This is my third photograph of Messier 33, the Triangulum galaxy. I’ve dedicated over 30 hours of exposure time on this image, which is 10 times more than my previous attempt, but it remains a very challenging object in light polluted skies. The extended spiral arms are very faint.
I captured a lot of H-alpha when the full Moon was up and close to M33. The emission nebula in M33 are clearly visible with 17.5 hours of H-alpha exposure.
To combine the H-alpha and LRGB data, I used a method developed by Vincent Peris. I’ve used this method before on several HaRGB images.
Narrowband filters pass both the emission line and some unwanted off-band light. To isolate the signal from the H-alpha emission line, the off-band signal must be removed.
I used the following PixelMath formula and my red filter data to perform a continuum correction on the H-alpha data.
H = Ha – Q*(R – med(R))
The Q-factor was determined iteratively, optimal correction removes the central glow and stars from the data. Only saturated stars remain visible, with minor artifacts.
This corrected H-alpha signal was added to the red channel and luminance channel. To simulate natural H-beta emission, 20% of the signal was also added to the blue channel.
The final result after stretching, color correction, local contrast adjustments and noise reduction:
Interestingly, I accidentally discovered supernova AT2019qml (Mag 18.6) while blinking through my luminance data! This nova was discovered only 2 days earlier and its brightness is still changing.
Technical details:
Dates: 29, 30 Aug 2019, 14, 17, 19, 20, 21 Sep 2019
Telescope: Teleskop-Service 80/480 triplet with 2″ TS 0.79X reducer/flattener and Baader 2″ UV/IR filter, FocusCube 2 autofocus
Camera: ZWO ASI1600MM-C with EFW-8 and ZWO 31mm filters
Mount: Skywatcher EQ6-R
Exposure: 30.4 hours
Ha: 210 x 300 sec (gain: 200, bin 1×1, -15C)
RGB: 24 x 300 sec each (gain: 74, bin 1×1, -15C)
L: 138 x 180 sec (gain: 74, bin 1×1, -15C)
Software: SGP, PHD2, EQMOD, PixInsight
SQM: ~19
Astro Virus 
This is an excellent image and I think you blended the Ha in perfectly! I’ll have to try your blending technique myself, as I’ve never seen it before. It results in a nice balanced image, even in your other galaxy images that include Ha.
I have practically the same equipment as you…do you think Lum is worth getting in a Bortle 8/9 (SQM ~18.22) zone? I so far only have ~18hrs Ha.
AJ
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Thanks AJ! I would recommend you give luminance and RGB a try. Adding H-alpha data to my older DSLR galaxy shots made a big difference, even with poor SNR in the DSLR data.
I am familiar with shooting from bottle 8/9 and collecting lots of luminance is the way to go for broadband objects. You can apply heavy noise reduction to the RGB data without much consequences on the final result, since contrast is coming from the luminance layer.
Managing expectations is also important: it will be hard to see the edge of the galaxy, but the core and brightest parts of the spiral arms should be visible trough the noise.
Finally, when you shoot from heavy light pollution, it is very beneficial to image only when the object is near zenith. That might restrict you to just a couple hours per night, but it’s definitely worth it. Try to schedule multiple objects per night and shoot each one when they are in the best spot in the sky. Let me know how it turns out! 🙂
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