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drksky
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Single Vs. Double Stack
      #5587193 - 12/24/12 12:57 PM

I was curious to see the difference between a single and double stack configuration, so I did a little experiment with our SolarMax II 90. Here are the results:


I think the tuning of the DS image was slightly off, but you can see the marked difference between the two.


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*skyguy*
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Re: Single Vs. Double Stack new [Re: drksky]
      #5587298 - 12/24/12 02:15 PM

I actually find the surface detail more interesting in the single filter image.

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drksky
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Re: Single Vs. Double Stack new [Re: *skyguy*]
      #5587472 - 12/24/12 04:22 PM

In this one, yes, I think so, too. But as I said, I think the DS tuning was off. But other DS images I've taken have looked much better. I think I pushed the etalon too far in this one and killed the detail in the active areas.

On the other hand, though, I do think that active areas can be more interesting through a SS as the spot detail shows better.


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rigel123
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Re: Single Vs. Double Stack new [Re: drksky]
      #5587501 - 12/24/12 04:45 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 $$$$. Evidently it must due to the number that have them!

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Pawel
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Re: Single Vs. Double Stack new [Re: rigel123]
      #5587653 - 12/24/12 07:14 PM

I own two SM90 front etalons and I see biiiig differnce between single and double stack. Belive me, it's worth

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marktownley
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Re: Single Vs. Double Stack new [Re: Pawel]
      #5587668 - 12/24/12 07:24 PM

I think a narrower bandpass is the way forward - trouble is its the $$$

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BYoesle
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Re: Single Vs. Double Stack new [Re: marktownley]
      #5587714 - 12/24/12 08:14 PM Attachment (105 downloads)

Double stacking air spaced etalons reduces the FWHM bandpass from 0.7 angstroms for the single etalon to less than 0.5 angstroms for the pair. This is a very significant benefit, as at 0.7 A FWHM, “parasitic” light form the photosphere is passed through, obscuring chromospheric detail.

I used Coronado SM 90's, and a single Baader D-ERF for each configuration. The images were processed identically, and demonstrate the single SM90 on the left vs. double stacked SM90's on the right. The added brightness of the disk on the right is due to photospheric light leaking through, significantly reducing the contrast of the chromospheric details. Also note that prominences are as easily visible at < 0.5 A. Indeed, due to the reduction of photospheric contamination, prominences generally appear equally if not more “prominent” at < 0.5 angstrom bandpass:


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BYoesle
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5587715 - 12/24/12 08:15 PM Attachment (81 downloads)

The image on the left clearly shows the “double limb” artifact - the disk of the photosphere leaking through at 0.7 A - rendering a translucent appearance to the chromosphere, and again reducing the contrast of chromospheric detail. Chromospheric detail is much more easily seen and imaged with the DS <0.5 A bandpass on the right. Both images again are identically processed.

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dbowlin
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5588147 - 12/25/12 07:05 AM

Great prensentation, BYoesie. Thanks, now I want to DS my SM 60. Will I get similar results? With what little viewing oppritunity I have had this year I don't know if it will be worth it?
Dale


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BYoesle
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Re: Single Vs. Double Stack new [Re: dbowlin]
      #5588263 - 12/25/12 09:58 AM

Hi Dale,

You will get identical results with regard to contrast improvement and detail visability. I don't know of anyone who properly double stacks who doesn't see a tremendous improvement. There is an "old-wives" tail about it reducing the brightness of prominences, but as you can see this is false, for obvious reasons. The image will be dimmer overall - however there are measures you can take to improve this, mainly removing one (or more) of the ERF's:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=solar&...


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dbowlin
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5588738 - 12/25/12 06:02 PM

Thanks BYoesle, I enjoyed your older post. Great food for thought. Half the fun is deciding what to do and how best to do it.
Dale


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David Knisely
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5588961 - 12/25/12 10:05 PM

Quote:

The image on the left clearly shows the “double limb” artifact - the disk of the photosphere leaking through at 0.7 A, rendering a translucent appearance to the chromosphere, and again reducing the contrast of chromospheric detail. Chromospheric detail is much more easily seen and imaged with the DS <0.5 A bandpass on the right. Both images again are identically processed.




Well, not quite. What the left image is showing on the limb is the Chromospheric "fringe"; the mass of fine spicules that, depending on how the filter is tuned, can show up as a thick band (sometimes called "the Spicule Forest"). Detuning the filter slightly can let the individual spicules show up in the filter, but the presence of the band on the limb does not necessarily mean that photospheric energy is leaking through. Zirin's book ASTROPHYSICS OF THE SUN on page 161 shows the effect of tuning a very narrow H-alpha filter on the spicule forest. A narrower bandpass filter set to the centerline of H-alpha will tend to screen out some of the slightly Doppler shifted detail (including some of the spicules), so the spicule forest band may begin to break down somewhat and be less prominent (or may show individual spicules that happen to be emitting in the passband of the filter). A broader filter will show less contrast for chromospheric disk detail but by its broadness, let in a little more off-band energy and may make the prominences a little easier to see than in a narrower filter. Thus, double stacking will show better disk detail contrast with a slight loss in the brightness and visible extent of some of the prominences. Clear skies to you.


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George9
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Re: Single Vs. Double Stack new [Re: David Knisely]
      #5589003 - 12/25/12 10:39 PM

David, I don't doubt that a broader filter can pull in some more energy from spicules or prominences that are slightly off band, but I still assume that the reason for the presence of a dimmer spicule band above a brighter disk in Bob's left photo is photospheric leakage (i.e.,the brighter disk is leaked photosphere), no? In the Zirin p. 161 figure caption, Zirin seems to claim it is actually "the general chromosphere" instead, not the photosphere.

In that same p. 161 figure, I cannot find a citation to the bandwidth. Are you sure it is very narrow?

Also, even if it is nominally narrow, say 0.4 A, what if it is a Gaussian filter with relatively high tails. Couldn't it still be photospheric leakage?

Versus say a double-stacked filter with sharper shoulders and lower tails and therefore less photospheric leakage (i.e., Bob's right photo).

Which actually brings up a separate question about double stacking. Is the improved contrast from double stacking really due to the bandwidth going from 0.7 to 0.5 (the half-height width), or is it really due to changing the shape of the transmission curve from Gaussian to something with squarer shoulders and therefore lower tails? (That is, is most of the leaked energy right near the centerline, or is it spread further away in the tails?) I remember going over this with David Lunt in 1999 or 2000, and we did the math and it seemed to be the tails, not the nominal bandwidth. So a 0.7 A filter with sharp shoulders would also have noticeably better contrast than a regular 0.7 A filter.

George


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David Knisely
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Re: Single Vs. Double Stack new [Re: George9]
      #5589038 - 12/25/12 11:12 PM

George9 wrote:

Quote:

David, I don't doubt that a broader filter can pull in some more energy from spicules or prominences that are slightly off band, but I still assume that the reason for the presence of a dimmer spicule band above a brighter disk in Bob's left photo is photospheric leakage (i.e.,the brighter disk is leaked photosphere), no?




I am more concerned with his choice of words here. That band is indeed the chromosphere seen in profile (ie: from the side). It is composed mainly of spicules and short fibrils or filaments and is not an "artifact" of the photosphere but a very real feature. In a narrower filter when tuned correctly, you can still see that fringe, although it will be notably less easy to see in a distinct way from the limb in the narrower filter, especially if the filter is tuned to the very centerline wavelength. The photospheric "leakage" is really continuum leakage and visually is better expressed by the lower contrast of the disk detail rather than the presence or absence of the fringe on the edge of the limb. Thus, calling it an "artifact" isn't really correct. Clear skies to you.


