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Spectrohelioscopes

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#1 asaint

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Posted 15 June 2004 - 12:56 PM

spectrohelioscopes

#2 David Knisely

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Posted 15 June 2004 - 04:14 PM

I saw this in a modified for on one of the Yahoo solar groups, and it *still* has some significant problems and misconceptions. Vieo states:

And it is not necessary to have 0.2A or 0.1A passband – even if you had such a very narrow passband, the solar detail would not gain in contrast, because atmospheric ‘seeing’ would introduce fuzziness into the image.


This is rather misleading. There *is* a gain in contrast by going from a 0.5 Angstrom to a 0.2 Angstrom filter, and this has little or nothing to do with seeing. If you are using a 3 inch aperture and the seeing is 1.5 arc seconds, you are working pretty close to the resolution of the instrument as far as spacial detail is concerned. However, the plain and simple fact is that as you screen out more and more contaminating light from other nearby portions of the spectrum, you do improve the contrast of the detail that is resolvable. I don't know where the point of diminishing returns is, but in a 0.2 Angstrom filter, the view is simply stunning (much higher contrast than with a 0.7 Angstrom bandwidth).

Comparison of performance for different passbands

1.0A passband, faint details not seen, brighter and darker details seen with difficulty.
0.9A passband, faint plage and filaments not seen, brighter and darker details barely seen.
0.8A passband, faint detail as plage or flare not easy to see, all stronger details easy to see.
0.7A passband, faint detail will be seen easily, also all other stuff easy.
0.6 and 0.5A passband, faint detail and all other stuff ok.


This is oversimplified and, for some features, incorrect. Passbands of 1.5 Angstroms or less are needed to see prominences on the limb, which is why you see both Thousand Oaks and Lumicon prominence filters with the 1.5 Angstrom passband. They show prominences reasonably well, although a passband of around 1 Angstrom might provide a little more contrast. These prominence filters, however, show little disk detail other than what is normally seen with a white-light filter. Major solar flares can somtimes be seen in the prominence filters, but minor ones will probably escape detection. To really see disk details in H-alpha requires a passband of *less* than one Angstrom. Its a continuum of increase in contrast and detail as you go narrower in passband (ie: the narrower the passband, the higher the contrast will tend to be). 0.7 Angstroms is a nice compromise between cost and detail, but if you *really* want to see the highest contrast with disk features, you have to go to under about 0.6 Angstroms. Solar Spectrum makes filters down to 0.2 Angstroms for this purpose.

The ERF is also employed in order to avoid over heating of the filter. The maximum size of an ERF is about 4”, as larger ERFs tend to overheat and crack.



This statement is ridiculous! ERF's are sitting out in front of the telescope. They are exposed only to basic non-concentrated light from the sun like any piece of glass sitting outside, so they are no more likely to overheat and crack than the windows in a house! They are available in sizes up to *five inches* from Daystar, and there is no reason why they could not be made larger (other than the increased cost of making the Energy Rejection Filter). Vieo repeats this mistake later in the article.

Coronado filters can be placed over the front of the objective or near the prime focus with telecentric optics. They can work with f/15 objectives, or other f/ratios, also using an energy rejection filter. Because the main part of the Coronado filter must be aperture sized, (the DayStar filter sits at prime focus, and thus is only an eyepiece aperture 30-34 mm in diameter on all models) a Coronado filter’s price goes up rapidly with the aperture.


Most of the Coronado filter etalons are *only* placed over the front of the telescope (they make special custom ones which can be placed near the focus, but they are not commonly available). They also can work at f/ratios considerably *shorter* than f/15!

Prices on the two classes of filters are comparable, at least for small apertures. The DayStar H-a filter, University model of 0.7A passband, will cost $4,200. An ATM model of 0.7A passband will be $2,500. A violet Ca II filter of 8A passband will be $3,300. Coronado filters are similarly expensive.


No, they are *not* price comparable at all! The moderate aperture etalons of the Coronado units are very expensive, so this restricts the aperture which can be used for a given cost. For example, my 90mm setup with a DayStar T-Scanner + ERF cost me about $1800. To duplicate it with a 90mm Coronado SolarMax would cost me nearly $6,000!! The advantage of the Coronado is that it can be used with a wide variety of f/ratios from short to long, but the DayStar requires at least f/30 to work properly.

