Starlight Xpress SXV-M25C - Manual (Page 22)

Handbook for the SXV-M25C Issue 1 June 2004
22
Taking pictures of the planets:
The SXV-M25 is not designed as a planetary camera, but it can be used quite
successfully in the ‘sub-frame’ mode. 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 a suitable focal length will be of the order of 5000mm,
or more. An SCT with a 3 or 5x Barlow lens will provide a reasonable image scale for
the planets – any vignetting is unimportant as only the field centre is required.
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 will 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).
The exposure time needed for good planetary images is such that the image histogram
has a peak value at around 200 and does not extend much above 220 (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 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.
Processing a planetary image:
Planetary images have one major advantage over deep sky images, when you come to
process them – they are MUCH brighter, with a correspondingly better signal to noise