepiece I'm already using? Is 30mm pushing it too far that I'd have trouble with the secondary mirror on the dob leaping into view?

I'm figuring it might not be the best option to use at home, but at my dark site, the skies are maybe mag 6 to 6.2 or so, which I'm hoping might be dark enough to stretch my eyeball wide enough! Or not...?

Really appreciate any and all experience/advice/opinions as always. Thanks! 

[scarp15]

I recently purchased a 31mm T5, which when I get to use in my 14" F4.6 dob and with a paracorr will create a 5.84mm exit pupil, 59X mag and 1.38 degree field. Without the paracorr this will become a 6.7mm exit pupil perhaps too much, though this will be used with a variety of nebula filters and at a dark sky location.  Your aim for a 30mm might be applicable, I am sure that others will soon add some contributions.

[John]

I used to use my 31mm Nagler with an F/4.8 10" newtonian and it worked pretty well. I didn't use a coma corrector so some coma was visible at the field edges but it was not too obvious. The exit pupil there was 6.5mm so I doubt that I was getting the full benefit of my aperture (I'm in my 50's). Using a 22mm Nagler seemed to make faint DSO's a little better defined because the background sky was darker so I tended to sacrifice a little field of view for that most of the time.

My 12" newt now is F/5.3 which brings the exit pupil with the big Nagler down to a more reasonable 5.8mm.

If you are youngish then I think 30/32mm as a longest focal length eyepiece in an F/4.7 scope might be worthwhile but have something a little shorter on hand so you have some options.

[Size9Hex]

Thank you for the speedy and helpful replies. Gives some good context to what might work and what might not.

John, did you have any trouble with the secondary getting in view when you used the 31mm in the newt? I guess that's my main worry with too big an exit pupil. If I understand correctly, the worst with a frac is that you effectively lose aperture while with a newt you see the secondary getting intrusive.

I read about a practical sounding method of testing pupil size recently which might help (or be of interest to others). Grab a set of allen keys, hold each one up close to your eye, and see which is the thickest key you can "see through" to pick out a star. Sky seems to be clearing tonight so I'll give it a go! :-)

[John]

I've not seen secondary shadow in any scope that I've used the 31mm Nagler in. I think F/4.5 is the fastest newt that I've used it with.

 

[michael.h.f.wilkinson]

I have used a 31T5 Nagler (a.k.a. Panzerfaust) in Olly's 20"F/4.1 Dob, so exit pupil in the order of 7.5mm. Worked well if I got my eye at the exit pupil propery. I have also used a 40mm TMB Paragon in my 80mm F/6 frac (6.67mm exit pupil) with no ill effect. You might lose some light, but the wide field is still great. I have only really seen the secondary in a Newtonian at wide exit pupil during daytime tests

[jetstream]

Thinking outside the box a bit here.... Instead of a longer eyepiece how about one that out contrasts the whole ES series? I use a 30mm ES 82 on nebs- great, bright eyepiece, love the FOV. However, other than the FOV difference, the 21mm Ethos provides better contrast. Under dark skies it is immediately noticeable, no guessing, no nuancing- its just better.

The ES 82's are VG with filters on neb, the 21E is just that much better and the darker the skies the better it gets..... I got that section in the Western Veil between 52 Cygni and TYC2687-801-1 that shows as 2 bright rails with a scooped out, kind of oblong section in between with the 21E/15"/10"/OIII/UHC.

[jetstream]

This borrowed image shows the outlined section, obviously co color but a great sight for sure, the 21E,17E can show this section very well.

[triton1]

The only trouble with the great 21E is the great price but a valid point though.

[John]

I agree with Gerry - I use the 21 Ethos quite a bit more than the 31 Nagler. With my 12" dob I can fit the whole of the Eastern or Western segments of the Veil into the field of view of the big Ethos and they make a stunning sight on a dark night.

This is a guide to the various segments of this complex and rewarding object:

http://www.deepskyforum.com/showthread.php?172-Object-of-the-Week-July-08-2012-Dissecting-the-Veil-Nebula

[Size9Hex]

Thanks for the latest replies everyone.

John, thanks for sharing that amazing link. So much to see!