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Spectral Joe
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Re: Single Vs. Double Stack new [Re: David Knisely]
      #5589120 - 12/26/12 12:53 AM

A thread from nearly a year ago that discusses the issue, with some references: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5057295/page...
The "band" is indeed the chromosphere, the "artifact" is the smaller disk, which is continuum light leaking through. It's hard for people to accept that their $6000 solar telescope doesn't "pass only the light of hydrogen alpha" like the advertisements say, but that's the way it is. A single etalon system, unless a much better (and more expensive) blocking filter is used, will pass enough contimuum to be noticed. Lunt, Coronado, and others know this. These instruments are marketed to the amateur community, who for the most part can't afford the equipment that has the top performance in this regard. The cost differential is large. When these etalon based systems were conceived I don't think anyone thought about high dynamic range CCD cameras and powerful image processing software being applied to them. Once these techniques are applied the faults become apparent. Never fear, you can always double stack. For more money, of course.


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George9
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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5589278 - 12/26/12 08:00 AM

That is a great thread. And yes that's what I meant by artifact. Thanks so much. George

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David Knisely
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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5589371 - 12/26/12 09:42 AM

Quote:

A thread from nearly a year ago that discusses the issue, with some references: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5057295/page...
The "band" is indeed the chromosphere, the "artifact" is the smaller disk, which is continuum light leaking through. It's hard for people to accept that their $6000 solar telescope doesn't "pass only the light of hydrogen alpha" like the advertisements say, but that's the way it is. A single etalon system, unless a much better (and more expensive) blocking filter is used, will pass enough contimuum to be noticed. Lunt, Coronado, and others know this. These instruments are marketed to the amateur community, who for the most part can't afford the equipment that has the top performance in this regard. The cost differential is large. When these etalon based systems were conceived I don't think anyone thought about high dynamic range CCD cameras and powerful image processing software being applied to them. Once these techniques are applied the faults become apparent. Never fear, you can always double stack. For more money, of course.




I so dislike the use of the terms "artifact" and "leaking through", as they imply that the filtering system isn't working properly when, in fact, it is. The passband shape of an etalon-based filtering system isn't infinitely sharp and the H-alpha line isn't square-wave sharp either. In fact, there is even just a little continuum energy from the sun at 6562.8 angstroms that is not being filtered out. What is happening here is purely a function of the passband width of the filter. The narrower the bandwidth, the greater the contrast of the disk detail tends to be. This is a combination of letting less continuum light in as well as screening out some of the slight Doppler shifted light detail. The fibril and filament disk structure tends to seem darker and a little sharper in narrower filters than in broader ones. It's just that simple. If you have a "leak", this implies that something well-off the primary passband's location is being let through, such as the unintended "leaks" found in some of the common nebula filters. This isn't the case in H-alpha solar filters. The primary passband is not leaking anything (it is just wider or narrower depending on the system's specifications or cost). As those with the old Daystar filters have sometimes found out, once the blocking or trimming filters start breaking down and actually "leaking" some of the other passbands of the "comb" created by the etalon, the H-alpha detail just goes away, so even a slight "leak" will kill the view and make it into just a very red version of the white light view. If you are seeing any chromospheric disk detail, the filter isn't really leaking anything. The same goes with calling some feature an "artifact", which in imaging and optical circles tends to mean something induced by the system that is not really there. This is not the case for the view of the spicule forest. It is a very real feature. Slight tuning variances can make it appear more distinct even in rather narrow filtering systems, so just because it is seen does not mean that this is some "artifact". If you want greater contrast, a double-stacked etalon can sometimes provide that, which is the plain and simple truth. Clear skies to you.


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marktownley
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Re: Single Vs. Double Stack new [Re: David Knisely]
      #5589828 - 12/26/12 02:19 PM

An interesting read indeed chaps, thanks!

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BYoesle
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Re: Single Vs. Double Stack new [Re: marktownley]
      #5589937 - 12/26/12 04:13 PM Attachment (38 downloads)

Quote:

Which actually brings up a separate question about double stacking. Is the improved contrast from double stacking really due to the bandwidth going from 0.7 to 0.5 (the half-height width), or is it really due to changing the shape of the transmission curve from Gaussian to something with squarer shoulders and therefore lower tails? (That is, is most of the leaked energy right near the centerline, or is it spread further away in the tails?) I remember going over this with David Lunt in 1999 or 2000, and we did the math and it seemed to be the tails, not the nominal bandwidth. So a 0.7 A filter with sharp shoulders would also have noticeably better contrast than a regular 0.7 A filter.




For reference, I thought I'd post an explanation on how double-stacking reduces bandwidth from David Lunt, original founder of Coronado Filters:

The result of two identical etalon filters in series is a convolution of the transmission bands of each. The single etalon has a passband shape which is Gaussian. If the bandwidth at 50% of maximum transmittance is w, then that at 10% of Tmax is 3.5w and that at 1% Tmax is 10w. The transmittance at any point in the spectrum of the stacked pair is T squared, where T is the transmittance of the single filter. The most important characteristic is that the bandwidth is reduced by the square root of 2. Given two etalons with bandwidths of 0.7A, the combined bandwidth becomes 0.5A, and the 1% bandwidth (or the "tails" of the passband) are reduced from 7A wide to ~1.8A. Thus the effect is to narrow the actual bandwidth and increase the visibility of chromospheric detail, while the steeper shape of the passband reduces the out of band transmission, thus significantly improving contrast. Empahsis added.

Also see this discussion:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3435927/page...


Quote:

I so dislike the use of the terms "artifact" and "leaking through", as they imply that the filtering system isn't working properly when, in fact, it is.



Sorry if I have offended anyone. I did not mean to imply that by use of the terms “leaking through” or “artifact” that a 0.7 FWHM filter is not working properly. Rather only that the filter system is indeed letting more off-band photospheric energy through, degrading contrast, and showing features that are not really a property or part of the chromosphere – such as more “spot detail,” as well as the “double limb” of the photosphere.

In the 50’s and 60’s there was some scientific debate as to whether or not the “double limb” was a true feature of the chromosphere (see the link in Spectral Joe's post). The “double limb” phenomena was determined to be an “artifact” of “parasitic” photospheric light contamination by blocking filters with insufficient narrowness to block side band harmonics. The double limb was determined in reality to be the border of the photosphere’s disk, and not a property or phenomena of the chromosphere. The double limb also appears identical with bandpasses over 0.5 A.

Quote:

In a narrower filter when tuned correctly, you can still see that fringe, although it will be notably less easy to see in a distinct way from the limb in the narrower filter, especially if the filter is tuned to the very centerline wavelength... This is not the case for the view of the spicule forest. It is a very real feature. Slight tuning variances can make it appear more distinct even in rather narrow filtering systems, so just because it is seen does not mean that this is some "artifact".




I think this may be an issue of semantics - a narrow band filter eliminates the light from the wavelengths adjacent to the H alpha emission line, and therefore the boundary between the photoshpere and chomosphere disappears. Contrast is improved, and non-doppler shifted prominences are unaffected and as easily visible as at a wider bandpass.

There is no hint of the double limb, "fringe," or a separate “spicule forest” in my system when properly tuned and optimized at < 0.5 A . Of course if it is tuned off-band, more light form the photosphere will appear, showing the double limb artifact, or 'effect' if you prefer. By tuning a narrow band filter off band, one will of course introduce more continuum light, revealing the edge of the photosphere, and thereby producing the "spicule layer," or "fringe." However, in reality it is still the entire chromosphere (including spicules) on edge delineated by the now revealed photospheric boundary.

