HoTech field flattener for ED80/QHY8pro. Have I got this right?

[jwoody]

Hello everyone.
I use a QHY8pro and a Skywatcher Evo 80ed scope but I am not happy with the misshapen stars in the corners. Looking around at field flatteners I have settled on the HoTech 2"SCA field flattener from here:
http://agenaastro.com/hotech-2-sca-field-flattener.html

It requires 56mm of back-focus to the camera CCD. According to the QHY8pro manual the CCD has 20mm of back focus therefore a 36mm spacer is required, no problem.
However, my QHY8pro differs from the one mentioned in the manual (maybe mine is an earlier revision, I am not sure). My camera doesn't have the tilt/centre adjustment or the hole for the desiccant tube.

So I am a little confused as to what extra spacing I require so maybe I should just get this :
http://agenaastro.com/baader-varilock-46-variable-length-t-2-extension-tube-29-46mm.html

From what I can see the combination of these two should do the job. Have I got this right or is my understanding of back-focus completely wrong.

Thanks
Jeremy

[acropolite]

Backfocus issues aside, if you have the standard Skywatcher focuser, it would be well worth considering the genuine Skywatcher item.

I've used the SW reducer/corrector on a full frame DSLR with stars perfect over most of the frame and just a slight amont of egginess at the diagonal extremes in some images only which I suspect are focus related.

[graham.hobart]

I have the old style QHY 8 (square) and I am using the Hotech flattener, send me an email on graham.hobart@bigpond.com and I will send you some pics and details
Graham.

[LewisM]

I have tried both the Hotech and the Skywatcher generic refractor reducer/corrector.

The Hotech sat unused. It doesn't live up to the concentricity claims either - the rubber cracked very fast!

Nuff said.

[jwoody]

Thank you for the replies. I did consider the Skywatcher FF/FR but I really only want the flat field and no reduction in effective focal length.
Cheers
Jeremy