Starlight Xpress Lodestar-C imager/guider - Manual (Page 6)

Lodestar C handbook Issue 1 May 2011
After colour conversion and some power law contrast stretching:
Post processing of your images is necessary, if you are to see the best results. Most astronomical
images have a very wide ‘dynamic range’ (brightness from very dim to very bright in the same image)
and so you may need to compress this range. This is quite easy to do by applying a ‘Power Law
Stretch’ with a power setting of less than 1. Call up ‘Power Law Stretch’ in the ‘Contrast’ menu and set
the slider to 0.3. Apply this and your image will brighten a lot and show much more dim detail. A
‘Normal’ stretch will allow you to restore the background to a darker level and you can apply the
power law again, if necessary. The picture of M42 above was processed in this way.
Guiding with the Lodestar C:
The Lodestar C is generally used with a separate guide telescope, or via an off-axis guider. It is
designed to be inserted into a standard 1.25” focuser assembly, but also has a 25mm ‘C’ type camera
lens thread in the front to allow standard CCTV lenses to be attached. Please note that the chip to lens
distance is only 12.5mm and so a ‘CS’ to ‘C’ extension will be needed to permit a standard C lens to
reach focus. Lenses designed for ‘CS’ mount cameras will focus without an extension tube, but many
of these lenses cannot fully illuminate a half-inch format CCD, so take care when selecting a lens for
this purpose.
A typical set up might consist of an inexpensive 80mm F5 refractor ‘piggybacked’ onto an SCT, with
the Lodestar fitted directly into its focus barrel. Many such telescopes are designed to be used with an
inverting prism ahead of the eyepiece and so the focuser may be too short when the Lodestar is inserted
directly. In this case, an extension tube may often be made from a cheap Barlow lens assembly with the
lens removed. This configuration can be seen in the picture below.