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Exit pupil issues in f4.5 Dob


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H folks,

Im having a few issues choosing the right combination of eyepieces for my 16inch Lightbridge at f4.5.

I currently have a very nice 28mm WO UWAN but I notice that when observing Im obviously not using the full 6.2mm exit pupil as out of focus stars have their edges clipped . Reading further into this Im beginning to realise that the suggestion that a 5mm max exit pupil is probably right for me given my age (early 40s) and the fact that when I view from my garden there is reasonable light pollution meaning Im never 100% dark adapted anyway and visits to truly dark sites to achieve this, with the scope in tow, are few and far between.

I started astronomy about 30 years ago and when a Plossl was considered a reasonable wide field EP so Ive always looked for a low power eyepiece in my collection, whereas now, with 100degree fields being possible, the option is there to up the magnification but at the same time to widen the FOV by choosing a 82 or 100degree field .......so keeping the same field but with a smaller exit pupil and higher mag.

So what would you suggest ? Im thinking of 3 eyepieces, plus barlow, ............... for widefield I like the FOV that the UWAN gives me but I need a smaller exit pupil, and the highest power I want to go to is 3.5-4mm (so 7-8mm with Barlow). Im not saying money is no object but Naglers secondhand are possible, and at a push , if absolutely necessary a 21mm Ethos (but are there cheaper alternatives for widefield vies with a reasonable exit pupil ie ES 100).

Cheers

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I haven't tried the alternative(s) but we had exit pupil issues with our 35mm Panoptic in our F4.4 16 inch Skywatcher (loss of contrast) and ended up going for the 21mm Ethos. 5mm exit pupil seems about the max for us too,

For the first time in my life, my widest eyepiece is my favourite :)

It's the only Ethos I have used a lot so not sure how it compares to the other ones, but I find the 100 degree field of view comfy to take in.

In our dob there's a fair bit of coma towards the edge with the Ethos without any corrector, so you might want a coma corrector/Paracorr as well to tidy up the edges depending on how much coma bothers you.

Edited by Luke
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What about the Meade/Maxvision 24mm 82 degree, I have one and it is avery good eyepiece, other than that there is only really the 26mm Nagler and the price tag. Either of those would take care of wide field. As for the other eyepieces I would say it really depends on what you like to observe.but if it were my scope I would go for a 14-16mm Nagler or Delos and a 8mm Delos with a Powermate to fill the other spots.

Alan.

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The average max pupil size at my age (51) is about 6.1mm, but the variability from person to person is very large. What you mention sounds not like an exit pupil issue but like a vignetting issue. Usually the secondary causes some vignetting at the edges of the FOV, in order to keep the central obstruction down. This effect is strongest in "planetary" Newtonians, and less in rich-field Newtonians.

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I agree that the clipping at the edge of field is unlikely to be an exit pupil problem but more likely vignetting or something in the light path causing out of focus stars to be affected at the edges. I previously had a 35mm Panoptic with a 12" f5.3 dob and this at home (light pollution is a problem being 9 miles from Manchester and three from Stockport) was on the edge for me and as I was buying a 16" f4, I had to consider a lower focal length. At the time there was a good deal on TV eyepieces so I bought a 26mm Nagler and this made a big difference at home with the f5.3 although less at a dark site. the field was approximately the same. I sold the Panoptic shortly afterwards.

in my 16" f4, I use a Paracorr so the focal ratio is effectively the same as yours (1600x1.15 = 1840 / 400 = f4.6) and the 26mm Nagler is superb in this scope even from home. Yes, the sky is a little grey / orange sometimes but it's usually OK. I suspect a 20mm Nagler might be more suited to home use (or a 21mm Ethos of course).

As you are embarking on a new buying 'spree' I'd suggest the following (no barlow required):

1) Paracorr or other barlow type coma corrector - this benefits all of your eyepieces and at your focal ratio is worth it.

2) 21mm Ethos

3) 12mm Nagler T4 (or T2 if you can get one)

4) 6-3mm Nagler zoom

In all honesty you may well just have the 21mm E in your focuser all night on many occasions. I presume you have the odd plossl or what have you and these can fill in at the short end. I personally prefer tightly bunch eyepieces for planetary but all but the Nagler zoom are cheap(ish) TV plossls, Radians and BGOs (all used of course). You don't need wider fields at the higher powers but others will not agree with this I suspect.

