Question about sky background removing

[Jacques Montier]

Hello all,

When the target is rather large (eg galaxy, nebula) exceeding the slit length, one can't directly remove the sky background with Isis.
So i used to record the spectrum of the sky backgound near the target.
Now i have some doubt about processing.
I think there could be two methods :

Method 1 :

- processing the target without removing the background -> spectrum 1

1.png (31.73 KiB) Viewed 2875 times

- processing the background images -> spectrum 2

2.png (33.37 KiB) Viewed 2875 times

- Arithmetic subtraction spectrum1 - spectrum 2 with Isis

subtraction.png (19.65 KiB) Viewed 2875 times

Method 2 :

- getting a master background from the background images

3.png (13.82 KiB) Viewed 2875 times

- processing the target images with the master background dark

4.png (31.82 KiB) Viewed 2875 times

My question (at last...) : which is the best way to go ?

Thank you in advance for your responses.

Best regards,

[JP Nougayrede]

Hello Jacques,
The first method makes sense, pretreating the target and closeby sky background, then substrating the target spectrum by the sky background spectrum. Not sure about the second one, as you want to substract the sky spectrum and not its image. With the second one, you also need to verify how Isis is going to scale the dark to the target.
My two cents..
Jean-Philippe

[Jacques Montier]

Thank you Jean-Philippe.
For the two cents, you accept Visa card ?