Positioning of filters for a refractor with focal-reducer/field-flattener combination

[Avocette]

Prompted by a post elsewhere, this question is on the pros and cons of placing multi-narrowband filters in front of the FR/FF or between it and the camera. In both my systems (ED80 and 61EDPH II) the initially convenient place to put 2” mounted filters is in front of the FR/FF where they can screw in without affecting the FF to camera back-focus distance. However, changing filters is tedious and risks finger marks or worse, so I have been contemplating buying a ZWO slide-in filter drawer and some extra filter holders which would replace the ZWO 20mm spacer in front of the camera. My concern with making the change, apart from the difficulties of getting hold of the hardware, was whether this might mess up back-focus. This advice from Altairastro leaves me confused…https://www.altairastro.help/info-instructions/faq/can-filters-be-placed-in-front-of-a-focal-reducer-flattener/

[david_taurus83]

That's the first I've heard of that? I've always stuck the LP filter in front of the reducer. Don't usually get any problems with halos. If you stick it behind the reducer then it will mess up the spacing and you'll have to compensate. Some say you add a spacer thats 1/3rd thickness of the filter glass. I say experiment either way.

[jacko61]

I have the William Optics reducer/flattener for my ZS73 and the instructions clearly state to place any filter in front of the reducer lens as can be seen in the FLO product page...  William Optics x0.8 Adjustable Reducer Flattener for ZS73 | First Light Optics

I'm pretty sure WO know what they're talking about.

Graeme