That the area of the chromosphere shown between the inner "double limb" shown at 0.7 A – the limb of the photosphere – and the remainder of the chromosphere lying beyond it, is sometimes mistakenly referred to by some as the "spicule layer/fringe" is understandable, but incorrect. It more accurately represents the entirety of the chromosphere lying above the photosphere (see diagram in the following post). Spicules themselves – features of the chromosphere - rise from the base of the chromosphere and penetrate the transition layer into the corona. These can be seen in the < 0.5 A double stacked animation link below just to the left of the surge prominence.

Again note there is no hint of the double limb from on-band light from the photosphere - or off-band light adjacent to the H alpha line - coming through, as it is far below the threshold (S/N ratio) needed for detection. Remember cooler Hydrogen in the photosphere is absorbing this wavelength far in excess of what the chromosphere is emitting, so with a sufficiently narrow bandpass the signal to noise ratio has rendered any off-band photospheric contibution invisible, and any on-band contibution from the photosphere is so weak as to also be invisible.

GIF https://dl.dropbox.com/u/31679363/June%2020%2C%202012%20Animation2%20gif.gif

AVI https://dl.dropbox.com/u/31679363/June%2020%2C%202012%20Animation1.avi


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BYoesle
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5590005 - 12/26/12 05:10 PM Attachment (42 downloads)

Another recent CN post concerning double stacking with good representations of the effects of narrower bandpasses can be found here:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=solar&...

Note particularly the single, double, and triple stacking comparison images by Jesus...


Representation of the chromosphere and spicules courtesy of NASA:


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Bill Cowles
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Re: Single Vs. Double Stack new [Re: dbowlin]
      #5590488 - 12/26/12 11:08 PM Attachment (48 downloads)

Stack...

Bill


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Andy Devey
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Re: Single Vs. Double Stack new [Re: Bill Cowles]
      #5591035 - 12/27/12 11:35 AM

Hi Guys

This really is an excellent and informative thread many thanks to you all

Regards

Andy


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David Knisely
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5591385 - 12/27/12 03:40 PM

BYoesle wrote:

Quote:

That the area of the chromosphere shown between the inner "double limb" shown at 0.7 A – the limb of the photosphere – and the remainder of the chromosphere lying beyond it, is sometimes mistakenly referred to by some as the "spicule layer/fringe" is understandable, but incorrect.




No, it is not really all that incorrect. That is what the feature has been called in the professional literature (spicule forest, "sierra", "burning prairie", ect.). To cite Prof. Harold Zirin (Astrophysics of the Sun, p. 161-169):

Fig. 7.5 caption: "Spicules at various points in H-alpha. In the line center (0 Angstroms) we see the spicule forest extending up to 7000 km, marked at lower heights by the general chromosphere. Beyond +/-1/2 Angstroms, the narrow emission of the general chromosphere disappears, and only the broad-line spicules remain."

Text on page 169: "Although spicules appear to form a thick forest in H-alpha pictures at the limb, Cragg, Howeard and Zirin (1963) recognized that they occur only at the edges of the magnetic network and subsequent pictures confirmed this fact. In the center of the disk, the spicules are seen to protrude in all directions from the network elements, tracing the connecting flux loops. The lower velocity of the IN (intra-network) structure produces a fairly narrow Doppler profile. Thus this widespread structure, which may be thought of as the general chromosphere, is not seen further from the H-alpha line center than +/- 0.6 angstroms. At the limb (Fig. 7.5) spicule and IN chromosphere are all mixed up. In centerline H-alpha there is a fairly continuous band about 7000 km high: since this disappears at +/-1/2 angstroms, we identify it with the IN chromosphere. Outside this wavelength only spicules are seen on disk or limb. Although long exposures with a coronagraph will show spicules extending up to 10000 km the fact that most spicules are tilted means that they are no higher than the IN chromosphere. Below 1500 km there is little structure visible in centerline, but at H-alpha +/- 0.3 angstroms, a dark band, due to a minimum in the Doppler broadening, can be detected."

In imaging circles, a visual "artifact" is defined as "anomalies during visual representation of e.g. digital graphics and imagery". The fringe-like feature seen in H-alpha filters on the limb of the sun is not an artifact. Its appearance will vary depending on where the filter is tuned and its FWHM bandwidth, but it is a real feature. Clear skies to you.


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BYoesle
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Re: Single Vs. Double Stack new [Re: David Knisely]
      #5591692 - 12/27/12 07:39 PM Attachment (36 downloads)

Hi David,

Thanks for the excellent citations you have referenced (I actually own and have read Zirin’s books as well.) No one has denied the presence or visibility of spicules in the chromosphere either at 0.7 or 0.5 A FWHM. Yet it is the entire chromosphere that is being shown by the presence of the photosphere at 0.7 A FWHM, not just spicules, spicule forest, ect.

I did specifically note in the next sentence following the quote above: “It more accurately represents the entirety of the chromosphere lying above the photosphere (see diagram in the following post).”

And obviously it is real.

I have not stated “the fringe like feature” on the limb of the sun is an artifact. I have stated the inner limb at 0.7 A FWHM is that of the photosphere, and in contradistinction, it is this photospheric light and limb that I and others have referred to as an artifact - not the chromosphere, or it’s components such as spicules, prominences, etc.

Initially it seemed you were referring to the layer above the photosphere (and perhaps still are) in my 0.7 A image as being more or less just the spicules: “the Chromospheric ‘fringe’; the mass of fine spicules that, depending on how the filter is tuned, can show up as a thick band (sometimes called "the Spicule Forest").” While this description “is not really all that incorrect,” it is incomplete. The layer simply is the entire chromosphere - including spicules, prominences, etc.

The photosphere too is real, and I suppose in this way it is not literally an “artifact” by the definition you are using. A better word might have been “defect” as it applies to the filter’s performance, if your desire is for it to only show the chromosphere. But it seems you would argue this too is false, as the filter is performing within specification. That would be true, and I believe that is the point.

I’m not trying to argue about semantics... I’m trying to clarify for CN’ers what they are seeing or imaging, what double stacking verses single stacking will show or not show, and to deal perhaps with expectations of the equipment they have and how it will perform.

Best wishes,


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George9
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5591981 - 12/27/12 11:39 PM

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.

George


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David Knisely
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5592100 - 12/28/12 02:21 AM

BYoesle wrote:

Quote:

The photosphere too is real, and I suppose in this way it is not literally an “artifact” by the definition you are using. A better word might have been “defect” as it applies to the filter’s performance, if your desire is for it to only show the chromosphere. But it seems you would argue this too is false, as the filter is performing within specification. That would be true, and I believe that is the point




Yes, it is not an artifact or a defect, so the use of that word would also definitely be incorrect. If you see this in a filter with a designed bandwidth of 0.7 angstroms, the filter is not defective. It is performing about as well as it can given the design limitations. It is the appearance of the sun with the given bandwidth that is the only question., and again, the narrower the filter's bandwidth specs are, the higher the contrast of disk detail tends to be. That is the only point to be made here. Clear skies to you.


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Re: Single Vs. Double Stack *DELETED* new [Re: David Knisely]
      #5592127 - 12/28/12 03:29 AM

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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5592778 - 12/28/12 02:25 PM



Bill


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Re: Single Vs. Double Stack new [Re: ValeryD]
      #5593469 - 12/28/12 10:37 PM Attachment (29 downloads)

Quote:

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.




Hi Valery,

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.


Quote:

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.

George




Hi George,

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:

Quote:

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:

Quote:

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. (b) 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. (c) A limb image on a different date with a pure filter (Rakuljic filter), with the inner limb barely detectable from the morphology.


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5593559 - 12/28/12 11:32 PM

Very very good, Bob.

I cannot find an image anywhere, but if you take two Gaussians [this should have been Lorentzians] and multiply them (i.e., two filters in series), you get the best output when their centerlines coincide. When you start to shift them, the output just becomes dimmer, not narrower.

