Eyepiece Selection: 20mm or 22mm Nagler?

[Rodstar]

I am planning to change my 22 Panoptic for either a 20 or 22 Nagler.

I am interested on any comments you may have about the pluses and minuses of each one. They cost pretty much the same, so money is not the issue. They are both very chunky eyepeices, and with my scope, weight considerations do not come into it, either.

The 20 Nagler will produce 125x magnification, the 22 Nagler 115x.

I have read some wonderful things about both EPs, but a few criticisms as well. For example, in relation to the 22, I understand it is a bit prone to kidney bean distortion, that edge of field performance is a bit iffy, and the click-stop eye adjustment is a bit annoying to some.

I would prefer to go with the 22 Nagler in terms of its framing of objects. I love my 22 Panoptic, but it suffers too much coma in my fast mirror, and sometimes I wish it had just a tad wider FOV (it has 36 arc minutes in my scope). With the wider true FOV with the Nagler, I would see 43 arc minutes with the 22 Nagler, compared with 39 arc minutes for the 20 Nagler. I would like the extra FOV if all else is similar.

What are your experiences and thoughts? Thanks for sharing.:thumbsup:

[casstony]

If you are content with eye relief of your 22 Pan, you would probably be happy with the 20 Nagler which is apparently superior optically to the 22mm.

I have the 22 Nagler as I prefer the extra eye relief. I never really noticed the less than perfect edge until I recently looked at it with and without a paracorr - the paracorr cleans the edge up nicely. I've had no trouble with kidney bean but I always observe seated so my eye is always steady - might be a problem for observers who stand? The click stop adjustment has been stiffened on more recent eyepieces and I don't find that it slips too easily.

[Rodstar]

Thanks for the comments, Tony. I was planning, ultimately, to get the Parracor as well, so your comments re edge of field are helpful.

[MarkN]

Tony

Is the view through the Parracor the same as normal but with the distorted edge cropped? There is an effective increase in focal length, ie narrower FOV; so I'm left wondering if I should just ignore the last bit of the field of view and save the $500 for other items.

[casstony]

Just to keep it perfectly clear, there is no cropping/vignetting of the apparent field of view. There is a 15% magnification increase so the true field decreases as a consequence, something you should take into account before purchase of subsequent eyepieces. A paracorr is probably a luxury with an f/5 dob and you should put money towards the eyepieces you want/need before spending on the Paracorr. (as a side note, I purchased my Paracorr from Astromart for about AU$315 shipped - new price is too much for me). Purchasing used allows you to minimize losses if you decide to resell the item - of course there are risks buying used as well.

[MarkN]

OK, that's cleared that up. Thanks Tony.

Mark.

[ausastronomer]

Rod,

I will chat with you about this at some length on the way home one day before you drop your coin.

For the benefit of others. In short, the 20mm T5 is a better eyepiece optically than the 22mm T4. It goes deeper due to better light transmission, has better contrast and is sharper on and off axis. Eye relief of the 20mm T5 is "similar" to your 13mm Nagler T6 that you owned and you would probably find it adequate. However, some proviso's. Once you get a few Pentax XW's in the shorter focal lengths there is a fair chance you may enjoy more eye relief than is offered by the 20mm T5. When combined with a paracorr the 22mm T4 improves remarkably in performance in a fast scope, as does the 17mm T4, which I own. The 22mm T4 gives a more "submersive" view than the 20mm T5 primarily due to the longer eye-relief and larger eye lens.

I think if you were comfortable with the eye-relief of the 13mm T6 that you owned, the 20mm T5 is a better eyepiece. If you like the idea of more comfortable eye-relief you will lose very little optically by going with the 22mm T4 and a paracorr. You won't necessarily need a paracorr with the 20mm T5. Consider this. If you plan to buy a paracorr and don't need to maximise the FOV at this focal length, the 20mm Pentax XW is a better eyepiece than either of the Naglers IMO. It certainly goes deeper with better contrast and throughput. I would give a lot of thought to going with a 31mm Nagler to maximise FOV at the low power end, then going to a 20mm Pentax XW, with a paracorr. I can let you try all of those combos in your scope and see what you think anyway, before buying.

