Starlight Xpress SXV-H9C - Manual (Page 14)

Handbook for the SXV-H9 Issue 1 June 2002
14
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
imaging. If you use a focal reducer, do not try to use it at maximum reduction, as the
large chip of the SXV-H9C will suffer from considerable ‘vignetting’ (dimming
towards the corners) and this will be difficult to remove from your images. To achieve
this, use only a short extension tube between the focal reducer lens and the camera.
The longer the extension tube used, the greater the focal reduction will be. As a guide,
most CCD astronomers try to maintain an image scale of about 2 arc seconds per
pixel for deep sky images. This matches the telescope resolution to the CCD
resolution and avoids ‘undersampling’ the image, which can result in square stars and
other unwanted effects. To calculate the focal length required for this condition to
exist, you can use the following simple equation:
F = Pixel size * 205920 / Resolution (in arc seconds)
In the case of the SXV-H9C and a 2 arc seconds per pixel resolution, we get
F = 0.00645 * 205920 / 2 = 664mm
For a 200mm SCT, this is an F ratio of 664 / 200 = F3.32, which is easily achieved
with the Meade converter and appropriate extension tube (as supplied with the
converter). Moderate deviations from this focal length will not have a drastic effect
and so any F ratio from about F3.3 to F5 will give good results.
The same equation can be used to calculate the amplification required for good
planetary images. However, in this case, the shorter exposures allow us to assume a
much better telescope resolution and 0.25 arc seconds per pixel is a good value to use.
The calculation now gives the following result:
F = 0.00645 * 205920 / 0.25 = 5354mm
This is approximately F27 when used with a 200mm SCT and so we will need a 2.8x
Barlow lens and the common 3x version will be good enough for all practical
purposes. Barlow lenses are less critical than focal reducers and most types can be
used with good results. However, if you are buying one especially for CCD imaging, I
recommend getting a 3x or 5x amplifier, or the planets will still be rather small in
your images.