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Weighing short & fat vs tall & skinny

Started by pkamm, 02/18/2005 12:23PM
Posted 02/18/2005 12:23PM Opening Post
I'm wrapping up another project and I've got some tough decisions ahead. Basically I've got 2 big scopes and one of 'em has to go. But which?

The contenders:
16" f/3.75-f/18 newt-cass (currently dob-mounted in newt mode only) - short & fat
10" f/8 planet-killer dob -- tall & skinny

The optics:
16" -- bit of a turned edge at the outer periphery -- not sure how much yet but know it's less than the outer 0.5" because at 15" (f/4.0) the star test is perfect. Well-defined diffraction rings, the surface appears extremely smooth, it appears many hours of polishing went into this mirror. Central hole is 6" diameter, for a C.O. of 38% by diameter at 16" or a whopping 40% at 15".

10" -- star test indicates perfect parabolic correction. Intra-focal and extra-focal images are identical, though diffraction rings not as pronounced -- in other words the mirror is figured very well but possibly could have been polished just a bit finer -- but it has given me the best planetary images of any instrument I've ever owned. 1/20th wave secondary, 1.5" (15% CO)

The pros/cons:

16" pros:
1) brightness
2) resolving limit
3) can use without ladder (very cool at this aperture level)
4) larger exit pupil is better for getting around floaters in my eyes
5) will be better at deep-sky if I get a trailer and start hauling it around
6) bulletproof, supersmooth optics inside the outer 1/2"

16" cons:
1) COMA! -- gotta buy a coma corrector $$
2) VERY fussy collimation
3) Cooldown time (mirror is 3" thick!)
4) huge C.O.

10" pros:
1) demonstrated sharpness and contrast
2) current eyepiece collection is built around this scope's characteristics
3) lighter
4) easy collimation

10" cons:
1) very long, need ladder

Should I do the "practical" thing and stick with my 10" f/8, or should I just surrender to aperture fever now, sell the 10" and buy a paracorr? :S
Posted 02/18/2005 12:37PM #1
I think you should decide according to the type observing you are intended to do. These scopes are different and will perform better at one thing than another. If you want good planetary, the 10" would be the way to go IMO. Better contrast and probably better details because of it and the better figure as you have stated.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 02/18/2005 03:41PM | Edited 02/18/2005 03:43PM #2
Hi Paul,

Have you thought about other mountings for these two scope ? Do you travel much with them ?

I would lean towards the 16", but then again, if you sell both, you can get most of the way towards a really nice 14" that would serve both purposes. I agree about the floaters, and aperture is the only way to cure that for now.

PS. I now look out the "back" of my 10" f6 Newtonian, but that's another story ...

Attached Image:

David Simons's attachment for post 88462

David Simons