ABE or DBE makes image very grainy

John Kmetz

Hi,

New to this forum.

Usually have this problem to some degree, but not this extreme. When trying to do ABE or DBE on this monochrome master light, the graininess increases dramatically.  The original seems so smooth but has a pretty high gradient do to my local light pollution. But afterwards, the grain is just awful. Wish I could preserve the original smoothness but remove the gradient. Have tried the default settings in ABE and DBE, and also manipulated most of the setting values without success. Is there a solution to the issue? The master is a stack of 180 x 60s subframes from an ASI1600MM-P at unity gain put through WBPP 2.0.

Any suggestions would be appreciated.

Thanks,

John

BlockHead

Hi John,

The sky brightness (your gradient) is a real signal. It is unwanted though. But it is relatively bright! So your "smooth" background is due to the unwanted light. When you subtract it using a tool like DBE- you are seeing the TRUE nature of your data. That noise IS your actual honest to goodness information. The noise isn't increasing (DBE isn't making your data look worse)- you are just now able to perceive all aspects of the your data without the unwanted signal of the sky brightness (and its gradient). 

Many first time astroimagers get this backwards. The sky brightness "covers" artifacts are only revealed once that pedestal sky brightness is removed. These artifacts are fainter than the sky... but this is also the level that shows the faintest aspects of the things that are being captured in the image. The fainter you go (or want to display) the more challenging things get. More exposures, optimizing optical DIQ... etc all go towards keeping that graininess at bay even *after* DBE. 

-the Blockhead

John Kmetz

Thanks, Adam. Even though the subframes were taken with little to no moon present, the avg. ADU's were high, ~ 3000. I think local haze must have been whitening the sky from reflected city lighting. Even my R, G, and B's show the same gradient. I tried lowering exposure time and gain settings without much success. Was hoping I could figure out some processing to lose the gradient and save the good stuff, but I guess I am limited by the data quality.

BlockHead

Again, you data quality is fine. What you are seeing with DBE is completely normal. 

No matter what site you image from, the sky is not dark (airglow). So you will always see this effect

of seeing more graininess after DBE. 

Funny thing... no one ever complains about this in terms of calibration. Although you may see more warm/hot pixels- there is *less* visible graininess before calibrating images. Only after calibrating the data does the true sky values start to show up- and then when you remove this signal (DBE), the very faintest (and noisiest) values can be seen.

-the Blockhead