Where does a 2" IR cut off filter fit in the image train?

[Ouroboros]

Where does a 2" IR Cut off filter go in the image train before a cooled OSC camera?  Does it need an M42 filter draw? I'm thinking of buying a ZWO one-shot colour camera with an APS-C size sensor.  

[alan potts]

I have mine on the front of the field flattener and in truth didn't think it mattered that much. When I use the reducer/flattener it has a filter draw after the reducer. This is then in a different place, can't say I see any difference apart from the extra FOV.

Alan

[fifeskies]

I put mine in the rear of the reducer on my GT81 as advised by WO, reducer is threaded to accept it

I don't think it matters a lot where it sits unless you get reflection problems, before or after should both work.

Rear of flattener/reducer you need to add 1/3 of filter thickness to the backfocus.

 

I will probably add a filter drawer (after the reducer) as it makes filter swapping much easier.

Edited July 28, 2021 by fifeskies

[The Lazy Astronomer]

I think ideally you want a filter as close to the sensor as possible, but this tool can be slightly reverse engineered to give you a maximum distance: https://astronomy.tools/calculators/ccd_filter_size

For your ED80 and an APS-C sensor, it looks like you could probably have the filter up to around 200mm from the sensor, so basically you can put it wherever you want. If you experience reflection problems, then you'd want to look at moving the filter closer though.

[Ouroboros]

Thanks all. I’m thinking that in order to get the right back focus (55mm when using the reducer on my ED80) it might be best to use a 21mm filter draw in place of the 21mm spacer. Would that be right? 

Whereas just adding the filter to the exit of the reducer will add a few mm to the back focus distance. Does that have to be catered for by using shorter spacers? Or isn’t it that critical? 

Edited July 28, 2021 by Ouroboros

[alan potts]

7 hours ago, Ouroboros said:

Thanks all. I’m thinking that in order to get the right back focus (55mm when using the reducer on my ED80) it might be best to use a 21mm filter draw in place of the 21mm spacer. Would that be right? 

Whereas just adding the filter to the exit of the reducer will add a few mm to the back focus distance. Does that have to be catered for by using shorter spacers? Or isn’t it that critical? 

21mm filter draws are available and now I believe FLO stock them by ZWO, so all should be well, I was not so lucky I got a different make as ZWO didn't make them a few years back, mine was a fair bit more costly.

Alan