Filters and reducers

[jambouk]

1. Are there benefits from having a filter between the reducer and the imaging sensor compared to having a filter pre-reducer, assuming both set ups had the correct spacing between the reducer and the sensor?

2. Does a filter add the actual depth of the filter casing (minus the male bit of thread), plus one third of the thickness of the filter material... So a 7mm deep filter, with 2mm of male thread, and say 3mm thick filter material would add 6mm (7-2 + (3/3)) to spacing equation if placed between the reducer and sensor?

Thanks

James

[lensman57]

1. Are there benefits from having a filter between the reducer and the imaging sensor compared to having a filter pre-reducer, assuming both set ups had the correct spacing between the reducer and the sensor?

2. Does a filter add the actual depth of the filter casing (minus the male bit of thread), plus one third of the thickness of the filter material... So a 7mm deep filter, with 2mm of male thread, and say 3mm thick filter material would add 6mm (7-2 + (3/3)) to spacing equation if placed between the reducer and sensor?

Thanks

James

The closer the filter to the sensor the better. If the filter is placed between the reducer and the sensor the optical length changes, compensating by  approximately 1/3 of the thickness of the filter glass to the back focus is normally correct.

A.G

[jambouk]

What is the problem with having a filter the telescope side of a reducer/flattener?

James

[IanL]

What is the problem with having a filter the telescope side of a reducer/flattener?

James

Are you thinking about an LP filter or something else here?  Putting an LP filter ahead of the reducer may well simplify any spacing issues since the change in spacing is eliminated (mechanical thickness of the filter assembly adjusted for optical thickness of the filter).

You do need to think about vignetting however.  With a small sensor, if you can put the filter close to a small sensor you can get away with a 1.25" filter, whereas if you put it ahead of the reducer you will need a 2" filter to avoid it.  There is some difference in cost between 1.25" and 2" filters - not a big deal if you're just buying a single LP filter, but a significant consideration if you need a full set of LRGB/Narrowband.

I put my 2" IDAS P2 ahead of my SW 0.85" reducer and it works fine - no reflection or significant vignette problems, but your mileage will vary with specific kit.  It definitely did not work between the camera and reducer as the increased spacing created elongated stars in the corners of my images.

[jambouk]

Ian. That's useful thank you. I just wasn't sure there was an optical problem other than vignetting, but your comments dismiss that worry for me.

Thanks.

James

[laser_jock99]

You'll have less issues with 'dust bunnies' if the filter is further away from the sensor. Filters are where the bunnies seem to live and breed!