Improved Results from Small GraXpert Processing Change

 A Small Change to DSO Image Processing 

Until recently my DSO images were processed using Autostakkert!3 for stack averaging, GraXpert for gradient removal and Corel Paintshop Pro 2021 (PSP21) for aesthetic touchups. This posting addresses the latter two apps and how they treat images. 

GraXpert numbered steps are 1. Load Image 2. Crop, 3. Background Extraction (gradient removal) 4. Denoising 5. Saving (Stretched Processed file). This sequence suggests one should click each step with any parameter setting if needed. Having assumed the latest denoising version would be the best and fastest I was surprised that version 3.0.1 could take 5 minutes to denoise one image adding incentive for noise reduction in PSP21. 

Having recently discovered Denoise version 2.00 produces satisfactory noise reduction in 20 to 30s I decided to try denoising in GraXpert. Unfortunately the sequence often resulted in excessively stretched images and histograms when imported into PSP21 if the normal sequence was followed.

A simple change to the GraXpert sequence fixed this issue giving me incentive to reprocess many past DSO images. Instead of using PSP21 for noise reduction the GraXpert process is changed as follows. After step #3, Background Extraction the image is saved to a stretched processed tiff file. Stretch is set to No Stretch and the stretched processed tiff file is reloaded. Then denoising is applied. 

Now, when loaded into PSP21 the processed image shows no excessive stretching and the noise is greatly reduced along with improved signal to noise. Thus more nebulosity can be pulled out of the otherwise noisy background. The three examples below show the advantage of the the modified sequence. 

Loaded image file, stretched 30%,  reduced gradients, denoised, saved stretched processed file, cropped in Paintshop Pro. Result: Added stretching (brightness) with added noise despite GraXpert denoising.

Loaded image file, removed gradients, skipped denoise, saved stretched processed image. Applied default digital noise removal in PSP21, then cropped. Result: Ineffective noise reduction despite application of PSP21 default digital noise removal. 

Loaded image file into GraXpert and stretched 30%. Removed gradients. Saved stretched, processed image. Set stretch to none then reloaded stretched processed file. GraXpert denoised. Saved processed image then loaded it into PSP21 and cropped. Result: Original stretching and greatly reduced noise yielding smooth dark sky. 

Summary: When using GraXpert to process DSO images save the stretched gradient processed image after step 3. Set stretching to None. Reload processed image. Apply denoising; version 2.00 recommended for speed. Save processed image.