Starlight Xpress SXV-M7C - Manual (Page 13)

Handbook for SXV-M7C Issue 1 June 2004
13
For a 200mm SCT, this is an F ratio of 844 / 200 = F4.22, which is easily achieved
with the Meade converter and appropriate extension tube (as supplied with the
converter). Moderate deviations from this focal length will not have a drastic effect
and so any F ratio from about F3.3 to F6 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 = 0.0082 * 205920 / 0.25 = 6754mm
This is approximately F34 when used with a 200mm SCT and so we will need a 3.4 x
Barlow lens. Such lenses are not available, but the common 3x version will be good
enough for all practical purposes.
An accessory that you will find valuable is the ‘M42 to T’ adaptor. These are short
tubes that carry an external M42 thread at one end and an internal ‘T’ thread at the
other end, and are available from most photographic supply shops. The T thread is the
same diameter as the M42 (42mm x 1mm) thread but has a pitch of 0.75mm and is
used for many astronomical accessories, such as telecompressors. An M42 to T
adaptor will allow you to easily interface with virtually any device in the astronomical
catalogue.
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.
STAR_M7C has a focus routine that will repeatedly download and display a 150 x
150 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 MX camera interface (File menu). Set the camera
mode to ‘Fast’ 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
now display a continuous series of 150 x 150 pixel images in the focus window and
you should see your selected star appear somewhere close to the centre. A ‘peak