It's lovely spending other people's money, although I do try to ensure I make reasonable, albeit expensive suggestions as a means of suggesting the sort of thing to get - there's always options.

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Good advice given. One other option is to simply use a low power wide field EP as a 'finder' and accept the issues, large exit pupil giving washed out field of view, maybe a bit fuzzy off axis, then switch EPs to medium or high power as needed. All scopes are a trade off one way or the other, including fast mirrors like an F4.5.

Regards, Ed.

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Just my 2 cents: I own a 8" f/5 Newton and I've bought a seconhanded Televue Plössl 32mm eyepiece, which gives me 6.4mm exit pupil. It can be close to the limit for my eye (I am 32 years old), especially on the suburban sky I usually observe. However, I've realized, that this eyepiece provides me the best view of large DSOs with low surface brightness, even I am not able to use all the light that comes to my eye (maybe?). And yes, the sky background is often very bright on the suburban sky!

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I agree with netol. You might lose a little light with a low power eyepiece that has an over-sized exit pupil, but you gain field of view. Maybe with an over-sized exit pupil you're effectively looking through a scope with an aperture a couple of inches smaller or what have you. That's really not so terrible, is it? The wide AFOV eyepieces are an option, as you say: they'll darken the background due to the smaller exit pupil and also provide a wide true field. The downside are weight, price, and that you have to move your head around to take in the whole field. From those points of view, a lower AFOV and larger f.l. eyepiece might be better. I think it all depends on what view you want to have. The point is that you should use the eyepiece that gives you the view you want and that you shouldn't worry about the "lost light".

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In our case, we hardly lost any field of view switching from the 35mm Panoptic to the 21mm Ethos:

"The 21mm's effective field stop diameter is 36.2mm, bringing its true field close to the 35mm Panoptic (38.7mm Field Stop diameter). Its true field is also larger than the 26mm Nagler with its 35mm effective field stop diameter. In an f/4 Dobsonian, the exit pupil is just 5.25mm using the 21mm Ethos instead of 8.75mm with a 35mm Panoptic, or 6.5mm with a 26mm Nagler." - http://www.televue.com/engine/TV3b_page.asp?id=20&Tab=EP_ETH-21.0

Sky at Night review here in case anyone is tempted: :evil:

http://www.skyatnightmagazine.com/review/eye-pieces/tele-vue-21mm-ethos-eyepiece

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Thanks for all your replies and suggestions.

Im a big fan of deep sky observing but with no tracking or goto I like the wide-field FOV as it helps keep the object in view for longer which makes star hopping easier as I can use the fainter anasterisms to guide me and I see a bigger chunk of sky. As I like the faint fuzzies I also dont really want to be loosing light by having an over large exit pupil.

There seems to be a lot of positive views of the Naglers, and as I wear glasses I like the suggestions of the T4 due to their reasonable eye relief and for high power views I can always take my glasses off and maybe use a T6. Would the ES82 range be close to the Naglers though or should I just spend the money on the second hand Nags and be done with it ?

I love the idea of an Ethos 21mm but given the price, the fact they dont come up often second hand , and I can get three second hand Naglers for the same price as a new one Im going to put that Idea on the back burner for now !

Its intrigued me about what has been said about vignetting and that may be one of the things Im seeing......the way I would describe it would be that when I take a bright star out of focus I normally see a bright disk with the secondary shadow visible in the middle.With the UWAN Im seeing the same image but I have to move my head around in order to see the edges of the out of focus disk, I cant see the whole disk in one go. Does this sound like vignetting ?

Cheers

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Hi

I have the 16" LB. when I see actual "square" edges to a defocussed star it means my light shroud is intruding into the light path and sure enough it always is. Once sorted the view is "circular" again

I love my ES 30mm 82* and ES 20mm 100* (sometimes but not always with a coma corrector) my ES 14mm 100* and my ES 11mm 82* EPs. Sometimes I Barlow the 14mm with GSO 2" Barlow.

Don't ever get "non circular" views except for reason above

HTH

Barry

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The Nagler T4 and both longer T5s (26 and 31 mm) are all fine for those who wear glasses when observing (like me) the longer focal length alternatives should also be fine, but at shorter focal lengths I found the T4s are near unbeatable for comfort. I replaced my Meade 14 mm UWA with the 12T4 purely for reasons of eye relief. At focal lengths shorter than 12 mm I went for Pentax XW and XF EPs, foregoing FOV for eye relief. Anything with less than 17 mm eye relief is uncomfortable for me.

