Starlight Xpress SXVR-M25C - Manual (Page 13)

Handbook for the SXVR-M25C Issue 1 June 2009
13
In the case of the SXVR-M25C and a 2 arc seconds per pixel resolution, we get
F = 0.0074 * 205920 / 2 = 664mm
For a 200mm SCT, this is an F ratio of 664 / 200 = F3.32, which is rather 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.
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 = 5354mm
This is approximately F27 when used with a 200mm SCT and so we will need a 2.8x
Barlow lens and the common 3x version will be good enough for all practical
purposes. 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 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_M25C_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
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 computer will now display a continuous series of 128 x 128 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.