Image of the Triangulum Galaxy (M33) taken on the nights of December 18/24/25, 2022.   I recently updated from imaging with a modified DSLR to a dedicated CMOS camera with cooling, and this image was the first I captured using my new ZWO ASI 533MC Pro.   Originally a OSC RGB image, and later enhanced with H-alpha narrowband data to create a NBRGB version.

Imaged with a William Optics GT81 at F4.7 (382 mm focal length) using Flat6A II field flattener/0.8x reducer, UVIRcut filter, and ZWO ASI 533MC Pro.  Total integration time of 5 hours: 150 subs at Gain = 100 with an exposure time of 120 sec.

Pre-processed using Pixinsight's weighted batch pre-processing script (WBPP) to perform calibration, debayering, registration, local normalization, integration, and drizzle integration.  Post-processing in Pixinsight:  background removal using ABE, spectrophotometric color calibration, blur x terminator, noise x terminator, masked stretch, curves transformation, HDR transformation using EasyHDR script, local histogram normalization, color saturation adjustment, and dark structure enhancement.

Original version:  M33 OSC RGB image

Rev B: M33 narrowband image, H-alpha data (56 subs x 300s at gain = 100)

Rev C:  M33 in NBRGB, OSC Red channel enhanced with H-alpha data

Rev D: starless NBRGB version

Rev E: starless narrowband H-alpha version

Notes on NBRGB processing:
> Started with fully  processed, nonlinear RGB and H-alpha images
> Tried several approaches starting with the Pixinsight NBRGB script and SHO-AIP script, but best results were obtained with 'alternate' NBRGB approach below
> Generated H-alpha enhanced R channel using the method described here:  https://www.chaoticnebula.com/pixinsight-nbrgb-combination-alternative-method/
1 - Split RGB image into separate R/G/B channels
2 - Linearfit Ha image using R channel from RGB image as reference
3 - Used Pixelmath generate an Ha enhanced R image by combining the R and Ha images using the 'screenblend' approach (pixel math expression:  ~(~R * ~Ha)
4 - Linear fit the Ha enhanced R image using the Green channel from the RGB image
5 - Generated the NBRGB image using the channel combination process on the Renhanced/G/B images
6 - Ran the color calibration process on the resulting image to obtain better overall color balance