However, there are a number of issues that presented themselves with double stacking, for which I have sought solutions in order to obtain optimum performance.
One of the first things that became apparent with double stacking was a considerable decrease in overall image brightness. A significant part of this could be attributed to the presence of two energy rejection filters – ERF’s – when only one is needed. Additionally, the standard ERF is made from a deep red colored glass, and has significant transmission disadvantages compared to optical glass.
Baader introduced a state-of-the-art ERF consisting of optical glass polished to 1/10 a wave with dielectric coatings (D-ERF), which has an even greater ability to block infrared wavelengths than traditional red glass ERF’s, which reduces or eliminates the heat load transmitted to the blocking filter assembly.* This is stated to prolong the life of the downstream filter components, which may also eliminate the premature “rusting” of the ITF - induced transmission filter - (I have had two previous failures of my BF30's ITF since its purchase in about 2003 from “thermal cycling.”) So I purchased the unmounted DERF 110 as being the best suited to the SM90/BF30 ( http://telescopes.ne...ic-coating.html ).
* NOTE: Lunt has also introduced a less expensive ERF with dielectric coatings on a red galss substrate with 1/4 wave flatness in order to reduce thermal loading ( http://www.lunt-sola...Hash=6251294854 ).
To properly optimize the double stack, the second etalon also needs to be first “clocked” or rotated to reduce ghost reflections. This will decrease the amount of subsequent tilting (too much of which can introduce a "sweet bar" of contrast non-uniformity) - then minimally tilted to just eliminate the ghost reflections from the field of view. See:
Unfortunately, rotating the second etalon (as described in the above link) on the standard double stacking adapter loosened the filter, making the adjustment subject to excessive mechanical play, and introduced the possibility of the etalon falling off!
The solution for the dim image and thermal overloading (picture bottom) was to remove first one, then the other, original ERF’s, and substitute a Baader DERF in its own specialized cell that could be threaded on to either filter for single stack or double stacked mode. The DERF itself is tilted 0.5 degree (as is the original ERF), and additionally can be “clocked” or rotated so as to adjust its tilt orientation to ensure no spurious DERF reflections are introduced in the double stacking rotation/tilting optimization process. The DERF filter holder is essentially two nested cells that allow the inner cell (holding the DERF) to be rotated and then fixed in the correct position.
The solution for double stacking “clocking” rotation was an adapter (picture top) which is a similarly designed nested cell to allow the inner cell to rotate, and then be fixed to hold the proper etalon clocking position.