Processing multiple exposure lengths

[edarter]

Hi, 

Looking for advice as to the right order for doing things when you need to merge different exposure lengths into an image. I'm currently working on the great orion nebula and have exposures at 240s and 30s. I've stacked the 240s exposures in APP and in the process of star colour calibration etc. I aim to do the same with the 30s images as well then merge them in ps, using the 30s as a layer to be revealed in the core only. 

My question is about that part though, do I get the rest of the image finalised first with the 240s subs and then layer in the core at the end (processed separately beforehand), or do I merge them early on and then treat it as one image for the rest of the processing steps? 

Hope that makes sense! 

Thanks

[vlaiv]

I would actually advise you to merge two images while still in linear stage before you start processing.

Take both images and remove background on both (mean background ADU should be 0). Then multiply short exposure stack with 6 (240/30 = 6) to equalize ADU values between them. Then do pixel math that is equal to this condition:

if (pixel>80%_max_possible) use short_stack_pixel; else use long_stack_pixel;

Or in another words - replace all the pixels that have value higher than 80% of max value in long stack with short stack. Ideally, you would replace only pixels that have max value - but because of stacking and flat calibration - clipped pixels can end up being less than max value - so you want to replace all of them that are "close" to max value (where close is arbitrarily chosen to be 80% and above - but you might as well use 85% or 87% - won't make much difference).

[edarter]

Thanks Vlaiv but I suspect these are PI functions? I don't have that, just APP and PS. Unless I've got the wrong end of the stick 🙂

 

[vlaiv]

2 minutes ago, edarter said:

Thanks Vlaiv but I suspect these are PI functions? I don't have that, just APP and PS. Unless I've got the wrong end of the stick 🙂

 

Actually, you can do that with open source software like ImageJ that lets you do all sorts of things with images - even writing your own plugins/macros if you have some programming skills.

[edarter]

OK, shall have a look... Cheers 🙂

[sn2006gy]

Does APP have HDR Composition?  In PI that's what I'd do for this particular topic.

[bottletopburly]

You can merge two images in Startools too