jsg If I am not using any filters, is it OK to leave the M42 filter drawer in place (21mm) without a filter, or should I remove it and replace it with the 21mm spacer? I'm wondering if it will affect field of view ...
Assuming both cases are in perfect focus, you will not see a change in FOV. The FOV is a function of the equivalent focal length of the OTA.
However, if you are using a reducer or flattener, your backfocus spacer needs to be retuned.
A glass filter increases the mechanical focus by 1/3 of the glass thickness. So, the good 3mm thick filters will increase the need of backfocus by 1mm -- removing the glass means that you also need to remove that extra mm.
I recommend not removing that filter completetly, but replace it with a parfocal IR-cut filter, since you need to perform some IR cut to get a good color balance (see the QE curve at the ZWO web site). That way, you also don't have to spend an entire night again to retune the precise backfocus.
Chen
