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NGC 4236, Barred Spiral Galaxy in Draco, LRGB, 3 May 2017

Contains: NGC 4236

Technical card

Resolution: 4333x3092

Dates: May 3, 2017May 4, 2017

ZWO Blue 31 mm: 22x300" -20C bin 2x2
ZWO Green 31 mm: 18x300" -20C bin 2x2
ZWO Luminance 31 mm: 60x300" -20C bin 1x1
ZWO Red 31 mm: 18x300" -20C bin 2x2

Integration: 9.8 hours

Avg. Moon age: 7.76 days

Avg. Moon phase: 54.03% job: 1566643

RA center: 184.160 degrees

DEC center: 69.484 degrees

Pixel scale: 0.860 arcsec/pixel

Orientation: 89.872 degrees

Field radius: 0.636 degrees

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


This galaxy is a little off the beaten path in that it seems not to be imaged as frequently, yet it has a large apparent size and I think it is pretty. It is quite dim and doesn't have a lot of internal structure. I thought it would present a good challenge to my LRGB skills and it was in the right part of the sky for a good try (though that part of my sky is encroached upon by a nasty light dome from Provo, Utah). I decided to go to 300 s subs for this image at the 0 gain 10 offset setting and I think this exposure looks better than the 180 s I have been using recently. Processing was challenging because there was not a lot of contrast between the galaxy and the sky glow, so I worked it pretty hard (maybe too much?). I have another version of this that brings out the galaxy a lot more, but I think I prefer this more subtle version that has less differential enhancement of the galaxy. In any event, I think it did turn out better than last year's effort. I understand this galaxy is gravitationally associated with M81 & M82, a couple more of my favorites. I just looked at a few other users' images of this galaxy and saw they identified lots of small faint fuzzies in the background, undoubtedly much farther away. Some show up in this image too, though perhaps not quite as well.

Date: 3-4 May 2017
Subject: NGC 4236, barred spiral galaxy in Draco
Scope: AT8IN + High Point Scientific coma corrector
Filters: ZWO 31 mm diameter unmounted L, R, G, B
Mount: EQ-6 (EQMOD 2.000j)+PEC
Guiding: Orion Thin Off-axis Guider + DSI +PHD (Win 10 ASCOM) using predictive PEC algorithm
Camera: ASI1600MM-Cool, -20 °C, Gain 0 Offset 10
Acquisition: Sequence Generator Pro
Exposure: 60x300 L, 18x300 R, 18x300 G, 22x300 B
Stacking: Neb 4.1.5, flats & darks, trans+rot align, Nebulosity 1.5σ stack and align.
Processing: StarTools 1.4.327. Stretched and deconvoluted the luminance. Combined RGB in Neb 4 and stretched, color balanced, and sharpened in StarTools. Layered luminance over RGB in Photoshop CC 2014, used Carboni’s Astronomy Tools deep space noise reduction several times, boosted the vibrance a lot and saturation a little. Applied GradientXTerminator medium/course. Used space noise reduction and some layer-masked levels to darken the background and remove some background color cast without dimming the galaxy. Topped off with AstroFrame



David Dearden



I think I prefer this more subtle version where I did not increase the contrast between the galaxy and the background as much.


After seeing the faint fuzzies in the background, I had to revise again. I also was not satisfied with the color balance; more R was needed, but I had not dealt with it adequately before because of the red color cast gradient. This time I used Gradient XTerminator early in the process to get rid of the color cast rather than trying to do so by darkening the background. I definitely like this version better.

Sky plot

Sky plot


NGC 4236, Barred Spiral Galaxy in Draco, LRGB, 3 May 2017, David Dearden

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