Stacking different exposure times ( eg: Ha RGB )

[Martin Dowd]

The ASTAP user guide mentions you can stack different exposure times but it is not recommended.
I want to image a Nebula with HaRGB under Bortle 8 skies during new moon period
Ha will be 3 minute subs
R will be 60sec subs
G will be 60 sec subs
B will be 60 sec subs
All subs same temperature -10C , all subs same Gain 100
Matching Flats
Matching Darks
Bias frames
Total expected integration time 5 to 10 hours

All processed in Startools via Compose R , G , B channels and Ha accent channel

Q: Can ASTAP stack and align each channel successfully

Await your advice and confirmation on the above

Thanks
Martin

[han.k]

Hi Martin,

That recommendation is for a single colour or mono image series. The main reason is that sigma clip stacking will not work so well. For combining different colours there is no disadvantage. You frames should stack well.

Cheers, Han

[Martin Dowd]

Hi Han,
Ok thanks
Im just stacking 4 individual stacks Ha , R , G and B with their respective calibration set from my 2600MM Mono camera and respective filters. All individual stacks with have same exposure times ( R,G and B all having all 60 sec ) and Ha being the odd one out at 3 mins . I was worried the R, G , B and Ha wouldnt align properly to each other under "Calibration and Alignment " method as Startools requires aligned data sets to load into Compose.

Martin

[han.k]

I have reframed the help file text as follows:

It is possible to mix different exposure times but it is not recommended for the frames of the same colour. The reason is that sigma clipping of pixel value outliers could work less efficient for frames with different exposure times. The frames will be combined with a weight factor relative to the exposure time but the image noise will not be fully linear with the exposure time. E.g. the read noise is fixed. Once the individual colour frames are combined then the exposure times are no longer relevant to create (L)RGB colour stacks .

So you could expose all red frames for 60 seconds and all green frames with 120 seconds and combine them to a (L)RGB. But combining red frames of 60 and 120 seconds is less desirable.

Han

[Martin Dowd]

Han
Thanks for the explaination

Martin