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I would be looking at the T4 Naglers. 22mm, 17mm and 12mm. These would be my choice.
The Nagler T4 and both longer T5s (26 and 31 mm) are all fine for those who wear glasses when observing (like me) the longer focal length alternatives should also be fine, but at shorter focal lengths I found the T4s are near unbeatable for comfort. I replaced my Meade 14 mm UWA with the 12T4 purely for reasons of eye relief. At focal lengths shorter than 12 mm I went for Pentax XW and XF EPs, foregoing FOV for eye relief. Anything with less than 17 mm eye relief is uncomfortable for me.

I couldn't agree more, I cherish my 22, 17, and 12 T4's in my f/4.5 18". To those, I added the 31T5 for a 1.2 degree TFV. I also feel very comfortable wearing glasses while I observe. Furthermore, while many insist on a Paracorr at this focal ratio, I've found minimal coma at the edges with these eyepieces.

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BTW, for shorter focal lengths the Delos range is also very interesting. I might replace the XF8.5 with a Delos 8mm at some time, and for my 80mm F/6 I would like a 6mm and 4.5mm at some time. I avoid Ethos and ES 100 deg EPs for reasons of insufficient eye relief.

I don't own an Ethos, but have looked through the 13mm and the 8mm. I don't use glasses, and had no problem at all seeing out to the field stop in all directions at the same time, without moving my head around, or 'looking round the corner'. I was aware of the field stop in my periphery vision, without actually looking directly at it.

But others, whose opinion I trust, do have issues seeing the whole field at once. I think one cause could be that each individual has different facial contours and eye socket depth. So its try before you buy if possible. I find that trying to see the whole field by holding the EP to my eye during the day, is not the same as using the EP at night attached to a telescope.

Maybe that's because my daytime pupil is dilated ? You may quite genuinely get different results than me.

Regards, Ed.

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I don't own an Ethos, but have looked through the 13mm and the 8mm. I don't use glasses, and had no problem at all seeing out to the field stop in all directions at the same time, without moving my head around, or 'looking round the corner'. I was aware of the field stop in my periphery vision, without actually looking directly at it.

But others, whose opinion I trust, do have issues seeing the whole field at once. I think one cause could be that each individual has different facial contours and eye socket depth. So its try before you buy if possible. I find that trying to see the whole field by holding the EP to my eye during the day, is not the same as using the EP at night attached to a telescope.

Maybe that's because my daytime pupil is dilated ? You may quite genuinely get different results than me.

Regards, Ed.

My comment on short eye relief pertains to those wearing glasses while observing only. The ES and Ethos ER of 15mm is too short, I (and many others) find; 17mm is a safe minimum. For those without glasses, 15mm is usually fine, except when the EP shape is very wide and flat, like the SW Nirvana or Panorama, and unlike the tapered shape of the Nagler 31T5, for example.

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Hi All,

Im sort of coming to a few decisions regarding an eyepiece set which hopefully will do what I want.

I know I originally wanted 3 eyepieces plus a barlow,a 5mm exit pupil or less, and a max of 3.5-4mm focal length - but as the discussions and research has moved on following your replies I've amended this slightly (ok significantly :shocked: ) and added the need for a decent eye relief as I wear glasses.

So my list is:

  1. Nagler T4 22mm / 19mm eye relief / 81x mag / 4.8mm exit pupil / 0.99 degree TFOV
  2. Nagler T4 17mm / 17mm eye relief / 105x mag / 3.7mm exit pupil / 0.77 degree TFOV
  3. Nagler T4 12mm / 17mm eye relief / 150x mag / 2.6mm exit pupil / 0.54 degree TFOV
  4. Delos 8mm / 20mm eye relief / 225x mag / 1.7 mm exit pupil / 0.31 degree TFOV
  5. Delos 6mm / 20mm eye relief / 300x mag / 1.3mm exit pupil / 0.24 degree TFoV
  6. Delos 4.5mm / 20mm eye relief / 400x mag / 1.0mm exit pupil / 0.17 degree TFOV

So although this is a nice range of eyepieces its still significantly more than I want to spend so are there any that you would leave off the list ie is the Delos 4.5mm a step too far at 400x mag,or are the Delos 8mm and 6mm too close to each other and I should just opt for the 6mm only plus the Naglers ?

