Focusing a refractor plus field flattener attached to DSLR camera

[Roger Dymock]

I am finding it difficult to focus properly across the whole image with the subject set-up (scope is a William Optics 71mm refractor). Using Live view on my Canon 550D camera to focus at the centre of the image I found that stars in the corners were out of focus.

Bench testing using pinholes in a black card at approx 8m from the telescope showed that focusing at the corners of the image gave a slightly better but not perfect result.

Chip to FF distance and length of FF plus T ring appear correct according to the WO Flattener 6A Usage Chart.

Any suggestions as to how to achieve a good focus would be much appreciated.

Regards

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[Richard Miles]

Hi Roger – Your issue isn’t so much trying to focus but rather ‘How to obtain a flatter field?’

One explanation can be that the focal plane of your camera is not exactly orthogonal with the optical axis. That may mean that the way the camera has been attached is skewed slightly.

[Roger Dymock]

Hi Richard,

Thanks for your comments. Ron Arbour’s article in this months JBAA reminded me that I have a Bahtinov mask suitable for my refractor. Bench testing showed that focusing for the ‘smallest blob’ using Live View is nowhere near as accurate as using the mask. Nick James suggested checking the focus across the whole image so I will try that using the mask asap.

I would rather it be a focus problemn than what you suggest which sounds more difficult to fix. Hopefully tonight or tomorrow night will provide clear conditions. 

Regards

[Author]

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