NGC 6188 Workflow

Description of the PixInsight workflow for processing the recently posted NGC 6188 image.

Data acquisition by Herbert Sauber, Werner Möhler, kindly offered for processing by the IAS e.V. community in May 2020

10micron GM 2000 HPS
Takashi FSQ-106ED
Baader filter L, R, G, B, Ha, O3, S2

7 x 300s R, 8 x 300s G, 8 x 300s B, 5 x 600s Ha, -20 °C
flats, darks, bias

RGB + Ha processing

Calibration / Integration

Calibration files

ImageIntegration of 9 bias frames, creating masterbias

Superbias for masterbias, creating mastersuperbias (not used in this processing)

ImageIntegration of 9 dark frames, creating UNCALIBRATED masterdark

ImageCalibration of 9 dark frames with masterbias

ImageIntegration of 9 calibrated dark frames, creating CALIBRATED masterdark

ImageCalibration of all flat frames with masterbias

ImageIntegration of 9 L flat frames, creating masterflat-L

ImageIntegration of 9 R flat frames, creating masterflat-R, CloneStamp with manual removal of residual stars (tracking not stopped)

ImageIntegration of 9 G flat frames, creating masterflat-G, CloneStamp with manual removal of residual stars (tracking not stopped)

ImageIntegration of 5 B flat frames (only those with tracking stopped), creating masterflat-B

ImageIntegration of 9 Ha flat frames, creating masterflat-Ha

ImageIntegration of 14 O3 flat frames, creating masterflat-O3

ImageIntegration of 12 S2 flat frames, creating masterflat-S2


ImageCalibration of 7 R subs with uncalibrated masterdark, masterflat-R

ImageCalibration of 8 G subs with uncalibrated masterdark, masterflat-G

ImageCalibration of 8 B subs with uncalibrated masterdark, masterflat-B

ImageCalibration of 5 Ha subs with masterbias, calibrated masterdark (scaling, optimize=on), masterflat-Ha

SubFrameSelector to check calibrated subs, RGB: FHWM 4.8 – 8.6 / Eccentricity 0.62 – 0.69, Ha: FWHM 3.8 – 4.1 / Eccentricity 0.60 – 0.67

StarAlignment all calibrated subs to G sub with best FWHM

ImageIntegration of 7 R / 8 G / 8 B / 5 Ha registered subs, Weights=Noise evaluation, Winsorized Sigman Clipping

Linear processing

DynamicCrop of all integrated channels to remove stacking edges

ChannelCombination to create RGB image

ImageSolver script with NGC 6188 coordinates for reference, focal distance=530, pixel size=9, limit magnitude=9 (default yields way too many stars!)

Tried calibration with Fluxx script, but the result were too red/purple for my liking, thus …

Autocolor script to neutralize background and balance colors, the result is a bit on the blue side

Extract L channel, permant autostretch with ScreenTransferFunction / HistogramTransformation

Additional HistogramTransformation to move the main “hump” to approx. 75 %, this is our RGB MMT mask

MultiscaleMedianTransformation with 8 layers and applied inverted RGB MMT mask for noise reduction

Clone Ha image, and apply the same processes as above to create Ha MMT mask

MultiscaleMedianTransformation of original Ha image with Ha MMT mask

Starless disgression

Clone RGB image after MMT

PixelMath mtf(0.007,$T) to stretch image

StarNet to create starless RGB image

PixelMath mtf(1-0.007,$T) to get image back to linear

Extract L channel, HistrogramTransformation with a strong stretch, not quite as strong as a standard STF, this is our starless mask

Non-linear processing

ArcsinhStretch of RGB image with stretch factor=150

HistogramTransformation of Ha image for a first stretch

HistogramTransformation of Ha image with applied starless mask for a second stretch, further brightening the nebula, but not bloating the stars

LRGBCombination with L=Ha image, lightness=0.5, saturation=0.3 applied to RGB image

RangeSelection to create range mask for main nebula areas

LocalHistogramEqualization with kernel radius=128, contrast limit=1.5, amount=0.33 and applied range mask, twice

DarkStructureEnhance script

SCNR to remove greenish background

Photoshop post-processing

Curves layer with slight S curve to increase brightness / contrast

Curves layer in color mode to remove bg color

Selective color layer to adjust red / purple colors, less cyan, more magent/yellow/black

Lightroom post-processing

Blacks -10, texture -10, clarity +10, dehaze +40