Fine focusing of a CCD camera is perhaps the single most important aspect of the entire imaging process. This is true for any type of photography, and should be considered the hallmark of a great image. I have covered my personal angles on focusing elsewhere. Here I will touch on them again briefly, and discuss some other tips I have learned.
Focusing at f/1.95 is easier than at f/10. Mirror shift (the propensity for the primary mirror to "wobble" slightly when the focus knob is turned) is less of a problem as the star images will appear to move less. Also, on a related note, collimation of the secondary is of no concern since it is not used in the Fastar mode. Some Fastar optical tube assemblies (FOTA's) could suffer from defects in primary mirror alignment, and if this is a problem service may be required. There is no provision for adjusting the primary mirror in the field. Occasionally, if your FOTA is stored in a horizontal state for extended periods of time, the mirror may shift slightly. I have noticed sometimes my images appear to be out of focus along one edge of the field after parking the scope for long periods. This can be eliminated by cycling the focus travel before beginning fine focus. Also, storing the FOTA in a vertical position may help.
Focusing is most easily performed using the CCDOPS peak method as described in the camera software manual. Basically, this method entails picking a medium brightness star, entering focus mode, and adjusting the focus until the level of light falling on the CCD chip 'peaks' at a particular value. This can take some time, and the value will constantly change. What you are looking for is a feel for the approximate peak value. When you are satisfied you have the peak, focus has been attained. This should be repeated whenever you think focus is not perfect, after moving the telescope to a radically different angle than before, or for every three degree change in ambient temperature. This temperature change can cause the tube itself to shrink or expand thus changing focus. It is also a good idea to enter critical focus by turning the focus knob counter clockwise. This helps to "load" the primary mirror by removing slack in the mechanism. Again, focus is critical!
One of the best accessories for helping with focus
is a motorized digital focus micrometer. They
are almost a necessity. These come in various designs, and allow
one to focus the FOTA by remote. This has the advantage of eliminating
vibrations from touching the focus knob with your hands. Also, you
have to be able to view the computer screen during focus procedure. This
is more easily accomplished sitting at the computer and using a hand controller.
My version was custom made by Jim Riffle of AstroWorks and features a digital
micrometer. A clutch mechanism allows manual focusing without having
to remove a motor or boot mechanism. Other versions are available
from companies such as JMI. They either attach to the existing focus
knob (as in the case of the JMI) or replace the knob altogether.
A hand controller is used to adjust focus. Some versions, like mine,
feature a digital readout. This can be helpful for returning to a
particular point of focus, and is especially useful for producing a table
of focus based upon what Fastar configuration you may be using (i.e., f/10,
f/6.3, etc.). I have received numerous email messages from imagers
distraught over focusing problems. Micrometers can help reduce many
of these woes, as will a bit of patience.