SCT & Refractor - Star Quality

[Stratis]

I shall weigh in on this one as I have a disproportionate amount of experience with all sorts of scopes.

I have owned and used, in the short space of two years:

• C9.25" SCT
• C8 SCT
• C5 SCT
• Altair 150mm Rutten-Maksutov
• SkyWatcher 127mm classic Maksutov
• 114mm Parabolic Newtonian
• 115mm triplet apochromatic refractor
• 102mm ED doublet refractor
• 90mm Maksutov
• 80mm ST80 achromatic refractor
• 66mm 'SD' doublet refractor

As you can see, the only family I have yet to explore are the exotic astrographs and the large dobsonians, the latter only because I don't really like having to keep pushing the damn thing.

My eyes are annoyingly sensitive to aberrations of all types, and I find most non-perfect star shapes highly distracting. I am unable to forget, when observing a binary pair through a less-than-ideal diffraction system, that something is subtly 'wrong' with the image.

Almost all of my early scopes were catadioptrics, a fact I chalk up to sheer market availability. The 127mm SkyWatcher was probably my first 'serious' scope and the star images disappointed me; it was an old blue-tube 127 with inferior coatings and with a few owners ahead of me. The stars, even after cooldown, were 'flared' even at perfect focus. I quickly sold that scope on after learning (via the 90mm baby Mak) that it was just inherent to the design of cheap, small Maks.

My SCT journey taught me that I could expect no better from an SCT at native focal length, stars still took on a slightly exaggerated appearance after several hours of viewing. I did notice with the SCTs that cooldown was vital, unless the 9.25 had at least 40mins to acclimate the star images were intolerable. Then, I discovered the magic of the f/6.3 focal reducer and never looked back. I now never use an SCT without one unless I am doing planetary or double-star viewing. The reducer gave the stars a much cleaner, sharper look (except at the field edge) and focus became much more satisfying. I still consider a 9.25" SCT at f/6.3 to be the best viewing I've ever done with with a 'cat'.

Then I got a 4" apo and... my world changed. Stars were like diamond pinheads, so, so much more appealing than the SCT to my critical retinas. Don't get me wrong, I love the SCT for deep sky and would take it over a big fat Dob any day, but for sheer pleasure nothing beats a refractor. Absolutely beautiful, and as so many others have said, much clearer and sharper. The downside is, DSOs simply arent that bright anymore, anything dimmer than M92 gets really quite hard to find much less observe. For DSOs, SCTs still have it for me.

My most recent acquisition is a 150mm Rutten-Maksutov; this Mak design is much more expensive to produce, but offers the designer much more freedom in flattening and sharpening the field. After leaving it for an entire hour to cool down, the 150 Rumak absolutely stomped on all of my SCTs and Maksutovs that went before in terms of sharpness; it wasn't a refractor-like view because the stars clearly retained the dark-fringed diffraction pattern from the central obstruction (cant beat physics), but it was as good as I feel things are ever going to get with an obstructed view without going for exotic designs employing meniscus or sub-aperture correcting lenses like many super-astrographs.

My next project is to try using a pure focal reducer to drop the f/12 Rumak down to f/9 or f/6.... it will hurt my field of view, but I am hoping to get closer to that refractor-like beauty in a compact package 

[John]

Interesting range of scopes you have tried

I reckon you would enjoy a maksutov-newtonian and possibly a slower long focus newtonian with top quality mirrors and a small central obstruction. Worth trying sometime at anyrate to continue your "journey" though the scope types   

My dob is fattish but also longish (12" F/5.3) but has turned out to be my most used scope. I have a couple of ED doublet refractors as well (102 and 120mm) for double stars and grab and go duties but my dob gets more "night air" than they do despite my admiration of refractor images.

It's fun trying out the different designs though and really the best way to find out what "floats your boat"

[Mr Spock]

With a large central obstruction and SCT is never going to produce a star image as sharp as a refractor. However, unless I have a ridiculous Euromillions win, I'm not getting a refractor with a resolving power of my C9.25 any time soon!

I used to have a 140mm Mak with ⅛th wave optics that gave star images as good as I've seen - when I'd waited 3 hours for it to cool down that is.

All I can say is, when I'm looking at an easy clean split of a 0.7" double star with my C9.25, you refractor owners can eat your heart out...

[Peter Drew]

I don't think a large obstruction, in itself, is the cause of less than "perfect" looking star images in catadioptric telescopes, all other things being good an obstruction removes light from the Airey disc and deposits it into the diffraction ring system. This reduces contrast but should not change the symmetry of the star image, good small Maks and SCT's that I have owned all gave "refractor like" images. The Achilles Heel of most catadioptrics is their larger than most refractor apertures when it comes to star images.      

[ollypenrice]

With a large central obstruction and SCT is never going to produce a star image as sharp as a refractor. However, unless I have a ridiculous Euromillions win, I'm not getting a refractor with a resolving power of my C9.25 any time soon!

I used to have a 140mm Mak with ⅛th wave optics that gave star images as good as I've seen - when I'd waited 3 hours for it to cool down that is.

All I can say is, when I'm looking at an easy clean split of a 0.7" double star with my C9.25, you refractor owners can eat your heart out...

There is a difference between enjoyng pin sharp stars for aesthetic reasons at 'enjoy the sky' magnification and technically splitting two boiling blobs at very high power. My heart remains uneaten!

lly

[Singlin]

Olly changing his heart viz a vie refractors is as probable as England winning the world cup         

[Mr Spock]

Who said anything about boiling?

[ollypenrice]

Who said anything about boiling?

I did. I never get the stellar quality from my 10 inch SCT that I do from my TEC refractor. The much missed BrianB once said something like, 'The image from an SCT may be scruffier but contains more information.' That's a pretty good observation in my view. Interestingly enough, most DS imagers using SCTs find it hard to keep the star size down. Then again, most planetary imagers find it hard to get anywhere near the efforts of Mr Peach without joining him in the use of SCTs. This paradoxical aspect of SCTs is interesting and I don't take an entrenched position. Damian Peach proves beyond doubt what an SCT can resolve on the planets but stars are curious entities, both visually and in imaging. I've always found stars to be sweeter in refractors at the eyepiece. That's not to say that, belted up to high powers, the SCT can't split doubles better. Two different points, I think.

Olly

[Pig]

I don't have a clue from an imaging perspective but visually clusters etc are much crisper and more pleasing in my little 80mm pea shooter than they were in my 9.25 SCT and the 6" SCT. I think a lot of this is down to the sheer number of stars that can be seen in wide field views.

My 6" is a lot better since I collimated it.