Handbook for the TRIUS PRO-694C Issue 1 September 2020
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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.
After the above procedures have been executed, the flat field will be ready for use.
Load up your image for processing, subtract the dark frame and then select ‘Apply
flat field’ in the ‘Merge’ menu. The result should be an image with very few signs of
the original artefacts and you can then process it in the normal way.
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The TRIUS PRO-694C guider port
The TRIUS PRO-694C is provided with a guide port for use with ST4 compatible
mounts. The Autoguider output port is a 6 way RJ11 socket, which is compatible with
the standard autoguider input of most telescope mounts. It provides 4 active-low opto-
isolator outputs and a common return line, capable of sinking a minimum of 5mA per
output. This socket may be used for telescope control if the TRIUS PRO-694C is
employed as an autoguider.
