Astrophotography Process

I have had several people ask how these images are created so here is a quick run down of what I currently have to do for every image.

The first step is to pick a target.  To be able to create the best image I need something that will be fairly high in the sky over the 3 hours that I will be imaging.  The atmosphere acts like a prism separating the colours of the target, which impacts the quality of the images taken.  The lower in the sky the target is, the larger the separation.  I also need to find something that will be in view of where I am observing from based on obstructions surrounding were the mount is set up.  Then you need to determine how bright and where the moon is going to be compared to the brightness and location of the target, pick a set of dates that work the best for the image and then hope for good seeing conditions.  Typically I try to have a list of targets that are valid for about 20 - 30 days before they are no longer approriate targets.

When you think a good evening is at hand, the telescope and all optics need to be set outside a minimum of 3 hours before imaging for all of the optics to adjust to the outside temperature.  Temperature differences within the telescope causes air currents that distort the image, and affects the focus as the temperature changes.

When it is still light out you start setting up the equipment.  First is the tripod.  It has a special orientation with one of the legs pointing south so that the mount head is orientated to the north.  I use a phone compass that compensates for the magnetic inclination for a rough alignment.  I then use a polar scope built into the mount to more accurately align the mounts position with polaris.  Then I place vibration absorbers under the feet of the tripod to help stabalize the mount for improved imaging, and adjust the tripod legs until the mount head is showing perfectly level.  Counter balance weights are then added to the mount after which the telescope is then attached.  The mount and the telescope are then adjusted for balance, slightly more to one side than the other to improve image quality by keeping all of the gears under tension.  All of the electonics are then connected and powered up with camera cooling enabled.  This keeps the imaging sensor at the same temperature to improvve image quality.

Once it is dark enough for the cameras to see some stars the focus is then accurately set, as it changes due to temperature and equipment attached.  Once complete the process of accurately polar aligning the mount ensues.  The purpose of this is to ensure as the mount rotates the telescope accurately tracks the stars as the earth rotates under them.  These adjustments are incredibly sensitive and this takes time and patience.  Once alignment is complete it is then necessary to calibarte the guiding camera.  This involves finding an appropriate star close to the meridian (the points in the sky 90 degrees from polaris) and running the calibration procedure that takes several minutes, repeating as necessary.

The chosen target is then selected in the software and the telescope is positioned to have the centre of the image within a few pixels of the centre of the imaging sensor, engaging the autoguiding system that keeps the telescope on target within 1 pixel.  The system takes images as programmed, doing focus checks based on temperature changes and time.  When the images are complete the telescope is positioned to point directly overhead.  A Light panel is then set on the top of the telescope and multiple light frames are taken, followed by turning the light panel off and taking multiple dark flat frames.  These are used in processing the images to remove vignetting and artifacts caused by dust on the lenses, filters or camera sensors.

The equipment is then disassembeld and stored in the reverse order.  In the winter I have had the outside of the tube encrusted in ice, and moisture condenses and freezes to everything when it comes in.  Some items go into plastic bags outside to minimize this.  The telescope itself has to go flat on the floor so that any moisture runs off the mirror instead of ponding on it.  In one case the mirror still had ice on it 6 hours after bring it inside.

The first phase of image processing is calibration  All of the images have known sources of noise removed from them using dark frames taken earlier, with the lightess of each pixel adjusted based on information collected in the flat and dark flat frames taken at the end of the imaging session.  Images are then gone through visually individually, removing any that have satellite trails, shooting stars, airplanes or other anomalies.  The images are then scanned and stastically analyzed for star count and size, eccentricity (roundness of the stars) and other factors.  Any anomalious images are then scanned to determine if they should be removed from the remaining processing.  The remaining images then go through an aligment process where every image is adjusted to match the image determined to be the best from the statistical processing.  These aligned images are then stacked (every pixel is essentially added from every frame and divided by the number of total frames, based on a weighting determined by the statistical values)  This generated single frame becomes the starting point for the remaining processing.  Although this process sounds straight forward, in every processing step there is a multitude of different methods to choose from, each generating slightly different results, so it can be repeated many times to find the one that works best with the collected images.  As I use what is called a one shot color camaer, my stacking process is actually more complicated than described, but the end result is the same.

At this point I also generate a stack which is the maximum over all images rather than the average. This allows me to see any lines, which would represent moving objects in relation to the fixed stars.  There are websites that you can use to determine if any known object would have been in your field of view over the observation time, so if you find something not listed it is new and you can go through a process of reporting it.

This is where the artistic side begins.  You take that final image and run it through a miriad of processes, deciding if any had an effect on improving the image, until you have your final piece of art.  This can take hours or days depending on the subject type, how good the imaging was, and other factors.

You then share and get peoples reactions.