Handbook for the TRIUS PRO-825C Issue 1 September 2020
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Summing can be done in the ‘Merge’ menu and involves loading the first processed
image, selecting a reference point (a star) then loading the second image and finding
the same star with the mouse. Once the reference is selected, you can either add
directly, or average the images together. Averaging is generally better, as you are
less likely to saturate the highlights of the picture. The signal-to-noise ratio will
improve at a rate proportional to the square root of the number of summations
(summing 4 images will double the signal-to-noise), but
different
exposures must be
used. Summing an image with itself will not improve the S/N ratio. Also note that
you cannot sum images before colour conversion, or the colour data will be
destroyed!
Although I have concentrated on the use of a telescope for deep-sky imaging, do
not forget that you have the option of using an ordinary camera lens for impressive
wide-field shots! A good quality 200mm F3.5 lens with an infrared blocking filter
will yield very nice images of large objects, such as M31, M42, M45 etc. If you
cannot obtain a large IR blocker for the front of the lens, it is quite acceptable to
place a small one behind the lens, inside the adaptor tube. You can even try using a
hydrogen-alpha filter to bring out nebulae, reduce light pollution and sharpen the
star images to pin- points.
Taking pictures of the planets:
Planetary imaging is in many ways quite different from deep sky imaging. Most
deep sky objects are faint and relatively large, so a short focal length and a long
exposure are needed, while planets are bright and very small, needing long focal
lengths and short exposures. High resolution is critical to achieving good results
and I have already shown how a suitable focal length can be calculated and
produced, using a Barlow lens.
Many camera users comment on the difficulty of finding the correct focus when
taking pictures of Jupiter etc. This is usually due to poor seeing conditions, which are
only too common, but may also be due in part to poor collimation of your telescope.
Please ensure that the optics are properly aligned as shown by star testing, or by
using one of the patent collimation aids that are widely available. It is also better to
use a star for initial focusing, as planetary detail is difficult to judge in bad seeing.
Although the star will also suffer from blurring, the eye can more easily gauge when
the most compact blur has been achieved!
You could begin by imaging lunar craters, or the planets, Jupiter, Saturn or Mars. The
rapid variations of seeing which accompany planetary imaging, will ruin the
definition of about 95% of your images and so I recommend setting the camera to
run in ‘Autosave’ mode. This will automatically take a sequence of images and save
them with sequential file names in your ‘Autosave’ directory. Dozens of images will
be saved, but only one or two will be satisfactory for further processing. The
‘Subframe’ mode of the SXV may be found useful for limiting the wasted area and
reducing the download time of small planetary images.
To start the Autosave process, call up the SXV Camera Interface and select the
‘Continuous Mode’ check box at the top (make sure the rest are unchecked). Now
