Starlight Xpress SXVF-H16 - Manual (Page 15)

Handbook for the SXVF-H16 Issue 1 August 2006
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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 often quite acceptable to place a small
one behind the lens, inside the adaptor tube.
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 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, but the colour content is low and so I
recommend 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.
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).
The exposure time needed for good planetary images is such that the image histogram
has a peak value at around 127 and does not extend much above 200 (Ignore the
major peak near zero, due to the dark background). If you use too short an exposure
time, the image noise level will be increased, and if too long a time is used you will
saturate the highlights and cause white patches on the decoded image. With the
recommended focal length, Jupiter and Mars will both need an exposure time of
between 0.1 and 1 seconds and Saturn will need between 0.5 and 2 seconds.