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Combining data from several images?

Astrosurf

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Astrosurf

Astrosurf

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This is something I've wanted to know but can never seem to find the info. If I had pics of sveral different-length subs and wanted to combine the data (no Layer Masks) in PS, what would I do? Is it in the blending Mode?

Alexxx

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tingting44

tingting44

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interested to know this too Astroturf

Like for example, some short exposures of M42 to reveal the core and longer ones to show the fainter dust clouds?

Try:  File/Automate/Merge To HDR.

ChrisH

this would not be ideal as you would not have no control over all the different exposure's would you?

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ChrisLX200

ChrisLX200

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interested to know this too Astroturf

this would not be ideal as you would not have no control over all the different exposure's would you?

Well you do if you look, but you would have to enter the exposure values manually. Not ideal. Better to avoid having a mixture of exposure values in the first place unless that is what was intended for the reason I outlined.

ChrisH.

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ollypenrice

ollypenrice

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Unless you know why you are taking short exposures, don't take them! They are not as good as long ones if they don't over expose the bright stuff, and they do that far less often than people suppose.

If you have a mixed set of exposure lengths for no good reason, just make a note not to do it again (tut tut!!) and then make the best of it!! So to make the best of it I would make separate stacks of each exposure length and co-register them to fit each other.

I would give them all a simple log stretch and black point cut back till they all had the same background value.

I'd paste one onto the other in Ps Layers and zoom in on the noisy bits then play with the opacity slider till I got the lowest noise, visually. Flatten.

If there were a third set of different subs I'd repeat the operation as above.

Next I'd continue to stretch in hopes that the fuller stack would allow it, which it should.

This business of short subs being needed to control bright bits is thrown about endlessly but I have used special short subs on about three images in the last eight years. When I look at my linear data from long subs I rarely find anything over exposed other than stellar cores and then I just remove the luminance from the cores and use RGB only for those bits. However, it is routine to make a softer stretch of the same data and layer it into a hard stretch designed to pull out the faint stuff. So multiple stretches of the same data, yes. Short exposures for the bright stuff? First look at the long subs to see if you need them. You probably won't.

Olly

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Astrosurf

Astrosurf

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Thanks guys.

Thanks Olly. But not sure what you mean by ' . . . co-register them to fit each other.' Plus, when you say 'I'd paste one onto the other in Ps Layers . . .' do you mean opening each stacked file and displaying them side-by-side, holding down the Shift key and dragging each image onto one so they're all registered as layers?

I assume there's no blending if you say just flatten them after processing? All the extra data from each will be combined to give a more data--rich image than each one separately? I was worried only the top layer would show!

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ollypenrice

ollypenrice

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Thanks guys.

Thanks Olly. But not sure what you mean by ' . . . co-register them to fit each other.' Plus, when you say 'I'd paste one onto the other in Ps Layers . . .' do you mean opening each stacked file and displaying them side-by-side, holding down the Shift key and dragging each image onto one so they're all registered as layers?

I assume there's no blending if you say just flatten them after processing? All the extra data from each will be combined to give a more data--rich image than each one separately? I was worried only the top layer would show!

Before you can paste one image on top of another in Photoshop they need to be aligned to each other. The proper term is co-registered. I do this in Registar or AstroArt but I guess most stacking programmes could be asked to do it.

The process in Ps for pasting one image on top of another is to make the one going on top active and then select and copy it (Ctrl A Ctrl C)  then make the one going underneath active and Ctrl V will paste the first one on top of it.

In Window open the layers palette and you'll see your two layers. The eyeball symbol makes the layer visible or invisible, the blue highlight bar shows which one is currently active and the blend mode (which defaults to Normal, but there's a drop down) defines how Ps will make the layers interact. Normal is fine for this. The opacity slider decides how much of the upper layer is applied to the lower. So suppose you have your shorter set on top, it will be noisier than your longer set below. However, it should still improve your signal to noise so lower the opacity until you get the lowest looking noise level. Then Ctrl E will flatten them.

There is no masking in this. By all means send me a PM if you're not clear about any of this.

Olly

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