WO 0.8x field curvature?

iceman (Mike) · #1 30-10-2007, 07:15 AM

Hi guys

Last night was the first light for the WO 0.8x reducer/flattener. First thing I noticed, was some sort of field curvature/rotation on the corners.

Here's what I was hoping to frame last night, NGC253 with the glob in the bottom right, but when I saw the curving stars, I decided to stick to NGC253 alone in the middle of the frame where the stars were sharper.

This is a 2 minute exposure taken in jpeg, just adjusted the levels so you could see it. It seems to only be in the top left and bottom right corners.

I don't remember getting that curvature without the WO reducer. What am I doing wrong? Or is just my tracking? Looking again now, maybe this image doesn't show it well enough.. the stars in the middle are still elongated a bit (from bad tracking), but the ones in the corner appear much worse.

What's the thumb-screw on the WO unit do? Why does it need to rotate around, and it also appears to be able to slide in/out just a little bit when the thumb screw is loose.

Thanks

mch62 (Mark) · #2 30-10-2007, 07:41 AM

Mike what scope are you using it on as it may be because the reducer was designed for a particular f ratio on a refractor and in particular a WO .
I don't see you having a refractor in your list and assume you may be trying it on your Newtonian.

I use one on my WO 66 and it works perfectly on that scope.
Mark

iceman (Mike) · #3 30-10-2007, 07:43 AM

Hi Mark
Thanks for the reminder to update my equipment list

I'm using it on a Saxon ED80, which is 80mm f/7.5.

mch62 (Mark) · #4 30-10-2007, 07:51 AM

What is your distance from the reducer to the imaging plane as it has to be within a specific distance = or - a mm.
I can't remember what it is of the top of my head but i think there is info a WO site??
Mark

mch62 (Mark) · #5 30-10-2007, 07:58 AM

PS the one designed for the z66 f5.9 has a 56mm distance and the other unit for the larger WO scopes is I think 46mm.

Mark

acropolite (Phil) · #6 30-10-2007, 08:33 AM

By the look of your image all the stars show elongation, looks to me like it might be alignment or tracking problems. Ponders has a lot of experience with that FR, he would better be able to give you an idea of what to expect at the edges.

[1ponders] (Paul) · #7 30-10-2007, 08:47 AM

Definately shouldn't be getting that Mike. Here's a full size shot, cropped from the very extreme top left hand corner. Your's looks much more likely to be field rotation.

My imaging train is 300D/T ring/WO FR/Focuser and that's it. The slightly distorted stars you can see in the corners are only in the very extreme corners. The rest is smooth and flat.

[1ponders] (Paul) · #8 30-10-2007, 08:50 AM

You could also have had your guide scope slipping or your guide camera.

[1ponders] (Paul) · #9 30-10-2007, 09:59 AM

Mike, on looking at your image again it appears that the stars are all pointing in the one direction. Are you using a guide scope. If so then this could well indicate that your guidescope is slipping in it's bracket.

If it is unguided then I would say that your camera is slipping in the focuser or the SC sleeve on the FR is slipping.

The thumbscrew is to allow you to rotate the camera if you have the FR attached to the thread of an SCT. Keep your thumbscrew tight for use with a refractor. If you have an Orion Prime Focus adapter remove the silver barrel and replace the black one from the WO and see if that gives your focuser a better grip to prevent slipping.

Geoff45 (Geoff) · #10 30-10-2007, 09:08 PM

Quote:

Originally Posted by [1ponders]

Mike, on looking at your image again it appears that the stars are all pointing in the one direction. Are you using a guide scope. If so then this could well indicate that your guidescope is slipping in it's bracket.

I agree. Field curvature would show elongations away from the centre, but yours are all in the same direction. Definitely a guiding problem.
Geoff

iceman (Mike) · #11 31-10-2007, 07:02 AM

I think you're all right. Unfortunately the image above wasn't the one that I noticed it most in - I deleted a 3 minute exposure image of an area almost on the meridian, which wasn't guided in any way. I think the field rotation was the issue in that case.

The final NGC253 image didn't show any problem so I'll assume all is well and look forward to next time

Thanks for your help.