Handbook for the TRIUS SX-825 Issue 1 January 2015
15
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 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 ratio. This means that aggressive
sharpening filters may be used without making the result look very noisy and so some of the effects of
poor seeing can be neutralised.
