Guiding is probably the one area of long exposure astrophotography that drives a lot of stress for the astro imager. We can currently do long exposure, deep sky astro photography successfully and relatively cheaply through the use of a guide camera and the associated software using only a basic mount system. There are mounts that are able to track targets for deep sky astro imaging without the use of a guide camera and associated software, but they are quite expensive to acquire and usually well out of the range of the financial resources for a beginning astro imager.
Guiding is used to make corrections to the tracking imperfections inherent in your mount. These errors are low frequency errors in your mount’s gearing system and contain both periodic errors and random errors. Also, as hard as we try we are still not able to get the perfect polar alignment. Then there’s the problem with atmospheric refraction playing tricks by shifting the apparent position of your targets.
The basic idea for guiding is to use a separate imaging sensor in the field of view of the your primary camera’s imaging sensor. The separate guiding camera is used to monitor a star’s movement in your target's field of view and give commands back to the mount to hold that star in the same fixed position on the guide camera’s imaging sensor. If the mount’s polar alignment is solid, the only movement corrections that need to be made are in the axis of right ascension, adjusting the mount’s tracking speed to stay consistent with the earth’s rotation. To prevent problems with slack in the gears or with backlash, the guiding software will usually make all the adjustments in the same direction.
PHD-2 is the standard for guiding software, created by a man named Craig Stark who open sourced the software so it could be used by as many different platforms as possible. It is almost laughably easy to use which is incorporated in the name itself - PHD stands for “Push Here Dummy”. There are a lot of tutorials out there explaining how to use it; the best one I’ve seen comes from a YouTube video on the Star Stuff Channel by Dylan O’Donnell.
The stress I mentioned comes from all the time I spend monitoring the guide graph when I’m executing an imaging session. If your guiding is off, your stars will probably not be the pinpoints of light you need to have and could create problems with your target image data as you're acquiring it that will be difficult to correct. As easy as PHD-2 is to use, here are some suggestions that can set you up for success:
- Read through the manual. This will save you a lot of painful lessons later.
- Use the setup wizard to get your equipment and equipment profile properly set up. If problems develop later on, running the setup wizard will usually help clear these problems.
- Take the time to get your guide scope focused properly. If the stars aren’t focused properly, you will have problems getting a good guiding calibration and subsequently any good guiding.
- Do not allow the program to select the star for you; select your own star. I’ve found that the program almost always selects a sub standard star that will be saturated. When this happens, you’ll wind up with poor stars in your images.
- The default settings are generally pretty good - DO NOT fiddle with them unless you completely understand what you're doing and why you're doing it. Use a 3 second exposure time for sub exposures.
- Make sure you build a solid dark library and rebuild it every few months.