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  #21  
Old 13-03-2021, 08:55 AM
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Originally Posted by astro744 View Post
Do not dismiss low power viewing just because larger exit pupils bring out the worst in ones eyes. Either use long eye relief eyepieces and wear glasses when observing or get the Tele Vue Dioptrx (available in various strengths).

Generally pick the magnification that best frames the target but for general sweeping of the night sky nothing beats low power on a 4Ē flat field Nagler-Petzal (except maybe a 5Ē Nagler-Petzal). Either of these telescopes are wonderful for low powers with either a 41mm Panoptic to maximise true field or a 31 mm Nagler for a bit more apparent field but a little less true field. The Tele Vue NP telescope have a fiat field so you donít have to worry about curvature of field provided you use highly corrected eyepiece that Tele Vue has to offer. There may be other eyepieces that work well with such telescopes.

Also any short refractor can be used for low power sweeping and depending on focal length and eyepiece used you will see some curvature of field and most of the time itís not too detrimental but certainty telescope/eyepiece combinations will work well and others not so well. Dark skies are recommended for low power viewing to maintain contrast.

See the article (extract) on the joys of low power viewing at:
https://www.televue.com/engine/TV3b_...=Advice&id=164

If you can source ďThe Best of Amateur Telescope Making Journal, Volume 1, Willmann-Bell publisher, it has the full article as well as many others. Volume 2 is highly recommended too.

Thanks very much for that article it was very interesting.

Greg.
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  #22  
Old 13-03-2021, 03:30 PM
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Originally Posted by gregbradley View Post
Thanks very much for that article it was very interesting.

Greg.
No worries. The other article I was thinking of is one called ďThe Perfect Telescope is...

https://www.televue.com/engine/TV3b_...=Advice&id=101

But plenty of good advice on eyepieces and telescopes here:

https://www.televue.com/engine/TV3b_page.asp?id=154

The article on Majesty factor describes contrast, exit pupil and field of view nicely with lots of good info there:

https://www.televue.com/engine/TV3b_...=Advice&id=114


The ultimate goal (for low power sweeping) is for a wide field with smaller exit pupil for better contrast (and less human eye noticeable astigmatism) but with a large field stop diameter to give a larger true field. The introduction of the 21mm Ethos was a big step toward that goal.

The field stop diameter of the 21E is 36.2mm, only 2.5mm less than the 35mm Panoptic. You get almost the same true field at nearly twice the power and half the exit pupil, (not quite 2:1 at 1.7:1 but you get the idea). Field of view for 540mm is 3.8 deg for 21E compared to 4.1 deg for 35P.

Tele Vue eyepiece specifications here:

https://www.televue.com/engine/TV3b_page.asp?id=214

Many an astrophotographer spends hundreds or thousands on a good camera because they can see the difference a better one makes and recommends such instruments on this and other forums. If your interest is not visual then itís easy to not think the same way when it comes to choosing eyepieces.

I am enjoying reading about your venture into the visual world and the experience with different eyepieces. Iíve kind of been there, done that and am happy using an assortment of Clave, Brandon and Tele Vue.
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  #23  
Old 15-03-2021, 02:24 AM
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I read so many eyepiece evaluations by people with small refractors, and discussions that hinge on how a particular eyepiece does in a small refractor on planetary viewing, or very widefield, ultra low power, sweeping.

But, having owned telescopes of 80-320mm and viewed through instruments up to 1.5m, it has always seemed to me that the larger the instrument, the more you see.

When I went from 6" to 8", I could see more deep sky objects, more details in them, and more planetary and lunar details than I could before. When I moved from an 8" to a 12.5", the same thing happened again--I could see more deep-sky objects, more details in them, and more lunar and planetary details than I could before.

I have owned over 350 different eyepieces and used about 100 more over the years, and I really like high-end eyepieces and what they bring to the observing experience. But never once, in 25 personal scopes, and all those eyepieces, has an eyepiece prevented me from seeing all that the scope could show. Seeing has. Transparency has. Darkness of sky has. My own experience has. And the aperture of the scope has. But no eyepiece has. Ever.

Some are sharper than others, yes. Some are better corrected at the edges than others, yes. But all have been adequately sharp in the center of the field to allow me to see what conditions and the aperture allow.

The point here is that people who invest big money in eyepieces, expecting to see more, are putting their funds in the wrong place. They should be looking at a larger aperture. A 10" will show you more than an 8" will. And an 8" will show you more than a 6" will.

I suppose we all top out at an aperture that is the most we can handle, and that will vary from person to person. That's OK. But with 8" apertures available for little more than pocket change these days, and they're all pretty small and light, really, why spend big dollars for tiny little scopes? And I say this as the owner of a 4" triplet apo scope that I use for low power views. But if I were forced to choose between my 12.5" and the 4", the 4" would go in a second because the ONLY thing it does better is to yield wider fields.

Not better views of the Moon or planets. It would only take a second to see the difference that aperture makes on those objects. Not star clusters, ALL of which are better in the larger aperture. Not nebulae, not galaxies, not even double stars.

One of the things I see among beginners is that they want to look at all the different objects--Moon, planets, everything. Yet they look at a cheap plastic 80mm refractor on a computerized mount when for nearly the same price they could have an 8" dob. There are two problems the beginner has--finding objects, and seeing objects. The 8" can help in both cases.
The 80mm? Not so much.

Then they buy the 80mm, and obsess about how another eyepiece may allow them to see something that is invisible in the eyepieces they own.
It just doesn't work that way. The 8" with cheap eyepieces will find and see more than the 80mm with the world's finest eyepieces.

So, if you enjoy eyepieces, buy whatever you want. But, use the largest scope you have to use them in. Unless you're content looking at the same 50 deep sky objects over and over again for the rest of your life, use the small scopes for imaging, and use the larger scopes for visual use.
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  #24  
Old 15-03-2021, 08:11 AM
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Nice writeup Don.

I would add a couple of minor points.

Smaller scopes are less susceptible to the seeing. I have a CDK17 which I use for imaging but I did use it once for visual. It was horrible. The seeing just made any view really bad as the seeing was being magnified. Perhaps a low power eyepiece would help there. I'll have to try it again with a wide view eyepiece and a night of at least decent seeing.

APO's versus compound mirrored scopes. APOs give sharper pinpoint stars and more pleasant views than compound scopes. But of course APOs aperture maxes out around 6 inches cost-wise. I don't know you gain as much past 6 inches with an APO.

APO' seem to cut through light pollution better as well as perform better in poorer seeing.

Greg.
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  #25  
Old 15-03-2021, 08:31 AM
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My newtonian gives refractor-like star images, so I don't see a difference there.
But, where seeing is concerned, the 4" does see better seeing but that is simply because it cannot resolve the seeing fluctuations. If the average seeing is 1.1" the 4" won't see a change, but the 12.5" will see mediocre seeing.
The seeing is the same in both scopes (as long as equally collimated and cooled), but the 12.5" resolves the fluctuations better.
And if the fluctuation enables a resolution of, say, 0.3", the 12.5" and 4" will both see excellent seeing, but the 12.5" will resolve much smaller details. Seeing where I observe is always fluctuating and is often much better than 1", so usually surpasses the capabilities of the 4" in resolution.
It has been 17 years since I owned a catadioptric scope (unless you consider a newtonian with a coma corrector a catadioptric), so I cannot compare. The large secondary does have an effect in reduction of resolution, but even there, a larger aperture will show you more.
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  #26  
Old 15-03-2021, 03:11 PM
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Thanks Don. An interesting discussion.

Greg.
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