focal reducer vignetting

[space3]

im imaging with a celestron 925 and a Canon 6D, im getting really bad vignetting when using the Celestron 

focal reducer,  I thought it maybe the full frame camera causing it, but I tried a crop sensor lastnight and Im getting 

the same result, 

It seems fine when I remove the reducer, so does anyone know is their something I can use to fix this,

 a different reducer of something else that will correct it, and let me still use the reducer

Thanks 

Derek

[wxsatuser]

Think that the vignetting would be expected.

Some good flats needed.

[Alien 13]

I think reducers also reduce the image circle which is fine on a refractor or similar if its a foot across (corrected circle is much much less) but is limited on SCT and Maks.

Alan

[ollypenrice]

Is your chip-to-reducer distance correct? The distance is not all  that critical on an SCT but I would look it up on the Celestron website and see if you are about right.

Very few telescopes will cover full frame chips. The majority won't do so.

Try to measure the vignetting. Just take a flat and them open it in a suitable programe which allows you to mouse over it so as to get an ADU value for the pixel under the cursor. In our 'most vignetted' rig we have about 19000 ADU in the corners and 23,500 in the middle. That's really quite a lot but flats deal with it quite happily.

Olly

[space3]

Thanks for the replys everyone, I will play with the distance between the reducer and chip and see what happens