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Long Focal Length VS. Short At The Eyepiece

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#1 mustgobigger

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Posted 28 January 2013 - 05:54 AM

with the rash of quality long fl refractors going up
for sale recently and reading all the arguments about color correction, better glass...Ect. maybe some here can share
thier experiences at the eyepiece IN focus doing visual
with both long and short refractors of same aperture.
your setup can be doublet...triplet.. whatever you like
and focal lengths anywhere from f/5 up to f/20 or longer.
i personally have not owned 2 at the same time to do
a direct comparison but have had a few long refractors
that i would have visually put up against any other
telescope in the same aperture.
one that comes to mind was a 60mm goto F/20 ...best views
in 60mm class i have used visually ever.
i also bet i would get different answers in the refractor
forum.
show some pictures if you have them of both your long and short telescope and your personal observations.
thanks,
Brian

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#2 Astrojensen

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Posted 28 January 2013 - 06:24 AM

I have compared a TV 76 f/6.3 apochromat to my Zeiss 85mm f/19 A doublet apochromat from 1903, stopped down to 75mm. The long focus Zeiss totally blew away the TV 76 on lunar-planetary targets and double stars. Views of Mars were incomparably more detailed in the Zeiss. I also strongly preferred my 80/1200mm Vixen achromat over the TV 76, despite the false color. It showed details easier than the TV 76, but not as easy as the Zeiss.

The comparison was not a single-night occasion, but was carried out over several months. Never once did the TV 76 showed anything on the planets that the 80mm Vixen didn't show as well or (in most cases) much better. The TV 76 was much more portable and a better deep-sky telescope, though.


Clear skies!
Thomas, Denmark

#3 mustgobigger

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Posted 28 January 2013 - 06:38 AM

thanks thomas,
just the type of comparisons i was looking for...no matter
the winner as its personal opinions that count.
do you have a picture of the Zeiss....f/19 WoW.
hope to hear from many more here.

#4 Jon Isaacs

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Posted 28 January 2013 - 07:50 AM

The long focus Zeiss totally blew away the TV 76 on lunar-planetary targets and double stars.



My 16 inch does a much better job on small, faint galaxies any 4 inch I have ever tried. I might quality that as "blow away." Two three inch scopes... humm...

Jon

#5 terraclarke

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Posted 28 January 2013 - 11:21 AM

Brian,
Funny you should ask. You can read my views on the subject in the refractor forum where over the weekend I posted my opinions on the same subject when I compare my AT72ED (72mm F6) to my Sears #6344 (76mm F16). I'm sure you can guess what I said.

#6 starman876

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Posted 28 January 2013 - 11:40 AM

the first time I used a 75mm Unitron I realized that these long focus achromats were just as good if not better than the short focal length ED scopes I had. I had a Stellarvue 80mm that was good, but the Unitron was better on planets. However, I also have a Celestron 4" F9 Flourite that blows away every long focal length scope I have looked through. What I am saying is depending how much you spend on the optics has a lot to do with image quality at the eyepiece. TMB triplets and the Zeiss APQ triplets are fantastic scopes and blow away most other scopes. AP (as you well know) makes some fantastic optics at short focal lengths. However, the early long focul length achromats were fantastic performers for their price range and dollar for dollar cannot be matched until you spend a lot more money. I still have the Celestron Flourite and enjoy it, but not that often. The scope looks like it was kept in a time capsule and I am afraid that I might scratch it. I do enjoy the portability of something that good I can carry in one hand out to the mount.

#7 actionhac

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Posted 28 January 2013 - 11:45 AM

Hi Brian.
The only size I've done any serious comparison is 4 inch.
I thought if I could decide which one I liked the best I could sell the rest.
Problem is, time and time again its undecided so here I am with 3 absolutely superb 4 inchers.

Edmund Scientific 4"f15 I've owned 3 of these over the years and all three have been outstanding.

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#8 actionhac

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Posted 28 January 2013 - 11:50 AM

Unitron 4"f15 I dreamed of owning one of these and finally bought this one recently. This Unitron is multi-coated.

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#9 starman876

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Posted 28 January 2013 - 11:50 AM

One more thing I would like to ad is that many of these high end short focal length scopes can be pushed to very high powers without image break down. My 6" F8 AP I have pushed to 480 power on good nights. I have heard from other people that the newer high end shorter focal length triplets can be pushed even higher. Makers like AP, TMB, Tak and so forth. Once again dollars come into the picture.

#10 terraclarke

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Posted 28 January 2013 - 12:04 PM

Johann, I had the Vixen equivalent of your Celestron, the Vixen fluorite F9. I bought it about 12 years ago this spring. Sold it four years later for financial reasons. Can you feel my pain? :foreheadslap:

Terra

#11 actionhac

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Posted 28 January 2013 - 12:07 PM

And here's the last of the 3 4 inchers:

Celestron C102F 4"f9 Made by Vixen for Celestron.
This scope was advertised by Celestron as having "incredible color correction and ultra-high contrast" and that couldn't be truer. I learned all about Fluorite with this scope and it blew me into the modern world.
It does require a barlow to get to high powers with the short focal length. I never liked barlows but today's barlows are superb and you don't even know you are using one.
This telescope is actually old technology now being just a doublet, but what a doublet!
This stubby performs. It took me a while to accept this telescope, and especially using a barlow but it is on par with the Edmund and Unitron.