CS-John B

[wavelandscott]

While not an expert at all, I was wondering why you wanted to be in the 20 or 22 range...I think Ausastronomer is right in suggesting that the 31 mmT5 might be a better go for low power wide field...

Of those Naglers I have tried, I like the T5s better than the T4s (I put the T6s in between) but I am not too concerned about eye relief...and have not used either with a Paracorr.

Take John up on his offer before you spend your "cold hard cash"...

On a side note, you know deep down in your heart that you are going to end up with a case full of premium eyepieces anyway to take full advantage of your fine telescope...to not do so would be a crime...so you might as well hunker down and get started...

Were I you, I'd start with a Paracorr and then (after using John's eyepeices and experience) start filling your eyepiece case as quickly as your wife and family will allow.

Clear Skies!

[Rodstar]

Thanks John and Scott. Food for thought.

Scott, I am very find of the magnification range of 110-125x. It seems just right for a lot of objects. My long term plan was the 31 nagler, something around the 20-22 mark, and then Pentax XW's in the 14, 10 and 7mm range.

John, I was interested in what you said on the phone a moment ago about the 16mm area being a natural point down from the 31 Nagler. I might just have to do some more experimenting to work out more carefully what I am really after.

[wavelandscott]

I understand liking the 110-125x range...me too

I am quite fond of the 16 mm T5 but some find the eye relief "tight"...a 17 T4 might be another alternative...

[AstroJunk]

17mm T4 for me. If I only had one eyepiece it would be that one - and I use it 80% of the time. Fabulous in the 20".

I use a 35mm Panoptic for low powers, only because it's a more reasonable size than the 31mm Nagler - but a paracorr is a must to flatten the field (I think a paracor is a must on any f5).

Back to your question - Keep the panny and buy a paracorr :thumbsup:

[Don Pensack]

If you see coma in your 22 Panoptic, you will certainly see it with the 20 or 22 Naglers, which have wider fields of view.

A Paracorr can certainly tune that up for you and, at your focal ratio, essentially eliminate coma.

Some people are very bothered by coma (I am one), and even correct it to beyond f/6 f/ratios. Others don't mind it at f/4.5. You will know which type you are by whether you notice it in either the 22 or the 20.

As for the choice between the 20 and 22, I agree with John that the 20 is a little sharper, and lighter. I own a 22 because I do a lot of star parties and I prefer not to have to clean my eyepieces all the time. For my own preferences, though, I think the 31-20-13 combination is just about optimum.

I prefer wider apparent fields than the Pentax XWs (John's favorites), but I like Pentaxes too. I have used a 14mm XW in my f/5 scope with Paracorr and liked it a lot. I observe without glasses, so I found the eye relief obnoxiously long, but the eyecup at maximum height worked fine. Whatever is your preference, you aren't going to lose whether the 20Nagler, 22 Nagler, or 20XW is in your case. One caveat, though...the 20XW has more field curvature than any others in the XW line. You may want to try that one before buying (ditto on the 17mm Nagler).

Otherwise, see if you can compare the eyepieces before buying. If not, choose the optimum field of view and/or magnification for your scope and eyes.

[Rodstar]

Thanks for your input, Don and Jonathan.

Don, I always appreciate the thought which goes in to your answers. I can't stand coma...it is driving me batty at the moment with the 22 Panoptic, which I would estimate becomes noticeable in all but the central 10% of the FOV. As I have a particular penchant for open clusters, this is a major issue which I am immediately resolving by ordering the Paracorr. It will be interesting to see how the 22 Panoptic performs with a Paracorr, as in all respects other than coma, I have always loved using it (it performed beautifully in my 10" LX200).

