charen
Pooh-Bah
Reged: 05/28/05
Posts: 1446
Loc: New Zealand
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I am wondering if the issue of CO and inferior optical quality is also a contributing factor.
Re. CO:
'The commercial SCTs represent the greatest compromises in design factors. The central obstructions of the typical 8" SCT range from 34% to 43%. W.H. Pickering demonstrated that the brighter diffraction rings caused by CO's were more susceptible to the effects of mediocre seeing where the entire spurious disc expanded in size compared to an unobstructed system.' [ P.Laughton.]
A 'large' obstruction degrades the image and contrast - fact - and poor seeing conditions will effect the deterioration more.
Re. optical quality:
'Because the typical SCT is mass produced with a low target price, the wave error target is 1/4 wave at the primary mirror. There is also the wave error created by the corrector and secondary to be considered. Often, more often than anyone would care to admit, the overall wave error on many commercial SCTs is 1/3 to 1/2 wave'. [ P.Laughton.]
'A 33% (by diameter) obstruction is as destructive as a 1/4-wave error. If your optics already have a 1/4-wave error, and most commercial reflectors do, then your total error with a 33% central obstruction is 1/2-wave. Most of us can definitely tell a 1/2-wave error. Obstructed telescopes require top-notch figures on the mirrors to achieve 1/4-wave performance.' [D.Ibis].
Are we sometimes asking too much optically speaking from the our mass produced SCTs ?
Having said that miscollimation and inadequate cooling will play a bigger role then C.O. or poor build quality in giving inferior images.
Just some thoughts.
Edited by charen (12/29/07 07:23 PM)
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Jeff Lee
professor emeritus
Reged: 09/17/06
Posts: 709
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Charen:
No, today's SCT (and this means almost all from the late 1990's) are very good scopes. But during the Halley Comet years they were some dogs. Collimation and cooling are two reasons why folks who haven't owned a SCT and get one often have issues.
Honestly most of those quotes given today's optics are erroneous.
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Jeff Lee
C90,C5,C8, 10 x 50's
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charen
Pooh-Bah
Reged: 05/28/05
Posts: 1446
Loc: New Zealand
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Hi ! thanks for the reply. I did not mean to be negative. It is good to have a discussion about these issues. I have a Celestron SE8i which I enjoy - but I am also aware of its 'limitations' at times - cool down time, collimation and when the seeing and tranperancy is 'limited' the image is reflected in that also - the 'bloated star' look.
On those poor seeing times my ED100 does give slighly improved [but dimmer images ] esp. of the brighter planets - most other times the 8in. does give impressive views on all other objects.
SCT are 'compromise' scopes - but good compromises - the weight and compact design , the light gathering power, the ease of use etc.
If I had to keep one it would be the SCT but I am in the lucky position to have both.
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30 binos.
Celestron C8
Skywatcher Equinox ED120 / Goto HEQ5.
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cbwerner
scholastic sledgehammer
Reged: 08/27/05
Posts: 846
Loc: Maidens, VA
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I actually started with SCTs for many years and then had a very nice refractor. Totally different animals. The sharpness, contrast and clarity of the refractor views totally blew me away. I think that if you set your SCT expectations by refractor views, you will be disappointed. Different tools with different strengths.
But with SCTs collimation is king. During the Shoemaker collision with Jupiter I observed with both a C8 and a C5 one night. The 5" totally blew the 8" away - it was all about their relative states of collimation.
Bob's Knobs and some extended collimation time. 
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Chris Werner
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Stephen65
scholastic sledgehammer
Reged: 04/14/07
Posts: 941
Loc: Melbourne, Australia
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When my Mewlon-250 arrived I did the Robin Casady daylight test and it seemed to be well collimated.
When I looked through it after letting it cool for a couple of hours it also looked sharp and contrasty, if not quite APO-refractor like then reasonably close and better than I had ever got my 10" Newt. Mewlons hold collimation well and I figured it had survived the bumps and knocks of transport in the condition it was sent.
