SCT Image Quality: Maybe a little OT

Hi All,

A friend and I were talking the other day about all the fantastic planetary images getting posted here and that many of them are taken with SCT optical designs. I have always been under the impression that the SCT optical design is not that great for high resolution work because of the large central obstruction. That a SCT is a good all-purpose type scope but is not what one wants to use as a "planet killer". So what gives? The details on the webcam shots of Jupiter thru some of the SCT used by group members here are stunning! No contrast or resolution problems there. Any ideas as to why the large central obstruction appears to have no detrimental effect on the video images? Is it just a visual effect? What do you guys see visually with your SCTs when you look at Jupiter and Saturn? Just wondering what the thoughts are.

Thanks for any replies.

John Miele

Hi John,

Visually, images of Saturn and Jupiter are quite stunning in my C11 (and it's old, 22 years old), given good seeing conditions first and foremost. This topic intrigues me too, the best resource I found is an article written by Thierry Legault. He summarized the effects of a central obstruction very nicely, showing potential effects on planetary imaging:

http://www.astrosurf.com/legault/obstruction.html

Still, looking at some of Damain Peach's C11 and LX200 Saturn's (as well as some others on this board) really make me think the SCT central obstruction issue is a non-issue in terms of planetary imaging:

http://www.theplanets.cjb.net/

Clear Skies,

--DJ

OK, let's be honest here, we all know that by logic, NO obstruction SHOULD be better than ANY obstruction...

Now, in REAL-LIFE, the difference is NOT that striking...
Yes, there's a difference, but not something ABISMAL, something subtle, visible, but nothing that I would personally call "Shocking"...

Visually, my 10" SCT gives me awesome views (properly collimated - cooled), specially under the "almost always steady" Florida skies..., and even though I've looked through Refractors, and loved the views, I did not see something that would make me think about selling my SCT... not at all.

Also, lets make sure we understand that webcam imaging stacks hundreds, thousands of images, and the end results are a combination of these... not a very good way to compare image quality here, unless we had somebody that did this with an SCT or something else, and a refractor, side by side...




Clear skies!



Ivan

The issue of central obstruction aside, I think the most critical factors for high resolution imaging are adequate signal/noise ratio and minimizing the amount of smearing due to atmospheric turbulence. A larger aperture simply provides more photons allowing the use of faster shutter speeds and lower gains resulting in better resolution of fine datails. In a nutshell, aperture wins.