Filter direction

[volcanotop]

a basic one.   Are mounted 1.25 or 2 inch filters designed to work

A)thread pointing to the sky.

or

 b) thread pointing to the camera.  

With different imaging trains i can fit them either way.   Is there a right or wrong? Or doesnt it matter?

when looking at filters visually one side is more mirrored and one more coloured.  I guess mirror side to the sky? (Thread to the camera?)

[symmetal]

Filter wheels are made such that the thread points towards the camera. Likewise when fitting a filter onto an eyepiece. The more reflective (mirrored) side should then be pointing towards the sky, which is what you want to minimize reflections in your images or views.

As you say some imaging trains force you to mount them the other way way round. The 2" UV/IR cut in my ZS61 used for imaging with a OSC camera has to go thread towards the sky. However both sides of this filter seem to have the same reflectivity so it doesn't really matter which way round it is.

For unmounted filters there should be an arrow on the rim of the filter (for those filters that matter) and the arrow should point towards the sky.

Here's an answer from Baader-Planetarium on the subject.

Alan