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SCT's don't make good planetary scopes.

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#26 Gaz O'C

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Posted 12 March 2008 - 02:20 PM

I've often heard the statement "inch for inch, refractors are best." But inch for inch isn't really a very meaningful metric IMHO. Dollar for dollar or pound for pound sound like they would be much more reasonable ones.


Dollar for dollar I'd take a 6-7" Mak/Cass or a 8"-10" Newt over a 8" SCT for visual planetary work. Planetary imaging is another matter though, thats where SCTs really excell.

#27 NeoDinian

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Posted 12 March 2008 - 02:38 PM

While it may be true that there are better choices than an SCT for planetary (Mak, Long fl Refractor), the SCT is a GREAT performer when properly collimated...

And without checking every post, I would say that MOST images taken of planetary are done with a webcam through an SCT...

#28 Rick Woods

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Posted 12 March 2008 - 02:45 PM

I doubt there would ever be a night that a premium 11" or 14" f/10-f/11 APO triplet would NOT be better than a C11 or C14 respectively on planets.

Well, I think we can all agree on THAT one!

My only point is that the proof of my senses tells me that SCT's can indeed be good planetary telescopes.

#29 Gaz O'C

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Posted 12 March 2008 - 02:49 PM

And without checking every post, I would say that MOST images taken of planetary are done with a webcam through an SCT...


Thats not really a good argument IMHO. Visually SCTs are poor on low contrast planetary detail, when you are imaging you are stacking hundreds of images, stretching the histogram, playing with the levels etc to compensate and bring all that out. The human eye doesn't have those luxuries. The reason that most plantetary images are taken with a SCT is that they are cheap for the aperture, have a large(ish) focal ratio and are easy to mount, not because they give good visual performance.

#30 Rick Woods

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Posted 12 March 2008 - 03:16 PM


And without checking every post, I would say that MOST images taken of planetary are done with a webcam through an SCT...


Thats not really a good argument IMHO. Visually SCTs are poor on low contrast planetary detail, when you are imaging you are stacking hundreds of images, stretching the histogram, playing with the levels etc to compensate and bring all that out. The human eye doesn't have those luxuries. The reason that most plantetary images are taken with a SCT is that they are cheap for the aperture, have a large(ish) focal ratio and are easy to mount, not because they give good visual performance.

But, they do give good visual performance.

#31 TG

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Posted 12 March 2008 - 03:21 PM

I've never been a big fan of any of my SCTs planetary performance. Inch for inch give me a Newt or a Mak any day.


Even a Mak with a 33% CO?

Regards,

Tanveer.

#32 Gaz O'C

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Posted 12 March 2008 - 03:21 PM


Rick, I can't really argue against something you are just telling me is true. I can only speak from my own experience....I'm not saying they are bad scopes, just that IMHO there are much better visual planetary scopes out there. Inch for inch and dollar for dollar.

#33 Gaz O'C

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Posted 12 March 2008 - 03:25 PM


I've never been a big fan of any of my SCTs planetary performance. Inch for inch give me a Newt or a Mak any day.


Even a Mak with a 33% CO?

Regards,

Tanveer.


The CO on a SCT is just one factor, But, as I've pointed out in another post in this thread, there are other factors in mass produced SCTs that compromise planetary views. Its easier to mass produce a very good Mak than it is to produce a very good SCT.

#34 David Knisely

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Posted 12 March 2008 - 03:47 PM

Yeah, right!
Who started that idea, anyway?


It's true that for a given aperture an SCT is a relatively poor planetary performer compared to several other designs. The SCT advantage is that above 8" or so the cost of those other designs (plus the cost of the mountings required) is so very much higher that they become unrealistic alternatives for most folks. When compared to other telescopes of significantly less aperture, the SCT looks pretty good.


