OAG on my EdgeHD

[boz_m]

Quite a specific question here. I capitulated and bought an off axis guider (OAG) for my 8” EdgeHD as I had troubles with differential flexure with my guide scope. I am aware however that the EdgeHD has specific backfocus requirements of 105mm when using the 0.7x reducer. I had all the right extension bits to get this but the OAG adds 16mm.

How hard and fast is the 105mm requirement? What effect will I get if I go outside this?

If a problem I will have to look for a spacer that is the correct length which will be tough. Cheers

[glend]

Alex, you can get variable spacers, they are screw tube arrangements with a locking ring. These allow you to experiment with spacing. Btw, I have seen posts on CN that suggest some Edge HD8 owners found that addition spacing was optimal for their cameras, so don't be afraid to add space if needed.

[hamptonskies]

It depends on the chip size you are using and how much distortion you are willing to accept in the corners. There is a graph (hopefully attached) in the Edge promo paper showing the effect of backfocus on distortion as you go further away from the centre. The bigger the chip, the bigger the effect by increasing the backfocus.
Greg