[Shiraz (Ray)]

Hi Guys.

Actually a cone of light is the ideal way to align a focusing mirror - that's why the Barlowed laser was invented for primary mirror collimation.

But this device is not intended as a primary alignment tool. It is mainly for the person who has a standard f4 Newt with a CC, collimates it perfectly with whatever system and then finds that the ##$% thing still will not produce round stars on a CCD. If this story is not familiar, you will not gain anything from this discussion (and you probably don't have an f4 scope) - unless you have a non-standard scope like Justin. If it sounds familiar, you either have misalignment problems that are messing up the star PSFs or you have dud optics. This device can help you see any misalignments that are not exposed by the collimation process, before you race off and buy new mirrors.

As an example, my f4 system has a centered secondary and it relies on the offset collimation process where the optical axis of the primary is angled off from the axis of the tube to introduce the appropriate secondary offset. Works fine for visual and collimates perfectly under a star test, but the light column is offset far enough that it catches the edge of the OTA tube when imaging over a largish CCD - adding significant diffracted energy. Dim stars are fairly nice round shapes, but bright stars have odd-shaped PSF skirts which vary over the FofV (example crop below). A few minutes of playing around with the alignment tool showed what was going on and what has to be done to fix the problem - the secondary has to be properly offset and I will have to make a new secondary mirror holder. In less than an hour I had solved a problem which had been pretty much a mystery for a year.

Finally, this device actually provides an excellent way to align the secondary vanes - the projection method puts the vane shadows in the same plane as a ruler, so there is no possible parallax error - and any twist in the vanes shows up as an increase in the width of the projected shadow - dead easy and very accurate.

Regards Ray