How to merge images from different nights - Forums

Topic starter

Oct 1, 2016

Hey guys!
I was wondering how your workflow looks like if you merge images from different nights/months under completely different conditions. While processing my gigapixel mosaic, I had to combine images from five nights and sometimes I really have trouble to get nice transitions. The problem seems to be a different color distribution, even though I used the same camera and stacking parameters for all tiles.
Do you have any suggestion for making such tiles homogeneous?

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Salvatore Iovene

Admin

IOTD Staff

Frequent contributor

Oct 1, 2016 · Edited: Oct 1, 2016

By the fact that you're assembling color images, I'm going to assume you image with a DSLR or a full color CCD.
I would try to split the RGB channels and make 3 separate mosaics with the same settings, once for each color channel.

If you use PixInsight, and I'm going from memory here because I don't have it installed on this computer, you can do two things:

normalize each component of your mosaic, i.e. all Red channels to the first Red channel[/*]
then use the PixInsight mosaic tools to make the transition smooth[/*]

Finally assemble the 3 mosaics into an RGB image.

Sounds very laborious but I think it should work.

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Wei-Hao Wang

Oct 2, 2016

You will encounter this problem even if you are mosaicking images taken in the same night under the same condition, as long as each mosaic panel has sufficiently long exposure (longer than half hour or so).

The reason behind this is sky background gradient, caused by either natural sky glow or light pollution. The gradient will change with time, as your target moves across different part of the sky. This prevents seamless mosaic.

What you need to do is to remove sky background and its gradient. To make this easier, your flat-fielding needs to be as good as possible. Then you can use tools like GradientXterminator in Photoshop, or DBE or ABE in PixInsight, or any other methods, to remove the gradient. Once this is done, you can normalize the brightness/contrast/color of your mosaic panels and stitch them together.

The normalization and stitching parts are easy. Many programs can do this (Photoshop Photomerge, Registar, combination of various tools in PixInsight, and many panorama programs). Some of them are automatic while some of them require manual control. The key is the sky gradient removal.

Cheers,
Wei-Hao

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Jeroen Moonen

Oct 2, 2016

I agree with all the answers above. Pixinsight (PI) is THE program you want to use creating mosaics. It's indeed best to split the channels as Salvatore mentioned. After splitting the channels you first create a mosaic from all the panes of the same color channel. Using this "basic" mosaic you align the panes again, now using star alignment in PI with "frame adaption". After that PI can combine these frames to a mosaic using GradientMergeMosaic. What this does is smooting out any differences between frames using a gradient mask.

There's an exccellent tutorial on Youtube: https://www.youtube.com/watch?v=H2r3ZYqCaP8
Also there is a great tutorial here: http://trappedphotons.com/blog/?p=994

Clear Skies!

Jeroen

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Björn Hoffmann

Topic starter

Oct 8, 2016

Okay, thank you for your helpful tips!
I will first try to separate the color channels and to make three mosaics. Lots of work with over 50 tiles..

Zac Dawson

Oct 8, 2016

I like how this forum is turning out😊.
I am in the process of putting together my second mosaic. I chose Heart and Soul because it one of the things I found star hopping when I got my first telescope. Anyway it is going to be a 16 pane mosaic and so far not to bad just a lot of work compared to my moon one.
I think I can get away with posting some progress in the Bin but the final will not be done until I get at least 3 more nights of data. I run 2 imaging trains at the same time with two different camera's and I couldn't do it without PI and Maxim DL.

Zac Dawson

Oct 8, 2016

Stefan Westphal

Oct 23, 2016

Hi Björn,
Very important is how Wei Hao has already written a very accurate flat-fielding for each panel. In addition, you have to stretch each panel exactly the same before you merge. Then I assemble my mosaics with Photomerge. It works very well.
Greetings,
Stefan

Björn Hoffmann

Topic starter

Oct 23, 2016

I suppose my flatfields are the main problem in my workflow. I think I have to get a flatfield box for my gear. The "white t-shirt on white laptop display" method does not seem to be the best choice here.

I follow a very strict routine during stretching, so I think this is fine. Let's see when I find some time to process all 50 panels again..

Stefan Westphal

Oct 24, 2016

A flatfield box is a good idea. At first I did Sky-Flats which was not a good solution. Since I use a Flatfieldbox the final images are very good.

Wei-Hao Wang

Oct 24, 2016

I use both. I have a LED light box for flat. It flickers, so I have to use shutters slower than 1/10 sec. It works well for my scope, but not quite so on wide-angle lenses (<50mm). So I always try to use sky flat on lenses. With my telescope, I also try to use sky whenever possible. I always point the scope/lens to different directions on the sky and also rotate them and take many flats. The sky-flat results are always better than the light box, but very often the light box is also good enough.

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darkstar3d

Dec 16, 2016

I have PixInsight and love it. If you haven't purchased it yet, Microsoft ICE is free and does a great job for mosaics.

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