Handbook for the TRIUS PRO 694 Issue 19th April 2019
15
more sophisticated software can automatically align planetary images and you may
find these programs (e.g. ‘Registax’) to be very useful.
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Taking and using a flat field:
Flat fields are images that 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. 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