Another factor to consider is that even slight heat build-up in an H alpha filter can shift the passband about 0.1A to the red or the violet. So with a 0.7a passband, you have a little leeway in adjustment. If the filter has passband about 0.5A, a slight shift can be noticed, for the solar detail will appear slightly different. Surge prominances and surge filaments have a Doppler shift of about 2.0A on average, and will not be seen in the center of a 0.6A passband, let alone 0.8A passband. Active prominances and filaments average about 0.4A Doppler shift, and will be partly seen in 0.8A passband. Alternatively, quiescent prominances and filaments have Doppler shift of 0.04A on average, will be seen easily in any of the passbands.


The temperature fluctuation is a moot point. DayStar filters are either heated well above ambient temperature in a precisely temperature controlled oven to avoid this problem, or are equipped with the "tilt" option (ie: the T-Scanner) to compensate for temperature variations, as well as allowing these filters to be "tuned" to track doppler-shifted features. The Coronado filters work at ambient temperatures and are also available with a "tilt" tuning option.

The plain and simple fact is that the Spectrohelioscope is less expensive than some of the dedicated H-alpha filters, but it is also more bulky and does not have the fine resolution that the H-alpha filters tend to have. Clear skies to you.

#3 Don W

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Posted 15 June 2004 - 04:28 PM

Very well put, David. I applaud Mr. Vieo for his efforts in popularizing inexpensive Spectrohelioscopes, but his other information is, shall we say, weak. I've observed through a borrowed Lumicon 1.5A system, owned a Daystar ATM .7A, and now own a Coronado Solarmax 40 and a P.S.T. Been doing this H-Alpha stuff for a while. My experiences agree with your statements, not his.

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Posted 15 June 2004 - 11:08 PM

Hi David. I thought I should quickly respond of your observations, as I was the messenger for Fred Veio’s observations on H-alpha filters and spectrohelioscopes. Fred does all of his Internet access at his local public library, so can’t always respond as quickly as he would like, which is one reason that I have tried to make his experiences with spectrohelioscopes more accessible.


1.0A passband, faint details not seen, brighter and darker details seen with difficulty.
0.9A passband, faint plage and filaments not seen, brighter and darker details barely seen.
0.8A passband, faint detail as plage or flare not easy to see, all stronger details easy to see.
0.7A passband, faint detail will be seen easily, also all other stuff easy.
0.6 and 0.5A passband, faint detail and all other stuff ok.


This is oversimplified and, for some features, incorrect. Passbands of 1.5 Angstroms or less are needed to see prominences on the limb, which is why you see both Thousand Oaks and Lumicon prominence filters with the 1.5 Angstrom passband. They show prominences reasonably well, although a passband of around 1 Angstrom might provide a…




Technically you are correct, though the 1.5 angstrom filters are typically sold as ‘solar prominence’ filters, and they are really a different product than the Daystar and Coronado filters. Even with a 1.5 A bandpass the prominences are easy to pick out because of the contrasting background; but a 1.5A filter is useless for observing surface detail. Fred was giving guidelines for observing activity on the solar disk; your post reiterates much of what he has said, so I think you are both in agreement.

________________________________________

The ERF is also employed in order to avoid over heating of the filter. The maximum size of an ERF is about 4”, as larger ERFs tend to overheat and crack.

________________________________________
This statement is ridiculous! ERF's are sitting out in front of the telescope.



You can flame me for this one, as I added this comment after a phone conversation with Del Woods (the owner of Daystar). It’s a misstatement on my part (not Fred’s) which I will ask Todd to correct. This should have read that ERF apertures over 4-5” can overheat the etalon, causing it to crack. It’s not a certainty, but you are reaching the thermal limits of a 30-35mm piece of glass placed at prime focus, even a tough one like a Daystar etalon. Figure that the ERF rejects around 80% of the light below red. A lot of infrared comes through, and the intensity at prime focus goes up by the square of the aperture.

________________________________________

Coronado filters can be placed over the front of the objective or near the prime focus with telecentric optics. They can work with f/15 objectives, or other f/ratios, also using an energy rejection filter. Because the main part of the Coronado filter must be aperture sized, (the DayStar filter sits at prime focus, and thus is only an eyepiece aperture 30-34 mm in diameter on all models) a Coronado filter’s price goes up rapidly with the aperture.
________________________________________


Most of the Coronado filter etalons are *only* placed over the front of the telescope (they make special custom ones which can be placed near the focus, but they are not commonly available). They also can work at f/ratios considerably *shorter* than f/15!




You are getting a bit picky and I’m not quite sure I see the problem here; I can probably reword this kfor clarity along the lines you suggest— “only” and “shorter than”. OK, I’ll send Todd revision taking these into account as well.

