Adding to last years data...

[SteveA]

A very basic question regarding stacking images.

When combining data from several imaging sessions what’s the best option; stack all of the individual subframes from each session, or, if having stacked subframes after each session can I then stack each stack….if you get my drift! I’ve got some images of M100 from last spring and having just taken some more over the last couple of nights I would like to add to last years images without having to start from scratch.

Steve

[ollypenrice]

Just stack the stacks. I do this a lot and Dennis, I think, did a test and concluded likewise. If the stacks are of very different depths then many software packages allow you to weight the blend accordingly. I do this in both AstroArt and Registar. For this kind of thing, and many others, I really recommend Registar. If you change camera and/or scope you will be able to combine data near instantaneously still.

Olly

[roundycat]

Yes, the cheap and easy way is to combine the night's data using a sigma reject routine and then, eventually, combine the separate stacks using average. If you combine each stack in MaxIm I believe it will normalise them (optional) so basically the background will be set at the same level for each stack.

If you align each night's master file you can do the adding up in Photoshop. Open one, drag in another whilst holding down the Shift key to centre the new layer and set opacity to 50% and flatten. You should use Curves on each separate master to set the background of each one as close as possible before dragging them all into the one file.

Don't be tempted to do the aligning in PS, it cannot do it to anything better than a one pixel accuracy; that is not good enough.

Dennis

[SteveA]

Many thanks guys......I'll give that go.

Steve