Well, I can't really call SCT performance "poor", even in a relative sense. It may be somewhat inferior to that of similar sized refractors or well-figures small secondary Newtonians, but it is still more than adequate to provide decent satisfying views of the planets. I had an excellent night with my NexStar 9.25 on Saturn, as it gave an outstanding sharp and clear image at 392x (nearly as good as I get with my custom-mirror 10 inch Newtonian). The northern and southern belts were visible as well as the polar darkening, the Cassini Division and the Crepe ring. I have even seen Enceladus in the 9.25 on a number of occasions, something which even my 10 inch occasionally might have trouble doing, so it is definitely a workhorse even for planetary viewing (blows my 4 inch refractor out of the water). My 10 inch will beat the NexStar 9.25 SCT I reviewed for Cloudynights, but the performance of the SCT was close enough to that of my 10 inch that I bought the review instrument (and am very happy I did so). Clear skies to you.

#35 bcuddihee

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Posted 12 March 2008 - 04:21 PM

I agree with your assessment David. It just seems to me that the majority of SCT bashing is unwarranted and uninformed.
I guess, what it seems like, is that those of us with recent vintage mass produced SCT's are getting some very high quality views, probably a lot better than SCT's of an earlier vintage, contributing to their rather poor reputation. Smoother optics, better coatings like UHTC and XLT, really squeeze the most photons into these scopes. Lighter OTA's help contribute to faster cool down times and aid in their portability. Also, when talking about central obstruction.. the CO of an 8 inch f4.9 newt is 29% as opposed to 31% for the C8. Not much difference there. The secondary obstruction for a 5 inch f12 mak is 31%. The INTES 6511 has a 31%, so the CO argument is kind of a mute point to me. Mass produced SCT's probably..and this point is debatable, can offer the kind of nth degree performance that custom optics can offer, be it a newt or APO, but they can get you 95% of the way there, for a small fraction of the price. I for one am quite satisfied and smiling..all the way to the bank.

#36 Gaz O'C

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Posted 12 March 2008 - 04:44 PM

I really don't see any 'bashing' going on, just people expressing opinions based on their experiences?

The CO (as I keep saying) is a factor but not a massive one when comparing SCTs with Newts and Maks. Its easier to mass produce a Newt or a Mak/Cass to decent specs than it is a SCT. The corrector (and secondary??) needs to be matched with the primary which is hard to do in a production environment . The primary is f2 ish on a SCT rather than f3 on a Mak. Most production SCT's have primaries of 1/4 wave at best (if you are lucky!), probably the same as production Maks but inferior to mass produced Newts. these are all things to take into consideration. I don't mean indisernable differences in between custom Newts and mass produced SCTs, I mean differences easily noticeable at the eyepiece in between examples of the two mass produced designs.

#37 bcuddihee

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Posted 12 March 2008 - 05:21 PM

No disrespect here but I still don't get it. There are gives and takes on any design but what I am seeing at the eyepiece doesn't gel with what your position. A blanket statement that a mass produced SCT figure is inferior to a mass produced newt, (Most production SCT's have primaries of 1/4 wave at best (if you are lucky!), probably the same as production Maks but inferior to mass produced Newts.) is really not true. I have read many posts in the reflector forum praising the optical quality of their instrument and just as many saying that it was inferior. Thats the nature of mass production. All I am saying is that its quite possible that the quality of SCT's may indeed be on the rise..something that you may not be quite willing to accept.

#38 mblack

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Posted 12 March 2008 - 05:32 PM

I think the big advantage of SCTs versus other designs is aperture and portability in combination, per dollar.


Couldn't say it better, and I just went back down the C11 road.

The C11 slots nicely in aperture between the 7" MCT and 15" Obsession, and I cannot touch its aperture and compactness with any other design at it's $2000 price of entry.


Agree again. I missed my C11 on globulars more than planets, but my 140mm refractor is my bright target scope.

I've had two meade (8" & 10") and two celestron (9.25 & 11") SCT's. All had nice optics, no dogs in the group. Everybody who owns an sct needs to become an expert on checking & collimating the optic, particularly at the end of a viewing session, given still air.

After plunking down $ on a new C11, you could say i'm a fan of this design.

#39 mblack

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Posted 12 March 2008 - 05:57 PM

Most production SCT's have primaries of 1/4 wave at best (if you are lucky!), probably the same as production Maks but inferior to mass produced Newts.


Yes. If you take the time to look at tests posted on mass production sct's they are a bit troubling. To some.

