Grain (from filter??) across picture

[djdurance]

Hi Folks,

I'm a bit new to this but seem to be stuck in trying to get rid of this "grain" that goes across the imagine. There is a lot more to be done to bring out M31 but the grain is there. This was stacked in DSS with 19 lightsx90seconds, 10 darks, some bias, and flats as well. I've worked at this for a bit in Photoshop and APP. Any thoughts on how to remove this grain? If I continue to stretch this it becomes much more noticeable. The original picture comes out a big "green" which is (I believe) due to the Svbony CLS filter I am using. Shot with a Canon 600d, 70-300mm lens, at 300mm F7.1.

 

 

[Seelive]

The "grain" is noise so you will need to apply some filtering to reduce it. I don't use Photoshop or APP but I'm sure they must have some noise reduction / filtering tools.

[IanL]

It's walking noise. You need to dither between subs by about 15-20 pixels. (I.e. Re-point the scope slightly for each exposure). If you're guiding then it's easy as there are settings in the software to do it automatically. If you're just using a tracking mount and no guiding, then you're going to have to do it manually - maybe re-point every two subs or so - you just need to nudge the RA and Dec axes a small amount in random directions each time.

The problem is that the camera's pixels all have slightly different responses to light, plus there are other sources of noise. Over the course of the series of exposures your tracking is not perfect, so the camera's field of view is gradually moving across the sky along the axis from top left to bottom right of the image (or vice-versa). When you stack the noise pattern builds up to the streaks you see. When you dither, the noise pattern moves around randomly and so doesn't produce this objectionable pattern.

Whilst calibrating (darks or defect map) can help, as can noise reduction, you'll never eliminate the pattern unless you dither.

[djdurance]

This is very helpful! Thank you! I'll try this tonight.