Streaky stars in DSS comet stack

[jambouk]

The attached image has been grossly stretched to show the issue I am encountering. The background stars have streaks when stacking in DSS to get both stars and comet sharp. The comet drifts from top to bottom in my subs from the first to last sub, and as polar alignment wasn't perfect, the stars drift slightly left to right between first and last sub. I presume the streaks are the tracks of the stars in relation to the comet's path, but is there a way to reduce the streaks in DSS? Would better darks, better flats help, or even better lights? Or is there a setting I can use to reduce the impact the streaks have? Grateful for any advice to rectify this data, or when imaging a comet in the future.

James

P.S. I think the banding left to right in the bottom third of the sub is from my flats. 

 

[Elp]

Have you done a comet only stack, and a stars only stack to start off with?

Use a stringent kappa sigma rejection factor (low numbers with little range between bottom and top values) to minimise the trailing.

You will still have some trailing in the comet only stack which will require manual post processing and blending with the stars only image.

[jambouk]

On 08/03/2023 at 13:48, Elp said:

Use a stringent kappa sigma rejection factor (low numbers with little range between bottom and top values) to minimise the trailing.

Thank you.

Do I need to apply this to the lights, the flats or the darks?

Do I use Kapp[a-Sigmal clipping or Median Kappa-Sigma clipping?

When opting for either of these two, I can change the values for "Kappa:" and for "Number of iterations:" - the default setting is 2 for Kappa and 5 for iterations. What values should I try? Dropping Kappt to 1 or increasing to 3 seem to make no difference. I've not altered iterations yet.  What is the range I can potentially opt for?

[Elp]

It's applied to the lights. Try a few values and see the result. When I did it last the best result was values of 1 and 2 respectively. 0.5 and 1 also gave similar results. Your result will differ as it depends on how much your starfield moves, if you've dithered etc.

But the main issue with the above image looks like you haven't stacked a comet only image and a stars only image, you've tried to stack it as one which is not how you do it as a stack for a comet which moves a lot.

Edited March 10, 2023 by Elp

[jambouk]

Thanks, I'll give that a go. I want both the comet and the stars to be sharp, hence using DSS to stack both, using the third stacking option "Stars and Comet". There is not enough detail in one sub to show the comet, and I don't like the streaky stars if just stacking on the comet. 

[Elp]

If your starfield has moved a lot and the comet it won't work. People either process one image, stack only a handful, or stack a lot and process the comet and stars separately then combine them together and post process some more. It took me a few tries to do it when I did and was actually more involved than doing a DSO image. However you do it, good luck. It's always great to capture unique objects out there.