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6" refractor resolving power.

Started by jjacobso, 12/23/2002 10:36PM
Posted 12/23/2002 10:36PM Opening Post
Here is a good question for all of you experts.

I am looking at two different scopes. One is an f/5 8" newtonian. The other is a 6" f/8 refractor (synta.) One of these will replace my 8" SCT f/10.

My question is this: In my 8" SCT, I can resolve globular clusters to the core. Will I be able to do this with the 6" achromat. I understand that resolving power is mainly a function of aperture, but I also understand that refractors have no central obstruction, and yield much higher contrast than other optical systems (also, they are just plain sharper.) Does the better optical system of the refractor make up for the smaller size in regards to resolution? In other words, will I be able to resolve most globulars into stars to the core?

I know this seems like a silly point to base the purchase of a scope on, but globulars have become my favorite deep sky objects to observe, since graduating to the 8" SCT.

Thanks in advance,

Posted 12/24/2002 06:23AM #1

Why are you getting rid of something that does what you like best? Neither of the scopes you're looking at will be as good as what you have (for globulars, anyway). If you want better globular performance, you need to move up in aperture. True, a high quality refractor (the Synta isn't) will do (slightly) better, but the image will be "dimmer". I sold a C-8 that I'd had for 20 years to get a nice 120mm refractor and was pleased with planetary performance, but you will lose on globulars and deep sky with a smaller aperture. If you go to the "fast" reflector, you'll lose the definition you have at f/10.


Posted 12/24/2002 09:38AM #2
I've got to vote with Pat and Rick. If you're going to make changes, go up in aperture, not down. One of my first experiences with globulars was a side-by-side comparison of a new TV102 and my old ETX125 on M13. The ETX's image was much more interesting than the Televue's; it was beginning to resolve the cluster in a serious way. The Televue didn't come close. Apparently central obstruction doesn't mean squat on globulars; aperture rules.

For a real eyeful of globular I now use a C9.25; it really shines. But I'm sure a C11 or a 12" LX200 would do even better. If you can't compare before you buy, try to hang onto the C8 for awhile after you get your new scope. Side-by-side comparisons on the objects you favor under your own peculiar viewing circumstances can tell you a lot. As a beginner, it told me the TV102 was a step backward on DSO's and helped me put all the refractor hype in proper perspective.
Bob Young

Having a big time on a small scale grin
Posted 12/25/2002 12:22PM #3
Great thread and since its Christmas - I agree with everyone!!
Couple of points:
1. Not all glogulars are created equal. More aperture means you can resolve fainter stars. Some globs are very rich in 13 - 16th magnitude stars (e.g.,M-4, M-5 and Omega Centauri) others (e.g.,M-13) have nearly half their stars in the 16 - 22 magnitude range. There is a big difference between my 6" refractor and my 9 1/4 SCT when viewing globs with most of their stars over 12th magnitude. The difference isn't so significant (still noticeable however) with clusters with more stars below say 10th magnitude. However, even my C-11 won't resolve 15th magnitude or fainter. According to Celestron, their C-14 will resolve to 15.3 magnitude. So resolving to the core is not going to happen with a 6, 8, 10, 12 or even 14 inch scopes.
2. Why are you viewing globular clusters? Spectragraphic analysis? Preparing Hertzsprung-Russell diagrams? Good for you. I look at them because they are beautiful. They are beautiful in 3, 4, 5 and 6" refractors, in 9 1/4 and 11" SCT's, in 5 and 6" MAK's and 7x50 binoculars.
3. Purchasing decisions when it comes to scopes are totally irrational. All of us who even read this thread are nuts. Those of us who join in are complete loons. If you don't believe me - ask your wives. So, buy whatever scope makes you happy (and doesn't bust the bank, too much) and enjoy! Regardless of what scope you buy, after 2:00 a.m. -- all globulars are beautiful.
Merry Christmas to all and to all -- Clear skies!!