Sounds like folks are using using large exit pupils without issue which is nice to know, but really interesting to read also about your experiences with a shorter length Ethos. I admit I really hadn't considered the question of whether contrast between different designs/brands might make more difference than the exit pupil. Do you think this contrast is a more important factor than the focal length? When I moved from a 14mm to 24mm it made a world of difference and I suppose I wondered whether a longer EP again would give further gains.

And I fear an Ethos might cause quite a bit of disharmony in the household. At that price it's difficult to sneak one in under the radar! 

 

 

[Mark at Beaufort]

Paul I had a 10" f4.7 Dob and used several EPs to obtain the best views of DSOs. I had Nagler 22mm and 26mm - the latter being very good. I also used a 38mm Panaview which gave a very bright view and so I mainly used this EP for star hopping. In the end my preferred EP was the 21mm Ethos. For various reasons I sold my collection of Ethos EPs and bought ES in their place.

However, I recent obtained a 20mm Myraid 100 degree EP which in my 12" f/5 Dob is very good and from what I have read the difference between the Myraid and Ethos is not huge.

If you buy a Myraid or even the ES 100 degree EP your exit pupil will be around 4.2 and a FOV of 1 degree 40 minutes pretty good for galaxies and nebulae.

 

[YKSE]

5 hours ago, Size9Hex said:

 Do you think this contrast is a more important factor than the focal length?

Paul,

I would think that's most probably the key.

We can try this analogy: download the picture Gerry has posted above, open it, looking at some specific faint very small features, then enlarge the picture slowly. You'll find the some features getting easier seen than the original size, despite the original has best resolution.

When using a telescope on extended DSO, it's just like looking at the faint features in a picture, increased magnification doesn't increase its contrast against the sky, it's the size increase makes it easier seen, despite it gets fainter with magnification. The perceived better contrast in higher magnification partly enforced by the fact that more faint stars seen.

[jetstream]

Off topic, but related, is the puzzling "contrast" -for lack of a better word- difference between eyepieces of near the same focal length (or same) using filters. I have a 17E,16T5,18ES 82,18 BCO, 18mm Leica zoom and a Zeiss zoom capable of 18mm and have been comparing them for quite a while. First I must say they are all VG eyepieces with filters, but there are differences in nebula contrast between them.

If magnification is solely responsible for this how is the difference accounted for? As I say, between the highest contrast eyepieces and the lowest in this group there is an immediate, noticeable difference, and the difference grows with increasing sky darkness and transparency.

Instead of contrast, how about the word (new ) "standoutness"? lol!

PS- the biggest difference is between the 17E and 16T5 which is also contrary to the mag issue...

[John]

Could coatings, element numbers, baffling design or similar make a difference to the extent of "standoutness" of nebulae and other similar DSOs ?

It's a great term Gerry, nicely invented !

 

 

[jetstream]

You know after reading Aceys contrast paper and trying to pound that into my head, the words contrast and brightness have taken on different meanings for me. I'm thinking the contrast only changes with sky darkness as the " brightness" of the object is fixed.

Hence the new word if anyone has a better word it is more than welcome.

After staring at nebula and trying to detect them, with and without filters, I'm still puzzled at why things can look different in different eyepieces, with presumably minimal aberration and varying distortion etc. I think all the factors you list play a big part John- another thing that baffles me is how an eyepiece can excel on lunar/planetary and yet fall behind on DSO ie my 12.5mm Tak ortho VS the 12.5 Docter UWA vs the 10 BCO. The Tak excels on the planets but is noticeably behind both these other EP's on galaxies. Yet the 10E has better standoutness than 10BCO on galaxies, but the 10BCO goes deeper...

I can say this... whatever Televue is doing with the Ethos series works, they are the best DSO eyepieces out there IMHO.

[Size9Hex]

Really interesting stuff. Hadn't quite expected the thread to take this direction when I posted, but it's all good stuff and not off topic at all. I suppose what I was really asking originally was if/how I could get a better view of nebulae with different kit (and was just presuming the answer might be to zoom out to a brighter view), so talk of standouty eyepieces is definitely relevant.

Based on reputation and price I wouldn't have expected two Televues (17E and 16T5) to be the two with the biggest difference to each other, with the ES eyepiece somewhere between the two by the sound of it. An Ethos would be a huge investment. I find the idea of standoutyness in an eyepiece pretty appealing, but how large a factor do you think it is it compared to varying transparency from night to night? I'm slowly learning that two seemingly excellent looking nights can still vary a lot in terms of the faintest targets I see.