Monday, February 11, 2019

February 6, 2019:  Understanding vignetting and post processing astrophotos

I've been plagued for months with the light region in the image (below).  I was using Deep Sky Stacker to produce the images and skipped the flat and bias images.  Turned out to be a huge mistake.   The flat images are obtained by shooting an evenly illuminated subject (sky, through a paper towel or tee shirt) at the same ISO used for the light (astrophotos) images.  The bias image are obtained by shooting a dark field with the lens cap on - typically 1/8000 sec, to measure the dark current associated with no exposure (hence the high shutter speed). Dark images are also taken at the same settings as the light images with the lens cap on.  This gives a measure of the dark current associated with the exposure and ISO used for the light images. 

This article discusses how to adjust for vignetting: Photoshop techniques to correct for vignetting

DSS image of NGC2024 and IC434 - I did not understand the white region in the middle
An image of the morning sky as seen through a paper towel covering the front of the lens

Stretching the histogram of the above flat image reveals dark corners - vignetting
Deep Sky Stacker corrects for vignetting (using the flat images) and here is the result - a stack of 150 images of NGC 2024 (flame nebula) and IC434 (horsehead nebula) - no highlighted area in the middle  

Another Example:   M31
M31 with significant vignetting
M31 with flat compensation followed by GIMP autobalance (histogram stretch)

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