George

Edited by George9 (12/30/12 10:44 PM)


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Re: Single Vs. Double Stack *DELETED* new [Re: George9]
      #5593753 - 12/29/12 03:04 AM

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Re: Single Vs. Double Stack *DELETED* new [Re: ValeryD]
      #5593898 - 12/29/12 08:22 AM

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Re: Single Vs. Double Stack new [Re: bob71741]
      #5594208 - 12/29/12 11:34 AM

For Lorentzian, as you shift the filters with respect to each other, it not only becomes dimmer, it also becomes wider. I tried to show it in the attached figures. The first shows (on an arbitrary frequency scale centered at 0) a single stack (SS) and double stack (DS center), the latter being the square of the first.

The second figure shows two SS's each offset in opposite directions but with the same bandwidth. The product is "DS shift," and "DS shift norm" is the same curve normalized to better see the width.

The third figure compares DS center to DS shift. The normalized version demonstrates that it gets wider.

As always, I could be wrong, but these were the kind of figures I was looking for. Sorry about the wrong function earlier.

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5594210 - 12/29/12 11:35 AM Attachment (28 downloads)

Figure did not attached. Let me try again. George

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Re: Single Vs. Double Stack new [Re: George9]
      #5595139 - 12/29/12 09:01 PM Attachment (25 downloads)

Very nice diagrams George!

Quote:

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



Answer:

Hi George,

... 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.

Bob




The top illustration shown below exhibits the transmission shapes to scale for a single H alpha etalon with 0.7 A FWHM (green) verses 0.5 A FWHM (red) pair of double stacked etlons (superimposed on a representation of the sun’s absorption spectrum). Your diagrams too show very well the changes that occur with double stacking per David Lunt’s description:

Quote:

... Thus the effect is to narrow the actual bandwidth and increase the visibility of chromospheric detail, while the steeper shape of the passband reduces the out of band transmission, thus significantly improving contrast.




The much broader “tails” and (and to a lesser degree wider bandpass) demonstrates why the photosphere becomes visible at 0.7 A, while it is invisible with a < 0.5 A DS system with it’s steeper sides and suppressed tails.

But also of interest (and related), when viewed with the emission spectrum (bottom) of the of the chromosphere at the same scale (distance between the “horns” is +/- 0.7 A = 1.4 A), we may see why the “general chomosphere” is all that can be seen with the narrower bandpass filter when it is "on-band" (as in filtergram c above), while at a wider bandpass the photosphereic disk appears, along with evidence of the network of Doppler-shifted spicules (filtergram b above). When a < 0.5 A filter is tuned off-band to the red or blue wing, the Doppler-shifted spicules should reveal themselves clearly, albeit with less continuum (photospheric) interference than the wider bandpass.

I will try to image this at < 0.5 A blue shifted (all I can do s tilt my filters to the blue wing) when the weather improves this spring, but some others (perhaps Bill in sunny Utah for instance ) might be able to show this to us much sooner.

Quote:

The H-alpha spectrum is most interesting (Figure 2)... Bright emission horns about twice the brightness of the central core appear at either wing. The structures seen in the horns are due to the slit crossing several spicules... The photospheric absorption line continues into the chromosphere without change, but bright emission horns of brightness equal to the photosphere appear at +/- 0.7A extending up to about 4000 Km. With good seeing, these break up into emission from individual spicules, extending to greater line shifts than the static chromosphere. At greater heights the horns disappear...

Johannesson, A. & Zirin, H. (1996) The Pole-Equator Variation of Solar Chromospheric Height. Astrophysical Journal, 1996, Volume 471, pp. 510-520




Lower image: Astrophysical Journal, 1996, Volume 471, The Pole-Equator Variation of Solar Chromospheric Height, Fig. 2:


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5595178 - 12/29/12 09:42 PM

I use a Lunt 152 LS to which I have added a Coronado 90 mm etalon up front to provide < 0.5 Angstrom bandwidth. The views are great as long as I don't dial up the magnification too high. Using an eyepiece less than 8 mm then the central obstruction of the Coronado begins to make its presence known.

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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5595194 - 12/29/12 09:53 PM

By far, the most significant contributor to the visible photosphere in a single etalon system is the poorly suppressed etalon modes blue and red of the desired mode. The combined transmission of these can easily exceed 5%, and they are located well outside any part of the H alpha absorption, where the intensity is 5 times that in the line core. In a phone discussion with one of the GONG design staff earlier this year the subject came up, relating to why imaging disk and proms was difficult with a single stack system. The NSO staffer (who's been doing this since before most of us were born) said that in single etalon systems the sideband leakage would present exactly the situation observed, and that professionals would be using these instead of the expensive alternatives if the performance was the same.
So why does a double stack eliminate the problem? Manufacturing tolerances. A difference of just 20 microns in the spacing in an air spaced etalon, or less of a variation in the thickness of a solid etalon, will shift the offending sidebands to the point where they fall beyond the skirts of the passband of a similar etalon (of the original spacing). This, combined with the usual blocking filter, results in a near total elimination of the unwanted light passed in these sidebands, if two slightly different etalons are stacked together. Evidence shows that the thickness is not that well controlled. Simply reducing the bandpass from 0.7 to 0.7 Angstroms isn't enough, but math shows that the sideband leakage is there, and that eliminating it makes enough difference to explain what is observed.


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5595199 - 12/29/12 09:55 PM Attachment (17 downloads)

Sunny Utah?

Bill


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Re: Single Vs. Double Stack new [Re: Bill Cowles]
      #5595375 - 12/30/12 12:10 AM

Here's a movie, scanning from 2 Angstroms blue of H alpha to 2 Angstroms red, 0.35 Angstrom bandwidth. The transition from photosphere to chromosphere is smooth, you can see the image "swell" as it passes through the line center. The seeing was bad and variable, as you can see from the ripples on the limb, but you can get the idea.



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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5595425 - 12/30/12 12:49 AM

I have learned a great amount in this thread. Thanks again, Bob.

I always wondered what manufacturers meant when they said they were "matching" two filters to make sure that they could be double stacked. Based on Joe's post, were they just making sure that the sidebands did not happen to line up?

David Lunt worried that my ASP-60 and a friend's ASP-60 would not make a good double stack, but it worked well. Perhaps the fact that they were manufactured a year apart actually helped.

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5595451 - 12/30/12 01:28 AM

George, your plots are correct, if you're still wondering. I've been lazy in using Gaussians to show the effect of stacking filters. I had a Gaussian spreadsheet already built and didn't want to take the time to do it right. I spent some time today to make a Lorentzian version and it agrees with yours.

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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5595463 - 12/30/12 01:50 AM Attachment (34 downloads)

Wow Joe - that’s a spectacular video! I note how in the red and blue wings chromospheric disk details like filaments (as well as prominences) disappear, but the spicule forest of bright patches and mottles really pops out - just as they do (i.e. spicules) at the limb:

Quote:

...we can now see that Secchi’s needles (the “spicules”) are arrayed in long picket fences... Spicules lie at the borders of supergranule cells...

Zirker, J.B. (2002) Journey from the Center of the Sun (p. 142). New Jersey, Princeton University Press.




Hi George: The inside story on ‘matching’ of front mounted etalons can be found here:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=solar&...

Then see this post for my solution:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=solar&...


Quote:

In a phone discussion with one of the GONG design staff earlier this year the subject came up, relating to why imaging disk and proms was difficult with a single stack system. The NSO staffer (who's been doing this since before most of us were born) said that in single etalon systems the sideband leakage would present exactly the situation observed...

