Merging different exposures into one image

[Moonshed]

I am waiting for a clear night to take a pic of what will be my first DSO, which will be M31. I have been thinking ahead and believe I have everything figured out but for one detail. I will be taking short exposures for the central core and longer ones to get the finer detail in the arms. Having done that how do I put them together? Do I seperatly stack them and make two images and use PS to layer them? Or do I stack them all together and let DSS figure it out for me? Your help would be appreciated. I am sure my first DSO will not be brilliant and will contain lots of faults but I want to give it my best shot, so any advice at all for capturing this image would be greatly appreciated. Thanks. Keith.

[D4N]

Stack separately, PI has a HDR combine process that is great for this but if you don't have PI then layers in Photoshop should work.

[Moonshed]

Okay, thanks for that, I will stack them separately and layer them in PS.

[SilverAstro]

24 minutes ago, Moonshed said:

Okay, thanks for that, I will stack them separately and layer them in PS.

HDR, exposure blend and layer mask are phrases I've seen used for Gimp, not yet done it mysel worth a google ? good luck

[uhb1966]

theres also a hdr mode in dss... worth trying!

[ollypenrice]

I wouldn't be too sure that multiple exposure lengths are obligatory. You might be very surprised by what lies buried in what initially looks like over exposed data. Before deciding whether or not your longer subs have over exposed the core you should look at them in linear (unstretched) form. You are going to get, in all probability, a white blob in the middle of the galaxy. But how big is it? If it is a huge saturated (ie  fully exposed) blob then you need shorter subs. But if it is a fairly small blob then shorter subs are not going to bring anything new to it. We all struggle with this target but when I went after the very faint outer glow I used 30 minute luminance subs with a good CCD camera from a dark site. Remarkably enough these subs let me get very nearly as far into the core as I got by the complex process of shooting a set of subs at longer focal length and with short exposure. M31 is not the Trapezium in M42. The Trapezium does need short subs to be blended with long. M31 is not such a certain case.

You cannot beat this tutorial for blending long and short subs. http://www.astropix.com/HTML/J_DIGIT/LAYMASK.HTM If you use a ready made HDR package you give control of your image to someone else. If you follow Gerry Lodigruss it will be your image!

My 30 minute L subs provided nearly all of the data in the core of this image. Long FL, short exposure, subs got me a bit further in but the effect was underwhelming.

Olly

[Pat Warborg]

Wow!!! How many hours of exposure went into this image?

Pat

[ollypenrice]

1 minute ago, Pat Warborg said:

Wow!!! How many hours of exposure went into this image?

Pat

Ah, now you have me. A lot. 50 hours or so (for a two panel mosaic in HaLRGB). However, it was an experimental and ongoing project as we tried to figure out how to image this monster. Based on what we learned I think we could do the same image in a third of that time. For example we found that 30 minute luminance subs thrashed 15 minute ones for that faint outer glow. It was a 'no contest' win for the long subs.

All I would say is that there is an orthodoxy that soon appears on this and certain other targets and I think experiment beats orthodoxy. I certainly want to improve this version by a large margin. The blues are too cyan and the resolution is very low at 3.5"PP. The resolution is easy to deal with by changing camera or focal length but I am entirely stuck on the colour issue. I'm happy with the outer glow, though.

Olly