Jonathan, given that our scopes are pretty much identical, I am interested to hear more about the 35 Panoptic vis a vis 31 Nagler choice. I have been puzzling over this selection myself, bearing in mind the major price difference, and that the difference in actual FOV is fairly minor. The reviews on this issue have held as a consensus that the 31 Nagler wins out overwhelmingly in terms of contrast and light throughput. Have you had a chance to compare the two eyepieces on those particular characteristics?

[Satchmo]

I think the 35mm is a bit of as waste on an F5 as most people unless a teenage eye in a very dark sky won't have more than a 5mm pupil, which is effectively stopping your scope down to a 15", when trying to shunt through a 7mm pupil which the 35mm EP would deliver. The 31mm will waste less light for the same true field.

If you are only after maximum true field then 35mm is a cheaper alternative to the 31mm Nagler. Another option would be go a cheaper 70 degree 40mm clone. At 8mm effective pupil , the field edge will look remarkeably good because a 5mm eye pupil is stopping the scope down to F7.5.

A good way to resolve the issue of attained pupil size would be to get someone to snap a flash photo of your eye while you hold a ruler next to it for scale, on close up setting, and do it a realistic setting like Kulnura. Sky and Telescope used to sell a little cardboard guage you could stare through which enabled you to test your pupil size. It may be possible to make one , I think it involved pairs of pinholes from memory.

Hope this helps.

Edit: If using the Paracor the problem wont be as bad of course due to the 1.15X amplification factor. Your F5 effectively becomes an F 5.75 :-)

[AstroJunk]

$$$ did it for me rather than science. Low powers aren't very high up my priority ladder so I spent the cash on the medium-power stuff and some nice filters. Similarly I don't get the chance to indulge high powers so I bought TMB planetaries (which turned out to be fantastic - but that's another story!).

If your budgetary priorities allow, then I'm sure the 31 is better.

As for exit pupil - I thought 6mm was more like it for a middle aged bloke? Is there a doctor on the forum??

It matters little - what you loose in light hardly matters at that magnification. The biggest problem with low mags occurs when the central obstruction becomes readily apparent, and this is not the case with the 35mm Panoptic.

[Satchmo]

It would be interesting to find a simple test by which people could survey their pupil diameters. I always err on the side smaller side, as I doubt most observers have the ambient conditions to achieve a maximum pupil. The eye is pretty poor quality at maximum aperture : contrast and sharpness are well down, so I rarely use those mags despite th efield of view.

Binoscopes make even more sense when you consider that you can have 40% less focal length for an equivelent monocular aperture allowing you to see twice the sky area when comparing true field at the focal plane. Just because you have a large scope doesn't mean you want to give up on the big pretty stuff, and I find a 25" F5 allready getting too restrictive in field for a lot of the showcase objects.

[Glenn Dawes]

Hi guys,

I have a 10" f5 dob (standard B&T so I thing that makes it a GSO type).
I love wide fields but hate coma. My standard used to be a 20mm erfle. Then I discovered the Meade series 5000 SWA 16mm (this was young Michael's recommendation at B&T and thank him for this - a good choice).

Now I'm getting greedy. I want to up the mag and keep the field size to a max and am thinking along the lines of a Nagler VI, 7 or 9mm. Any thoughts or experience with this combination of scope/ep? Any distinction between the 2 (except for the higher power)?

Regards

Glenn

[ausastronomer]

Hi Mark,

You can do this yourself without any fancy equipment and get a very close estimate of your pupil's maximum dilation. You need a set of allen keys or twist drills of known diameter.

