Starlight Xpress SXV-M25C - Manual (Page 17)

Handbook for the SXV-M25C Issue 1 June 2004
17
‘Flat fields’ are often recommended for optimising the results from your CCD
camera. The purpose of a flat field is to compensate for uneven illumination and
sensitivity of the CCD. In the case of the very large chip of the SXV-M25, it is likely
that you will need to use flats to eliminate optical vignetting and so it is a good idea to
get used to taking and using them. There are several ways that a flat field can be
recorded, but it surprisingly difficult to generate a ‘perfect’ flat field image. In
essence, you need to take a picture of an evenly illuminated surface so that the
resulting picture contains only the dust shadows and optical shading of the CCD field.
This can then be divided into the image data and used to cancel out the defects.
Achieving the even illumination necessary for a good flat field is quite difficult. Some
users construct a special ‘light box’ to drop over the objective of the ‘scope and this is
probably the best method, but some easier techniques will work. One common
method is to stretch a white fabric sheet across the telescope aperture and take an
image of the daylight sky with this in place. A white ‘Tee shirt’ is the classic item for
use as a diffuser and so such a flat is often known as a ‘Tee shirt flat’.
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.
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.
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. A Tee shirt can be used, but 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.