[Joe]




This is another reason for double stacking to reduce bandwidth (as noted above). Besides improving contrast, removing the light of the photospheric disk equalizes the brightness of the disk and the prominences, making capturing both in the same exposure much easier, and eliminates the need to combine separate exposures for each - note the filaprom @ ~ 10:30:

June 15, 2012 21:37 UTC


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5595497 - 12/30/12 02:40 AM Attachment (30 downloads)

When you process a properly exposed 0.7 A disk filtergram to bring out disk detail with increased contrast, you usually end up wiping out the limb chromosphere and prominenecs. That’s why you may be required to make two separate exposures, or use more involved processing to obtain both disk and prominence details:

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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5595528 - 12/30/12 03:36 AM

Spectral Joe wrote:

Quote:

Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!




I love that sig! A friend of mine lives up in the wilds of the Paul Bunyan State Forest in Minnesota on Mantrap Lake and I went to visit him one year. We had his 10 inch f/5 Newtonian out at the lake shore (three inch off-axis ERF and "Barlowed" to f/31) with his DayStar T-Scanner and were watching a post-flare arcade develop on the limb when some people came by on a boat and shouted out, "You'll go blind!!". We just smiled and went back to the scope. Clear skies to you.


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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5595918 - 12/30/12 11:14 AM

Quote:

George, your plots are correct, if you're still wondering. I've been lazy in using Gaussians to show the effect of stacking filters. I had a Gaussian spreadsheet already built and didn't want to take the time to do it right. I spent some time today to make a Lorentzian version and it agrees with yours.




The thing is, when I incorrectly did the Gaussian the day before, I discovered that as you shift, it becomes dimmer but not wider. That is, when you normalize, all the curves coincide. I guess it's a moot point unless there are also Gaussian filters out there, but it's an interesting property. I knew it was supposed to get wider, but I didn't make the connection until seeing Valery's post. Bottom line, I guess, is that Gaussian turns out not to be a good shortcut.

Thanks for the double stacking links, Bob. So the matching is more mechanical. I remember seeing your great innovation when you first posted it. I just put up with the two dark ERFs and the TMax. Although when one of my filters decontacted after 12 years and I had to hacksaw the ASP-60 apart to fix it, I made it so I could leave one ERF off, and that brightened the double stack.

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5596152 - 12/30/12 01:14 PM Attachment (24 downloads)

Hacksaw?! You’re a brave man George!

Quote:

I made it so I could leave one ERF off, and that brightened the double stack.






Just in case others are as rusty on their math and statistics as I am, I thought I’d post what the shapes for Gaussian (“normal” or “Bell”) distributions verses Lorentzian (“Cauchy”) distributions look like (upper diagram).

It seems that unless you “do the math,” it can be hard to tell the difference given the variables of amplitude and deviation (lower diagram) - where the Gaussaian vs. the Lorentzian distribution looks almost identical to the DS v. SS profiles above...


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5596600 - 12/30/12 05:25 PM Attachment (23 downloads)

Your bottom curve really shows it well. I threw in a double and quadruple stack, all at the same FWHM. It looks suspiciously like a large number of Lorentzians will approach Gaussian.

(I had to hacksaw. Lunt said neither they, nor Meade, nor Isle of Man would be able to do it cost-effectively, so I had nothing to lose. Luckily the spacers remained stuck to one element.)

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5596818 - 12/30/12 07:41 PM

excellent post...thanks for starting and continuing...

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Re: Single Vs. Double Stack new [Re: George9]
      #5596876 - 12/30/12 08:18 PM Attachment (26 downloads)

Thanks for the plots George.

I was wondering if you might run the following bandpasses and post them as you have above at the same amplitude:

Lorentzian - 0.8, 0.7, 0.5, and 0.35 Angstroms FWHM respectively (this last value is what I assume results from a triple stacked series of 0.7 FWHM etalons). Is quad stacking actually a 0.24 A FWHM bandpass?

It would be nice to have a comparison for what Jim Ferreira is imaging at 0.8 A (which obviously shows a near-photospheric disk and lots of Doppler-shifted as well as on band prominences), as well as what I and others are seeing at 0.7 A and 0.5 A, and what Jesus Munoz has shown with his SS, DS, and TS images seen here:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=solar&...

From what Jesus’ images show in the thread above, and interpolating a curve between your green Lorentzian ^2 curve (double stacking) and the red Lorentzian^4 curve (quad stacking), I’m not sure I see much benefit to triple stacking compared to double stacking - but then again the curves shown above are at the same FWHM. It would be nice to see some quantitative data or graphical representations, and to perhaps show what the narrower DayStar or Solar Spectrum filters actually accomplish - assuming these solid etalons provide similar shaped transmission curves.

Joe might press his awesome spectrohelioscope into service for showing some controlled comparison images to go along with these graphs - or perhaps he already has. It’s hard to find solid info like this anywhere...


Addendum

Finally had some sun today, but the seeing was horrible (1-2/10). Temp -1 C, solar altitude ~ 22 degrees (local noon), relative humidity 26%, winds SE 5-10 km/h. Close ups of the limb spicules off-band will have to wait for a much higher sun, and hopefully warmer temps. But I did get images of the chromosphere double stacked @ < 0.5 angstroms: top on-band @ 20:43 UTC, bottom ~ 0.7 A to the blue wing @ 20:44 UTC. Tuning was accomplished using the etalon closest to the objective (i.e both etalons were tilted together)...

The change off-band is obvious, showing the spicule (mottle) forest and much diminished general chromosphere features (Joe’s video is much better). Both images with identical exposures (5 ms), gain (575), gamma (1500), and processing. Paradoxically, the off-band image is dimmer - any ideas about this??? From the discussion above more continuum from the wing should be mkaing it through (especially considering tilting also widens the bandpass slightly), and therefore it should be brighter - no?

Quote:

The thing is, when I incorrectly did the Gaussian the day before, I discovered that as you shift, it becomes dimmer but not wider.

[George]




Hmmm.....


Edited by BYoesle (12/30/12 10:31 PM)


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5597171 - 12/30/12 11:46 PM Attachment (31 downloads)

Very good image despite the conditions. I gave it a visual try today, but seeing was bad for me, too.

Here is what I ran:

1. 0.8 FWHM of single stack
2. 0.7 FWHM of single staack
3. double stack of 0.7, which is 0.45052 FWHM
4. triple stack of 0.7, which is 0.35688 FWHM
5. quadruple stack of 0.7, which is 0.30449 FWHM

In the figure, "Lorentzian" is 0.7 unless otherwise specified. Remember that a real filter won't have 100% transmission, so each increase in stacking will actually be dimmer. I welcome someone to check my derived FWHM's. I just iterated to the answers. And very happy to replot if I misunderstood.

(On the Gaussian, yes it seemed odd. You could send one up to violet and one down to infrared and theoretically it would be just as narrow, but just very dim. Would need to double check that.)

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5597317 - 12/31/12 02:16 AM

Thank you very much George - these are very useful graphs and FWHM values!

As Joe noted, it looks as if the tail suppression is the more significant aspect compared to reduced bandwidth, although going from 0.7 to 0.45 A (FWHM - 0.5 on the verticle scale) with double stacking two etalons seems to give more “bang” compared to triple or quad stacking. As with most things it seems to be the law of diminishing returns... ERF removal would seem to be a must - hacksaw or no hacksaw.

I would think these shapes might possibly change a little bit for the solid etalons, although I don’t think they’re as susceptible to as much dimming with decreased bandwidth as via stacking multiple etalons...

Do you have any idea(s) as to why tilting a pair of DS etalons off-band would produce a dimmer image verses a brighter image? Going off-band would seem to allow more continuum in, and tilting an etalon should widen the bandpass (at least if it’s a Lorentzian), which would seem to be a double-whammy for giving you a brighter disk.

