Starlight Xpress SXVF-H9 - Manual (Page 11)

Handbook for the SXVF-H9 Issue 1 May 2007
11
imaging. If you use a focal reducer, do not try to use it at maximum reduction, as the
large chip of the SXVF-H9 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 SXVF-H9 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.
Achieving a good focus:
Your starting point will depend on the focus aids, if any, which you are using. With
the par-focal eyepiece, you should slip the eyepiece into the drawtube and focus
visually on a moderately bright star (about 3
rd
magnitude). Now withdraw the
eyepiece and carefully insert the camera nosepiece, until it is bottomed against the
drawtube end, and then lock it in place.
With the flip mirror unit, all that is needed is to swing the mirror down and adjust the
focus until the star is sharply defined and centred in the viewing eyepiece. Now lift
the mirror and you are ready to start imaging.
SXV_H9 has a focus routine that will repeatedly download and display a 128 x 128
pixel segment of the image at relatively high speed. This focus window may be