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M51, Whirlpool Galaxy, 0 Gain LRGB, 5 Apr 2017

Technical card

Resolution: 4619x3483

Dates: April 5, 2017

ZWO Blue 31 mm: 22x180" -20C bin 2x2
ZWO Green 31 mm: 20x180" -20C
ZWO Luminance 31 mm: 59x120" -20C bin 1x1
ZWO Red 31 mm: 20x180" -20C bin 2x2

Integration: 5.1 hours

Avg. Moon age: 8.65 days

Avg. Moon phase: 63.33% job: 1531652

RA center: 202.427 degrees

DEC center: 47.197 degrees

Pixel scale: 0.860 arcsec/pixel

Orientation: -90.314 degrees

Field radius: 0.691 degrees

Locations: Mapleton Lateral Observatory, Mapleton, Utah, United States


Conditions were much less than optimal for this image; intermittent high clouds mixed with thicker ones. However, it’s the best I’ve had in weeks so I decided to give it a try. On top of the weather problems, my home-brew stepper-based focuser is on the fritz (broken and/or shorted wires in the connector, for starters) so the automated focusing was working only intermittently if at all. I think I also found the source of my optical distortion with the AT8IN. The focuser is not set with the correct tension, and therefore sags pretty badly. I did not have high hopes for this image, but I am learning a little better how to handle my new camera. For this image, after seeing results others have obtained using the zero gain setting (maximum dynamic range), I decided to give that a try. I also saw some Cloudy Nights posts (by GeneralT001) listing optimum background levels to aim for at different gain settings. According to those numbers I’ve been WAY overexposing. I never thought it was possible to overexpose a deep sky astrophoto, but decided to try to hit the suggested optimum exposures, which are much shorter than I’m used to. This image is the result, and to my surprise I think it turned out better than my previous best of M51, which is one of my all-time favorite targets (at least in terms of how frequently I attempt it). Even with what seem to me to be very short exposures given the low gain setting, the tidal tails show up and the detail is better than before, especially in the pure luminance data (I lost a little when color was added). The color balance feels better in this image, although there may still be too much green. I wonder how this would look under good conditions.

Date: 5 Apr 2017
Subject: M51, Whirlpool Galaxy
Scope: AT8IN + High Point Scientific coma corrector
Filters: ZWO 31 mm diameter unmounted L, R, G, B
Mount: EQ-6 (EQMOD)+PEC
Guiding: Orion Thin Off-axis Guider + DSI +PHD (Win 10 ASCOM)
Camera: ASI1600MM-Cool, -20 °C, Gain 0 Offset 10
Acquisition: Sequence Generator Pro
Exposure: 59x120 L, 20x180 R, 20x180 G, 22x180 B
Stacking: Neb 4.1.5, flats & darks, trans+rot align, Nebulosity 1.5σ stack and align.
Processing: StarTools 1.4.327. Aggressively stretched and deconvoluted the luminance. Combined RGB in Neb 4 and stretched and attempted to color balance and correct in StarTools. Layered luminance over RGB in Photoshop CC 2014, boosted the saturation a little, and used Carboni’s Astronomy Tools deep space noise reduction, topped off with AstroFrame.



David Dearden



In this revision, stretched the RGB more and used HDR:Equalize rather than HDR:Optimize. Tried to bring up the tidal tails more, hopefully without loss of detail.


Same data, but in this one I used curves to boost the luminance a little more before applying it and darkened the background a little more. I’m not sure it’s an improvement, but it is an attempt at one.


Sorry to keep spamming with revisions, but I keep thinking I can improve this image. I noticed the ringing around many stars in my previous effort, decided I had to fix that, and one thing led to another. A little better star masking, a little less deconvolution of the luminance, and a little playing with the color balance to bring up the blue a little.


Last revision on this one (famous last words!). I reprocessed the luminance to try and preserve the detail and stretch a little harder without making the stars ring. And speaking of the stars, I used StarTools’ Repair module to try and fix the out-of-round stars. This is the result.

Sky plot

Sky plot


M51, Whirlpool Galaxy, 0 Gain LRGB, 5 Apr 2017, David Dearden

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