Starlight Xpress SXVR-H16 - Manual (Page 10)

Handbook for the SXVR-H16 Issue 1 June 2009
10
adjustable degree of automatic contrast stretching (for focusing on faint stars). To use
this mode, start up the software and select the H16 camera interface (File menu). Set
the camera mode to ‘Bin 1x1’ 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 with luck it should be
close to a sharp focus. If the focus is still poor, then it may appear as a pale disk of
light, sometimes 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 100
x 100 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 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 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’.