Voyaging through clear dark skies

M31

Image details: M31 does not need introduction but is a great target to test and evaluate the use of continuum subtraction techniques for Ha incorporation into RGB 

Date: October 3rd 2024

Location: Tiny Marsh, Ontario (traveller)

Image Batch:

            L: 30 x 60s ; R: 30 x 120s ; G: 30 x 120s; B: 30 x 120s : Ha 40 x 180s

Telescope : Takahashi FSQ85-EDP at 450mm

Mount: ZWO AM3

Camera : ZWO2600mm Pro operated at -10oC and Unity Gain, Offset 0

Filter wheel: ZWO EFW 

Fliters: ZWO L, R, G, B, Ha, OIII, SII

System Control: Stellarmate OS on a Rpi 5 

Guiding Camera :ZWO 120mm

Guiding Camera: OffAxis Guider with ZWO OAG-L

Guiding Software : EKOS internal guider

Calibration Frame Creation:
Master Dark frames were created from 20 dark frames of 60, 120, 180 or 300s in length at a temperature of -10oC. The raw frames were processed through PixInsight as part of a WBPP batch to create master darks. Dues to the timing of the images the same darks were used 

Master Flat Frames were created using an even illuminated field from a EL panel in a custom 3D printed frame controlled using Stellarmate OS (EKOS) using the automated flat routine to maintain ADU at 40,000. DarkFlat frames were also created. Raw flat frame data was processed using the WBPP in PixInsight  to create the master flat frames.

No Bias frames  

Processing: 

  • Raw frames were calibrated, integrated and any cropping performed automatically with WBPP in PixInsight. No cosmetic correction or pedestal was applied to the images. 
  • GraXpert was applied to all master images with standard parameters
  • RGB Data: 
    • RGB data was combined using LRGBCombination script
    • Simple correct applied to star shape using BlurXterminator
    • Image astrometric coordinate fixed using image solver
    • Colour Calibration was undertaken using Spectrophotometric Colour Calibration 
    • GHS was used to gently stretch the image ensuring the core was maintained
    • Before saturation the stars were removed from the image with StarXterminator
    • A range mask was created and applied to maintain the cor
    • Further GHS manipulation was undertaken to bring out the spirals and dust lanes
    • SCNR for green removal was undertaken, Average neutral protection  and 0.6 amount
    • Further GHS and Linear stretching to enhance nebulosity and manintain back point enhancing depth
    • Star re reapplied to the image using PixMath and “Combine(image 1, image2, Op_screen())
  • Luminance Data:
    • Lunimance was manipulated with GHS to enhance the nebula regions
    • After GraXpert, Noise and BlurXterminator were applied to the image, BlueXterminator settings matching those of the RGB data  
    • Stars were removed from the luminance image 
  • Combining L +RGB:
    • The starless L data was applied to the RGB image using LRGB combination script
    • The resulting image was again subjected to GHS and linear as well as slight noise removal with NoiseXterminator
  • Processing Ha Channel:
    • Ha data was manipulated as a single Grey Image
    • After GraXpert, Noise and BlurXterminator were applied to the image, BlueXterminator settings matching those of the RGB data  
    • The Ha channel was then processed using the SetiAstro ContinuumSubtraction Utility. 
      • Linear Ha and R data were used to generate the HaNB output
    • Starless HaNB image was then applied to the R channel of the L-RGB image 

After addition of the Ha to the L-RGB data any colour imbalance was corrected