Tutorial - "Deep Sky Stacker" and LRGB imaging

Deepsky Stacker and LRGB imaging tutorial.

This will be a tutorial on how to process LRGB images using only software that is totally free to download from the net. It assumes that you have gained relatively good data (not perfect) in RAW format from your CCD and are now all set to combine everything into a final composite image. I hope ya’ll find it helpful

First of all, let me show you how I set up my directory structure prior to imaging LRGB. I have a separate directory set up for each of the luminance, red, green and blue channels. A separate directory for darks, flats and bias frames, a separate combine directory and a final images directory. I have this directory structure saved somewhere on my laptop so that when I am out in the field I can easily copy, paste and rename this structure to the name of the object I am trying to capture and the date.

So! You have just come in from a great nights imaging and you have all your RAW fits images neatly saved into their proper directories. Opening up Deepsky Stacker (DSS) you notice all the processing functions on the left, together with the preview window and frame list on the right. DSS de-rotates, stacks, subtracts darks and bias frames automatically without any input from you. But in order to get the best results, there a just a couple of tricks that you need to do to improve the final result.

First load your luminance frames together with your darks, flats and bias frames and then “check all”. Go to “register checked pictures” and a window comes up with two tabs “actions” and “advanced”. We will come back to the “advanced” tab later but for now, you need to go into the “actions” tab and un-tick the “automatic detection of hot pixels” and “stack after registering” box as we are not ready to stack just yet. Make sure you tick “register already registered pictures” though and click Ok.

DSS now goes to work for a little while evaluating all your light frames. It counts the number of stars that it finds in each image and assigns each frame a score based on the number of stars it identifies and therefore the quality of each frame. The frame with the best score will be called the reference frame and this is the frame that DSS will use on which to stack all the other frames.

Now in order to get the best final result it is important that you only allow the best frames to be used in the stack. In this regard you need to be totally merciless and get rid of the ones which show trailing, camera shake and are otherwise mediocre in quality. I find that generally frames that have a score of less then half the reference frame should be discarded immediately. Go into “check above the threshold” and enter there, the score that is half the score of the reference frame to get rid of these runty frames.

Once this is done, it’s time to go back into “register checked pictures” again. Go into the “advanced” tab for some further fine-tuning. You now have a window which says “compute the number of detected stars”. Click this button and DSS goes to work for a while before returning with a magic number. What does this number mean? Well this is the number of stars that DSS can identify as being present in all the frames in the stack. What is the best number to have? Well if the number is less then 40 then DSS uses less efficient methods to stack the frames. DSS only uses the best 100 stars to stack anyway so there is no point in having a number greater then this. And also you want to be sure that DSS does not detect a whole load of false stars, hot pixels etc as this will not be good either. So in general you need a number of at least 40 to 50. Adjust the threshold slider so that you get as close as possible to this figure.

Now finally we are ready to stack. Go back to the “action” tab, and re-tick “automatic detection of hot pixels” and “stack after registering”. DSS goes to work again and comes back eventually with your finished luminance channel image. Save in your “combine” directory as l.fits.

Now you process each of your red, green and blue channels separately in the same way saving them in the combine directory as r.fits, g.fits and b.fits ready to combine later on. A trick I use is to make the reference frame from the luminance channel the reference frame in the stack of the R, G and B channels. Once loaded, right click on the luminance frame to force it to be used as the reference frame (an astrix appears to show that this is the case). But we definitely don’t want it to be actually included in the stack so make sure that you untick it on the left hand side before you proceed. Once done save the result as above in the combine directory.

Combine in IRIS

So now you should have four files in your combine directory l.fits, r.fits,g.fits and b.fits. In order to load these files into IRIS you need to rename the extensions of these files to “.fit” file. Once this is done, in IRIS click file> settings and change the working path to your combine directory that contains these four files. Click on the command function (left of the camera symbol) and cut and paste the following lines into the command prompt;

>load l
>coregister l r
>save i1
>coregister l g
>save i2
>coregister l b
>save i3
>load i1
>scalecolor i j 1 3
>trichro j1 j2 j3

Hit return after each line until you come to the scalecolor command. You need to draw a box around a non saturated star and hit return. Then continue to the end hitting return after each line. Repeat the scalecolour command until you are happy with the result and colour and save to the final directory. You should now have your finished LRGB colour image ready for further enhancement in an image processessing software if required.

Making this a Sticky as it WILL prove useful to find in the future for all like-minded enthusiasts. Thanks for this Dave. Keep it updated with any new bits of information will you please?

Seanie.

DSS Trouble Shooting

“I took a number of subs but no matter what I do DSS only stacks a few of them.”

Check the dx, dy and angle columns and see what values DSS has assigned to each sub. If you see ones where NC has been calculated, then for some reason DSS has been unable to work out the de-rotation and stacking values necessary to stack the sub onto the reference frame.

First off look at the quality of the your subs for trailing and focusing as DSS will not register these as stars. You are better off discarding these subs, as they will definitely have an adverse effect on your final image.

The second thing is in order for a sub to be stacked in DSS, there needs to be a minimum of eight stars that also appear in the reference frame. You can click on the red cross on the right of the screen to see all the detected stars on a sub shown by green circles. These stars are detected automatically by DSS you cant add them manually.

It is possible to change the “Raw DPP Settings” and increase the brightness values to enable more stars to be detected by DSS. Open the register settings box, click on RAW DDP settings and enter your new brightness setting there. You must re register your subs again with the new settings to calculate new scores, star detection threshold etc. Now check compute offsets and see if it has had an effect on the dx, dy, and angle columns.

Remember the star detection threshold here!!! It should be no more then about 100 for the best results. A significant value here means that the whole stacking process will take a very long time (possibly crash your pc if it runs out of memory, disk space etc) as each of the detected stars will be traced through each sub to see if there is a stacking alignment and the best 100 stars evaluated.

David

I'm new to Iris and LRGB imaging and I need some further explanation of your excellent tutorial.
In your section on " Combine in IRIS" you indicate to "cut and paste" lines into the command prompt eg Load l, coregister lr etc. this is probably a stupit question but I'm clearly missing something. Where do I cut and paste from?. I would, be grateful if you could be more specific as to exactly how I follow your instruction to cut and paste.

Many Thanks

David

Hi Jasper

You should highlight the following lines below using your mouse.

>load l
>coregister l r
>save i1
>coregister l g
>save i2
>coregister l b
>save i3
>load i1
>scalecolor i j 1 3
>trichro j1 j2 j3

Right click, and click copy.

Open up Iris and click on the comand function which is to the left of the the camera symbol.

A window should then open up for you to paste in these lines of text .

Then, making sure you have changed the working directory in Iris to the directory containing your four final images, hit return after each line.

When you get to the scale command then you need to highlight a white unsaturated star as a reference star before you continue to hit return after each line until the end. If the colours aren't correctly combined then go back to the scale command and pick a different star and repeat.

Hope this helps .

DB

Hi David

I have attempted the above but get the error message "unknown command" after I make the first paste ie load l

David