Poor collimation or something else

Hi all,

Confused by this but maybe someone with an SCT has seen this before?

After receiving my f3.3 reducer last night I put it on my 8" SCT to try some imaging. The speed of imaging was dramatically improved but I have a strange coma like abberation all of the image.

Now, I'm pretty sure I was collimated at F6.3 so I would imagine I was collimated at F3.3 but this was impossible to verify visually (couldn't get to focus at that focal length with any eyepiece). Hopefully I haven't got one of the bad batch that Meade produced recently.

Here you can see a slightly out of focus image of M13. Ever star is pretty horribly distorted.



Can anyone shed any light on this?

Many thanks,

Trev

almost looks like atmospheric refraction

Trevor - I'd have a go at collimating with the 3.3 in situ and see if that helps.

Any more details on the image? Exposure time, imager etc?

I've certainly seen similar in my 8" SCT at F6.3 and collimation helped.

Dave

Hi Dave,

It was a Sac10 3.3MP imager (3.4mm pixels), 180 second exposure with a 1.25 inch Hutech IDAS LPS filter. M31 was only about 30 degrees up at that stage.

I didn't try collimating at F3.3 because no eyepiece would reach focus but
I forgot I could use the NexImage on something like Vega to collimate at F3.3 so I'll try that tonight if it's clear. That way at least if I'm still having problems I can post an image of the diffraction rings.

Thanks,

Trevor

Hi Trev,

The exacting requirements for collimation go up exponentially as the f ratio comes down. To operate at f/3.3 you *really* need to have the collimation nailed right to the wire. Also you need to ensure that the CCD chip is exactly perpendicular to the focal plane. A slightly loose fitting nose piece may result in the camera being at a slight angle to the focal plane. However as Dave mentions you do need to collimate with the 3.3 reducer in place.

I tend to agree with Dave ... it appears that all the 'flaring' is in the same direction and might indicate that the reducer is not squared up with the telescope. When I have used this reducer with the 30mm spacer for F/3.3, I get similar comatic blurs, but they all pointed away from the center.

I've had the best luck just using this reducer without any spacer (15mm or 30mm). It seems to produce a focal ratio in the neighborhood of F/5.8. By the way, what configuration are you using now? (I'm assuming you are using to the Meade model?)