The front etalon (and attached D-ERF) was “pre-tilted” as usual to remove ghost images, and then the whole DS assembly was tilted to go off-band. Could it have something to do with how the D-ERF’s dielectric coatings act when tilted? They seem to be more reflective at an angle verses straight on (normal to the filter) - yet the tilting is not that great... total of a few degrees at the most.

Or would it more properly be inherent to the etalons themselves? For example, do their different initial tilts make them behave as if they are “shifted” rather than identical, thus producing the dimming you noted earlier for both Lorentzian and Gaussian transmissions? Would tilting them in unison somehow exacerbate the “shift- dimming”?

There certainly appears to be no significant brightening on-band, dimming off-band, or vice-versa, with Joe’s video via a hi-res diffraction grating. So it would seem to have something to do with the behavior of etalons, tilting, coatings, etc...


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5597452 - 12/31/12 06:34 AM

Quote:

I’m not sure I see much benefit to triple stacking compared to double stacking





Hi Bob et al,

I've had a bit of a play round with triple stacking. My results were very contrasty. This is with the TS40.


TS Full disk bw by Mark Townley, on Flickr

Banding and sweetspotting issues become an absolute pain and is tricky to get everything just right, so I agree that double stacking is a better and easier alternative. I would have explored this more this year but we've had the wettest year for over 100 years and solar moments have been at a premium...

Mark


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George9
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5597611 - 12/31/12 09:46 AM

Quote:

Do you have any idea(s) as to why tilting a pair of DS etalons off-band would produce a dimmer image verses a brighter image? Going off-band would seem to allow more continuum in, and tilting an etalon should widen the bandpass (at least if it’s a Lorentzian), which would seem to be a double-whammy for giving you a brighter disk.



Bob, I don't have a good explanation. My assumption was that tilting the pair inadvertently shifted them with respect to each other. E.g., if you tilt the front one to avoid the ghost (and you optimized the tilt for the best view), then tilting the pair could either untilt the front one with respect to the sun or further tilt it depending on the direction. My tilting double stack now belongs to a friend, so I can't try it, but I would just experiment with single stack tilting to note the behavior, and then add the second element to the already tilted first element, optimizing brightness. In other words, I suspect you had to do some fiddling to get your on-band double stack optimized, so to be fair you need to fiddle to optimize the off-band double stack.

For my current PT double stack, off band (continuum) is a little brighter than on band (H-alpha), but not enormously so.

George


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Re: Single Vs. Double Stack new [Re: marktownley]
      #5597620 - 12/31/12 09:50 AM

Excellent Mark!

Quote:

Banding and sweetspotting issues become an absolute pain and is tricky to get everything just right...




Looks like you nailed it

Any comments about prominences with TS? Jesus' image looks as if they lose a little intensity compared to SS and DS, but again processing needs to kept the same for valid comparisons - what’s your impression?


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Re: Single Vs. Double Stack new [Re: George9]
      #5597719 - 12/31/12 10:51 AM

Quote:

My assumption was that tilting the pair inadvertently shifted them with respect to each other. E.g., if you tilt the front one to avoid the ghost (and you optimized the tilt for the best view), then tilting the pair could either untilt the front one with respect to the sun or further tilt it depending on the direction.... but I would just experiment with single stack tilting to note the behavior, and then add the second element to the already tilted first element, optimizing brightness. In other words, I suspect you had to do some fiddling to get your on-band double stack optimized, so to be fair you need to fiddle to optimize the off-band double stack.

George




Thanks George - that makes a lot of sense.

I'm looking forward to that experiment when it gets a little warmer - handling a frozen DS etalon assembly with my bare hands was a little unnerving. Fiddling with two frozen etalons and an separate ERF bare handed would probably result in a visit to the hospital ER...


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Bill Cowles
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5598153 - 12/31/12 03:09 PM

Love your setup and images!

Bill


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Spectral Joe
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Re: Single Vs. Double Stack new [Re: Bill Cowles]
      #5598933 - 12/31/12 11:56 PM

With the recurring question of how to set filters (single, double, or any multiple) onto H alpha, I have a suggestion. Get a source of hydrogen emission that you have control of. You can get a hydrogen discharge tube (commonly referred to as a "spectrum tube") for between $20 and $40, depending on the vendor. You can get the power supply and mount for $150 to $200, or you can build your own. So, $170 to $240 sounds like a lot? Some people spend that on eyepieces. With the spectral source there is no question where you are tuned. This seems to be an issue with double stacking, I see images that clearly show chromospheric network features that are see in the wings of H alpha, the stack is tuned red or blue of the line core. As the passband gets narrower (with a properly tuned double stack) the details seen in the line wings fades, and different details are seen, ones that were hidden before. Tilt tuned systems may have trouble finding a tilt value that doesn't give annoying reflections, pressure tuning is better for stacked systems for that reason. But even pressure tuned systems will show a wavelength variation across the field, such are the woes of filter systems. Oh, one last thing before I get off my soapbox - go lightly with the Photoshop or Registax rolling pin, bakers know that overworking the dough or batter can render the result less tasty.

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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5599717 - 01/01/13 02:21 PM

Interesting suggestion, Joe. And it's always fun to have another device to work with. I guess you could learn how to optimize the filters, but you would presumably still need to do in-the-field tweaking (e.g., as temperature changes or as air leaks slowly out of the PT).

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5599763 - 01/01/13 02:50 PM

The thing is portable, and can be taken to the scope. One of the drawbacks to etalons is not knowing exactly where you're tuned, this is a way to find out. An yes, monitoring etalon conditions is a great idea, and achievable for the gadgeteer.

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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5602306 - 01/03/13 02:33 AM Attachment (14 downloads)

Bill

Quote:

With the recurring question of how to set filters (single, double, or any multiple) onto H alpha, I have a suggestion. Get a source of hydrogen emission that you have control of... With the spectral source there is no question where you are tuned...

[Joe]




Hi Joe -- excellent suggestions - the discharge tube would be a relatively inexpensive H alpha “accessory.” Perhaps a “discharge tube light box” (using more than one tube?) could be used in optimizing filter tilts and rotations the way a flat-fielding light box is used. This could help to not only get the filter system on-band, but to also help optimize field uniformity as noted by Mark for his triple stacking endeavor.


Quote:

Tilt tuned systems may have trouble finding a tilt value that doesn't give annoying reflections, pressure tuning is better for stacked systems for that reason. But even pressure tuned systems will show a wavelength variation across the field, such are the woes of filter systems.




Tilt verses pressure tuning is usually more a problem of tuning the primary internal etalon to be on-band. Tilting can introduce “banding” - creating a band of good contrast, and the area outside of it which falls off-band. Tilt-tuned induced banding is more problematic with internal etalons - Andy Lunt stated “I would guess that internal Etalons are about 5-6 times more sensitive to tilt [banding] than external Etalons...” http://luntsolarsystems.com/blog/internal-etalon-performance

Pressure tuning eliminates the problem of banding due to tilt tuning with internal etalons.

Internal etalons are also more sensitive to circular “sweet spots “ from the internal collimating (or telecentric) optics they use. This is due to the difficulty in getting field and instrument angles within the acceptance angle of the etalon for good across-the-filter bandpass performance. Adding “banding” via tilt-tuning exacerbates a sweet spot issue.

On the other hand, ghost reflections are created between multiple etalons (i.e. double stacking). These reflections are relatively easily removed from the field of view with a little extra tilting to the DS etalon. But this again can introduce banding. Pressure tuning would be great for the second etalon, as when it is tilted it goes slightly off-band, and pressure tuning could bring it back exactly on-band, but probably would not do much for reducing "banding." No one is making a pressure tuned front etalon, although at one time Lunt was looking at the possibility. However, one could design a sealed enclosure with pressure tuning to house an existing etalon, using the ERF for the front “window” and optical glass for the second...

