Can spherical aberation be caused by incorrect corrector placement

[mtodman (Matt)]

Hi all.
I have previously "upgraded" my C11 OTA by installing ventilation fans, mirror lock & flocking. This process required me to remove the primary mirror as well as the corrector plate / secondary. When I put everything back together, I did my best to ensure that the corrector plate was positioned as close as possible to its original position.
I recently performed a star test which seems to indicate that I'm suffering from spherical aberration (clear diffraction rings on one side of focus but blurred on the other). I have captured this in the following link:
https://www.youtube.com/watch?v=gmFUJRro8DE

My question is, could this spherical aberration be caused by a slightly incorrect placement of the corrector plate?

Any other ideas as to what could cause this?

Cheers

Matt

[multiweb (Marc)]

TBH your airy disc looks just fine. Spherical aberration would be caused by focusing on an artificial star too close to the scope so the distance between the secondary and the primary would be wrong. If your corrector was off your star test would look a lot worse so you're ok.

[mtodman (Matt)]

Thanks Marc.
So are you saying that the star test is not showing serious spherical aberration?
This star test was performed on Hadar. I have a Moonlite motorised focuser hanging of the back of the OTA which would effect the distance between the primary & secondary. Could this contribute to SA?

Cheers

Matt

[Shiraz (Ray)]

looks OK to me as well - small amount of SA, but still pretty good. The star test is quite brutal and can expose aberration that has no practical effect.

I would guess that it works very well when focused - looks like you have a good scope.

[DarkArts]

Looks fairly normal to me. I'd say it's a good scope.

With an offset corrector, you get what looks more like permanent astigmatism rather than spherical aberration.

[mtodman (Matt)]

Thanks very much for the replies.

[multiweb (Marc)]

Originally Posted by mtodman

Thanks Marc.
So are you saying that the star test is not showing serious spherical aberration?
This star test was performed on Hadar. I have a Moonlite motorised focuser hanging of the back of the OTA which would effect the distance between the primary & secondary. Could this contribute to SA?

Cheers

Matt

Yes. The airy disc intra/extra focal looks fine and typical of a good SCT.

There is a sweet spot for the distance between the secondary and the primary. Ideally the scope should be focused by moving the camera/eye piece at the back with the distance in between the mirrors being fixed.

In practice you move the primary up and down the baffle and that changes the back focus. I have the measurement somewhere. I'll post them when I find them. Top of my head 46cm approx. between mirrors for the C11.

PS: found it. This is for a standard C11, not EDGE. I'd assume it's still valid.

[mtodman (Matt)]

Great. Thanks Marc.