Handbook for the TRIUS SX-825 Issue 1 January 2015
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unlike some other designs, so it can also be used for tri-colour imaging. If you use a focal reducer,
using it at maximum reduction may cause the relatively large chip of the TRIUS SX-825 to suffer from
considerable ‘vignetting’ (dimming towards the corners) and this will be difficult to remove from your
images. Experiment with the distance between the reducer and the camera to optimise the results. The
longer the extension tube used, the greater the focal reduction will be. As a guide, most CCD
astronomers try to maintain an image scale of about 2 arc seconds per pixel for deep sky images. This
matches the telescope resolution to the CCD resolution and avoids ‘undersampling’ the image, which
can result in square stars and other unwanted effects. To calculate the focal length required for this
condition to exist, you can use the following simple equation:
F = Pixel size * 205920 / Resolution (in arc seconds)
In the case of the TRIUS SX-825 and a 2 arc seconds per pixel resolution, we get
F = 0.00454 * 205920 / 2 = 467mm
For a 200mm SCT, this is an F ratio of 467 / 200 = F2.34, which is much less than can be achieved
with the Meade converter and appropriate extension tube. However, moderate deviations from this
focal length will not have a drastic effect and so any F ratio from about F4.5 to F6.3 will give good
results. It is clear from this result that the ‘Starizona Hyperstar’ adaptor is very well suited to use with
the SX9, as it operates at around F1.95, so you might be interested in getting one of these.
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.
SXV_hmf_usb.exe 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
