RA clock-drive - attempting to dither

[KevinPSJ]

I've been struggling to eliminate noise in my images for a while and keep reading about dithering. I'm using tracking only - no guiding - and I don't even have a computer controlled mount. I just have a clock driven RA and a slow motion control on the Dec but I thought I'd give it a try.  Using my camera at prime focus I can reliably get 25-30s subs with no star trailing once I polar-aligned. Can't go above 30s though. I'm in a Bortle 4 area so the sky is pretty dark and on moonless nights I can see M31 naked eye and find M33 in my finderscope so the background sky isn't too much of an issue.

So on Saturday night I set everything up to image M33 Triangulum galaxy. I have an EQ3-2 mount with a Skywatcher 150P F/5 750mm scope. I attached a Canon 600D (astro-modded) at prime focus and used BackyardEOS to frame the image and then set up 60 x 30s lights.

I tried dithering by suspending the imaging every 5-7 frames and alternated pausing the RA drive, slewing west for a moment and very small shifts north and south. I then wait a few seconds and resume the imaging sequence. I could see M33 in the finderscope with averted vision so I was able to make sure I always kept the scope pointed in roughly the same position. At the end of the session I took 10 flats and for once didn't take any darks, just put the scope away.  

I found the manual dithering quite tricky - especially on the RA axis. Pausing the clock drive was ok to dither east but I have quite a bit of backlash on my RA axis so I'm not sure my dithering in the westward direction was successful. I do have to stay at the scope the whole time and keep count of number of frames so I remember to suspend the session every few frames to dither. 

In the morning I processed the data. First I stacked and calibrated in Siril. The data looks pretty good but there still seems to be some noisy horizontal stripes. I've attached an image of M31 I took about a week previously using darks instead (30x25s lights, 5 flats and 10 x 25s darks) for comparison and there's clearly an improvement. These images are stacked and calibrated and displayed using 'histogram' stretching in Siril - no post-processing.  

Am I on the right track here or wasting my time and better off sticking with darks? The tradeoff is longer imaging sessions but essentially passive: I can go inside to the warmth once the imaging starts and only need to come back out to cover the scope before the darks. I'm just not convinced that not dithering and using darks will really get rid of the noise.

[Elp]

In Siril do the banding reduction, vertical banding reduction also sometimes helps too but the horizontal one will get rid of the lines. Also do green noise reduction. Background extraction will remove most uneven gradients, crop the image for any stacking artifacts at the edges of the frame before you do any of the above. Looks like good data.

Most people run some sort of Denoise operation to get a very smooth look, my images are still noisy as a result despite dithering often and using an astro cooled camera. The dithering is to remove walking noise mainly where hot pixels appear in the same position on the sensor so if the camera remains static it would appear like dark/coloured rain like smears in the image. The 600d is prone to it a lot, I have one.

[900SL]

I live in a hot country so in summer I use LENR in camera (Nikon). I only get half the exposures but they are clean and no walking noise. At 40c it is well worth it, at normal temperatures I'm sure there are better options

[KevinPSJ]

Thanks Elp. I tried the banding reduction and it looked a lot better. Here's what I got when I processed the result using startools. I was able to stretch it a bit more without making the background look too grey.

 

@900SL it looks like Canon 600D also has a LENR option. I will do a trial using that option too and see if it makes things easier without compromising noise. I think there's probably a degree of experimentation here in finding (a) what works for my camera and (b) making life easy during the imaging session. I'm in south of England so generally thermal noise due to environment is not an issue - but my camera is not cooled and sensor temp is usually reporting in the mid 20s C after an imaging session so it's not exactly cold!