Starlight Xpress SXVR-H9C - Manual (Page 25)

Handbook for the SXVR-H9C Issue 1 June 2009
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telescope to provide an evenly illuminated surface. These can work well, but I prefer a
simpler method, as follows:
Most imaging sessions begin or end in twilight and so the dusk or dawn sky can
provide a distributed source of light for a flat field. However, using the sky directly is
likely to result in recording many unwanted stars, or patches of cloud etc., so a
diffuser needs to be added to the telescope. An ideal material is Mylar plastic drafting
film, obtained from an office supplies warehouse. It is strong and water resistant and
can be easily replaced if damaged. Stretch a piece of the film loosely across the
aperture of your telescope and point the instrument high in the sky, to avoid any
gradient in the light near the horizon. Now take several images with exposure times
adjusted to give a bright, but not overloaded, picture. A histogram peaking at around
128 is ideal. Averaging flat fields together is a good way to reduce their noise
contribution and so recording 4, or more, images is a good idea.
To use your flat fields, they must first have a dark frame subtracted. Although this
may appear to be unimportant with such brightly lit and short exposures, there is the
‘bias offset’ of the camera in each image and this can produce an error in the final
correction. As we are mainly interested in the bias, any very short exposure dark
frame will give a good result. The dark subtracted images should then be averaged
together before use.
After the above procedures have been executed, the flat field will be ready for use.
Load up your image for processing, subtract the dark frame and then select ‘Apply
flat field’ in the ‘Merge’ menu. The result should be an image with very few signs of
the original artefacts and you can then process it in the normal way.
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The SXVR-H9C accessory ports