[avandonk]

You are correct Chris the image train flexes differently depending on where the mount is pointed. The wood frame has eliminated about 80% of this but flexes itself in certain orientations. The aluminium version with adjustable nylon flat pads on all four sides of the camera should fix this.

A solid bracket that holds the filter wheel is also on the drawing board. It will attach from the three threaded holes in the CFW (see picture) to the top of the camera holder. The difference in focus positions due to filters is less than 100 micron so this will not cause any problems.

I am sure this problem of image train flexure occurs in all systems but only becomes obvious with fast systems. The Atlas focuser weighs 1.3kg (3lb), the filter wheel weighs 3.1 kg (7lb) and the PL16803 weighs 3.0 kg (6.7lb). For a total of more than 8kg (18lb) with adapter.

My best guess is I have flexure in the mounting plate of the RH200 and in the filter wheel interfaces. I have measured this with a dial indicator and it smoothly varies by up to 0.5 to 0.7 mm at the sensor position depending on mount orientation. This corresponds to 77 pixels movement vertically from the dovetail! There is also an orthogonal movement to this or laterally on the sensor. In the real world there is no such thing as a rigid body.

This is why so many swear by the use of OAG's rather than put up with the so called flexure of their guidescope. The flexure is in their image train!

The thingy image does not show this as Alpha Cent was at the zenith and the geometry did not change much during the exposure run. I suspect the 'ring' which is produced by Alpha Cent. is due to light leakage around the baffles. I made sure to insert all filters the 'correct' way. I intend to extend the dew shield so the optics only 'see' the area being imaged on the sensor. This should also increase contrast by eliminating scatter of light not contributing to the image. The haloes around bright stars are partially or wholly caused by diffraction from the dew shield inner lip. A laser cut aperture in the dew shield extension should minimise this. I may even make it a square aperture with rounded corners and curved sides aligned with the sensor orientation. The movie industry has known about this for years have a look at the elaborate lens hoods they use.

I am sure there are things I have not thought of. The real fun is taming this system so it produces the results I want. This is a totally new game and we do not understand all the rules yet!

Bert