But...when I did a proper star test - mag 3 star at 450x at the zenith at 1am after hours of cooling, I saw that the diffraction rings were slightly non-concentric, very similar looking to the 3 arc minute deviation on page 118 of Suiter's book.
A bit of fiddling with the push/pull screws fixed that and I honestly didn't expect to see much difference because it already looked good. However, after that tweak it was noticeably better - stars were even tighter and I learned a lesson about the importance of being fanatical about collimation.
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Megrez 90
FLT 132
Mewlon 250
Lots of XWs, an Ethos, Supermonos and some other EPs
Edited by Stephen65 (12/29/07 11:39 PM)
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a__l
professor emeritus
Reged: 11/24/07
Posts: 522
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My Nexstar 8SE is lemon.
Collimation Ok!
There are no details on Jupiter for all summer period.:- (
Eyepieces: Tak -5 LE, Naglers 9T6, 13T6.
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Home made 24" F/3.3 Steve Kennedy Optics
Home made 18" F/3.9 Santel optics (Russia)
SIPS, Denk Super Systems, Ethos eyepieces 21,17,13,10,8...
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Charles Funk
member
Reged: 09/06/05
Posts: 25
Loc: W. Tennessee
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I think Jupiter was placed badly for all of us this year. I wouldnt base my judgement of a scope on Jupiter of 07. The views for me were so bad, so often, I just quit trying and moved on to other objects a little higher in the sky.
I have one of these terrible critters a C8, also have a 12.5 dob, my best view of Jupiter to date was through the C8, was like a hubble picture, but that was a special night with excellent seeing and havent had another night like it since, but for that one moment, on that one night, the C8 reigned king. I can still "see" the clarity of that view in my mind, forever etched I suppose. That was a few years back however, Jupiter was much better placed for viewing then.
The three most important conditions for a telescope: collimation/seeing/and patience.
Id bet with a little of all three, you will see your scopes potential. Depending on your location, there may only be a few nights per year when optimum viewing conditions present themselves, you will know them when you see them.
Charles
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80 ED Orion
C8 ASGT
12.5 Discovery
20x90 Obies/p-gram
"Never weld in a flannel shirt!"
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bcuddihee
Carpal Tunnel
Reged: 11/04/06
Posts: 2031
Loc: Cincinnati Ohio
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I have an 8SE and I am very pleased so far. Its images are comparable to an Antares 8 inch Newt that I had last year. The newt was great but much more difficult to collimate than the SCT. I usually collimate on Polaris with the the SCT at approx 340x and then barlow to check at 680x. With that said the airy discs seen on star images are not quite as perfect and distinct as the 80 zsfd I owned last year, but.. the scope and mount weighs less than the EQ mounted 80, and the views of DSO'S are soooo much better. I am also getting nice sharp images or Mars with quite a bit of surface features visible as well.
By the way the central obstruction on the 8se is stated to be 31.3%. No to shabby in my book, and compares very well if not better than most MCT's of similar FL, on the market.
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B Cuddihee
1968 Jason Empire 60X700mm refractor
1977 Jason Empire 313 discoveror EQ
Celestron Nexstar8SE "The Bumblebee", with Feathertouch Microfocuser, and
Celestron CPC Heavy Duty Tripod.
Antares 89mm finder
Stellarvue 50mm "Sparrowhawk" finder
Denk bino's with Power x switch.
Pair of Edmund 15 and 28 plossls.
Pair of Edmund 28 RKE'S
Pair of 32mm Brandons.
Pair of 24mm Brandons.
Pair of Smart Astronomy 19 EF's.
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Jeff Lee
professor emeritus
Reged: 09/17/06
Posts: 709
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Charen:
I did not mean to sound like I was focusing on you. Its that these quotes could be quite accurate if describing some of the early SCT's. C and M just got their acts together and have produced some very good scopes in the last 10 years or so.
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Jeff Lee
C90,C5,C8, 10 x 50's
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