I wonder, how much would be added to the cost of an sct if the optics were more in line with a Santel or Intes-Micro Mak Cass for example. I'd love a 10" IntesMicro MakCass :grin: But in my case i'm looking at $2200 for the C11 vs. ~ $8,000 for the MakCass.

To get back on topic, that $8,000 10" MakCass is going to outperform any equivalent SCT on planets, or any other target. Superior optics will guarantee that.

But to my eyes the SCT has been a very good design, once you accept it's limitations.

#40 scrubmonkey

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Posted 12 March 2008 - 06:12 PM

I'll report tonight. I've been a diehard newt fan since I was grinding glass at Edmund Scientific twenty years ago. Have an Astroview 6EQ, Just got (delivered an hour ago) my new Celestron C6 XLT starbright coatings. Aperture being technically equal, the only upgrade on the SCT is the 2" diag w/dielectric coatings. Focal length is different, 750mm for the newt and 1500mm for the C6. But I guarantee you lugging the C6 outside will be easier than the Astroview 6.

#41 bcuddihee

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Posted 12 March 2008 - 06:28 PM

Just make sure you check and adjust collimation if either scope needs it.
Thanks BC

#42 Eddgie

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Posted 12 March 2008 - 06:30 PM

Well, there are a great many reasons why SCTs are not considered to be the best planetary scopes.

To me, though it is simply an aperture thing.

The reason that SCTs make such great planetary scopes (in my own opinion) is because you can get a GREAT FREAKING BIG ONE on a tracking mount!!!.

To me, the SINGLE biggest advantage of the SCT design is simply that you can mount it on a tracking mount of sufficiently less mass than most other designs and still have a solid high power observing platform.

I use high powers for MOST of my observing, and the reason I love my C14 is that I can track my targets and stay in a freaking chair.

But I have no doubt that a really superb 12 inch Newt could easily outshoot the C14 on Planets.

The SCT also has a bit bigger diffraction limited field than most faster newts, but if you are tracking, it doesn't matter, does it.

Scts are good. Newtonians are better. Refractors are best.

But I have a 6" AP refractor, and I have a 14" SCT. and I suspect that when Jupiter gets high this year, it will be the C14 that gets used the most on it.

As the Ford guys used to say when they were beating the pants off of Ferrari in the 60s, "there is no substitute for Cubic Inches".

Arf arf.

#43 jrbarnett

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Posted 12 March 2008 - 07:01 PM

100% agree.

Aperture provides resolution, plain and simple.

Added aperture can overcome design disadvantages when it comes to viewing planets, and as you say it is comfortable to sit behind a big SCT.

Regards,

Jim

#44 mblack

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Posted 12 March 2008 - 07:14 PM

The reason that SCTs make such great planetary scopes (in my own opinion) is because you can get a GREAT FREAKING BIG ONE on a tracking mount!!!.


you sir, are a wordsmith.

and that thought is right on :rockon:

#45 NeoDinian

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Posted 12 March 2008 - 07:44 PM

Well, this is taking a turn...

Heres the deal... If we're putting down the SCT as a "Mass-produced" unit, then you MUST compare the "Mass-produced" SCT to an EQUALLY "Mass-produced" other scope... Comparing the "Mass-Produced" SCT to a practically hand made APO is NOT a fair shot.

Anyways, all that said, in my original reply I DID point out that with proper collimation, blah, blah, blah... I myself LOVE my 10" SCT. I've seen in good detail banding, color and storms on Jupiter and Saturn, and I've seen dust storms and polar caps on Mars. No loss in detail with my scope. And yeah, I understand that a planetary image is a stack of MANY images to produce that pic, but I've NEVER seen as much detail as any pic in ANY scope, or ANY design. But my 10" SCT delivers (in my opinion) MORE details on planetary than I've seen through a Custom built 10" f/13 Astro-Physics... Of course my Collimation was DEAD ON on those nights, and seeing was outstanding. For some reason however, the 10" AP did a far better job on the Lunar features at high mag. My SCT I need to keep at a medium magnification on the Moon or it washes out.

