Experiments with imaging M27

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timh
Posts: 272
Joined: Mon Aug 26, 2019 5:50 pm

Experiments with imaging M27

Post by timh »

A quick explanatory foreword.

The notion here was A) to take a long (110s) subframe-based exposure image of M27 using a UHC filter to capture the near full-extent of the HA/ OIII and B) to also create a very short frame-based exposure luminance image of M27. The supposition was that B) will offer the greater sharpness and detection of stars (because of the absence of a UHC filter) but that A) will offer the better SNR and detection of faint planetary detail. The challenges were twofold. First to establish suitable exposure, gain and sampling for B) so as to see a worthwhile improvement in sharpness over longer exposures but, at the same time, still retain acceptable SNR (i.e. for the M27 core using an 8 inch telescope at F 4.5). Secondly, having created images A) and B) the challenge was then to find a suitable way of processing them together so as to produce a combined image retaining the best features of each.

For Image A)
SW PDS200 Newtonian telescope, F 5.0 using a Baader MkIII flattener, Astronomik UHC filter, Baader steeltrack focuser, Pegasus Cube2 focuser controller, Ioptron CEM70 mount, SW Startravel 80 guidescope and ASI 120mm camera plus PHD2 guiding. ASI 294 MC main camera at pixel size 4.63 uM sampling at 0.95 pixels per arc sec. Moonless sky Bortle 4-5
30 x 110s images at gain 124 at -10C (corrected with 100 flats and 33 darks) captured in Sharpcap after filtering by FWHM (< 2.8 arcsec ) preprocessed and stacked in PixInsight. Processing via background removal , noise reduction, Photometric colour calibration and star size reduction before stretching to non-linear and final adjustment using curves.

For Image B)
SW PDS200 Newtonian telescope, F 4.5 using an SW 0.9X reducer/ flattener, ZWO IR/UV filter, Baader steeltrack focuser, Pegasus Cube2 focuser controller, Ioptron CEM70 mount, SW Startravel 80 guidescope and ASI 120mm camera plus PHD2 guiding. ASI 294 MM main camera at pixel size 2.315 uM sampling at 0.53 pixels per arc sec. Moonless sky Bortle 6
1026 x 3s images (2072 x 1410 pixel sized) at gain 285 at -10C (corrected with 100 flats and darks) captured in Sharpcap after filtering by FWHM (< 2.0 arscec) preprocessed and stacked in PixInsight. Processing via slight background removal noise reduction etc before stretching to non-linear.

Analysis using PixInsight subframe selector confirmed the visual impression that the integrated short frame linear image B) - before stretching - was indeed somewhat sharper than the equivalent for image A), the estimated FWHM values being 1.93 and 2.73 respectively.

Further combined images were produced via alternative routes of processing in PixInsight i.e.

Image C. Here the luminance of image A) was simply substituted with that of image B). While this resulted in a more contrasty and punchy central part of the image most of the detail at the periphery was lost.

Image D. Here the luminance of image A) was extracted, recalibrated using LIN to scale to image B) and then the two luminances combined using the MAX function in PIXMATH. The resultant luminance returns the maximum value of A) and B) at every point. This combined luminance was then replaced into image A). This seemed to produce a better overall image than in C.

Image E. Here the luminance of image A) was extracted, recalibrated using LIN to scale to image B) and then the two luminances combined into a single high dynamic range composition using the HDRC tool in PixInsight. It was important to smooth the masking and set the transition properly to avoid artifacts. This luminance was then replaced into image A.

Overall I think that something was probably gained over Image A) by adding in the additional short frame luminance with probably image E providing the best compromise?


Tim
Attachments
M27_AB.JPG
M27_AB.JPG (120.3 KiB) Viewed 106 times
M27_AC.JPG
M27_AC.JPG (118.28 KiB) Viewed 106 times
M27_DE.JPG
M27_DE.JPG (111.51 KiB) Viewed 106 times
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turfpit
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Re: Experiments with imaging M27

Post by turfpit »

A very thorough write-up Tim plus a good M27.

Dave
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