Starlight Xpress SXVF-H16 - Manual (Page 17)

Handbook for the SXVF-H16 Issue 1 August 2006
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As a finishing touch, the application of a Median filter or a Weighted Mean Low Pass
filter can be useful to smooth out the high frequency noise after a strong Unsharp
Mask.
As with deep-sky images, it is advantageous to sum planetary images together to
improve the signal to noise ratio. In this case, the ‘averaging’ option should always be
used, or the result is likely to exceed the dynamic range of the software and saturate
the highlights. Aligning the images is always something of a problem, as there are
rarely any stars to use when imaging the planets, but Jupiter’s satellites can be useful
reference points. Otherwise, you will have to find a well-defined feature on the planet,
or estimate where the centre of the disk is located. Some more sophisticated software
can automatically align many planetary images and I recommend ‘Registax’ for this.
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Other features of SXV_H16
‘Slew & Sum’ imaging:
The SXVF-H16 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.
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.
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