Starlight Xpress SXVR-H9C - Manual (Page 14)

Handbook for the SXVR-H9C Issue 1 June 2009
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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 are happy with the focus quality
achieved, you might like to trim the settings of your par-focal or flip mirror eyepiece
to match the current camera position.
Although you can reach a good focus by the above method, many observers prefer to
use additional aids, such as Hartmann or Bahnitov masks (an objective cover with
several 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 (I will deal with
planets later). 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 in a convenient
directory.
Most competitive brands of CCD camera require a ‘dark frame’ to be subtracted from
your images to achieve the best results. A dark frame is simply a picture which was
taken with the same exposure as your ‘light frame’, but with the telescope objective
covered, so that no light can enter. It records only the ‘hot pixels’ and thermal
gradients of your CCD, so that these defects are largely removed when the dark frame
is subtracted from the light frame. The SXVR-H9C CCD is quite different from those
used in other brands of camera and generates an extremely low level of dark noise.
Indeed, it is so low that subtracting a dark frame can actually INCREASE the noise in
your images! This is because the statistical noise of the dark frame can exceed the
‘pattern noise’ from warm pixels and hence add to that of the subtracted result. If your