Vignetting

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Vignetting issues with the Meade F6.3 Focal Reducer

I get so few imaging opportunities that up until now I've not looked at setup issues and just grabbed as many images as I could.

Recently I've noticed some significant vignetting issues that I would like to solve.

I use a 10 inch LX200 with a JMI NGFS focuser and a Meade F6.3 focal reducer. The imaging camera is a Starlight Xpress SXV-H9.

This is what I have found so far:-

Usual imaging configuration as I believe many other people use:-

LX200 then NGFS then F6.3FR then extension tube then CCD camera.

Below is a typical image and flat from this setup. Please note, changing the extension tube length varies the FL but the 'Donut' remains!

 

Alternate imaging configuration:-

LX200 then F6.3FR then NGFS then extension tube then CCD camera.

Below is a typical image and flat from this setup.

 

 

In my opinion the alternate configuration gives a much better result and I would say is a typical vignetted field.

I do not understand why placing the FR after the NGFS has such a severe effect. I have also tried the JMI large format adapter which made no difference. In both configuration I have tried to keep the FR to CCD distance to about 90mm.

My questions therefore are:

What causes the 'Donut' vignetting of the first configuration?
Why doesn't there seem to be any other people reporting this effect?