Handbook for the TRIUS SX-825 Issue 1 January 2015
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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 TRIUS SX-825 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 test pictures have an exposure time of less than about 10 minutes
(as above), then don’t bother with a dark frame, just ‘kill’ any hot pixels with your processing software.
In TRIUS SX-825, the ‘Median filter’ can do this, but other software (e.g. Maxim DL) will provide a
‘hot pixel killer’ that can be mapped to specific locations in the image, or methods such as ‘Sigma
combine’ may be used.
In the unlikely event that you feel that dark frame really is necessary, please proceed as follows:
To take a dark frame, just cover the telescope objective with the lens cap and take another exposure
with the same length as that of the light frame. This image will be a picture of the dark signal generated
during your exposure and it should be saved with your image for use in processing the picture. If many
such darks are recorded and averaged together, the statistical noise will be reduced, but the gains to be
had are rather small compared with the effort involved.
As variations in ambient temperature will affect the dark signal, it is best to take the dark frames within
a few minutes of capturing your images. For the same reason, it is not wise to use ‘old’ dark frames if
you want the best possible results, however, some software allows you to scale library dark frames to
match the image (e.g. AstroArt) and this can be useful as a time saver.
‘Bias frames’ are somewhat more useful than dark frames when using the TRIUS SX-825. A bias
frame is essentially a zero exposure dark frame and records any minor readout defects that the CCD
may possess, so a ‘bias frame subtraction’ can clean up any ‘warm columns’ or shadings that are
created during readout. To record a bias frame, cover the camera aperture and take a 1000
th
of a second
