Handbook for the TRIUS PRO-825C Issue 1 September 2020
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128 x 128 pixel segment of the image at relatively high speed. This focus window
may be positioned anywhere in the camera field and can be displayed with an
adjustable degree of automatic contrast stretching (for focusing on faint stars). To
use this mode, start up the software and select the SXV camera interface (File
menu). Set the camera mode to Binned 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 it should be
close to a sharp focus. If the focus is still poor, then it may appear as a pale disk of
light, often 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
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 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.
