Handbook for the TRIUS PRO-694C Issue 1 September 2020
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exposure times shorter than the drive error period. The resulting image has more noise
than a single exposure of the same total length, but this method of imaging is still an
effective way of making long exposures without a guider.
To take an S&S image, go to the camera interface window and select an exposure
time for one image of the sequence. Do not use a very short exposure time, as the
read-out noise will become dominant. About 30 seconds is a reasonable minimum.
Now go to the ‘Multiple Exposure Options’ and select a number of exposures to take.
You can also select to average the images, rather than adding them, and there is a
‘Alternative Slew Mode’ available, which uses the correlation of image areas, rather
than a single star. This mode can be better in dense star fields.
Another option is ‘Auto remove dark frame’. This is advisable with S&S images, as
the slewing will mis-register the images with a single dark frame that is applied to the
finished sequence. To use this option, you will need a dark frame, taken with the same
exposure time as a single image from the sequence. This is stored on drive C with the
name ‘dark.def’
Now click on ‘Take Picture’ and the sequence will begin.
Using the ‘Binned’ modes:
N.B. Binning will destroy any colour data – do not use binning for images that
you want to convert to true colour!
Up to this point, I have assumed that the full resolution, imaging mode is being used.
This is fine for most purposes, but it will often provide more resolution than the
optical system, or the seeing, allows. ‘Binned 2x2’ mode sums groups of 4 pixels into
one output pixel, thus creating a pixel image with 4 times the effective sensitivity.
Using 2x2 binning, you can considerably improve the sensitivity of the TRIUS PRO-
694C without losing a great deal of resolving power, so you may like to use this mode
for many faint deep-sky objects. Other binning modes (3x3 and 4x4) are available and
will further increase the image brightness and reduce its resolution. However,
generally, these are more useful for finding faint objects, than for imaging.
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
