Crescent-shaped stars with focal reducer

[carldr]

Hi all,

Due to a (brief) break in the cloud, I managed to get outside for the first time with my Celestron C9.25 SCT to image a DSO.

However, when I was using the Celestron f/6.3 focal reducer, I was getting images like the attached image of Aldeberan. If it's not too obvious in the size of image attached here, you can see the full, original version at http://carldr.com/tmp/aldeberan.jpg. You'll notice the stars away from the centre are crescent-shaped.

Removing the focal reducer fixed the problem, as evidenced by the attached photo of M42 (Original at http://carldr.com/tmp/orion-nebula.jpg.) Unfortunately, I didn't think at the time to take a photo of the same target. The rest of the setup remained the same.

When I removed the focal reducer, there wasn't any sign of condensation. I had just cleaned the collector plate of the SCT, and since it was the first time I'd done that, I first assumed I'd cocked it up, but as you can see, without the focal reducer things are fine.

Unfortunately, the clouds rolled in before I could try again with the focal reducer.

I use an unmodified Canon 500D at prime focus. My collimation seems fine, and the seeing during the evening was pretty poor.

Has anyone seen this before? Could it be down to condensation on the focal reducer, or some other part of the image train, or is a real problem with some part of my equipment?

Any ideas would be gratefully received.

Many thanks,

Carl.

[FraserClarke]

Looks like a real optics effect, though I'm not sure exactly what. Have a feeling it might be astigmatism (though I'm not sure if this should be rotationally symmetric around the optical axis?).

May sound daft, but did you have the corrector in the right way round? I'm not sure if it is even possible to mount these the wrong way round, but maybe worth checking.

Also, did you have the corrector at the right distance from the focal plane? Their performance can be quite sensitive to optical separation, and certainly in some cases astigmatism will result from incorrectly separated optics.

Another thought; was the telescope a long way from it's nominal focus position?? With SCTs you can throw the focus a very long distance, but it does affect the performance (though I would seriously doubt by this much!)

Just some ideas! Don't take any of these too seriously...

[Dangerous-Dave]

Sounds like incorrect spacing between the reducer and sensor to me.

Have a look at this thread:

http://stargazerslounge.com/equipment-help/67685-f6-3-focal-reducer-distance.html

[carldr]

Sounds like incorrect spacing between the reducer and sensor to me.

Have a look at this thread:

http://stargazerslounge.com/equipment-help/67685-f6-3-focal-reducer-distance.html

Ah, I bet that is exactly it.

I didn't think about the distance between the reducer and sensor as being important, although now you've mentioned it, I do remember checking the distance between the TeleVue x0.8 reducer and sensor would be correct before I bought it.

Thanks for your help guys, and if it is clear tomorrow night as is forecasted, I'll confirm (or not) that it was that.

Many Thanks,

Carl.