Handbook for the SXVR-H16
Issue 1 June 2009
9
that any pixels that are to the left of the ‘black’ pointer are set to black and any pixels
to the right of the ‘white’ pointer are set to white. The pixels with values between the
pointers are modified to fit the new brightness distribution. Try experimenting with
the pointer positions until the image has a pleasing brightness and ‘crispness’.
At this point, you will have a working knowledge of how to take and process an
SXVR-H16 image. It is time to move on to astronomical imaging, which has its own,
unique, set of problems!
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Astronomical Imaging with the SXVR-H16
1) Getting the image onto the CCD:
It is essential to set up a good optical match between your H16 and your telescope.
The H16 has a very large CCD area and so many of the popular ‘SCT’ ‘scopes are
unable to provide good quality star images over the large chip. Because of this
limitation, the H16 was designed for use with a wide field highly corrected refractor,
such as the Takahashi FSQ106 or similar, but some flat-field reflectors will be OK. A
particularly good option is the ‘Hyperstar’ adaptor from ‘Starizona’, which works
well with the larger SCT ‘scopes.
As a general guide, most CCD astronomers try to maintain an image scale of about 2
arc seconds per pixel for deep sky images. This matches the telescope resolution to
the CCD resolution and avoids ‘undersampling’ the image, which can result in square
stars and other unwanted effects. To calculate the optimum focal length required for
this condition to exist, you can use the following simple equation:
F = Pixel size * 205920 / Resolution (in arc seconds)
In the case of the SXVR-H16 and a 2 arc seconds per pixel resolution, we get
F = 0.0074 * 205920 / 2
= 761mm
Because of the large CCD size used in the H16, field vignetting and field curvature
will be a problem with many general purpose telescopes. The larger SCTs and many
of the new ‘APO’ refractors will not suffer so badly from this issue, but you may have
to compromise on vignetting and usable field size when imaging with a less highly
corrected instrument. Application of a ‘flat field’ to your images will help to remove
the edge shading, but the star images may well be badly distorted around the
periphery of the image, due to field curvature.
Achieving a good focus:
The SXV_H16 software has a focus routine that will repeatedly download and display
a 100 x 100 pixel segment of the image at a relatively high speed. This focus window
may be positioned anywhere in the camera field and can be displayed with an
