Handbook for the TRIUS SX-825 Issue 1 January 2015
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estimate where the centre of the disk is located. Some 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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Other features of the TRIUS SX-825 hardware and software
‘Slew & Sum’ imaging:
The TRIUS SX-825 can be used in an automatic image-stacking mode, called ‘Slew & Sum’. The
camera is set to take several sequential exposures, which are automatically ‘slewed’ into alignment and
then summed together by the software. This mode can help to overcome a poor RA drive by summing
images that have 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:
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 SX-825 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 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.
