Handbook for the TRIUS PRO-694C Issue 1 September 2020
15
The same equation can be used to calculate the amplification required for good
planetary images. However, in this case, the shorter exposures allow us to assume a
much better telescope resolution and 0.25 arc seconds per pixel is a good value to
use. The calculation now gives the following result:
F = 0.00545 * 205920 / 0.25 = 4489mm
This is approximately F22 when used with a 200mm SCT and so we will need a 2 or
3x Barlow lens. Barlow lenses are less critical than focal reducers and most types can
be used with good results. However, if you are buying one especially for CCD
imaging, I recommend getting a 3x or even 5x amplifier, or the planets will still be
rather small in your images.
Achieving a good focus:
Your starting point will depend on the focus aids, if any, which you are using. With
the par-focal eyepiece, you should slip the eyepiece into the drawtube and focus
visually on a moderately bright star (about 3
rd
magnitude). Now withdraw the
eyepiece and carefully insert the camera nosepiece, until it is bottomed against the
drawtube end, and then lock it in place.
Starlight Vision has a focus routine that will repeatedly download and display a 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
