Starlight Xpress SXV-M8C - Manual (Page 15)

Handbook for the SXV-M8C Issue 1 Jan 2005
15
imaging. If you use a focal reducer, do not try to use it at maximum reduction, as the
large chip of the SXV-M8C 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-M8C and a 2 arc seconds per pixel resolution, we get
F = 0.00313 * 205920 / 2 = 324mm
For a 200mm SCT, this is an F ratio of 324 / 200 = F1.6, which is only possible with
something like the ‘Fastar’ adaptor. Hence images with normal SCT optics are going
to be considerably ‘oversampled’. This can often be acceptable, but it is clear that the
SXV-M8C is better suited to small refractors and lenses than long focus SCT
telescopes.
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 = 0.00313 * 205920 / 0.25 = 2578mm
This is approximately F13 when used with a 200mm SCT and so we could use the
native F10 focal length with good results. A Barlow lens can be used to increase the
image scale, if necessary.
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_M8C 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
positioned anywhere in the camera field and can be displayed with an adjustable
degree of automatic contrast stretching (for focusing on faint stars). To use this mode,