Starlight Xpress SXVF-H16 - Manual (Page 10)

Handbook for the SXVF-H16 Issue 1 August 2006
10
In the case of the SXVF-H16 and a 2 arc seconds per pixel resolution, we get
F = 0.0074 * 205920 / 2 = 762mm
For a 200mm SCT, this is an F ratio of 762 / 200 = F3.8, which is too short to be
achieved with the Meade F6.3 converter, but a slightly longer focal length will not be
a problem. Any F ratio from about F4 to F6 will give good results and you might try
experimenting with the camera to reducer spacing to optimise the performance.
Because of the large CCD size used in the H16, field vignetting will be a problem
with many ‘scopes when used with a converter. The larger SCTs and many of the new
‘APO’ refractors will not suffer from this issue, but you may have to compromise on
vignetting when using a small SCT. Application of a ‘flat field’ to your images will
help to remove the edge shading, but the star images may well be distorted around the
periphery of the image, due to field curvature.
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 = 0.0074 * 205920 / 0.25 = 6088mm
This is approximately F30 when used with a 200mm SCT and so we will need a 3 x
Barlow lens to get a reasonable size of planetary image.
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 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_H16 has a focus routine that will repeatedly download and display a 100 x 100
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 H16 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 it should be close to a sharp focus. If
the focus is still poor, then it may appear as a pale disk of light 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