Handbook for the SXVR-H694C
Issue 1 March 2012
13
the distance between the reducer and the camera to optimise the results. 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 SXVR-H694C and a 2 arc seconds per pixel resolution, we get
F = 0.00454 * 205920 / 2
= 467mm
For a 200mm SCT, this is an F ratio of 467 / 200 = F2.34, which is much less than can
be achieved with the Meade converter and appropriate extension tube. However,
moderate deviations from this focal length will not have a drastic effect and so any F
ratio from about F4.5 to F6.3 will give good results. It is clear from this result that the
‘Starizona Hyperstar’ adaptor is very well suited to use with the H694C, as it operates
at around F1.95, so you might be interested in getting one of these.
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 = 0.00545 * 205920 / 0.25 = 4489mm
This is approximately F22 when used with a 200mm SCT and so we will need a 2 or
3x Barlow lens. 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 even 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.
SXV_hmf_usb.exe 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, start up the software and select the SXV camera interface (File menu). Set
the camera mode to Binned 1x1 and select an exposure time of 1 second. Press ‘Take
Picture’ and wait for the image to download. There is a good chance that your
selected star will appear somewhere within the image frame and it should be close to
