To best understand how narrow-band etalon filters work, you must first understand that everything hinges on the incident ray angles that pass through the filter. Each filter is designed to work at an optimum center wave length (CWL), and when light diverges away from “normal” (perpendicular) to the filter, the CWL will shift to shorter wavelengths. Once the angle exceeds the “acceptance angle” of the filter, the CWL has shifted far enough for the filter to be considered “off-band.”
The ideal narrow-band solar telescope would have a filter which is relatively small to keep costs low, and present all light rays going into the filter normal to the filter so that all parts of the filter remain on the CWL:
Unfortunately, this ideal telescope-filter system is impossible in reality. The sun is a relatively large object subtending 0.5 degree, and when placed on the optical axis, the edge of the sun’s disc will therefore present the objective with a “field angle” of 0.25 degree. There is no way to reduce the field angle to be smaller than this. Therefore placing the etalon filter ahead of the objective will give the best possible filter performance. The field angles are as small as they can be, and there are no instrument angles to contend with. These filters are designed with a slightly “high” CWL so that they may be tilt-tuned with little ill effect in order to be exactly on the H alpha wavelength, which is necessary due to changes in atmospheric pressure and temperature which can change the filter's CWL. Unfortunately, etalon filter costs go up dramatically with increased size (e.g. surface area). The traditional maximum typically found is about 150 mm, and 100 mm etalon filters are not cheap.
In order to make solar telescopes more affordable, a smaller etalon filter can be placed in the optical system after the objective. However, this introduces more complexity in the light rays presented to the etalon in the form of “instrument angles.” Therefore additional optical elements are employed to better control the angle of the rays presented to the etalon.
The diagram below will be useful in understanding the discussion that follows (and forgive me if some details are incorrect - I'm not a optical engineer, and didn't stay at Holiday Inn last night ;-)
In the collimator system, a negative lens is placed before focus, or a positive lens is placed after focus. This lens renders a “collimated” or parallel bundle of instrument rays to the filter, and after that a positive lens is used to re-focus the image where it can be viewed or imaged. The focal length of the re-focus lens is usually adjusted in order to render the effective focal length back to that of the objective alone.
While it can be seen that the collimator produces no incident angle to the etalon on the optical axis, as one moves farther off-axis, field angle magnification increases, and at some point across the etalon the field angles will exceed the acceptance angle of the filter. This increase in field angle magnification also makes it more difficult to use tilt tuning, and pressure tuning is generally considered a better method to adjust the CWL. It is also generally considered that for good full-disc on-band contrast performance, the focal length of the collimator lens should be no less than half the focal length of the objective, which results in no more than a 2x magnification of the field angles (just as an eyepiece does – same thing in fact.)
It can also be seen that the size of the etalon is also inversely proportional to the magnification, and therefore ideally is no less than ½ the objective diameter. Any shorter a focal length for the collimator (and the resultant smaller etalon) results in less than ideal contrast uniformity across the full-disc as the filter shifts more quickly off-band with greater field angle magnifications. This results in the well-known “sweet spot” phenomena observed where portions of the disc shift to a continuum view of the photosphere verses the chromosphere. This can be acceptable for those interested only in narrow close up views of portions of the solar disc, and is often the case with DIY solar telescope projects which use smaller collimators and etalons.
An even smaller etalon filter can be used, but now the optical system usually changes to the “telecentric” system in order to achieve acceptable etalon performance. In the telecentric system, a positive or negative lens is again used to essentially collimate the light rays similar to the collimator lens system. However, after this a relatively weak positive lens is used to focus the image using a sufficiently narrow converging light cone to keep the angles under the acceptance angle of the smaller downstream etalon near the focus. At the same time the telecentric system ensures that the off-axis light cones are also brought to a focus in the same manner, with the cone axis axis normal to the filter. This results in a uniform spectral performance across the etalon. An f 30 or even longer focal ratio light-cone is required for these systems. Remember again that the narrower the filter band-pass, the narrower (e.g. the longer an f ratio) this light cone must have in order to achieve the bandpass specification of the etalon.
The caveat with these filter systems is that generic “telecentric” barlow lenses, or a telecentric lens system used with the wrong focal length objective – e.g. “non-optimized” – may not result in true telecentric performance. Non-optimized telecentric systems therefore may not present the filter with an appropriately narrow light cone to achieve the specified filter band-pass (f 30 or greater). They may also increase angles from tilted peripheral field angle cones, and therefore exhibit a “sweet spot” in addition to being significantly band-pass widened on axis.
Not understanding the principles of how these filters work can result in solar enthusiasts spending a lot of money on a really narrow bandpass filter, only to realize the performance of a much cheaper and wider bandpass filter due to lack of telecentric system optimization.
Edited by BYoesle, 04 February 2017 - 10:49 AM.