Tuesday, 22 March 2011

CaK Full Disk 19th March & Fixing Astigmatism

cak full disk lrg, originally uploaded by Mark Townley.

Following the realignment procedure of the prism in the CaK PST to reduce / remove astigmatism from this lovely scope I was keen to re-image the sun at these wavelengths. The image of the sun here shows that while quality is significantly improved on those of last year, looking carefully at the southern polar regions there is still some 'smearing' visible. I am aware of my fix of realigning the prism on the PST by using sticky back velcro is by no means a permanent method, however it has served as a good test of the rationale behind the fix.

When the focus on the PST is racked all the way in this forces the prism to push against the interior of the black box body of the unit. Normally this would not be an issue if the the interior was square, however it is not, there is a sloght recess around the eyepeice holder area; as such, when the prism pushes against it causes the prism to be pushed out of it's square position.

So, now, it's the fix part 2. I intend to reopen the box, and either fit a spacer in the recess so that its surface sits flush with the rest of the interior of the PST, or, resit the prism on the block that fixes to the focus thread so that when the focus is racked all the way in the prism doesn't touch the interior body of the unit. Not sure which method will work the best, but will be sure to check both out and report back!