Because of the aforementioned sweet spot issues that can occur with internal etalons - and greater susceptibility to banding if tilt-tuned - they seem more likely to exhibit banding and/or sweet spots when double stacked (e.g. the bandwidth is reduced making any sweet spot and banding more evident.)

Using two external (front) etalons seems to minimize the magnitude of both sweet spots and banding, as there are no instrument angles, and the field angles are just those of the sun itself ~ 0.25 degree. Of course it's the most expensive way to go, and the aperture of commercial etalons is currently limited to 100 mm. Tilting the etalons (front to remove ghosts, objective to tune), can produce some banding, which is a more subtle gradient than is seen with internal etalon based systems. However, when properly adjusted, double stacking two front etalons (or evidently even three triple stacked front etalons as shown by Mark's SM40's) can give good contrast uniformity with little evidence of banding or a sweet spot.

DS SM90-SM90/BF30:


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5603066 - 01/03/13 01:57 PM

Wow! Excellent discssion!

Jim


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5603454 - 01/03/13 06:08 PM

Quote:

(or evidently even three triple stacked front etalons as shown by Mark's SM40's)




This was with 2 external double stacked 40mm etalons and a internal 20mm etalon


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Re: Single Vs. Double Stack new [Re: marktownley]
      #5603589 - 01/03/13 07:42 PM

Ah-ha. Thanks Mark. This explains your comment related to "sweetspotting issues" being "tricky to get right" - and makes your results even more extraordinary.

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Scott BeithAdministrator
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5604922 - 01/04/13 02:31 PM

This has been a great thread and the results are pretty darn impressive!

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Re: Single Vs. Double Stack new [Re: Scott Beith]
      #5605184 - 01/04/13 04:44 PM

I just hope there is not going to be a test.

Bill


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Re: Single Vs. Double Stack new [Re: Bill Cowles]
      #5605405 - 01/04/13 07:14 PM

Tuning any combination of bandpass filters is going to be tricky, especially if you aren't sure of what to tune to. If it's of any use to anyone, this is what the core of H alpha looks like at 0.35 Angstrom bandpass:



Taken during a rare moment of good seeing on 4/24/09, 1.1 arcsec per pixel.

The hydrogen lamp looks like this in use:



It's easily mounted in any sort of enclosure you prefer. I use it for alignment and bandwidth checking.


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5605526 - 01/04/13 08:28 PM

Quote:

Using two external (front) etalons seems to minimize the magnitude of both sweet spots and banding, as there are no instrument angles, and the field angles are just those of the sun itself ~ 0.25 degree. Of course it's the most expensive way to go, and the aperture of commercial etalons is currently limited to 100 mm. Tilting the etalons (front to remove ghosts, objective to tune), can produce some banding, which is a more subtle gradient than is seen with internal etalon based systems. However, when properly adjusted, double stacking two front etalons (or evidently even three triple stacked front etalons as shown by Mark's SM40's) can give good contrast uniformity with little evidence of banding or a sweet spot.





Nice shots, Bob and Joe.

The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding. The weak point is the reflections, covered on other threads. The SS is pretty close to perfect in terms of sweet spot, banding, and reflections.

My external Coronado ASP-60s were free of sweet spots or banding, just as Bob describes, until I bought a binoviewer. Then I had severe banding. I never quite figured it out. Perhaps the Denkmeier OCS (more or less a Barlow that goes in front of the blocking filter so you can achieve focus) changed the light cone so that the blocking filter induced banding. Tuning the blocking filter did move the band up and down (the blocking filters used to be tilt tuned).

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5605999 - 01/05/13 04:06 AM

Quote:

The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding.




Is this a result of the etalons being 'oversized' to what you might expect? ie. only the central area of an etalon is being utilised...


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Re: Single Vs. Double Stack new [Re: marktownley]
      #5606592 - 01/05/13 01:07 PM

Ditto, great close up detail and discharge tube pic Joe.


Quote:

Perhaps the Denkmeier OCS (more or less a Barlow that goes in front of the blocking filter so you can achieve focus) changed the light cone so that the blocking filter induced banding. Tuning the blocking filter did move the band up and down (the blocking filters used to be tilt tuned).




The blocking filter is also a interference filter, therefore the incident angle of the rays it receives will affect it's performance. Per David Lunt (in an old Coronado blog):

"...the focal ratio ideally should be greater than f10 to maintain performance of the blocking filter....but in fact you will see good results at considerably shorter f ratios than this."

My understanding is that a standard barlow will increase the field angles (bad). This could be made worse with tilting as described above, and might indeed produce banding. Using a TeleVue Powermate would be better, since it has "telecentric" properties which will minimize the increase in field angles.


Quote:

"The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding."

Is this a result of the etalons being 'oversized' to what you might expect?




I’m in no way an optics expert, but from what I have been able to learn here’s how etalon size relates to sweet spots:

Typically the sweet spot with a collimated etalon is caused by a magnification of the field angles of an extended object. If the field angle exceeds the acceptance angle of the etalon (i.e the angle needed to meet the filter's band pass specification), that portion of the image (in this case the limb of the sun) will begin to fall off-band. A decease in contrast (a loss of chromosphere detail) or prominence visibility will be noted outside the sweet spot.

For the sun’s limb, the field angle starts out being ~ 0.25 degree (i.e 1/2 the sun's angular diameter, or when optically centered the center-to-limb angle). For an etalon mounted on front of the objective, this is the field angle, and amounts to an f 108 optical cone convergence. Generally there is a very wide sweet spot at this angle and configuration.

In a collimator based internal etalon system, the field angle magnification is the ratio of the objective to collimator lens focal lengths. Because of the optical geometry, this is also roughly proportional the ratio of objective diameter to the etalon (working) diameter. Therefore, the larger the internal etalon, the less the magnification of the field angles, and the better the on-band performance.

For example, the Lunt LS80 uses an internal 50 mm etalon, and dividing 80 by 50 = 1.6. Hence the internal field angle of the suns limb can be generally determined to be 0.25 x 1.6 = 0.4 degree. For the LS100, which uses the same 50 mm etalon, the collimated field angle is 0.25 x (100/50) = 0.5 degree, and so on. So we can see that the smaller the etalon is compared to the objective (i.e. the collimator lens FL and geometry) the greater the field angle magnification, hence the smaller the sweet spot of good on-band performance.

Of note for narrower bandwidth filters is that they also have a narrower acceptance angle to remain on-band. This perhaps can explain why a single stacked system with no evidence of a sweet spot may develop a sweet spot (and possibly banding from tilting the second etalon) when double stacked, due to the constraints of the system to have a narrower acceptance angle for proper on-band performance.

Again I’m not an expert, so if I’ve erred, I hope someone will correct me.


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5607601 - 01/06/13 12:12 AM

I'm trying to figure out what is causing the banding while using the binoviewer. Nothing past the etalon will affect its bandpass. Is it actually a band or a central hot spot? If it a spot I suspect it's a relay imaging issue with the design of the binoviewer.

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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5607772 - 01/06/13 04:59 AM

Quote:

I'm trying to figure out what is causing the banding while using the binoviewer. Nothing past the etalon will affect its bandpass. Is it actually a band or a central hot spot? If it a spot I suspect it's a relay imaging issue with the design of the binoviewer.




Newtons rings maybe?


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Re: Single Vs. Double Stack new [Re: marktownley]
      #5607836 - 01/06/13 07:27 AM

Perhaps the design of the blocking filter used with the ASP 60 might be part of the banding issue George noted when used with the accessory optics...