Go to a dark observing site, and give your eyes about 30 minutes to fully dilate. When dark adapted and ready to test, try to block the light from a star by putting increasingly large sizes of allen keys or drills a couple of inches in front of your eye. Start a couple of millimetres less than what you expect your pupil dilation to be. When the allen key or drill is smaller than your pupil, you will see the star as double. When the wrench matches your pupil, the star will wink in and out as you move the wrench in front of your eye. Only after you find the right size allen key or drill should you turn on your red torch and find out its size. I am 48 yrs old and mine is about 6.2 mm under really dark skies. Many things affect your pupil dillation including age, general health, smoker or non smoker, heavy drinker, high blood pressure, blood sugar levels etc etc etc. As you correctly point out, it is better to err on the side of conservatism and go with an undersized exit pupil. In other words, stay around 6mm to 6.5mm maximum, if high contrast views with black sky background, are your goal. Thus, a 31mm Nagler or 30mm Pentax XW in an F5 scope is ideal IMO.

CS-John B

[iceman]

Glenn, you should probably start a new thread so it doesn't get lost in here..

But I'd go for a 7mm Pentax XW!

[AstroJunk]

Which amazingly fits the formula: 8.1 - (age/25) !

I don't know where the formula cames from; I was given it and it seems to be ok.

[Satchmo]

Thanks John, That was the kind of simple test I'm after . I'll bring a 6mm drill as a simple pass/fail up to Kulnurra next time. At 46 , I'll be very happy if my pupil still reached 6mm, and 5mm was too pessemistic.
One has to remember too how much alcohol can effect your night vision. I once saw that the Milky Way disappear after half a bottle of 'South Pacific Star Party' label Port .:lol:

[Glenn Dawes]

Wait a minute - this means at the age of 202 years 6 months I'll be legally blind - NOT HAPPY!

Glenn

[Don Pensack]

I recently switched from a 35 Panoptic to a 31 Nagler and noticed the image became noticeably brighter. The change from a 6.1mm exit pupil to a 5.4mm exit pupil was/is mostly responsible.

I used a Sky & Telescope Pupil Gauge to judge my exit pupil.
It consists of a series of small holes in two rows not quite parallel. Each pair of holes is 0.5mm closer than the pair above it, all the way from 10mm apart to just touching. You hold it up to the sky and move down the rows of pairs until the gap between the holes appears to disappear.

Using this, I discovered my pupil size, when looking at the sky, is 4.5mm.

Another technique is to take the magnitude scale from the bottom of a star atlas and cut it in half so you have a row of half-moon scallops. Hold it up to the eye so your eye's pupil makes the half moon into a round moon. Looking in a mirror, move the row of star images back and forth until it matches your pupil size. Darken the room completely and wait a couple minutes to turn on your red LED observing light. Illuminate the room just enough to see the pupil gauge near your eye and move the gauge back and forth until the half moon scallop just matches your pupil diameter. Turn the light off, wait a few seconds and turn it back on. Still matched? Good, that's your pupil diameter. In this manner, I obtained a 5.0mm pupil. which means that looking at the night sky actually causes my pupil to contract a little bit from looking at the ground (something I already knew).

Eliminating peripheral light can allow the pupil to dilate to maximum--hence, the black cloth over the head technique used by people in previous centuries.

At any rate, I had my answer as to why the 31 Nagler seemed brighter--the 35 Panoptic was stopping my scope down because its exit pupil was significantly larger than my dark adapted pupil.

For those of us who are older than a half century, matching the low power eyepiece to our pupil diameter makes a lot of sense.

Other advantasge of the 31 Nagler: it does have a larger true field of view than the 35 Panoptic (~10% wider), it has a darker sky image (higher magnification) which improves contrast, and it has less field curvature (so star images are tighter at the edge when the center is focused--the 35 Pan requires focusing on a star about 25% of the way out to get the entire field in good focus if your eye cannot accommodate the curvature when you focus on the center--this varies from person to person).

If your eye can accommodate the exit pupil, and the mild field curvature, the 35 Panoptic is a very nice low-power eyepiece. Though I ended up with the 31 Nagler, I used a 35 Panoptic for years without reservation. Either way, you don't lose.