Single Vs. Double Stack
Posted 06 January 2013 - 08:42 PM
"First it should be said that no normally achievable temperature will adversely affect the filter on a permanent basis. The AS1 series will easily withstand 200 degC.
The construction of these filters is such that, when they are optimised for operation in vacuum, the configuration of the assembly has an effective coefficient of expansion of 8x10e-8, corresponding to a passband shift in wavelength of ~1Angstrom/2,000degC. This design was originally conceived for a space project in which the satellite went in and out of the Earth's shadow encountering wildly varying temperatures.
The performance in air is more complicated. There are two factors involved,- the physical expansion of the etalon support structure and the change in refractive index of the air with temperature,- dn/dt. This value is -9.5x10e-7 and, as it is negative, means that the air gap becomes effectively narrower with increasing temperature. To combat this, a material is used for the support structure with a higher coefficient of expansion than that used for vacuum,- approximately 5x10e-7. As this value is positive, it can be chosen to balance the effect of changing refractive index of air. In the ASP-60 and AS1-90 models, this balance results in a coefficient of passband shift of approximately 1A/350degC. Thus, it can be seen that, provided the filter comes to equilibrium with ambient conditions, the shift of the passband is negligible for any normally experienced temperature.
However, there is a side effect that can be observed under certain conditions. Any change in temperature will affect the refractive index of the air more rapidly than the larger mass of the structure of the filter. Thus, if the ambient temperature is ramping quickly, the structural mass will lag behind the refractive index change of the air and the filter will drift slightly to shorter wavelength. It will resume its normal performance when it comes to equilibrium. It should be noted, however, that it could be possible for the ambient temperature to be changing enough that this point of equilibrium is never fully achieved.
The crux of the matter is that the filter is extremely thermally stable for a wide range of temperatures,- but not quite so in a rapidly changing temperature. Even so, this effect is not great;- amounting to a drift of about 0.25A in 40degC (72degF)."
Posted 06 January 2013 - 10:40 PM
So good find - and great info!
Coronado’s first solar filters basically took a prominence filter, which was an ERF plus a PROM15T (prominence, 15mm, maybe T for tilting?) and inserted an etalon behind the ERF. The prominence filter by itself showed no solar disk details... Over time I guess they switched to a blocking filter designed for the etalon. I don’t know, but I suspect that the new blocking filters were wider: still narrow enough to avoid other etalon orders but wide enough to avoid having to tune it with temperature variation (and perhaps cheaper to produce).
Yes, the Coronado blog confirms the PROM15T used a 2A bandpass prominence filter as described, while the later blocking filters used a 6A bandpass blocker, which apparently is well within the range needed for an etalon with a FSR (i.e. free spectral range - the distance between the side-band harmonic peaks) of 10A.
Posted 09 January 2013 - 01:56 PM
I would definitely want to "Try Before I Buy" due to the cost of a DS. It would have to be a huge improvement for me to spend that kind of $$$$.
In my opinion the substantial improvement in overall performance from reducing the bandwidth via double stacking filters is quite worth the added expense. This thread has covered a lot of ground, but I’d like to get back to and summarize the essence of what happens when you double stack air-spaced H alpha solar filters.
1. When you DS two 0.7 Angstrom filters, the “full width half modulation” (FWHM) bandpass is reduced form 0.7 Angstrom for the single filter to less than 0.5 Angstrom - per George’s graphs and research ~ 0.45 Angstrom.
2. The “tails” of the transmission curve of the filter system are greatly suppressed. This reduces to invisibility out-of-band continuum light coming from the photosphere. Again reference George’s graph’s.
The effect of these changes is to increase contrast of the features seen on the chomospheric disk, which is exceptionally obvious visually (represented by the top image shown below.)
Second, a “pure” view of the limb and prominence or filament/prominence detail is achieved without interference from the brighter “double limb” and disk of the photosphere (bottom image below). If one wishes to observe off-band features such as spicules in more detail, this can be easily achieved by tuning the filter system slightly off-band.
Third, and perhaps more subjectively, I and others have observed the scatter of light seems reduced, rendering the sky background darker. This may also represent a reduction in continuum light transmittance.
A disadvantage of double stacking is the additional expense to get the improved performance. I don’t know of many endeavors where this does not apply.
Another disadvantage is an overall decrease in image brightness. This is due to two factors - the peak transmission is reduced via the two etalons in series, and the presence of two energy rejection filters. The latter cause can be corrected relatively easily by judicious removal of one of the ERFs without any safety concerns - but may void your warranty.* To be honest the decrease in disk brightness was a blessing to me for visual use compared to a single filter (once I remove at least one of the ERF’s), and I do not notice any decrease in the visibility of prominences which are not Doppler-shifted.
Speaking of which - another possible disadvantage for some might be the decrease in the visibility of Doppler-shifted events due to the narrower band-pass. This again can be remedied by a slight de-tuning of the filter system. If this is your main interest, you might be better off with a wider bandpass system or a prominence only filter.
But for most of those who have tried it - “once you’ve double stacked, you won’t go back.”
* Note - Solarscope DS etalons may not have the redundant ERF. If so they can’t be used as a stand alone filter. Solarscope also supplies a second blocking filter with less ND filtering for use when double stacking due to the decreased peak transmission of two etalons, which is nice for imaging as well as observing. I found that removing the RG (red galss) ERF's and replacing them with a single Baader D-ERF (which uses dielectric coatings instead of colored glass) likely does the same thing due to its increased transmission.
Posted 09 January 2013 - 05:35 PM
I agree wholeheartedly!
“once you’ve double stacked, you won’t go back.”
Posted 12 January 2013 - 05:27 PM
Posted 09 March 2013 - 07:18 PM
Question: 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. George
... 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. Bob
As follow up to the discussion above, and George’s question about capturing the spicule layer off-band; today I could take advantage of better seeing than in December and was able to capture the spicule layer at 0.5 Angstrom FWHM.
The top image shows the on-band performance, with good prominence detail, macro-spicules, and no hint of the “double limb” of the photosphere showing through. The bottom image shows the filter system tilt-tuned off-band, now clearly showing the spicule layer, with the limb of the photosphere now apparent as the filter is blue shifted off the H alpha line ~ 0.5 A.
Note: reprocessed the image to bring out more spicule detail:
Posted 10 March 2013 - 07:39 PM
Is that the same point on the sun at around the same time? I.e., did the prominence disappear almost completely off band? So the prominence is not moving much in our line of sight, but those off-band spicules happen to be pointed towards us, and if you go on band, you are actually seeing different spicules (versus the spicules each having broader emission)?
Posted 10 March 2013 - 10:11 PM
Yes it is indeed the same area on the sun's limb, with the top prominence image taken at 23:00 UTC, and the bottom spicule image taken at 22:59 UTC. May not be perfectly lined up - but close.
I'd agree with your interpretation, with the caveat that the large spicules in the prominance image are "macro" spicules projecting above the "spicule layer." I will have to wait for better seeing to get a better resolved pair of images. And as you noted above, tweaking the secondary filters tilt did make the off-band image brighter - confirming your hypothesis - thanks again for your input
BTW - the last image on my March 9, 2013 Sun "polar prominence" images shows the on-band prominence a bit earlier at 22:51 UTC.