True Technology Custom-Wheel MK III
Applications and Technical data
A great way to use the Stellacam EX deep-sky video camera, is to combine it with a wide version of the True Technology Custom-Wheel (which has a quality front aluminised diagonal mirror in the filter number 1 position), or even with a more basic flip-mirror unit. Not only does this provide you with a 'finder' which is vastly more sensitive than the human eye, but of course will unleash the FULL POWER of your main instrument on to the Stellacam EX, for recording and displaying as normal. In an Observatory scenario, this is an invaluable bonus, providing direct viewing video for all.
The computer controlled filter wheel provides automated LRGB sequence imaging via MaxIm or AstroArt software, using your main cooled deep sky CCD imaging camera. Returning to filter number one position at any time, brings the Stellacam EX immediately into play. Note that light to the main imaging camera is blocked off in the filter number one position, so that while the Stellacam EX is being used, one can take dark frames on the cooled imaging camera. This is another bonus for those cameras which do not possess an integrated shutter.
The image above shows a computerised wide Custom-Wheel, with the Stellacam EX on top, and a Starlight MX916 CCD camera at right. These are attached to a Takahashi Sky 90 refractor.
Upgrades from MK II wheels:
* Double power motor
* Wide Wheels now have stainless steel countersunk screws retaining side panels to bring them into line with the narrow version, which has always had this fixing
* Hand unit now has both True Tech and SBIG+ (pulse mode) control as standard (only cables for SBIG mode are extra)
Positional accuracy of wheel
The two images above may look identical, but they were taken with the combination shown at the top of this page, of a tripod head some 15 metres away. In the first image the wheel was brought to position one in the 'up' direction, ie from the last filter position on the wheel (in this case No 6). In the second image the wheel was brought to position 1 in the 'down' direction, ie from position No 2. When you examine the images with a mouse pointer you will find an error of 25 pixels, or approximately 170 microns on the CCD chip. However:
1. This is the REFLECTION from the diagonal mirror, and thus the error is DOUBLE that of a ray going straight through a filter to an imaging CCD camera mounted in the normal position, and
2. The radius of the curve from the centre of the wheel to the Stellacam chip was 125mm, whereas the radius of the centre of a filter from the centre of the wheel is only 43.5mm.
By simple geometry, the angular diplacement of the centre of a filter is half of 43.5/125 x 170 microns,
= 30 microns. This is less than 5 pixels shift, and represents a backlash on the spring plunger at the wheel edge of 69/43.5 x 30 = 47.5, say 50 microns, or only 1/20'th of a millimetre.
We suggest that this is very good indeed, and in repeated tests with more than one wheel, we got virtually identical results.
In the REAL WORLD, where one is either controlling the wheel from a computer program or manually, any particular filter will ALWAYS be approached from one direction only, so there will be ZERO positional error.