Handbook for the TRIUS PRO-825C Issue 1 September 2020
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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
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
‘bias offset’ of the camera in each image and this can produce an error in the final
correction. As we are mainly interested in the bias, any very short exposure dark
frame will give a good result. The dark subtracted images should then be averaged
together before use.
