Handbook for the SXVR-H16
Issue 1 June 2009
14
adjacent to it. The isolated nature of the warm pixels allows the median filter to work
very well and most imagers will be quite happy with the results of this simple
procedure.
Another excellent way of removing the warm pixels is to generate a ‘hot pixel map’
and apply this to the image. This is not available in the SX software at the time of
writing, but Maxim DL can use this method. The basic procedure is to capture a long
exposure dark frame and then ‘slice off’ all values below a threshold level which is
selected so as to leave only the warm pixels in the resulting image. This ‘map’ is then
applied so that every warm pixel co-ordinate in the map causes that pixel in the image
to be replaced by the median of those pixels around it. The result is similar to that of
using a median filter, but only the warm pixels are modified – the rest of the pixels are
unaffected.
Yet another option is to sum several images that have a small degree of ‘dither’ in
their star alignment. If the images are realigned on the stars for summing, the warm
pixels will not now be aligned and will average down to a lower level. If the summing
is done by using a ‘statistical’ summing technique, such as median or sigma
combining, then the warm pixels will be removed altogether. This method needs more
powerful image processing than is available in SXV_Hmf, but both AstroArt and
Maxim DL can do it.
The result of applying a median filter and contrast stretch to the raw image
Another thing to try is the summing several images for a better signal to noise ratio.
Summing can be done in the ‘Merge’ menu and involves loading the first (finished)
image, selecting a reference point (a star) then loading the second image and finding
the same star with the mouse. Once the reference is selected, you can either add
directly, or average the images together. Averaging is generally better, as you are less
