I last photographed M3 in 2019 (https://www.astrobin.com/402551/). This older image combined luminance data from the Mt. Lemmon Sky Center's Schulman telescope with color data taken using my C8 from my backyard. My goal for this project was to see what I could do with my Astro-Physics GTX 130, five more years of experience and modern image processing techniques. 

The base of the image is 323 x 180 sec sub-exposures. To maximize resolution, I used drizzle integration to double the effective resolution. With good seeing I am right on the edge of being under sampled so this seems reasonable to me. Additionally, I set my rejection criteria very high attempting to keep only the sharpest images. I enhanced these base images with some shorter subs with the goal of avoiding saturated star cores. I started with 10 second subs but found these were still a little too long. I ended up taking a set of 5 second subs which worked very well. I combined the 5 second stack with the 180 second stack to create an HDR composition with no blown out star cores. 

The resulting HDR composition had crazy dynamic range, so stretching properly was a challenge. I found MaskedStretch did the best job flattening this dynamic range while also preserving color. I tested MaskedStretch on just the 180 subs stack but the resulting image had significant problems with star core saturation. The HDR data in comparison showed no core saturation confirming using HDR was the right approach. Normally I would use MaskedStretch only on color data and handle luminance or synthetic luminance with a different stretching methodology. But the dynamic range in this globular cluster is so high, MaskedStretch alone produced the image I was looking for - one that portrays a very bright core but with detail preserved and also showing the wide extent of stars in the cluster. 

I did run into one problem using MaskedStretch with my HDR data. No matter what I did I was completely clipping my background. In the end, it turned out the default clipping fraction (0.0005) is much too high for my 64-bit HDR image. In the end I set this parameter to 0, disabling the shadow clipping feature resulting in a background with some texture as I expected. 

I used SpectrophotometricColorCalibration to ensure the star colors in the cluster are as close to realistic as possible. After this color calibration early in processing, I was very careful not to disturb the color balance. I did boost color saturation a bit however. 

I like the resulting image better than my earlier attempt even though it included data taken through a much larger aperture telescope. This latest image does showcase what an extraordinary instrument the AP Starfire GTX 130mm is.