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#12 bremms

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Posted 28 January 2013 - 12:16 PM

There are a LOT of the F5-8 ED doublets in the 70-90mm range. That being said, my Sears 76mm 6339A (Earlier Royal Astro version of Terra's 6344) gives the nicest views of planets, moon and doubles than any other 3"/80mm I've looked through. It's a little better than my 82mm F15 Jaegers. My Vixen A80 Mwt (Japanese made version) is close too, but at F11 it has more color.

#13 bremms

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Posted 28 January 2013 - 12:42 PM

What it really comes down to is the smoothness, figure and correction of the lens. My Sears scope just has a superb lens. It's not inherently better than a shorter focus ED refractor. It is easier to get higher powers on a long focus refractor. The steeper curves on the short focus scopes are harder to make. Each refractor lens seems to have a little "personality". Seem to have gravitated toward refractors lately. On the chance of getting flamed... A well executed long focus newt in the same price range will show MUCH more detail than a refractor. A 3" telescope is still a 3" telescope. Compared to a good 6" or 8" F8 newtonian??? NO contest. Still love small refractors though.

#14 Grava T

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Posted 28 January 2013 - 12:46 PM

I only have two very different focal length refractors to compare and there is no competition. The 77mm f1200mm beats the 80mm f400mm every time for high power planetary observations but this seems kind of obvious given the longer focal length of the Japanese telescope. The ST80 does better for wide field views but there is sooo much false colour and the stars are just not as sharp. Even with a barlow to double or triple the f ratio of the 80mm it still can't keep up with the classic refractor. I guess it's kind of an unfair comparison.

I have not had the chance to compare my 60mm f910 to my 60mm f800 yet so I think that will be interesting.

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#15 mustgobigger

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Posted 28 January 2013 - 12:50 PM

this post was meant for any size long VS. short ...
if you have a 8" refractor to compare let us know
your findings.
my 60mm post was just a example.
i have owned a lot of these long ones posted already
but very few of the quality short ones.
keep em coming this is interesting.. :jump:



What it really comes down to is the smoothness, figure and correction of the lens. My Sears scope just has a superb lens. It's not inherently better than a shorter focus ED refractor. It is easier to get higher powers on a long focus refractor. The steeper curves on the short focus scopes are harder to make. Each refractor lens seems to have a little "personality". Seem to have gravitated toward refractors lately. On the chance of getting flamed... A well executed long focus newt in the same price range will show MUCH more detail than a refractor. A 3" telescope is still a 3" telescope. Compared to a good 6" or 8" F8 newtonian??? NO contest. Still love small refractors though.



#16 bremms

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Posted 28 January 2013 - 01:00 PM

Almost universally the long focus refractors I looked through were better than their shorter focus counterparts. One exception was an 6" F8 AP Starfire which was clearly better than a 6" F15 Brashear refractor we were using. It's an AP though.. Just sayin'

#17 starman876

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Posted 28 January 2013 - 01:17 PM

another example of a good short focal length refractor is the Brandon 94mm with the AP lens. The one I have is a great example of this scope. Short focal length and excellent optics. Once again, it is a triplet.

#18 starman876

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Posted 28 January 2013 - 01:18 PM

Terra

I feel your pain. I did the same thing when I had aperture fever and traded a 5" F12 AP for a large newtionian. That was something I have always regreted.

#19 terraclarke

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Posted 28 January 2013 - 01:19 PM

And just one other thing that I wanted to mention (forgive me if one of you already said this and I overlooked it) is focal depth. F15s and thereabouts are just so darned easy to focus, their old single speed R&P focusers do just great. With anything down in the F6 range, you just have to have a duel speed Focuser (at least in my opinion). Even so, it's not always easy to get a really sharp focus. And the long focus scopes also function pretty darned well with simple EPs like orthos, plossls, and kellners. Don't get me wrong, I love my shorty scopes, I've got 3 of em. But they're for quick looks and wide looks, not for close looks.

#20 terraclarke

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Posted 28 January 2013 - 01:25 PM

Welcome back Johann, we've missed you the past week.

Terra

#21 starman876

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Posted 28 January 2013 - 01:27 PM

I would say the flourite Celestron is much better than the Unitron and Edmunds. Having the Unitrons and have had the Edmunds I can say the Flourite out performs them both. Of course that is my opinion :jump:

#22 starman876

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Posted 28 January 2013 - 01:34 PM

Thanks Terra. Santa has sent you something today.

#23 terraclarke

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Posted 28 January 2013 - 01:45 PM

Johann,

:jump:

Terra

#24 terraclarke

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Posted 28 January 2013 - 01:53 PM

Johann,

How is your fluorite lens holding up? Mine used to scare me to death. The crown element was made of pure calcium fluorite. That's the same as the mineral apatite which is 4 on Moh's hardness scale which makes it harder than finger nails and pennys but softer than window glass. I was always afraid to wipe the lens with anything for fear that the least bit of grit would scratch it. I would hardly ever use even a lens brush and would never take it out if there was the least chance of dew.

#25 starman876

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Posted 28 January 2013 - 02:01 PM

Terra
I do not take the flourite out very often for fear of damaging it. The flourite is flawless to this date as is the rest of the scope. Mine also came with the mount that Celestron shipped with it all in the original shipping box with all the original accessories.. I was lucky to find this scope in this condition and hopefully someday will pass it on to someone else in this condition.






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