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What is best software for producing Mosaics from sets of overlapping images?


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I have 2 sets of HSO images made by different scopes on different nights ... the 2 sets are of an overlapping area of Cygnus.

I have been trying for the last weeks or so to get PixInsight to make a decent job of making a 2 panel Mosaic frames for each filter set .... and I am failing.

When I "blink" the the final HSO mosaics I can see that the stars are not aligned properly ....  especially around the edges of the images but not too bad in the centre of each mosaic :-((( There is some sort of unwanted distortion going off somewhere along the line.

Can anybody suggest another bit of software will do a better ( and more intuitive ? ) job of making these mosaics??

Mike

 

 

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Problem with wide field images is lens distortion.

This does not usually happen with telescope images because FOV is too small - few degrees at most. As soon as you start doing FOVs that are larger than that - lens distortion kicks in.

It is in fact not due to lens but due to projection. We are trying to image sphere (celestial sphere where coordinates are in angles) onto flat 2d surface of chip. It is a bit like trying to show globe on 2d map - some distortion is necessary.

Here is FOV with 4/3 camera and ~400mm FL scope (2.6° x 2°):

image.png.99c845a7391997300880ba7adf949b5a.png

Look at background RA/DEC grid - sides of sensor are almost parallel to Equatorial grid in the background, although distortion is slowly creeping in (bottom DEC line is not quite parallel to bottom edge of sensor - but a bit bent).

Same thing now with APS-C sensor and photo lens:

image.png.138428677dfe1d19ae88b5cbe2198842.png

This is now FOV spanning dozen of degrees in RA and DEC - look at level of distortion - RA lines are not parallel to sides and that is obvious - on left side they are tilted inward to the right and on right side - again inward to the left this time.

In any case - above is explanation of what is happening. I'm going to list software that can address this - and also caveats of using each:

1. APP. This is payed software that is used for stacking - as far as I know, it can stitch panels into mosaics and deal with lens distortion. I haven't used it myself - that is something that I just saw (or think I saw) in software description

2. Microsoft ICE. This works like charm - but two issues. First, it is no longer available for download (at least it wasn't few months ago - maybe they put it back online again). Second issue is that it works only with 16bit images and you need your images pre stretched for this to work. I prefer this done while data is still linear. It also won't deal with gradients properly (it might correct them just enough to seamlessly join subs).

You can find download links via Way Back Machine and I think that people posted here saved versions of ICE (3.1 if I'm not mistaken) - so do search to see what comes up

3. Hugin. This should do the same as Microsoft ICE - except it is readily available for download as it is open source:

http://hugin.sourceforge.net/

Haven't used it, but I think that same applies - need to pre stretch panels and deal with gradients independently of software.

4. ImageJ/Fiji has number of plugins that stitch images together. This is scientific software and should deal with linear data properly. It won't deal with gradients, but it is likely that it will transform data and let you combine it later. Not sure if it handles perspective issues.

https://imagej.net/plugins/image-stitching

I think that most of above (except maybe APP) require same focal length / pixel scale so first step would be to adjust pixel scale on one of the images (but I'm not really sure).

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For free Microsoft did a great app called ICE image composition editor (I've read reports it might have been withdrawn it's a W10 program)

Infinity Photo I think has done a youtube on an astro stitch.

Was the reference frame used on each matching stack perhaps different, does PI let you pick the reference frame for stacking.

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14 hours ago, vlaiv said:

Problem with wide field images is lens distortion.

This does not usually happen with telescope images because FOV is too small - few degrees at most. As soon as you start doing FOVs that are larger than that - lens distortion kicks in.

It is in fact not due to lens but due to projection. We are trying to image sphere (celestial sphere where coordinates are in angles) onto flat 2d surface of chip. It is a bit like trying to show globe on 2d map - some distortion is necessary.

