M1 But With An Equipment Question

This was taken with a C11, prime focus,Pyxis camera rotator, AO-8,CFW-10 and an ST8-XME (efl 3370mm). I had just finished collimating the scope and wanted to take a quick picture to confirm the collimation. It was cold out (-5 F) and during the exposue, the camera setup slipped but I never lost tracking. This is the image that came back. What I see in this image is that "sweet spot" of the C11 is not centered on the chip. Stars in the upper left are almost round while the lower part are clearly not. Do you see the same thing and, if so, how does one get the "sweet spot" centered on the chip?

Chris
Cedar MN

Attached Image:

It would be nice to see full frame.
This looks more like field rotation to me.
I see that you have field rotator.
Is it functioning normal ?
Have you previously got good stars ?

Chris Nisbet said:

...during the exposue, the camera setup slipped but I never lost tracking.

Chris,

Is this the image that resulted after the slip that you mention? If so, chances are that's the cause of what I'm seeing. As Tapio mentioned, this looks more like rotation than collimation issues. I would rely more on a test shot where I know the imaging train did not move at all during the exposure. To check collimation, I would normally shoot very short exposures of a star field (10 seconds or less). To go longer, especially guided exposures, would only introduce other factors into the results.

HTH,
Eric

It appears as field rotation. The center of the ST8 imaging chip is of center a little (almost 2/3 down from the top) and the guide chip is above the imaging chip. The image will rotate around the star that is being used to guide with. Should the star selected for guiding is in the upper portion of the guide chip it will be a long way from the FOV center. The center of the rotation would not appear on the imaging chip. See the attached image which will show where the FOV is on an ST8/ST7 SBIG camera.

I also use my CCD to collimate my C11 and to do a quick drift alignment. To collimate you must find the center of the imaging chip to center the out of focus star.

These techniques seem to work for me, hope this helps.

Sam

Attached Image:

If it was coma or field curvature I would think that the stars would be pointing away from the center (sweet spot) of the field. Your stars are circling it...at least on the bottom of the frame. Here is a shot I took of M1 a while back and the stars at the edge point toward the center.

Joe

Attached Image:

Thanks for all your responses. Here is a shot of a flat. I think the optical center of my imaging train is not centered on the chip. Is that correct?

Attached Image: