My usual astrophotographic process
- I connect my Canon DSLR camera to a Windows laptop running AstroPhotography Tool (APT, obtainable for a very small price from www.astroplace.net). I set the camera to bulb (B) mode. APT connects to the camera and fully controls it.
- Having pointed the camera/telescope to my required target, in APT I use live mode, zoomed in, and then the focussing aid to manually focus on a star in the field. The following image shows what this looks like.
The idea is to minimise the FWHM value (Full Width at Half Maximum of the profile) and that is quite easy to do.
- I use short enough multiple exposures that drive errors will not be a problem in the final combined image (GRIP later gives me a record of the drive errors between frames so I can check). APT captures the RAW images directly to the PC in a directory named for the date (eg, 2015_10_24). After one of my observing sessions there will typically be several hundred RAW (.cr2) files of about 30 Mbytes each. All subsequent steps are done indoors the next day.
- (If jrawio were up to date, so GRIP could read the RAW files from my newish type of camera directly, I would not need this step.) I open the new directory in a free application that comes with Canon cameras: DPP (Digital Photo Professional). I select all the thumbnails and batch process them to convert them to 16-bit TIFF format at 300dpi, without embedded colour profiles (so they will be simple RGB images). This step takes getting on for an hour on my 4-processor Windows 7 (64-bit) laptop. I set DPP to put the files in a subdirectory called TIFF.
- I step through all of the images in DPP (set to show thumbnail and summary details for each image). This enables me to do 2 things. First I can find which frames are the beginning and end of a multi-exposure sequence that will be combined to make one photo. Secondly, it enables me to identify certain frames that should be omitted from the combination process for various reasons. Typical reasons include: obvious meteor or satellite tracks, blurring due to telescope movement (eg, on a windy night), fleeting cloud fragments (often seen as brighter than usual background). I record frame numbers and relevant metadata (exposure settings, start and end times) in a spreadsheet so I can label the photos accurately on my web site after processing. An extract from the spreadsheet is as follows.
- I move each multi-exposure set of TIFF files, apart from any visually rejected ones, into a subdirectory of its own, named after the subject (eg, M45 or CasAlpha). This makes for easier management for the subsequent steps and for archiving later.
- I run my own GRIP program. In the batch menu I select "Astro combine into 1 image" (or sometimes the comet option). From the dialogue I use the "Browse" button to select the subdirectory containing the set of TIFF files, Ctrl-A to select all of them and proceed. The "Proceed" button of the main dialogue then starts the process. It takes about a minute per frame to combine them all into one, at the end of which a FITS file is saved, 32 bits per colour channel.
- At this stage I can delete the input TIFF files and just keep the result file (.fits) in the subdirectory. I will later archive the RAW files but there is no point in also archiving the much larger TIFF versions of the same images (130 Mbytes each).
- If the purpose of the photo is to do photometry then that's it. We must not alter the contrast in any way before opening the FITS image in GRIP again to estimate magnitudes. However, if we want the best presentation of deep-sky nebulae then there is more processing to be done, as in the following steps.
- From the Levels menu of the image window select the Curves option (keyboard Ctrl+M). This creates a window showing a histogram of the contents of the 32-bits-per-channel image. It is usually possible to see quite clearly where the background peak is in each channel (red, green, blue). A button is available for neutralising the background (because light pollution is likely to give it a colour cast, typically brown). Then a curve (well, straight-line segments) can be set to maximise the contrast just above the background level, where our faint deep sky objects are likely to lie. Save the result as a new FITS file (keep the original result from stacking, give the new file name a suffix so it ends _crv.fits).
- There are other possible processing options in GRIP, while the image is still 32 bits per channel, such as colour saturation. Then on the Levels menu convert to 16 bits per channel and save the result as a TIFF file - Photoshop can then read it.
- Open the new TIFF file in Photoshop.
- On the filter menu select Sharpen/Unsharp mask. In most cases select the largest possible radius (250px in Photoshop CS4) and experiment with the percentage somewhere around 150%. The result of this operation is often quite dramatic. NB: Do not use the other end of the scale, with just a few pixels radius, as you would for everyday (non-astro) photos.
- That is usually enough. Save the result as another TIFF file, in case a print (at 300dpi) is ever required. Also convert to 8 bits per channel, crop/resize to something smaller at 72dpi, and save as JPEG for use on the web.