Starlight Xpress SXVR-H18 - Manual (Page 10)

Handbook for the SXVR-H18 Issue 1 March 2010
10
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 objects. 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, using the ‘Bin 1x1’ mode 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. Now you need a ‘dark frame’, if the best results are to be extracted from
your raw image. To take this, just cover the telescope objective with the lens cap, or
drop the flip mirror to block the light path to the CCD (make sure that this is light
tight), and take another 60 second exposure. 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. The SXVR-H18 generates relatively little dark signal and so
dark frames are not essential for short exposures of less than a few minutes, but it is a
good idea to record at least one for each exposure time used during an imaging
session. As variations in ambient temperature can 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 Maxim DL).
‘Flat fields’ are often recommended for optimising the results from your CCD
camera, but these are generally less important than dark frames, especially if you
make sure that the optical window of the camera is kept dust-free. The purpose of a
flat field is to compensate for uneven illumination and sensitivity of the CCD and it is
better to avoid the need for one by keeping the optics clean and unvignetted. I will