Starlight Xpress SXV-M25C - Manual (Page 14)

Handbook for the SXV-M25C Issue 1 June 2004
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filter for the reduction of light pollution and aberration effects and it is a valuable
accessory to any imaging system.
It is fairly easy to find the correct focus setting for the camera when using a standard
SLR lens, but quite a different matter when the SXV-M25C is attached to a telescope!
The problem is that most telescopes have a large range of focus adjustment and the
CCD needs to be quite close to the correct position before you can discern details well
enough to optimise the focus setting. Fortunately, the relatively short focal length and
wide field of view associated with suitable telescopes for the M25, makes this easier
than with longer focus systems and smaller CCD chips. Attach the M25 to your
optical system with either a T thread adaptor or by clamping to the 2 inch diameter
nose piece provided with the camera. Any filters should be at least 48mm in diameter
and mounted just ahead of the camera (or on the objective, where appropriate).
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-M25C and a 2 arc seconds per pixel resolution, we get
F = 0.0074 * 205920 / 2 = 761mm
This is a very rough guide and so the 500mm FSQ is well suited, as are most other
short focus refractors.
Achieving a good focus:
Point the telescope at a bright star and focus the optics to an estimate of the correct
focus position. A good way to determine this is to space a sheet of frosted glass,
plastic or grease-proofed paper about 18mm behind the drawtube aperture and focus
for a sharp star image on the screen. Substituting the camera for the screen will put a
fairly well focused star image onto the CCD.
The SXV_M25C 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 4x4 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, often 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