Telescope Reviews: Band-Pass of Single and Double Stacked Filters

I first learned the terms instrument angle and field angle from Coronado technical articles by David Lunt.

I think the salient point for me when it comes to choosing a H-alpha system design is that there are always trade-offs between bandpass (contrast) uniformity, aperture, and cost, ease of configuration, and the use(s) for which the system is intended.

When it comes to mounting an etalon behind the objective, things become more difficult/complicated in order to achieve reasonably uniform bandwidth across a full-disc (or larger) field comparable to what is obtainable with the front mounted etalon. The professional science folks generally are not too concerned with full disc (or larger) views, so collimated and/or telecentric optics are ideal for the purpose of bandpass uniformity across a relatively small field of view and high resolution imaging. But for amateur viewing at relatively low magnifications, contrast uniformity across a large field of view seems to be of merit.

I may be wrong (most of you are more versed in optics than I) but it appears a telecentric system would have to be designed for a particular objective to work optimally -- I believe the Baader 4 x telecentric is specifically designed to work with a 100-150 mm f8 objective. Likewise, an internal collimator lens would have to be specifically designed and optimized for a specific objective and placed very precisely to have optimal instrument and field angles transmitted through the etalon.

My DayStar filter always showed more uniform bandpass at a reduced aperture and f 30 than I could achieve with a larger aperture and off-the-shelf telecnetric system -- that's why I eventually moved to a Coronado front mounted etalon to get both larger aperture and contrast uniformity across the full field of view.

I note that the Lunt internal negative collimator systems seem to perform better (better design, tighter tolerances, or ?), since there seems to be far fewer reports of a "sweet spot" phenomena than with the Coronado scopes with internal etalons.

Solar Scope seems to have taken the "purist" approach of H-alpha to its extreme -- unobstructed front-mounted etalons, and double stacking for reduced bandwidth only via a second front-mounted etalon = very expensive.

Then there’s the issue of the blocking filter failure. The eyepiece-side filters are less expensive for a given bandpass, but require the aforementioned telecentrics, and are ~ $700 USD to refurbish when the blockers fail. And there's the Coronado rusting blocker ITF’s, which has been a real nuisance, but relatively inexpensive to repair/replace. We might not have had a long enough experience with the Lunt or Solar Scope blocking filters to know if they too won't eventually need periodic replacement.

For aperture greater than 150mm, internal etalons are the only real choices, especially for a bandpass below 0.7 A. These would seem to require a custom designed collimator or telecentric for use with a specific objective (and a large ERF) OR just a large aperture long f ratio system (i.e. 6 inch f 30 folded refractor, etc). But then you have to have to use a focal reducer of some kind to get a full disc view with standard focal length eyepieces...

Seems like the search for the Holy Grail in order to get large (150mm +) aperture, uniform bandpass, reasonable size, and somewhat “affordable” cost. It looked to be the Lunt LS 200 double-pass-etalon reflector (!) at one time, but alas, not yet.

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Bob Yoesle

You are a child of the universe,
no less than the trees and the stars...
And whether or not it is clear to you,
no doubt the universe is unfolding as it should.

Desiderata