Improved Results from Small GraXpert Processing Change

My DSO images are processed using Autostakkert!3 for stack averaging, GraXpert for gradient removal and Corel Paintshop Pro 2021 (PSP21) for aesthetic touchups. This posting addresses how GraXpert processing. 

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 setting any parameters 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. 

It turns out Denoise version 2.00 delivers  satisfactory noise reduction in 20 to 30s. 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 fixes 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. More nebulosity can be pulled out of the otherwise noisy background. The three examples below show the advantage of the the modified sequence. Older DSO images are being reprocessed using this modification.

Update: Images from original posts were replaced by current images.

Example 1: Cropped image of Messier 94. Processing: GraXpert 30% stretch, AI gradient removal, denoise. Image shows excessive stretching indicated by brightness and noise in dark sky areas.

Example 2: Processing is the same as Example 1 except for the following change. After A.I. gradient removal image is saved and stretching is set to None(0%). Then image is reloaded and denoise is applied.

Example 3 - Processing: Stretched 30%, A.I. gradient removal, GraXpert denoising skipped. Default digital noise removal in PSP21.  Background sky shows more mottling and stars are slightly less sharp than in example 2. 

Globular Clusters

This is the time of the year to image globular clusters, some of my favorite targets. The bulk of them tend to be low in the southern sky which is a challenge considering seeing and refraction. M13 was included in the previous post. Several others acquired on the same night appear below. M55 was acquired on a later night. Images were averaged from 30x30s subframes acquired at 120 unity gain. Configuration was Celestron 8 , Starizona NightOwl 0.4x focal reducer, ZWO ASI294MC Pro CMOS camera. Update: Images reprocessed with GraXpert stretching and residual gradient removal.

Messier 3

Messier 4

Messier 5

Messier 10

Messier 12

Messier 13

Messier 55