I recently finished the Stretch Acadamy update to Adam Block's wonderful PixInsight Fundamentas videos. I not only learned about two useful new process plug-ins for PixInsight, but also how to use them strategically to optimize the stretched light curve. Managing the light intensity curve is a key problem in postprocessing astronomical images. Long stacked exposures can capture the million to one linear dynamic range of many astronomical scenes, but making the key information captured viewable on screens or prints with much lower dynamic range is difficult. A standard JPEG digital image catures a dynamaic range of less than 1000:1 which is more than many displays or prints are capable of, but only 0.1% of the range of many astro images. Astronophotographers call this process stretching the image.

Herbert Walter's MaskedStretch does a wonderful inital stretch of the image. Contrast is low, but data is preserved making a great starting point for enhancing contrast in important areas. The idea is similar to masked stretching that I've done before. Previously I would use a blurred intenesity mask and a relatively large stretch for bright areas. MaskStretch iteratively does small stretches with an intensity mask, 100 is the default value. The result is fast and reliable.

Mike Cranfield and David Payne's Generalized Hyperbolic Stretch allows you to target specific areas of the stretched light curve for contrast enhancement (increased slope). This piecewise stretching, with targeted masking if needed, allows you to bring out detail in specific areas of the image.

To test this data on my own image, I used my M31 data from northern Arizona in 2020. I've reprocessed this data a half dozen times improving with each. My last challenge was to bring out detail in both the fringes and bright central areas of the galaxy.

M31 under the dark skies of the Coconino Plateau in northern Arizona. I've reprocessed this a half dozen times and this is my best. Single axis Vixen Polarie mount. Seventy minutes of 30" exposures of Andromeda on 2020-09-14. William Optics RedCat 250/51mm Petzval refractor and ZWO ASI533MC cooled astrophotography camera. Controlled by a ZWO ASIAir Pro. All on a single axis Vixen Polarie mount. Processed in PixInsight with Masked & Generalized Hyperbolic Stretch, Blur/Noise/StarXTerminato, and final crop, exposure, and color balance in Photoshop.

In response to a question from my friend Ed LaBelle, here are some smapshots of the evolution of this image reduced in size about 10x to fit on the page.

No stretching

First is the stacked 70 minutes of exposure without any stretching at all. Only a few of the brightest stars are visible. If instead of an hour of exposure, I had a dozens hours the center of the galaxy would be a faint fuzzy spot.

Linear stretching

Next the image is brightened by lowering the white point to about 0.5% of the original. The green cast is because the one shot color camera sensor has twice as many green pixels as blue or red.

Non-linear auto stretch

Finally I've applied the automatic unlinked mid-point stretch that PixInsight's ScreenTransferFunction uses. This is the sort of processing you might have in a live view rolling stack camera mode. Colors are very washed out.