Starlight Xpress SXV-M25C - Manual (Page 15)

Handbook for the SXV-M25C Issue 1 June 2004
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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. 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 have determined the position of best focus, you might like to
mark the drawtube barrel so that the correct setting can be found quickly in future.
Although you can reach a good focus by the above method, many observers prefer to
use additional aids, such as Hartmann masks (an objective cover with two or three
spaced holes) or diffraction bars (narrow parallel rods across the telescope aperture).
These make the point of precise focus easier to determine by creating ‘double images’
or bright diffraction spikes around stars, which merge at the setting of exact focus.
The 12-16 bit slider control allows you to adjust the contrast of the focus frame for
best visibility of the star image. It defaults to maximum stretch (12 bits), which is
generally ideal for stars, but a lower stretch value is better for focusing on planets.
Taking your first astronomical image:
I will assume that you are now set up with a focused camera attached to a telescope
with an operating sidereal drive. If so, you are now in a position to take a moderately
long exposure of some interesting deep-sky astronomical object. As most drives are
not very accurate beyond a minute or two of exposure time, I suggest that you find a
fairly bright object to image, such as M42, M13, M27 or M57. There are many others
to choose from, but these are good examples.
Use the finder to align on your chosen object and then centre accurately by using the
focus frame and a short exposure of between 1 and 5 seconds. The ’12-16 bit’ slider
in the focus frame allows you to adjust the image contrast if you find that the object is
too faint with a short exposure. Once properly centred and focused, take an exposure
of about 60 seconds, and observe the result. Initially, the image may appear rather
barren and show only a few stars, however, there is a great deal of data hidden from
view. You can get to see a lot of this, without affecting the image data, if you go to
the ‘View’ menu and select ‘Auto Contrast Stretch Image’. The faint image data will
then appear in considerable detail and I think that you will be impressed by the result!
If you are happy with the image, go to the ‘File’ menu and save it as a Tiff file in a
convenient directory.