Wednesday, October 09, 2013

Astro-Tech 8" Imaging Newtonian, Part IV

Found out that collimation is still shifting even with the relocated locking knobs, so I decided to do the next step and replace the collimation springs with stiffer ones.

Looked through RS Online and found this "Spring, compression, music wire, 11.25 dia x 29.5mm" (part number 121-157). These springs are about the same diameter as the stock Astro-Tech springs, and are 29.5mm long. Spring rate is 4.51 N/mm and maximum force (when compressed to 10.8mm) is 85.42 N which is quite substantial.

These springs cost S$ 0.60 each (minimum purchase is a pack of 10) and have nice flats ground into both ends. They were pretty hard to place into the mirror cell, but I resisted the urge to cut them shorter with a cut-off wheel and eventually managed to get them in place.

Side by side with the stock collimation springs, it's rather obvious that the stock springs are woefully inadequate.

Now when in place and the primary mirror collimated, the springs are compressed to a length of about 12mm. This translates to a holding force of (29.5mm - 12mm) x 4.51N or about 79N per spring.  That is very substantial, the equivalent of about 24kg across the three springs.  It is still possible to shift the mirror when pushing it by hand, but I am hoping that with these new springs there will no longer be any collimation shift with position.