Starlight Xpress SXV-M8C - Manual (Page 20)

Handbook for the SXV-M8C Issue 1 Jan 2005
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(summing 4 images will double the signal-to-noise), but different exposures must be
used. Summing an image with itself will not change the S/N ratio!
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 135 or 200mm F3.5 lens with an infrared blocking filter
and a ‘minus violet’ 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
also try using a light pollution filter to bring out nebulae, reduce light pollution and
sharpen the star images. The Hutech/IDAS P2 type is particularly effective with a
colour camera.
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, if
necessary, by using a Barlow lens. Planetary images are taken in High Resolution
Interlaced mode.
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 judge 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
check the ‘Autosave Image’ checkbox near the bottom of the window. If you now
click on ‘Take Picture’ the automatic sequence will begin and will not stop until you
press a computer key. The images will be saved in FITs format with sequential names
such as ‘Img23, Img24….’ and will be found in the ‘Autosave’ directory (or a sub-
directory of Autosave, set up in the program defaults menu).