One-sided diffraction spikes?

[Stub Mandrel]

I managed to get one half of the Double Cluster, but all the bright stars show a single-sided diffraction spike.

I checked the subs and it's on them all and the same, so its a real effect, and not caused by a tracking error.

[blinky]

Hmmmm..... Every sub the same - It really looks like guiding errors or something causing the mount to jump.

[Physopto]

Could it have any thing to do with mirror pinch or collimation?

Derek

[blinky]

I dont think its anything to do with collimation or mirror pinch, the stars look good other than the little spike.I would try and look at the guiding graph and see if you can spot anything

[Stub Mandrel]

I'm not convinced its my (unguided) tracking - although this did have the scope pointing close to the zenith. I just restored a few dumped subs from my recycle bin and the spike is there as well as the tracking error.

If it was a collimation issue, it came on suddenly - I had no problems with the Pieades and it was a simple move to get to the cluster.

What is odd is that the spikes are lined up with three much smaller ones. Perhaps the mirror 'flopped' when the scope went vertical?

This so-so stack of the Plieades taken immediately before shows even spikes:

[Physopto]

Do you think there could have been a drop in temperature over a period. Causing contraction and hence a mirror movement when you went vertical. If so it may have shown up as some slight slackness in the mirror holding mechanism. I am asking because it is as you say a one sided phenomenon. Very frustrating when you have had good results.

Derek

[Stub Mandrel]

Yes it had become very cold. I will check the mirror in a bit, but it will have warmed up by now.

> Very frustrating when you have had good results.

Fortunately it wasn't a very good image ;-)