Handbook for the SXVR-H18
Issue 1 March 2010
9
As a general 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 optimum 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-H18 and a 2 arc seconds per pixel resolution, we get
F = 0.0054 * 205920 / 2
= 556mm
Because of the large CCD size used in the H18, field vignetting and field curvature
will be a problem with many general purpose telescopes. The larger SCTs and many
of the new ‘APO’ refractors will not suffer so badly from this issue, but you may have
to compromise on vignetting and usable field size when imaging with a less highly
corrected instrument. Application of a ‘flat field’ to your images will help to remove
the edge shading, but the star images may well be badly distorted around the
periphery of the image, due to field curvature.
Achieving a good focus:
The SXV_H18 software has a focus routine that will repeatedly download and display
a 100 x 100 pixel segment of the image at a 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 H18 camera interface (File menu). Set
the camera mode to ‘Bin 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 with luck it should be
close to a sharp focus. If the focus is still poor, then it may appear as a pale disk of
light, sometimes with a dark centre (the secondary mirror shadow in an SCT, or
Newtonian). Now select the ‘File’ menu again and click on ‘Focus frame centre’; you
can now use the mouse pointer to click on the star image and the new focus frame co-
ordinates will be displayed. Now return to the camera interface window and click on
‘Start’ in the Focus frame. The computer will now display a continuous series of 100
x 100 pixel images in the focus window and you should see your selected star appear
somewhere close to the centre. A ‘peak value’ (the value of the brightest pixel) will
also be shown in the adjacent text box and this can be used as an indication of the
focus accuracy. Although the peak value is sensitive to vibration and seeing, it tends
towards a maximum as the focus is optimised. Carefully adjust the focus control on
your telescope until the image is as sharp as possible and the peak value reaches a
maximum. Wait for any vibration to die down before accepting the reading as reliable
and watch out for bursts of bad seeing, which reduce the apparent focus quality. Quite
often, the peak value will increase to the point where it is ‘off scale’ at 4095 and in
this case you must halt the focus sequence and select a shorter exposure if you wish to
use the peak value as an indicator. Once you are happy with the focus quality
achieved, you might like to trim the settings of your par-focal or flip mirror eyepiece
to match the current camera position. Although you can reach a good focus by the
