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Reducers have a specified distance from the metal most edge fo the reducer to the chip of the camera - its called the metal back distance.
Depending on the reducer this will have a specified tolerance - for example a Tak reducer may have a metal back distance of 106.7mm +/-6mm. So you have to be within 6mm of 106.7 for it to work.
They also have a corrected image circle which has to be larger than the size of the chip you are imaging with.
I use a MMOAG from Astrodon with a Planewave CDK17 at 2950mm focal length and I use an ST402ME as a guide camera. I have also used a Lodestar which is OK but mine puts a white horizontal line in the image which can mess up the auto select for a guidestar function of CCDSoft and also can mess up calibrating the autoguider (the software picks the bright line instead of the star). The ST402 of course is cooled and is not that big and works quite well and is quite sensitive.
I don't think you could put the reducer after the OAG as the chip and guide camera need to be in focus together at the same time. Perhaps you could with the right spacer for the guide camera otherwise you may not be able to achieve focus with your guide camera whilst the imaging camera is in focus.
My Planewave has a corrector before the MMOAG and camera. The AP reducer is the go. I wouldn't try these other more dodgy brands. You'll more than likely find out they don't do the job properly.
The AP67 reducer is a well known and respected reducer that isn't too expensive (by AP standards anyway).
Not 100% sure if a reducer would work with an Edge HD (best to get advice from those who use them) as it already has a flattener and flatteners and reducers don't normally work together.
Greg.
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