Starlight Xpress SXVF-H9 - Manual (Page 10)

Handbook for the SXVF-H9 Issue 1 May 2007
10
In this case, the camera and eyepiece are made par-focal with each other by locking
up the mirror, focusing the camera on an easy object, such as a moderately bright star
and then flipping the mirror down to view the same star with the eyepiece. Once the
eyepiece has been locked into the correct position, you can use it to focus on the
image by lowering the flip mirror and operating the telescope focus wheel until the
image is sharp. When the mirror is raised, the image will fall onto the CCD surface
and should be accurately in focus. Most flip mirror units allow several adjustments to
be made, so that the image can be centred properly in the eyepiece and CCD fields,
which are not necessarily coincident when you first buy your unit!
Opinions vary as to the utility of flip mirrors. They are a convenient way to find and
focus, but they add quite a bit of extra length between the camera and telescope. This
can be very inconvenient with Newtonians, and not a lot better with SCTs, especially
if the assembly is somewhat flexible. They also make it difficult to use a focal reducer
with your camera, as the rapidly converging light cone from a reducer cannot reach all
the way through the flip mirror unit to the CCD surface. If you are using one of the
popular F3.3 compressors for deep sky imaging, you will NOT be able to include a
flip mirror unit in front of your camera and using a par-focal eyepiece is your best
option.
Whichever device you use, it is necessary to set up a good optical match between your
camera and the telescope. Most SCTs have a focal ratio of around F10, which is too
high for most deep sky objects and too low for the planets! This problem is quite easy
to overcome, if you have access to a focal reducer (for deep sky) and a Barlow lens
for planetary work. The Meade F3.3 focal reducer is very useful for CCD imaging and
I can recommend it from personal experience. It does not require a yellow filter for
aberration correction, unlike some other designs, so it can also be used for tri-colour