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Handbook for SXV-M5C Issue 1 August 2004
19
read-out noise will become dominant. About 30 seconds is a reasonable minimum.
Now go to the ‘Multiple Exposure Options’ and select a number of exposures to take.
You can also select to average the images, rather than adding them, and there is a
‘Alternative Slew Mode’ available, which uses the correlation of image areas, rather
than a single star. This mode can be better in dense star fields.
Another option is ‘Auto remove dark frame’. This is advisable with S&S images, as
the slewing will mis-register the images with a single dark frame that is applied to the
finished sequence. To use this option, you will need a dark frame, taken with the same
exposure time as a single image from the sequence. This is stored on drive C with the
name ‘dark.def’
Now click on ‘Take Photo’ and the sequence will begin.
Taking and using a flat field:
Flat fields are images, which display only the variations of illumination and
sensitivity of the CCD and are used to mathematically modify a wanted image in such
a way that the errors are removed. Common flat field errors are due to dust motes on
the camera window and vignetting effects in the optical system of the telescope. Dust
motes act as ‘inverse pinholes’ and cast out-of-focus images of the telescope aperture
onto the CCD chip, where they appear as shadow ‘do-nuts’. Most optical systems
show some vignetting at the edges of the field, especially when focal reducers are
used. This causes a brighter centre to show in images, especially when there is a lot of
sky light to illuminate the field.
If dust motes are your main problem, it is best to clean the camera window, rather
than to rely on a flat field to remove the do-nuts. Flat fields always increase the noise
in an image and so physical dust removal is the best option. If you have serious
vignetting, first check whether the optical system can be improved. The most likely
cause of this problem is trying to use too powerful a degree of optical compression
with a focal reducer and you might want to try moving the camera closer to the
reducer lens.
If you really do need to use a flat field for image correction, then it must be taken with
care. It is most important that the optical system MUST NOT be disturbed between
taking your original images and taking the flat field. Any relative changes of focus
and rotation etc. will upset the match between flat field and image and the result will
be poor correction of the errors. The other necessity for recording a good flat field is a
source of very even illumination for the telescope field. This is surprisingly difficult
to achieve and many designs of light source have appeared in the literature and on the
Web. These usually consist of a large wooden box, containing several lamps and an
internal coating of matt white paint, which is placed over the objective of the
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
draughting 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