I believe there was expressed a wrong point of view how the narrower band wide can be achieved.
If both identical etalons will be adjusted that the vertexes of their bandwides will fully coincide then we have the same bandwide. And narrower bandwide can be achieved ONLY if we will shift one bandwide vs second one. In this case we will have significantly lower maximum of transmittance, narrower bandwide and much supressed wings.
In such way we can achieve the same narrow bandwide even with, say, two 0.9 etalons. However maximum transmittance will be lower and wings suppression will be lesser.
This is incorrect. I have very successfully used two identical bandwidth filters to narrow the bandwidth as stated by David Lunt in the post above. The ideal is to have one filter be on band with no tilt, and to have a second filter that has the identical bandwidth/center when slightly tilted to remove ghost reflections between the two etalons.
Now I get it (I think). I had misunderstood what Zirin meant by the "general chromosphere." Figure 7.5 is neat because the shots are close in time and the spicules are the same from frequency to frequency. You can see individual spicules uncovering themselves as you go off band.
So the bottom line is that the lower disk you see is indeed the photosphere, and it marks the base of the chromosphere, and the spicules start more or less at the base of the chromosphere, although we only see their tops because of the intranetwork (non-spicule) structure, which Zirin calls the general chromosphere.
Bob, have you tried to duplicate Zirin's photo, using a SS filter so that the photosphere is visible and trying to see closer to the base of the spicules by going slightly off-band? That might be a fun experiment.
The “general chromosphere” as Zirin refers to it, is shown in the on-band images of Fugure 7.5. It is also as demonstrated by the images I made above for both the 0.7 and < 0.5 A bandpasses which were taken on-band
Spicules are made more visible with off-band tuning, and this of course introduces some more continuum energy into the image, showing the limb of the photosphere. This is what David was referring to.
So while I haven’t tried to image spicules @ 0.7 A with off-band tuning, they would be just as easily imaged with a 0.5 A filter with off-band tuning. When a < 0.5 A filter is used on-band, only the top of a few larger or macrospicules become visible, as in the animation above. The vast “spicule forest” on the limb remains hidden by the general chromosphere when the filter is on-band.
In another publication, Johannesson and Zirin noted that when using a very narrow <0.5 A filter:
The image through this pure filter [the Rakuljic (Rakuljic & Leyva 1993 ) holographic filter] does not show the spurious inner limb, which was shown by White and Simon (1966) to be due to continuum passing through the Lyot filter sidebands...
... Images in the wing of H-alpha show Doppler-shifted moving features in the chromosphere, namely spicules. Despite the evidence of the Dunn-Zirker images and various eclipse measurements, there is a general belief among solar physicists that the general chromosphere extends only to 2000 Km, as in the VAL model (Vernazza, Avrett & Loeser 1981). It is thought that the higher H-alpha emission is due to spicules. Our result establishes the qualitatively obvious fact that, except for macrospicules, the general chromosphere extends above the average spicules.
The off-band (spicule) limb is always lower than the centerline limb by an average value of 500 Km (0.7 arcsec). Because of the self reversal of the chromospheric H-alpha line, the off-band chromosphere is twice as bright as the centerline. As a result, the gradient of the off-band limb profile is considerably steeper just above the photosphere. Thus the popular view that spicules rise above the chromosphere is incorrect, except insofar as the macrospicules are concerned.
Johannesson, A. & Zirin, H. (1996) The Pole-Equator Variation of Solar Chromospheric Height. Astrophysical Journal, 1996, Volume 471, pp. 510-520
So to get back to the more salient issue of this post’s topic, and the point of narrowing the bandpass via double stacking:
Narrow passbands of 0.5 A or less... are required to take advantage of the highest opacity in the cores of Ha, Hb, and Ca H and K and thus achieve adequate contrast to study the disk structure.
Foukal, P.V. (1990) Solar Astrophysics (p. 293). New York: Wiley & Sons.
It is therefore established that a 0.7 A FWHM filter will admit continuum light from the photosphere when it is centered on-band. This reveals the limb of the photosphere showing a boundary between the photosphere and the general chromosphere. This is an inherent property of a filter with the wider bandpass. It should not be confused with a filter tuned off-band to show greater spicule details.
It is furthermore established that a < 0.5 A FWHM filter will not admit continuum energy from the photosphere when tuned on-band, and shows only the general chromosphere, without showing the limb of the photosphere. When tuned off-band, it too will admit continuum energy, revealing the disk of the photosphere.
It is furthermore established the chromosphereic spicules are made more visible when the filter is tuned into the wings of the H alpha line. This will also introduce continuum energy, revealing the limb of the photosphere, irrespective of bandpass.
Filtergrams below from the Astrophysical Journal, 1996, Volume 471, The Pole-Equator Variation of Solar Chromospheric Height
, Fig. 1:(a) An off-band chromospheric image. Several arrows mark the locations of dark and bright spicules crossing the limb. The spicule is brighter than the sky, but normally is in absorption against the disk, even near the extreme limb. ( A centerline image at the same position angle but a few hours later showing the inner limb due to sidebands of the Zeiss (Lyot) filter. © A limb image on a different date with a pure filter (Rakuljic filter), with the inner limb barely detectable from the morphology.