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Equipment Discussions >> Cats & Casses

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Eddgie
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Reged: 02/01/06

Re: Q7 vs C8 on planetary? new [Re: Mirzam]
      #5520195 - 11/14/12 09:32 PM

In fact it does, but you have to get about 300mm of back focus before the change in correction enough to be easily visible in the image.

And if the scope is slightly undercorrected at stated focal length, a lot of back focus can actually improve the correction.

But if the scope is perfectly corrected at stated focal lenght, 300mm of back focus can cause about 1/5th wave of spherical abberation (as I recall... Rohr did this measurment on Astro-Foren once and has a graph for back focus on the C11... I seem to recall that it had to be about 300mm before there was enough SA that the average observer could easily see).

I am sure that you can find it if you check Astron-Foren.de.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Q7 vs C8 on planetary? new [Re: Eddgie]
      #5520229 - 11/14/12 10:02 PM

Eddgie, all your points are well taken. The SCT is quite possibly the most successful or popular design around. Like most folks, I have owned my share and been quite happy.

I have yet to put my finger on the reason the Mak has reinvigorated my love for observing, especially planets. There is something about the design that offers them that niche at a given aperture and CO. So far, the best answer seems to be the way they treat SA and and other aberrations, including their smoothness. It might well be the best mass produced scope I have owned in terms of optical quality and resulting crisp images.

No doubt a 10 and 11" SCTs would have prevailed. And an 8" should prevail, too. But, a very well made 7" Mak might just hold it's own with an 8" scope in terms of planetary contrast. It's a very close call, especially if the Mak MTF curve is not further degraded by any notable aberration.

With both scopes having roughly the same CO, any aberration advantage in either might well equal or overwhelm any gain on 1" in aperture through most of the spatial frequency range. That would depend on the care taken during manufacture, one would have to be very well corrected and smooth and the other average or less. At this point, the MTF curves defining resolution above the Dawes limit could be about the same. Even if one were slightly better, that small improvement likely would be not seen at the eyepiece.

You know, thinking about it, asking a question like this is really one of those probably too close to call or to make a blanket statement that generally holds true. Comparing a 7" SCT of average build would be easier to say one way or another (but then there would be no question, probably.) There is just something sweet about a Mak on planets, I'm still trying to put my finger on what it is.

Yes, the Schmidt is a fascinating design, too. The shape of the corrector does neat things to the wavefront, less correction near 70% zone, and progressively more toward marginal and paraxial rays. The meniscus is much simpler and does not provide this level of zonal correction. I suppose, though, that once the Schmidt surfaces are corrected to within the diffraction limit they are set and have no significant higher order spherical. By virtue of it's steeply curved meniscus and very significant HSA, the Mak has to be better corrected (balanced or aspheric.)

This seems to suggest a Mak has tighter tolerances and less slop in the surfaces, and this level of tolerance might account for their /perceived/ consistency across samples. Maybe that's the edge, and maybe that edge can put a Q7 on par with an 8" scope in terms of planetary.

Anyway, really don't like these comparison threads, but the Mak design interests me.

Edited by Asbytec (11/14/12 10:22 PM)


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azure1961p
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Re: Q7 vs C8 on planetary? new [Re: Asbytec]
      #5520252 - 11/14/12 10:22 PM

Interestingly, what would seem to bypass this comparison discussion is F
ccd imaging. I ve seen some 7" apo Jupiter images and they can't hold a candle to a 10" f/4 reflector. Imagers seem to sidestep the visual contrast querks and nuances and beeline it straight to the angular resolution advantage and design be damned. The 6" apo details in ccd images - despite wonderful inherent contrast have details that seem stubby and smoothed compared to a C11s. In the electronic imaging realm the contrast is adjusted with sweeping control in processing that seems to not care a bother for CO and SA. It creates an interesting paradox of sorts about modern imaging and cutting edge contrast sensitivety. On one hand its arguably better than human vision on the other hand it seems callously indifferent between those finer contrast we observe even casually between differing Scopes. Angular resolution seems king while contrast resolution is *negotiable*.

It'd seem like a better imagery would indeed seperate the Q7s from the C8s but the processing software seems to over power this, even handedly.

Pete

Edited by azure1961p (11/14/12 10:24 PM)


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orion61

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Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Q7 vs C8 on planetary? new [Re: azure1961p]
      #5521082 - 11/15/12 12:01 PM

Very interesting post here thank you for sharing, that C11 looks like it would be a pretty good performer.
I have had over a dozen SCT 8's go through my hands over the years. Fairly good is the majority of the quality with Celestron generally winning the overall optical tests.
This is why I have been for the ability to have these tested and graded, (think of baseball cards)This way you know exactly what you are getting.
Intes has been doing this and the better ones fetching beter prices.
I have finally settled on an old Celestron Pacific Orange tube.
As I got them in I had kept the best one, I found this one a number of years ago and havent had one out perform it yet.
I have had a bit brighter ones, prettier ones,
I have also had some come through that left me scratching my head wondering how in the world it ever made out of QC?
I also know 1st hand Celestron used to have a few held back and when Dealers took a tour they were offered these for sale as a personal scope. This story came to me from a Dealer who still has his that he picked up at the factory.
It is indeed a better than average tube.
Heck the one I have doesn't even have multicoatings but I have kept it over Starbright and one XLT tube.
I can say tho the optics coming out of China are better in a more consistant basis than any others I have seen.
I have a C6se that is just killer.


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Q7 vs C8 on planetary? new [Re: orion61]
      #5522100 - 11/15/12 11:12 PM

You know I just may get an old orange tube c-pacific. I HAD one that was actually pretty good and I let it go. The mechanics on the pacifics I love though the image shift can be a little wonky.

I'm glad you like your c6se mine is terrific too. I honestly can't fault the beast. The C11 is probably one of the best lunar planetary Scopes out there and for allthe reasons eddgie mentions. Apparently the 9.25 has a strong fan base too.

Back on the Q7 though I do recall a review in the review section under man's comparing a Q7 to a Meade 7mak. The Q7sounded like it needed itscoatings renewed and enhanced.

Its projection to be sure but I don't think a good C8 would suffer as bad as might be expected. Just a hunch.

Pete


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sqrlman
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Reged: 01/02/08

Re: Q7 vs C8 on planetary? new [Re: Darren Drake]
      #5522161 - 11/16/12 12:19 AM

Quote:

Has anyone ever compared a Questar 7 to an excellent C8? I would think that while the C8 has a little more secondary obsstruction % (I think) that its increased aperture would make up for that on planetary viewing. Any actual experiences out there?




I would like to compare my excellent C8 to a Q7 but I don't know anyone that owns a Q7. I have compared it to some other Mak-Cassegrains. TEC 8" F20, Intes-Micro 715, Intes 9" F13.5, Orion 180 Mak. The made in the USA C8 beat all of them on planets. There is nothing inherently wrong with the SCT design. If they are made right they can be excellent telescopes.

Steve


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