Astigmatism in GSO Ritchey Chrétien, anyone?

[ollypenrice]

I've been helping a guest with an 8 inch GSO RC which he bought second hand, but haven't I worked on one of these before. We've set up an artificial star test and have succeeded in getting to the point of having clearly visible diffraction rings. Extra-focal, the rings look fairly concentric. Intra-focal they flatten into ovals, suggestive of astigmatism to me. There is no flattener or reducer in the train, this is an all reflecting system, and the effect is precisely replicated in two TeleVue eyepieces, so it won't be that. It applies edge to edge in the eyepiece.

Has anyone else seen this in the GSO RC? And can anyone think of an alternative to astigmatism as the cause?

Thanks,

Olly

 

 

[zicklurky]

I think the collimation may be slightly out. I know RC's are prone to astigmatism when the primary and secondary are misaligned.

I found an interesting read here: http://www.supernovae.be/rc-collimation

[ollypenrice]

44 minutes ago, zicklurky said:

I think the collimation may be slightly out. I know RC's are prone to astigmatism when the primary and secondary are misaligned.

I found an interesting read here: http://www.supernovae.be/rc-collimation

Many thanks. The article does indeed make it clear that mis-collimation in RCs creates astigmatic symptoms. This is rather what I feared!

Olly

[Peter Drew]

I've just read the article myself and a few quotes made me wince. I've no practical experience of RC's but considerable experience of Schmidt type systems including Schmidt cameras where collimation and mechanical tolerances and adjustments are critical. I appreciate that miscollimated RC optics can introduce astigmatism, but nothing introduces it as easily as a tight primary. Lateral displacement of RC optics will soon introduce coma which shouldn't be corrected by tilting a component, the same goes for APO triplets. I would also have thought that GSO's original cell/baffle tube design would have been preferable provided that greater care was taken with the accuracy, having an independently adjustable focuser, although a means of correcting an out of tolerance mechanical detail, does add extra uncertainties to the mix. Finally, to have a cell that drops position when loosened for want of a suitable register machined into the rear cell beggars belief.

 

With regard to the RC in this post, I would have expected the astigmatic star shape to cross over either side of focus if inherent to the system, can I suggest that the OTA is defocused to exhibit the astigmatism and then rotated to determine whether the plane of the astigmatism rotates with it, this will prove whether it is in or external to the OTA.