Starlight Xpress SXV-M8C - Manual (Page 27)

Handbook for the SXV-M8C Issue 1 Jan 2005
27
its resolution. However, generally, these are more useful for finding faint objects, than
for imaging, as the colour information is lost in all these modes.
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 of the telescope field and good colour matching of
the light source to the sky light. 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 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