#46 amys

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Posted 12 March 2008 - 07:53 PM

I've often thought that I wish I still had my NexStar 8i so that I could compare it side-by-side to my Intes-Micro 703D. Now that I understand more about collimation and seeing, I wonder if the soft images I was getting from the SCT, particularly on the planets, was due to imperfect collimation and/or lousy seeing. But here's the thing: I know my Mak is not yet perfectly collimated and the seeing is rarely good here. Still, the views I had of Jupiter last year through the Mak, when collimation was more off than it is now, were much more detailed than the views of Jupiter through the 8i.

So I throw out this question: is it possible that SCTs are more sensitive to imperfect collimation than Maks? If so, why? Just curious.

#47 Jon Isaacs

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Posted 12 March 2008 - 08:04 PM

the CO of an 8 inch f4.9 newt is 29% as opposed to 31% for the C8.



I have seen this before. I would like to measure the CO of a new Celestron 8 inch SCT because in the past they have had secondaries that were around 34% and the Meade's are/were a bit larger IMMSMW.

By comparison, I happen to have an early Konus 8inch F/5 Newtonian. This is the Synta version which was the same as the early Orion 8 inch F/5. Last night I removed the secondary and measured it with calipers at 2.00 inches so the CO is right at 25%. There may be 8inch F/5 with a 29% CO and there maybe 8 inch SCTs with a 31.7% CO. That difference is small. The difference between 25% and >31% is not so small.

- In general I think planetary views are about the scope being ready and more importantly there being good seeing. A scope with decent optics, cooled and collimated will give good views if the seeing is good.

Roland Christen has some interesting comments on AM about Maks vs SCTs today...


Jon

#48 mblack

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Posted 12 March 2008 - 08:11 PM

Hi Amy,

You've asked a really loaded question, so I'll try to stay on topic.

Collimation is critical. MakCass, SchmidtCass, Newt, MakNewt, all of these designs require nearly perfect collimation for planetary viewing. Contrast is king on planets, and reflecting scopes just won't deliver without collimation done to very exacting standards.

Your 703 has the advantage of exceptionally figured optics. Once you get the collimation dialed in, you may never need to fuss with it again.

What was this thread about anyway? :lol:

#49 Jon Isaacs

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Posted 12 March 2008 - 08:20 PM

Don't know if it qualifies as "really superb" but it qualifies as "pretty darn good" and it does track the planets and does have a 21% CO and three fans so it cools nicely. And it weighs about 300lbs... 12.5 inch F/6 is a nice place to be.

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#50 jrbarnett

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Posted 12 March 2008 - 08:23 PM

Actually it would surprise me if the recent vintage, Chinese made SCTs are better than the vintage ones considering that the vintage ones were made here, in smaller volumes, and were much, much more expensive than similar sized one available today, meaning that there was margin to be had and an incentive, therefore, to invest more labor.

Also, the science about large CO's not being optimal isn't theoretical or opinion. It is fact. There is no question, for example, that the star images produced by a 6" MCT with a 31% CO differ noticeably (for the worse) from the star images seen through a 6" MN with only a 17% CO. If you care about the science as to why this is factually the case, here's a good basic discussion of the effects of CO and on airy disk size and the amount of energy scattered from the airy disk to the diffraction rings. This scattering represents light that isn't going to where it optimally is supposed to go - your eye as a focused image.

http://astro.geekjoy...sixinchers.html

I've owned and used many examples of each design (Newtonian, SCT, MCT and refractor) at all quality levels (cheap, commercial, premium), and in my experience the science and the experience coincide. I cannot tell you how many dozens of times a member of the public (non-astronomer) will comment that the view though a 10" or 12" commecial Dob or 4" commercial-grade APO is "sharper", "clearer" or "better" than the planetary view through an adjacent 12", 8" or 6" SCT. Hardly "5%" or "Nth degree" differences when a complete neophyte with no pre-conceived notions, and no experience, easily sees the difference predicted by the science.

I love SCTs for their virtues - the unique combination of affordability, portability,mountability and availability. I don't count optical superiority by design as one of the virtues.

Regards,

Jim


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