Filter Placement Question...

Filters are normally threaded onto the front of the eyepiece in a position that places them near the focal plane of the eyepiece.

The other night as I was observing, I began to remove the eyepiece to put on a Minus-Violet filter when I wondered why bother, why not just place the filter on top of the eyepiece. It seemed to work just fine.

Obviously this requires eyepieces with sufficient eye relief and a design that keeps them in place but given those constraints, it seems that placing the filter in the exit pupil might have some advantages because it is filtering the already magnified image, thus this approach might be less sensitive to the optical quality of the filter...

Just thinking, wondering, comments appreciated...

jon

Jon,

From the standpoint of aberrations and such, after the eyepiece is the better way, no question. The only issue is eye relief.
--
Mike

I alway position the filter in first,
before adding the coffee.



UO MK's are threaded for filters on both ends. Blinking for planetaries is done from the eyeball end of the ep. My Lumicons change the focus point ever so slightly when threaded into the bottom of an ep. Top placement might do something too, but it still works.

The only downside that I suspect is keeping the filters as clean when on top.