Thanks,

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Personally I'd substitute an Ethos 13mm in place of the 12mm and 17mm Nagler T4's. The 4.5mm Delos won't get used that much perhaps although it's nice to have a good quality ultra high power EP for when conditions allow.

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Personally I'd substitute an Ethos 13mm in place of the 12mm and 17mm Nagler T4's. The 4.5mm Delos won't get used that much perhaps although it's nice to have a good quality ultra high power EP for when conditions allow.

The eye relief issue pops up: only 15mm. Personally that would drive me nuts. You keep banging your glasses into the EP. On a Dob that means moving the scope unwittingly, and on my C8 on EQ mount it means vibrations which ruin the view.

The Meade UWA 24mm is rated at 17mm eye relief. These are now sold as MaxVision 24mm 82 deg EPs for very little, especially compared to the 22T4. I have no experience with that particular one, but the 68 deg is very good indeed

http://www.explorescientific.de/maxvision-82deg-okular-24mm-p-25559.html?language=gb

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The Meade UWA 24mm is rated at 17mm eye relief. These are now sold as MaxVision 24mm 82 deg EPs for very little, especially compared to the 22T4. I have no experience with that particular one, but the 68 deg is very good indeed

I tried a look through the 16mm maxivision michael and the views were pretty poor imo. Remembering that i wear glasses too, even with the eyeguard fully retracted I was only seeing around 50 to 60% of the FOV, eye fully pushed up to the guard.

Hi All,

Im sort of coming to a few decisions regarding an eyepiece set which hopefully will do what I want.

I know I originally wanted 3 eyepieces plus a barlow,a 5mm exit pupil or less, and a max of 3.5-4mm focal length - but as the discussions and research has moved on following your replies I've amended this slightly (ok significantly :shocked: ) and added the need for a decent eye relief as I wear glasses.

So my list is:

  1. Nagler T4 22mm / 19mm eye relief / 81x mag / 4.8mm exit pupil / 0.99 degree TFOV
  2. Nagler T4 17mm / 17mm eye relief / 105x mag / 3.7mm exit pupil / 0.77 degree TFOV
  3. Nagler T4 12mm / 17mm eye relief / 150x mag / 2.6mm exit pupil / 0.54 degree TFOV
  4. Delos 8mm / 20mm eye relief / 225x mag / 1.7 mm exit pupil / 0.31 degree TFOV
  5. Delos 6mm / 20mm eye relief / 300x mag / 1.3mm exit pupil / 0.24 degree TFoV
  6. Delos 4.5mm / 20mm eye relief / 400x mag / 1.0mm exit pupil / 0.17 degree TFOV

One way of further trimming your costs significantly (after not getting the 4.5mm Delos) with very little compromise in quality is to replace the 22 and 17mm Naglers with Vixen LVW of the same size. Admittedly there is less FOV (65) than the Naglers but you will be able to get the full TFOV without having to move your head about. You could also replace the Delos 8mm with the same Vixen size.

http://www.firstligh...-eyepieces.html

TFOV 22mm 0.79

TFOV 17mm 0.61

TFOV 8mm 0.29

Saving made depending where you buy from and taking into account the Delos offer at the moment is circa 425 quid. Even keeping the Delos 8mm and just replacing Naglers saves you about 375.

However a Delos at 12 and 17mm would also be much less expensive than the Naglers and even less compromise on quality and FOV (72)

TFOV 17mm 0.68

TFOV 12mm 0.48

Still saving you around 325 and still allows you to keep the 22mm Nagler for the spacewalking experience.

(Saving enough in either option to stick a 10mm Delos into the collection :grin: )

ps; I have ordered a Vixen 13mm to try it out as a potential replacement for other EP of same size, I have birthday funds to spend and Vixen on top of the list, I will let you know how I find it if you are interested.....

Edited by bomberbaz
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I tried a look through the 16mm maxivision michael and the views were pretty poor imo. Remembering that i wear glasses too, even with the eyeguard fully retracted I was only seeing around 50 to 60% of the FOV, eye fully pushed up to the guard.

The 16mm has far shorter eye relief than the 24. The 24 68o is comfortable, and sharp. I have not looked through a 24mm 82, but have heard very good reports in F/5 scopes. I used a 14mm Meade S5K UWA in an F/4.1 scope and it was very close in performance to a 17T4.

The 22, 17, and 12T4 allow me to see the entire FOV without moving my head (as does the 31T5 "Panzerfaust"). For many DSOs I really like the extra 12 deg compared to my XWs.

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