What is the correct method of integrating subs of different exposure lengths?

[MikeOates]

What is the correct method of integrating subs of different exposures lengths?

I am not talking about HDR, but just different exposures from different nights, say 1800s subs on a good night with excellent tracking, but 600s subs on another night because it was a bit windy and longer exposures would have trailed.

I just tried integrating them all in one go, setting the reference to a longer exposure, but it produced artefacts in the core of the galaxy, grey pixels.

The example is an integration (not processed, just calibrated) of 14 x 1800s and 10 x 600s of M81 in Ha. This is enlarged to 300%

Thank you,

Mike

[pfile]

that behavior could be caused by your rejection parameters. are the 1800s exposures blown out at the core?

basically the right way to do it is what you did; ImageIntegration will weight the images by SNR.

rob

[MikeOates]

Hi Rob,

No the 1800s exposures when integrated on their own only gets to K: 0.68 in the core also single subs with the same value, so no they are not saturated.

Copying from the console during integration, a typical 600s exposure was:
G:/Astronomy/M81-82/20140308/Light/reg_ha/m81_m82_600s_ha_bin1-25c_20140308_07_c_cc_r.fit
* Retrieved data from file cache.
Scale factors   :   2.79564
Zero offset     :  +3.130653e-002
Noise estimates :  7.7660e-004
Weight          :     0.78697
and 1800s sub this:
G:/Astronomy/M81-82/Master Project/reg/m81_m82_1800s_ha_bin1-25c_20140323_05_c_cc_r.fit
* Retrieved data from file cache.
Scale factors   :   1.74467
Zero offset     :  +2.198543e-002
Noise estimates :  1.2270e-003
Weight          :     0.80946
and the reference sub at 1800s was:
G:/Astronomy/M81-82/Master Project/reg/m81_m82_1800s_ha_bin1-25c_20140312_01_c_cc_r.fit
* Retrieved data from file cache.
Scale factors   :   1.00000
Zero offset     :  +0.000000e+000
Noise estimates :  1.9260e-003
Weight          :     1.00000

The ImageIntegrations settings are shown below:

Thanks,

Mike

[TobiasLindemann]

Hi Mike,
I may be wrong, but think you should try "linear fit clipping" as the rejection algorithm. See pixel rejection here:http://pixinsight.com/doc/tools/ImageIntegration/ImageIntegration.html#description_003 Especially look at the two graphs "sigma clipping" and "linear fit clipping".

Tobias

[MikeOates]

Tobias,

Thank you, yes you are right, I did some experimenting today and found out that Linear Fit Clipping worked well. And I am glad you can confirm that. I have seen the integration page you linked to, but I am afraid most of that just goes right over my head. So below I show the ImageIntegration settings I used and a 300% enlargment of the core to match the above image after some processing.

Thanks,

Mike

[pfile]

i guess no matter the rejection algorithm it's a good idea to look carefully at the rejection maps to see if too much (or not enough) is being rejected. if things are going right you should pretty much just see hot pixels (assuming you were dithering) and cosmic ray hits in the rejection maps. sometimes you need to stretch the rejection images a lot to see everything, though.

rob

[MikeOates]

Hi Rob,

Yes, in fact looking at the rejection map showed all those pixels as being rejected, the problem was I did not know why and what the rejections would be like with other rejection methods and there are so many extra settings to set as well. Not being good at math (actually very bad at math) meant I did not have the faintest idea where to start so it was a matter of make a change and try it etc. etc.

Anyway, that issue is now sorted and I can move on.

Regards,

Mike