From: Alan Friedman <alan@greatarrow.com>

Date: June 15, 2006 3:33:02 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Image scale (was: Monochrome Unibrain results)

Hi Milton - I am working on a talk that I'm giving later tonight for Niagara Centre RASC, so some of this is fresh in my mind.

On Jun 15, 2006, at 4:47 PM, Milton Aupperle wrote:

Then you probably need to reduce the pixel matching tolerance when using the longer focal length, especially with average /  mediocre seeing and high gains.

Also using a larger pixel area does not always work as well as smaller area for matching on features. This is especially true for low contrast images with high gains, because you wind up matching the background gain noise rather than feature of interest. So it's better to use say a 32x32 or 64x64 pixel area and partially contain a "feature" a larger area with flat noisy data. However what constitutes low contrast is debatable. 

All good advice. I had set matching tolerance at the lowest level, but at a large image scale, the combination of noise, low contrast detail and image shift/morphing due to the atmosphere works together to cause trouble. You might find this comparison interesting:

http://www.geocities.com/alanfgag/jupiterscalecomp.jpg

these are single frames through an IR pass filter - on the left with 10" aperture at 7 meter fl, on the right, 6" aperture at 3 meter fl. The bottom images are stacks of about 200 such frames process with unsharp mask. 

At the larger image scale, sharpness varies by region within a single frame and this region moves around a lot from frame to frame. Multiple region alignment (supported in Astro IIDC v3.0) can be used to advantage to improve resolution of the final result. At the smaller image scale, it makes little difference over a single alignment - and the detail in the festoons is contrasty enough to get a very good stack alignment on a 32x32 pixel square placed in the center of the disk. 

I never really realized it, but that image has a resolution of around 0.5 arc seconds, which is twice the theoretical resolving power of my MAK 5.1" aperture scope . The disk is 74 pixels tall and at that time Mars was 20 arc seconds, so each pixel represents 0.27 arc seconds.

Remember that Dawes limit addresses the limits of resolution for two point sources of light. Processed planetary images often capture finer resolution than this. If my memory serves correctlyt, the Cassini division is .75 arc seconds and the Encke division, though not able to be resolved at .05 arc seconds, it is routinely recorded through contrast enhancement in good amateur images. 

I will be interested to see if detail is recorded on Ganymede at the shorter focal length. Capturing this and certain other features (such as recording the location of the Encke division on saturn) have seemed to need a long focal length - but maybe not...

That will definitely be interesting to see.

Rod Kennedy (Australia) caught some detail on Ganymede

Yep - seeing quite a bit of Ganymede albedo these days - I thought it was cool when I captured this transit last year:

http://www.buytelescopes.com/gallery/view_photo.asp?pid=5839&c=17165&page=2

but now you have to capture craters on Europa to get attention. A lot of planetary imagers out there are getting real good.

... I think it's the software!

8^)

Alan