Combining images taken on different nights?

[LMLeitch]

Hi,
I have images of IC1318 taken over 3 nights. I used the Batch Preprocessor script to calibrate the images of each night. The next step was to get master combination of each of the nights. I tried using the Star Alignment tool to align the red master from night 1, 2, and 3. One or two of the combined images would lose most all of its contrast. Next I tried using the Star Alignment tool to align all of the registered RGB (one color at a time) and still the results were terrible. The masters of each night looked good.

How should this task be handled?

TIA

Larry Leitch

[pfile]

are you sure the calibrated subs are okay? i've never seen staralignment botch an image up.

i'm not sure though what you did - you integrated each night's work, then aligned each night to one of them, then integrated those 3 nights?

why not just align all nights to the same reference image and integrate them all together?

rob

[Jules]

What rob says works for me. Although I have occasionally had a star alignment failure when the subs has been meridian flipped.

Julian

[LMLeitch]

Those replies were quick!

I took two approaches:
1.  align *master* red from night 1, 2, 3 and then repeat for green night 1,2,3 and then blue master nights 1,2,3
2. align the red registered *subs* for night 1,2,3 and then repeat for green and blue.

The result in both cases was at least one three-night master which had very low contrast.

The masters from each single night looked good.

One mistake that I did discover was that early on I was selecting a Lum frame for the reference frame so that the RGB would already be aligned to the lum. This upsized the RGB and caused poor quality results. The difference between picking a lum reference and a rgb reference was very noticeable.

Larry

[Jules]

Larry

Out of interest have you checked your calibration masters, dark, bias, flat etc?

Your darks are the same time and bin as you images, flats binned same as images etc.

Sorry in advance if I am teaching you to suck eggs.

Julian

[LMLeitch]

Suck eggs - never heard that before! ;-)

I just processed all of the red sub-exposures from each night in one grand calibrate, register, normalize, noise reject, and combine in CCD-Stack and the result looks good.

The flats were created in Maxim (best calibration tool that I have found) and the rest of the processing was done in CCD-Stack.

The individual night's images done in Pixinsight all look ok. It is just when I try to combine nights that I run into problems.

Thanks to all for such speedy responses!

Larry

[topboxman]

Never heard of it either so I Googled for it:

http://en.wikipedia.org/wiki/Teaching_grandmother_to_suck_eggs

Peter

[pfile]

is it possible to upload those 3 masters somewhere? i wonder if all we're talking about here is either a normalization issue when integrating all 3 together.

rob

p.s. i have heard that expression many times but never knew what it meant, thought it was just random (considering i probably heard it the first time in a Ren & Stimpy episode there was a good chance it had no meaning!)

[Jules]

Thats exactly what sucking eggs is about.

I used to calibrate in Maxim and PI, now the batch preprocess script is up and running I do it all in PI.

As Rob said it would be interesting to see those masters. I collect my flats through an automated process and every once in a while a 'dodgy' one is thrown into the mix which causes problems. I do look at every cal frame now before I make my masters.

Julian

[MikeOates]

Hi Larry,

Ok, for clarity, just in case we are talking about a different workflow. What you should be doing is along the lines of:

Pre-process as you have been, but don’t integrate, or if you are, don’t use those files. Do this separately for each night’s images, assuming you took separate flats for each night/filter.

Open StarAlignment.

Ignoring any registered images produced by the BatchPreprocess, select a calibrated image to align to (the reference image). By calibrated I mean, it has had the appropriate flat, bias & dark frames applied. The important bit now is this should not be changed to any other image during the whole processing of this object, even months in the future should you want to add more data. A luminance reference may be a better reference image if you took any.

Click on Add Files to add all the subs from all the 3 nights in one go. This may mean pressing the Add Files button 3 times if your red filtered subs are located in different folders.

Set an output folder

Execute StarAlignment

Now repeat the above process for the green & blue subs, but keeping the same red reference image.

Once all the 3 filtered subs from the 3 nights are registered, then run ImageIntegration to combine all the red subs from all three nights in one operation, repeat for the green & blue.

For more info on the settings for integration see:
http://pixinsight.com/tutorials/master-frames/en.html

Alternatively to the above workflow, so long as you select the same reference image when using the BatchPreprocess for each of the filtered subs it should work. Doing it this way means you can now just integrate the registered files created by the script, a bit quicker and easier.

The problem you may be having is not using the same reference image for all the images or I have completely missunderstood the problem.

I hope that helps,

Mike