Here is FOV with 4/3 camera and ~400mm FL scope (2.6° x 2°):

 

Look at background RA/DEC grid - sides of sensor are almost parallel to Equatorial grid in the background, although distortion is slowly creeping in (bottom DEC line is not quite parallel to bottom edge of sensor - but a bit bent).

Same thing now with APS-C sensor and photo lens:

 

This is now FOV spanning dozen of degrees in RA and DEC - look at level of distortion - RA lines are not parallel to sides and that is obvious - on left side they are tilted inward to the right and on right side - again inward to the left this time.

In any case - above is explanation of what is happening. I'm going to list software that can address this - and also caveats of using each:

1. APP. This is payed software that is used for stacking - as far as I know, it can stitch panels into mosaics and deal with lens distortion. I haven't used it myself - that is something that I just saw (or think I saw) in software description

2. Microsoft ICE. This works like charm - but two issues. First, it is no longer available for download (at least it wasn't few months ago - maybe they put it back online again). Second issue is that it works only with 16bit images and you need your images pre stretched for this to work. I prefer this done while data is still linear. It also won't deal with gradients properly (it might correct them just enough to seamlessly join subs).

You can find download links via Way Back Machine and I think that people posted here saved versions of ICE (3.1 if I'm not mistaken) - so do search to see what comes up

3. Hugin. This should do the same as Microsoft ICE - except it is readily available for download as it is open source:

http://hugin.sourceforge.net/

Haven't used it, but I think that same applies - need to pre stretch panels and deal with gradients independently of software.

4. ImageJ/Fiji has number of plugins that stitch images together. This is scientific software and should deal with linear data properly. It won't deal with gradients, but it is likely that it will transform data and let you combine it later. Not sure if it handles perspective issues.

https://imagej.net/plugins/image-stitching

I think that most of above (except maybe APP) require same focal length / pixel scale so first step would be to adjust pixel scale on one of the images (but I'm not really sure).

Thanks Vlaiv for your lengthy responce 🙂

I am working through your suggestions ... and I have just done an initial test using "MS ICE" ( which I downloaded a while back)  ... it is VERY easy to use.

I soon realised that ICE didn't like un-stretched images so I put in fully stretched versions .... and first impressions look very good indeed.

I have imported all three separate HSO mosaics into Photoshop and after assembling them into a single RGB file all details look to be perfectly lined up which is a big improvement on what I was getting with PixInsight.

I will continue to tinker with ICE and the other suggestions as time goes on 🙂

Again thanks

Mike

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18 hours ago, SlimPaling said:

Can anybody suggest another bit of software will do a better ( and more intuitive ? ) job of making these mosaics??

I've not used the APP mosaic tool, but I thought it was highly regarded. There are video tutorials: https://www.astropixelprocessor.com/video-tutorials/

Might be worth a visit.

Ian

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Earlier today I tried out "MS ICE" to see if it would manage to produce two pane mosaics for each of the HSO frames ..... I must say that it was pretty straight forward and I am more than happy with the resuts 🙂

Once MS ICE did the 3 separate HSO mosaics I went into PS and carried on as normal. I am attaching my final result below.

I am not sure if "MS ICE" can cope with mosaics made up from multiple rows & columns .... but I have also just been looking at what APP can do on the video that Ian has suggested ..... impressive !!! I will have to wait until I have a need for much larger mosaics before I spend the dosh though.

Mike

 

SADR Region Mosaic.jpg

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1 hour ago, powerlord said:

It's pretty old ms ice right enough. I'd be surprised if there are not better alternatives out there. Glad it worked though.

 

Where did you get ice from? Ms site says discontinued.

Try these two web pages ... "MS ICE" is available on both sites at this time ....

https://microsoft-image-composite-editor.en.softonic.com/

http://web.archive.org/web/20160421081257/http://research.microsoft.com/en-us/downloads/69699e5a-5c91-4b01-898c-ef012cbb07f7/default.aspx

Mike

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