[Shiraz (Ray)]

Tandum (Robin) recently started a very interesting thread on aligning an f3.8 Newt. Thinking about the issues of getting it just right with such an instrument, it seemed that most Newt alignment procedures get the mirrors and focuser properly harmonised, but largely ignore the alignment of the optical system with the tube itself. To get some info on this aspect, I built up this little gadget to put a bright LED at the focus of the optics (an LED torch should work OK if it has a wide enough beam). When a sheet of translucent paper is placed over the open end of the OTA, the light column and shadows of the secondary optics can be seen. This makes it very easy to see if the light column is central in the tube and:
- if any component is optically aligned, but still not in the optimum position for peripheral rays, particularly the secondary offset, skew or longitudinal positioning,
- if any components, or the OTA itself, intrude into the light column, causing diffraction and distortion in star shapes,
- if the spider vanes are twisted or otherwise misaligned; it is easy to very accurately align the spider using a ruler directly on the shadow pattern.

It is a very simple gadget and I assume that something like it has probably been used by others in some form or other. It is a comforting confirmation if the alignment is perfect, but I managed to improve the alignment of both the secondary and the spider vanes on my f4 within 5 minutes of getting it going - it provides information on aspects of alignment that are difficult to test in any other way. Probably also worth making a version with LEDs at the positions of the corners of the CCD chip to have a look at vignetting. Regards Ray