Quote:

The [ASP-60] etalon is in the front element. The diagonal (PROM-15T) contains the blocking/prominence filter. This does not 'tune' the whole filter. The narrow element (etalon) has a temperature coefficient of passband shift of <1 Angstrom/2000 degrees C. However, the blocking filter is somewhat temperature sensitive and needs compensating, - by tilt, - for large differences in ambient temperature. This filter isolates the H-alpha passband in the etalon and eliminates all the others. If it moves with temperature, without being compensated, it would allow one of the other etalon orders to come through and, thereby, lower the contrast.

[Old Coronado blog - emphasis added]






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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5608380 - 01/06/13 01:29 PM

Quote:



I’m in no way an optics expert, but from what I have been able to learn here’s how etalon size relates to sweet spots:

Typically the sweet spot with a collimated etalon is caused by a magnification of the field angles of an extended object. If the field angle exceeds the acceptance angle of the etalon (i.e the angle needed to meet the filter's band pass specification), that portion of the image (in this case the limb of the sun) will begin to fall off-band. A decease in contrast (a loss of chromosphere detail) or prominence visibility will be noted outside the sweet spot.

For the sun’s limb, the field angle starts out being ~ 0.25 degree (i.e 1/2 the sun's angular diameter, or when optically centered the center-to-limb angle). For an etalon mounted on front of the objective, this is the field angle, and amounts to an f 108 optical cone convergence. Generally there is a very wide sweet spot at this angle and configuration.

In a collimator based internal etalon system, the field angle magnification is the ratio of the objective to collimator lens focal lengths. Because of the optical geometry, this is also roughly proportional the ratio of objective diameter to the etalon (working) diameter. Therefore, the larger the internal etalon, the less the magnification of the field angles, and the better the on-band performance.

For example, the Lunt LS80 uses an internal 50 mm etalon, and dividing 80 by 50 = 1.6. Hence the internal field angle of the suns limb can be generally determined to be 0.25 x 1.6 = 0.4 degree. For the LS100, which uses the same 50 mm etalon, the collimated field angle is 0.25 x (100/50) = 0.5 degree, and so on. So we can see that the smaller the etalon is compared to the objective (i.e. the collimator lens FL and geometry) the greater the field angle magnification, hence the smaller the sweet spot of good on-band performance.

Of note for narrower bandwidth filters is that they also have a narrower acceptance angle to remain on-band. This perhaps can explain why a single stacked system with no evidence of a sweet spot may develop a sweet spot (and possibly banding from tilting the second etalon) when double stacked, due to the constraints of the system to have a narrower acceptance angle for proper on-band performance.

Again I’m not an expert, so if I’ve erred, I hope someone will correct me.




You may not be an expert Bob, but you can be on my pub quiz team anyday

Taken me a while to ingest all this and get my head around it, alongside some other stuff about it elsewhere on the web. I've never really given the whole field angles thing much thought, but it really does make a whole lot of sense and gives an interesting viewpoint to look at Ha systems:

Some of the best images i'm seeing to date on the web are coming out of LS80s - the pictures are stunning in terms of nice tight and even bandpass across some quite large chipped CCDs. Maybe our field angles have a lot more to do with it than we think.

I'm curious now as to the etalon diameters in the Coronado equipment... Hmmm


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Re: Single Vs. Double Stack new [Re: marktownley]
      #5608627 - 01/06/13 03:23 PM

Thanks Mark!

I found this article and blog discussion from several years ago by David Lunt be very helpful in understanding how important the issue of angles (among other factors) are to narrow band interference filter performance:

link


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5609073 - 01/06/13 07:17 PM

Wow, this thread just keeps staying interesting.

Joe, it’s a band, not a spot. Maybe a quarter or third FOV high, and it is as wide as the FOV. I can shift the band up or down in the field of view by tuning the blocking filter. Therefore, I don’t think it is actually the binoviewer. If I had the filter, I could just throw an eyepiece in place of the binoviewer with the OCS in it and try it; not sure why I never tried that before. The other thing I just realized is that the band was always horizontal (like the tilt of the blocking filter), so it wasn’t the etalon (whose angle varied depending on how tightly I screwed in the TMax).

Bob, I think your explanation is right on. The OCS is magnifying the field angles. Great job finding that document.

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. I remember David Lunt commenting that I should not need to tune the blocking filter with the etalon; just leave it untilted because the temperature effect was not that big. This worked until I got the binoviewer.

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).

In your document, David Lunt mentions the dark element in front of the blocking filter to reduce reflections, and how they later brightened the view with a lighter one. I remember he had me send my blocking filter to Isle of Man to swap in the brighter version. Makes we wonder about the DSII and whether an element could be put between the two etalons to reduce the reflections (reflections would pass three times, but the primary would pass only once), but I guess the view would become too dim.

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5609205 - 01/06/13 08:42 PM

Bob, I think the 1A/2000C was correct. Your finding that document prompted me to look at the old coronado solar blogs circa 2000 (via the Internet Wayback Machine). Here was David Lunt's answer to my question about temperature sensitivity (5/10/2000):


"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)."

George


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Re: Single Vs. Double Stack new [Re: George9]
      #5609386 - 01/06/13 10:40 PM

Thanks George, I will “re-correct” the text back to the original. The reason I thought it might have been a typo is that I recalled - and then confirmed - that the SM series of Tucson etalons made in the early 2000's, and the Isle of Man etalons currently made by Solarscope, have a specification for thermal stability stated to be < 1 Angstrom / 200 degrees C.

So good find - and great info!

Quote:

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.


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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5613776 - 01/09/13 01:56 PM Attachment (20 downloads)

... To be honest, when I double stacked my filter system, I was puzzled as to why my close up images of the limb of the sun didn’t show the “double limb” I was seeing in so many other close up images posted here. For a while I honestly thought my filter system may not be performing as well as it should. It wasn’t until Joe’s original question regarding double stacking that I realized what was actually taking place - a narrower bandpass eliminates continuum light coming through the filter system. This was confirmed by the additional research as shown above, and looking back it seemed quite obvious.

Quote:

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.


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frolinmod
Carpal Tunnel


Reged: 08/06/10

Loc: Southern California
Re: Single Vs. Double Stack new [Re: BYoesle]
      #5614141 - 01/09/13 05:35 PM

Quote:

“once you’ve double stacked, you won’t go back.”



I agree wholeheartedly!


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: frolinmod]
      #5614463 - 01/09/13 08:26 PM

Nice summary, and I love those photos. George

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PEterW
professor emeritus


Reged: 01/02/06

Loc: SW London, UK
Re: Single Vs. Double Stack new [Re: George9]
      #5619491 - 01/12/13 05:27 PM

..... So I will now have to start saving for that second filter.... Darn you all! VERY good thread discussion, thanks to everyone!

PEterW


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BYoesle
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Reged: 06/12/04

Loc: Goldendale, Washington USA
Re: Single Vs. Double Stack new [Re: PEterW]
      #5722853 - 03/09/13 07:18 PM Attachment (11 downloads)

Quote:

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:

Edited by BYoesle (03/10/13 11:49 PM)


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marktownley
Postmaster


Reged: 08/19/08

Loc: West Midlands, UK
Re: Single Vs. Double Stack new [Re: BYoesle]
      #5723362 - 03/10/13 04:36 AM

Excellent demonstration of the effect Bob, thanks

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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: marktownley]
      #5724722 - 03/10/13 08:39 PM

Wow wow. Great images, Bob.

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)?

George


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BYoesle
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Reged: 06/12/04

Loc: Goldendale, Washington USA
Re: Single Vs. Double Stack new [Re: George9]
      #5724912 - 03/10/13 11:11 PM

Hi George.

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.


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