From: Alan Friedman <alan@greatarrow.com>

Date: October 21, 2005 3:12:15 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: some thoughts on sampling

I thought I would post this note I sent in response to a question that came up at a recent presentation on my set-up for planetary imaging. - Alan 

At the meeting last week, Dennis asked a question which I didn't have 

an accurate answer for - what is the sampling ratio of my planetary 

set-up. For you non-ccders, this is a measure of the physical 

resolution of the set-up expressed in the number of pixels used to 

cover a one arc second slice of the subject.

I had a chance (rather a need, the seeing was poor each time I went 

out) this week to try out a few different settings and measure the 

results. Mars was just a shade under 20 arc seconds in diameter so the 

math was easy!

My usual set-up uses the 10" f14.6 scope with a Baader FFC (a fancy 

flourite barlow) followed by a filterwheel and camera, which yields 

about f48. I've had some good luck on good nights with this set-up for 

saturn and jupiter. At this setting mars is recorded with 250 pixels - 

.08"/pixel (or, 12.5 pixels/arc second)

For the Oct 19 mars dust image I used an old A/P 2X barlow in front of 

the filter wheel yielding about f29, or .13"/pixel (or 7.5 pixels/arc 

second).

For the mars image from this morning I used a 1.5" extension between 

the barlow and the filterwheel bringing the system to f34, or 

.11"/pixel (or 8.75 pixels/arc second).

My initial impressions - f48 is probably too much magnification for the 

usual 4-6/10 seeing around here. Comparing the f29 and f48 streams I 

found I could get a tiny bit more detail if I selected and aligned the 

f48 frames by hand. But I might also be able to achieve this if I took 

that time on the f29 frames. As it was, I was able to get good auto 

stacking results at f29 and complete the entire LRGB image in about an 

hour.

Remember that the albedo detail on mars is low contrast - the moon 

would be a different story. On almost any decent night the longer 

effective focal length would capture additional detail on the moon. But 

on mars, the f29 was just about as good - with much easier focusing 

because of the higher contrast image. I was also able to completely 

select, align and stack using automated software because of the 

predictable character of the sharp images at the lower magnification 

(the big images are softer and can fool the software into a poor 

selection of the best frames.)

Overall - I think f34 is a good compromise and probably about perfect 

for all but the best nights in WNY.

This is a long ramble - I hope some of you find it useful.

cheers-

Alan