It is sometimes the case that when filters are changed between imaging different channels, that there is some slight misalignment between channels. This could be because the scope may need to be refocused when the new filter is put in place and may have to be slewed to a bright star to do Bahtinov mask focusing if the filters are not exactly parfocal. It could be that if a manual filter wheel is used, or filters exchanged in a single filter holder, that the act of changing the filter could introduce some slight movement that can result in misalignment of the colour channels.
Whilst techniques such as the fully automatic locating and centering of an object in AstroDMx Capture will minimise misalignment after changing filters, it may not completely eliminate misalignment.
Using Deep Sky Stacker (DSS) it is possible to stack all three colour channels separately and also achieve perfect co-alignment of the three colour channels stacked images. This then facilitates the RGB composition of a colour image from the three monochrome stacked images.
This procedure applies equally to narrowband imaging as well as RGB imaging.
Screenshot of Deep Sky Stacker in action. The panel at the left hand side contains most of the actions described in this article
Method
Start with the monochrome images taken through the RED filter
Load the Red Picture files into DSS
Load the Dark files
Load the Flat files
Click Check all
Click Register Checked pictures
Make sure that Stack after registering is UNCHECKED
Using the Advanced tab, adjust the Star detection threshold so that about 100 stars are detected
Click OK and the images will be registered
Click Compute offsets
Click Stack checked pictures and then click OK
The images will be calibrated, stacked and the resulting stack saved as a 32 bit file called autosave.fit or autosave.tif depending on what format of images is being stacked. (this can also be specified).
If you look at the folders containing the Darks and the Flats you will find a file called MasterDark… and a file called MasterFlat… These will be needed later
It will facilitate matters if the autosave.tif is copied to the desktop at this stage and renamed to something like REDreference.tif
Restart DSS
Load the Green Picture files
Load the MasterDark file
Load the MasterFlat file
Click Check all
Click on the Load Picture files again
This time navigate to the REDreference.tif file on the Desktop and load the file. It will go to the bottom of the file list in DSS.
Right click on the REDreference.tif file and select Use as reference frame. Make sure that this reference frame is UNCHECKED
Click Register Checked pictures
Make sure that Stack after registering is UNCHECKED
The checked images will be registered
Check the reference frame and click Compute offsets.
This will compute all of the information to co-align the Green files with the Red reference file.
UNCHECK the reference frame
Click Stack checked pictures and then click OK
The images will be calibrated and stacked and the autosave.tif file for the Green filtered images will be saved to the Desktop.
Rename it to something like Greenstack.tif
Restart DSS and repeat the procedure exactly but this time with the Blue Picture files.
On the desktop you will now have three monochrome 32 bit stack files. One for each of the colour channels which are perfectly co-aligned.
These three files should be stretched with Curves so that they have a similar brightness.
These can be rgb-composed into a colour image which can then be further processed using a program such as the Gimp.
