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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 (47 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
sage


Reged: 12/11/04

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

Post deleted by ValeryD

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



Bill


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BYoesle
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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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George9
sage


Reged: 12/11/04

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

Post deleted by ValeryD

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bob71741
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Re: Single Vs. Double Stack *DELETED* new [Re: ValeryD]
      #5593898 - 12/29/12 08:22 AM

Post deleted by bob71741

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George9
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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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George9
sage


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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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BYoesle
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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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doctomster
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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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Spectral Joe
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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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Bill Cowles
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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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Spectral Joe
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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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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack [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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