Stacking filters.

[Franklin]

Any advice on filter stacking? Are there any rules on the best position and order in the light path. Most filters are threaded on both sides for stacking but what about either side of a diagonal? Any benefits doing that? Cut down internal reflections etc. Been experimenting and can't say I see much difference but I have read reference to this on CN. Thanks.

[Franklin]

Still thinking about this. Can't say I know much about optical theory but a 2" filter placed ahead of the diagonal will intercept the light path at a wider part of the optical plane so the light that is being filtered will pass through a greater area of the filter. As opposed to a 1.25" filter threaded into the eyepiece where the light cone is just about to reach focus and subsequently much smaller and more concentrated. From this it seems pointless to place a 2" filter just before a 1.25" eyepiece as I will only be utilizing the middle part of the filter. Does this make sense or have I lost my marbles?

[andrew s]

What type of filter are you thinking about? Coloured glass, narrow band broad band  interference filters, UV / IR blocking? What are you trying to do with them on what type of object?

We need some more context to comment.

Regards Andrew 

Edited August 22, 2020 by andrew s

[Louis D]

Stacking filters can lead to reflections bouncing back and forth between them causing ghost images of bright objects.

[Franklin]

The filters I have are all 2" ones, I used to have some of the smaller 1.25" type but they are so fiddly to remove in the dark and also having to change them more often when swapping eyepieces. The ones I have are Baader Neodymium, ES ND 0,9, Castel UHC and a WO VR-1 minus violet (scope is a 127mm f9.4 achro). Stacking is primarily with the VR-1 on brighter solar system objects. (Neodymuim & Minus Violet) Just wondered if there's any advantage/disadvantage to the placement of filters further away from the eyepiece and in what order? I can put filters in the nose of the diagonal (2" Burgess CED-1) and/or in the 2"-1.25" adaptor. Also when just using one filter, is it beneficial to place it closer or further from the eyepiece? Thanks.