Imaging same object over multiple nights & stacking

[Pankaj]

Hello everyone. I have seen many members on this forum that they image (collect data) the same object over multiple nights. In some cases they add data to a previous imaging session after a year of the first session.

Wanted some advice on whether the orientation of framing the object in the camera's FOV has to be precisely the same in every imaging session. This means that if I image the same object over multiple multiple nights to collect more data, will I have to make sure that the camera's orientation on the focuser is the same every night. Also, will the temperature variance impact the image and subsequently the stacking process in DSS ?

I image using a portable set set-up that consists of 130PDS on NEQ6 with a Canon 1200D and stack the images (Lights+Darks+Bias) in DSS.

[teoria_del_big_bang]

The short answer is No, it does not have to be perfect rotation or framing. The stacking software will still align all the images and stack them properly.

BUT, you need to get both the FOV and the angle as close as possible really to get a good final image, because you will probably have to crop the final image to remove the edges where the two sessions differed in the FOV so the closer you get it the less you need to crop.

Also, to some extent it depends on the object really.
I say depends on object because if the main target is quite small, and you would perhaps do a fair crop anyway to make the image look bigger in the final frame, then it doesn't matter so much, but if the target is something like Andromeda galaxy that usually is actually as big as, or even slightly bigger than the FOV then getting as lose to the original framing and angle is important.

If you have lots of different angles of the same target then this is what you are likely to end up with in the stacked image.

Now whether some processing, such as DBE in Pixinsight, can even out the background sky enough to make it acceptable I am not sure as I always make sure the framing is the same (within reason so I can just crop the very edges).

HTH

Steve

Edited March 4, 2022 by teoria_del_big_bang

[Pankaj]

On 04/03/2022 at 13:41, teoria_del_big_bang said:

The short answer is No, it does not have to be perfect rotation or framing. The stacking software will still align all the images and stack them properly.

BUT, you need to get both the FOV and the angle as close as possible really to get a good final image, because you will probably have to crop the final image to remove the edges where the two sessions differed in the FOV so the closer you get it the less you need to crop.

Also, to some extent it depends on the object really.
I say depends on object because if the main target is quite small, and you would perhaps do a fair crop anyway to make the image look bigger in the final frame, then it doesn't matter so much, but if the target is something like Andromeda galaxy that usually is actually as big as, or even slightly bigger than the FOV then getting as lose to the original framing and angle is important.

If you have lots of different angles of the same target then this is what you are likely to end up with in the stacked image.

Now whether some processing, such as DBE in Pixinsight, can even out the background sky enough to make it acceptable I am not sure as I always make sure the framing is the same (within reason so I can just crop the very edges).

HTH

Steve

Thank you Steve for the insight. I presume this will also apply to Darks and Bias frames. One more query please - will a temperature variance of 4-5 degree centigrade ruin the image while stacking in DSS ?

[teoria_del_big_bang]

If you do not have a cooled camera you can maintain the same temperature then there is really not much you can do about the temperature variance and I would not think it will make a massive difference.
I do not think anyone would go the the bother of taking separate darks and Bias frames throughout the night as the temperature drops.

Steve

[Richard_]

Are you using any image capture software with your setup (ie ASI Air, Mini-pc, laptop) or just an intervalometer? I know that the software NINA for Windows has a built in framing assistant which will help get your rotation spot on when compared with a previous image. 

Otherwise, you'll end up with the image shown by Steve where you get regions of different signal (ie the overlapping areas) so will have to crop. If you have a big enough sensor, this may not be too much of an issue since you have a lot to play with. 

[Pankaj]

I am just using an intervalvometer with a canon 1200d. I think I will try the next time imaging on two different nights. If the final image after stacking comes right, then it’ll be good for me. Else I will stack the images in Dss separately for two nights two create two stacked images.
Another query- will dss stack  the two images that are produced after the stacking process of two different sessions?

[barbulo]

7 hours ago, Pankaj said:

will dss stack  the two images that are produced after the stacking process of two different sessions?

I would stack all of them at the same time: leave all invariant frames (referring to bias frames only, since I wouldn’t bother taking darks since you cannot control the sensor’s temperature) in the Main Group tab and put each night’s frames (lights and flats) in separate tabs. DSS will do the job.