Adding Subs Over Time

[JoLo]

I have a question regarding adding additional subs collected over a period of time to an image you have already stacked and calibrated. I have seen this topic alluded to in various discussions on this and other forums, but am curious as to how I go about doing this.

As an example, I have about 60 mins of data on M31 (12 x 5 min). I calibrated (darks only at this point) and stacked the images, processing this through PS for curves, color, etc. I do have the original stacked image saved (both .tiff and .fits) per my homebrewed procedure.

Now, I want to go out this weekend and add a couple more hours of integration with the same integration time, and add shorter ones to help with the core blowout. Is the correct procedure to go ahead and collect more lights and darks, process them as a single unit, then stack them to the image I already have? What if I add flats to the new image set, will that screw things up? Is there a different way to go about this that I am missing? I also keep my original RAW files, so I guess I could go back and restack as a group, but the darks will be off for the two sessions, won't they?

I assume each imaging session should be regarded as a discrete image, with its own dark, flat and bias frames. Then, these discrete images can be combined to build your data up, up, up....

One more thing, I manually guide (autoguiding is a year or two off, by choice)....will I have problems with making sure the object is framed the same and can be stacked between images taken at different sessions? I am sure this issue is not as much an issue when using software to locate and frame your target. I use Maxim to stack, but I also have AIP4WIN that I use as well.

Thank you!

Joe

[ollypenrice]

Don't combine your short 'core' exposures with your long ones at the stacking stage. There would be no point. Make a 'final short' by stacking in the usual way and keep it linear for exporting into Photoshop.

How you combine your various nights' long exposure images doesn't seem to matter much. Some do an entirely new stack from scratch with all the data from all the nights.

Others (I'm one) make separate stacks for each night. For simplicity's sake I try to make these stacks have about the same number of component subs as each other. I then combine the various stacks using 'average.'

It will make little or no difference which way you do it, I think.

Now stretch open both the long and the short images as usual in Ps. In the short one, you can forget the faint stuff and in the long one you can forget the bright stuff. Concentrate on getting the best out of the bright stuff in the short stack and the best out of the faint stuff in the long stack.

Then just follow this excellent turorial on how to layer mask the short onto the long for a good blend.

Compositing 2 Different Exposures via Layer Masks

Olly

[Psychobilly]

If you have plenty of subs at each exposure setting then the entropy weighted stacking method in DSS can produce nice results...

[FraserClarke]

How you combine your various nights' long exposure images doesn't seem to matter much. Some do an entirely new stack from scratch with all the data from all the nights.

Others (I'm one) make separate stacks for each night. For simplicity's sake I try to make these stacks have about the same number of component subs as each other. I then combine the various stacks using 'average.'

It will make little or no difference which way you do it, I think.

From a purely statistical point, it is slightly better to combine all the individual images. But, if you have enough images in each night to make a decent stack each night, then I doubt you'd see any difference at all in reality -- and it's probably a lot easier to deal with.

[ollypenrice]

From a purely statistical point, it is slightly better to combine all the individual images. But, if you have enough images in each night to make a decent stack each night, then I doubt you'd see any difference at all in reality -- and it's probably a lot easier to deal with.

That's my empirical finding, certainly.

Olly

[JoLo]

Thank you all, very helpful, I appreciate it.

Joe

[JoLo]

I have one more follow up for you kind folks and that is the issue of framing night to night when manually guiding. Do you forsee big problems for Joe here....I am assuming, if i can reacquire the same star on which to guide, I will be good enough for stacking software. Any potential pitfalls here?

Thanks again.

Joe

[FraserClarke]

The software should be able to cope with aligning all the separate images, so that will take care of any night-to-night misalignments. Obviously though you want to get the framing as close as possible, so that you have as large an area as possible common to all the images. I'm usually happy with up to 10-15% difference in alignment.

[ollypenrice]

Some software copes with rotation, some (eg AstroArt 4) doesn't. DSS is very good at this, though for various reasons I use it rarely myself. If that is your software, fear not.

Olly

[Psychobilly]

I used to sketch the brighter stars from the previous nights data and use that for reference...

[martin_h]

I use Nebulosity 2 and the frame and focus routine bins the camera to its max...in my case 4x4, so I load an image from the previous session and scale it to 4x4 bin size then use "Al's reticule" Als Reticle (for your PC) - Page 4 - IceInSpace

to place a cross hair over a bright star or galaxy centre or some other obvious part, then, when framing tweek the mount until the object is under the cross hair, within obvious limitations I can place a star over the same one every night and not loose to much on the edges when stacked.

[blackparticle]

Using a DSLR, I've started keeping one of the previous nights subs on the memory card for reference and adjusting the next nights image to roughly the same spot.

Having said that, I've noticed DSS will mosaic the subs into a full frame image as long as some of the stars on the reference image are also present on the next nights worth of subs.

Nebulosity will also do this but the alignment process in DSS, I've found, is exceptionally intuitive.

[JoLo]

all good ideas, thanks again...I have never thought of keeping a frame on the picture for reference, but that makes perfect sense to me.

[James]

I too use Nebulosity 2, holding the Ctrl button (not sure what the PC equivalent is) and clicking on a star leaves a marker on it. I pick three stars on one of the last sessions images and just align the same stars to the three marks. That helps me judge the rotation of the field too if I've removed the camera in the meantime.

James