Making a four panel image

[Starlight 1]

When making more than one panel Image so as the title, four panel take away the overlap do this mean the picture is nearly 4 times its size or you apply less stretch .

I'll started doing M31  all the overlaps will be with in M31.  really what I am asking for is a explanation on what size the final image will be and Less stretch Will keep tighter stars hopefully.

[vlaiv]

Not really sure what you are asking, but let me try to help.

If you are set on making a mosaic (multiple "panel" image stitched together), first make sure that you have proper tiles for making mosaic. This means that framing for each tile needs to be such that there is slight overlap with adjacent tiles. If you decide on 4 tile mosaic this would mean that you need top-left, top-right, bottom-left and bottom-right tile. Top-left needs to overlap with top-right and bottom-left panel. Good overlap would be somewhere around 10% of frame - it really needs to have enough features to properly align and stitch tiles together (enough stars, similar to stacking - you can only stack if there are enough stars to align subs). Also, make sure that there are no holes in area covered with tiles.

Depending on equipment and software that you are using, there might be tools to help you frame / position your scope to take tiles.

Here is an example image form EQ Mosaic software (part of EQMod suite) as an example how to position tiles:

Now, when you have captured, calibrated and stacked your subs for each tile - you need software to do the stitching. Don't know any software that will do it out of the box (I'm sure that there has to be some, I just have not found it). I use ImageJ and MosaicJ plugin. It involves one more step prior to stitching and that is background equalization (making sure that background levels are the same, I do my imaging in quite heavy LP and background levels vary with part of the sky, so it is more than likely that tiles will have different background levels depending on time that they were recorded at).

Once you have your image stitched it will be almost 4 times the size of original (2x2 minus overlap) - now you can choose if you wish to keep the larger image size or you want to reduce your image. If you opt to reduce image size - it is best to use binning rather than "plain resize" - this will improve your SNR. Depending on your choice final image will either have original resolution (if you do not bin mosaic), or it will have two times less resolution than your setup is providing if you opt for 2x2 bin (if you shoot for example at 1.5"/pixel with 4000x3000 pixel sensor - mosaic will either have 8000x6000 size with resolution of 1.5"/pixel if you leave it as is, or 4000x3000 size with resolution of 3"/pixel if you bin it 2x2). You can bin it even higher if you have a large sensor and bin / reduce size anyway when you process single stack. In mosaic binning will of course have benefit of SNR increase and due to lower resolution, it will also provide "tighter" stars in pixel units (they will still have same FWHM in arcseconds, but since you are using more arcseconds per pixel - in pixel terms stars will be smaller).

At the end - just process mosaic as you would normally process single stack.

HTH

[Starlight 1]

You have work out just what I am asking for. I using SG-PRO so it done the hard work , here a stretch of one panel x 10 lum 120 sec. I just looking to see if the image was clean and Stars not looking to big.

 

[ollypenrice]

The stretch has nothing to do with the size. The size, as vlaiv says, is the original x the number of panels minus the overlap.

If this is your first mosaic I would go for a bigger overlap than 10%. I'd go for 20%. 

Getting a seamless mosaic gets harder the less overlap you have.

There are a number of issues which arise when you come to join the panels. Personally I run them through DBE, still linear, then give them to Registar to calibrate and combine.

Olly

[Starlight 1]

Thank Olly I did set it to 20% and not putting to much time in , was going to start with 2 panel so by trying 4 will give me a bit more to get my head round, It was very nice  of valiv for his long  post as well.

[Starlight 1]

Well I am starting to work it out. this is 2 of the 4 panels make this 20x120s lum.