I first became aware of these issues a few years ago. Like many other non-professionals, I was quite uninformed as to filter theory, real performance, and chromosphereic features -- so when I first noticed these differences I thought there might be something wrong with my filter system, as it wasn't showing this often-imaged - and mis-identified - "spicule layer." All the more motivation to get to the truth of the matter.
Shown below are "normalized" single and double stacked transmission curves, and demonstrating the seeming irrelevancy of filter bandpass specification with respect to elimination of parasitic continuum, while at the same time showing the real difference lies between the filter system transmission "tails" where the continuum is actually leaking through:
The identically processed images were also "normalized" to show equal brightness for prominence features, thereby clearly demonstrating the contribution of continuum to contrast degradation of disk features (and increased visibility of photosphere features - sunspots), and seemingly debunking the frequently-stated myth (even by the filter manufacturers) that a narrower bandpass (usually achieved via double stacking filters) decreases prominence details and visibility. This "decrease" in prominence visibility is apparently mostly due to the decreased overall brightness of the chromosphere, usually from the reduced transmission of a multiple cavity filter system or a narrower single cavity system.
Ken - in a recent post on digital heliospectroscopy verses filters - cited a reference which spelled out the situation clearly giving parameters to when this parasitic light form the photosphere begins to intrude into the chromospheric H alpha emission: http://solarchat.natca.net/viewtopic.ph ... 75#p167202
Note the reference to DSing a 0.125 A FWHM filter (which apparently also shows a double limb) with a 1 A "prefitler," which increased system finesse enough to eliminate on-band parasitic photospheric intrusion. However, rather than "suppressed bandwidth," (highly unlikely with a 1 A pre-filter) it is more likely that suppressed transmission "tails" eliminated the parasitic intrusion of continuum form the photosphere into the chomospheric H alpha emission. Christian Valadrich has also established continuum suppression using a 4 A filter with 0.6 A DayStar system: http://www.astrosurf.com/viladrich/astr ... ntrast.htmBray and Loughhead, "The Solar Chromosphere" p18-19 talk at some length about Parasitic light.
"The absence of the photospheric limb on narrowband photographs free from parasitic light shows...that in the CORE of Ha the chromosphere is optically thick along a tangential line-of-sight to the limb"
"High resolution observations show that the photospheric limb first begins to re-appear at Ha +/_ 0.65A....As one moves further out from the line centre, the photospheric limb rapidly becomes more prominent and appears as a sharp boundary crossed at irregular intervals by isolated chromospheric features."
This is well demonstrated in their accompanying plates 2.8 and 2.10.
They also note the use of a 1A prefilter on their 1/8 A Halle filter suppressed the bandwidth and parasitic photosphere.
Last comment: The actual wings of Ha extend 8A on either side of the core. It is obvious that ANY off-band (red/blue wing) data will be only be obtained close to the core "shoulder" - at less than +/-1.25A and not out further in the extended wings which will give, with any filter/ SHG bandwidth, access to the photospheric continuum light.
Thanks to this reference from Ken, I thought I would try and look to see what might actually be going on. First, there is the problem of using "normalized" transmission curves, which do not accurately represent the true situation when double stacking etalons, and the entire DS curve is stretched taller than it should be. Here, using normalized transmission values for SS verses DS, and the figures stated above for the +/- 0.065 nm limits for the intrusion of continuum, we have a value near 22% for single stacked transmission level, and about 8% for double stacked:
However, using a reasonable assumption of about a 60% peak transmission value for a typical DS etalon system compared to a SS, we can see a number of interesting areas:
Note that the non-normalized DS transmission curve crosses the +/- 0.065 nm threshold at ~ 0.048 (4.8%) instead of ~ 8%. This has led to a significantly larger and more accurate representation of area covered by the difference in the two transmission profiles (especially adjacent to the +/- 0.065 nm limit), and perhaps showing a larger amount of flux difference permitted to intrude into the chromospheric emission by the SS profile.
The region labeled 1 indicates an area where at under the +/- 0.065 nm limit, the SS curve apparently does not contribute significant additional continuum to the chromosphere. Area 2 indicates the region where additional continuum can leak into the chromosphere emission. However, from this we need to consider that at the point where the DS curve crosses the +/- 0.065 nm limit - area 3 - there is apparently insufficient flux (area under the curve) to make a noticeable contribution of continuum to the chromosphere. This leaves areas 4 and 5 as the remaining regions to consider, and establishes that a transmission level above 0.048 (4.8%), continuum may be highly suspect to be leaked. Moreover, the extended region in the SS area 4 stretches way out in either direction into the continuum regions, and also has significant area. Ken notes that at +/- 0.125 nm beyond the "core shoulder," continuum will dominate, and as can be seen there is significant area in region 4 beyond this limit, whereas in the DS configuration, area 3 is well within this limit and would seem to have no significant contribution. These extended areas are where the flux from the photosphere becomes quite large, especially as there is overall about a 100,000:1 relationship between the brightness of the photosphere and chromosphere.
It would be interesting to analyze what parts of areas 4 and 5 actually contribute most significantly to "parasitic" continuum leakage (e.g. transmission height vs. wavelength) -- and see a more rigorous analysis from a mathematical perspective -- but that is not my forte - mostly a visual guy here...