From: Milton Aupperle <milton@outcastsoft.com>

Date: July 22, 2008 10:29:50 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] double hump stacking histogram


Dear doodlebun;


On 22-Jul-08, at 8:36 AM, doodlebun wrote:


Thanks Milton. You explained the behavior of the save-as confusion. 

Last night we were imaging Jupiter trying to catch the shadow transit 

when we ran into these issues:  1. Patches of scattered hazy clouds and 

streaks traversed Jupiter. Therefore the brightness of Jupiter was 

dimming on and off. When the movie was processed the resulting 

histogram wasn't the usual Gaussian curve but a double hump at 66% 

confidence level. In the portion to the left of the pointer (the images 

that wouldn't be used) a bell curve appeared. Then to the right of the 

confidence pointer there was another much lower hump, like a mesa. Is 

this behavior typical of imaging a source that doesn't maintain a 

constant level of brightness?


Yes it could be. Sharpness of an image is basically contrast and contrast is also affected by brightness. If the change from one features edge is 0 to 100 in brightness and the clouds darken the image to 0 to 30, then the uncloudy feature will be brighter, have more contrast and appear sharper. So the lower hump should have less sharpness /  contrast as compared to the upper hump. However this doesn't tell the whole story of which is "sharper".


You said streaks of clouds, so when the clouds obscure Jupiter do they have sharp or diffuse edges? If they have sharp edges, then the upper hump may be when clouds cross Jupiter. If the clouds are diffuse and then likely the upper hump is the brighter / sharper portion. Also are the clouds brighter or darker than Jupiter? Under my light polluted skies her in Calgary during the winter months with snow cover, I have seen clouds become brighter than saturn's disk.


Would you recommend setting the histogram 

setting in the capture mode to "Auto", which would tell the program to 

constantly bring the brightness of each frame to a specified level?


You mean for Histogram expand, right?


Or 

would that just make things worse?


I would likely not do it, as it's going to be tough to pick sharpest frames and align the image if the gain / brightness jumps all over the place (Astro IIDC can compensate for brightness changes when aligning though). You might want to experiment when it isn't a once in a life time or important event just to see if it works or not.


Another possibility might be to try using the "Auto" exposure checkbox, which changes the exposure time, and won't be adjusting brightness using a gain / scalar like histogram expand would.


HTH..


Milton J. Aupperle

President

ASC - Aupperle Services and Contracting

Mac Software (Drivers, Components and Application) Specialist

#916, 742 Kingsmere Crescent S.W.

Calgary Alberta T2V2H8

1-(403)-453-1624

milton@outcastsoft.com

www.outcastsoft.com