________________________________________
Prices on the two classes of filters are comparable, at least for small apertures. The DayStar H-a filter, University model of 0.7A passband, will cost $4,200. An ATM model of 0.7A passband will be $2,500. A violet Ca II filter of 8A passband will be $3,300. Coronado filters are similarly expensive.
________________________________________


No, they are *not* price comparable at all! The moderate aperture etalons of the Coronado units are very expensive, so this restricts the aperture which can be used for a given cost. For example, my 90mm setup with a DayStar T-Scanner + ERF cost me about $1800. To duplicate it with a 90mm Coronado SolarMax would cost me nearly $6,000!! The advantage of the Coronado is that it can be used with a wide variety of f/ratios from short to long, but the DayStar requires at least f/30 to work properly.



Again, this seems a bit picky. The two companies largely compete in the same price space. Coronado’s systems have a problem in scaling up because the etalon becomes pricy; Daystar has trouble scaling up because the etalon heats up. But with a moon-sized object like the sun, 3-5” aperture is adequate for a lot of interesting observation.


________________________________________
Another factor to consider is that even slight heat build-up in an H alpha filter can shift the passband about 0.1A to the red or the violet…
________________________________________
The temperature fluctuation is a moot point. DayStar filters are either heated well above ambient temperature in a precisely temperature controlled oven to avoid this problem…



I'm afraid I agree with Fred on this point; plus, I think you are taking a parochial and picky stance. Remember that both Fred (California) and I (Hong Kong) are in locations where you are more likely to need to cool rather than heat the filter. Temperature fluctuations matter, not to the point of undermining the effectiveness of filters, but just affecting the character of their performance. That’s all that Fred was saying.



The plain and simple fact is that the Spectrohelioscope is less expensive than some of the dedicated H-alpha filters, but it is also more bulky and does not have the fine resolution that the H-alpha filters tend to have.


If you read Fred’s book, you will see that he goes to great lengths to explain the performance capabilities of spectrohelioscopes and emphasizes the importance of quality optics; a spectrohelioscope has many optical elements in a fairly (compared to a telescope) convoluted light path. Poor performance in practice is more often a consequence of mediocre optics than inherent flaws in the spectrohelioscope design. Spectrohelioscopes are comparatively bulky (you are correct), which is why they tend to use heliostats. That said, there are instruments presented on http://spectrohelioscope.net that perform in the .2-.3 A range that are quite affordable.

One of my primary motivations in getting this comparison onto Cloudy Nights was to let amateur solar astronomers know more about the spectrohelioscope options that are open to them. For that reason alone, I invite them to explore the material on http://spectrohelioscope.net and to draw their own conclusions. I do hope to see more amateurs taking me up on this.

Chris Westland
Professor, ISMT, University of Science & Technology, HK Clearwater Bay, Kowloon, Hong Kong
Tel: 852 2358 7643
Fax: 852 2358 2421
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URL:: ihome.ust.hk/~westland Alternate e-mail: chris.westland@hkmensa.org

#5 David Knisely

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Posted 16 June 2004 - 01:07 AM

westland posted:

Again, this seems a bit picky. The two companies largely compete in the same price space. Coronado’s have a problem of scaling up because the etalon becomes pricy; Daystar has trouble scaling up because the etalon heats up. But with a moon-sized objectp like the sun, 3-5” aperture is adequate for a lot of interesting observation.


No, the Coronado filters of significant size do not exactly compete in the same price range as I have demonstrated. The least expensive DayStar filter which I would consider would be the 0.8 Angstrom T-Scanner, and it is between $1400 and $2000 (exact price depending on what size ERF and the final bandwidth desired). The least expensive Coronado filter is a mere 40mm in diameter, yet is around $1099 (and you are limited to that aperture *only*). 40mm is fine for casual observing, but a lot more can be viewed much of the time with apertures of 2.5 to 5 inches, which is one reason I like the idea of the DayStar line (although if I were to replace mine, it would probably be a narrower Solar Spectrum filter). The most expensive DayStar filter is the University 0.5 Angstrom model ($5600 without ERF). An ERF up to 5 inches would be usable, so a resolution equivalent to that would be possible (I have seen some days when morning resolution can get to around 1 arc second). The most expensive Coronado filter is the AS1-140, which is so expensive that you have to ask them for a price! The SolarMax 90 for my setup (1300 mm focal length) would cost about $5320, which is a lot more than I spent for my 0.7 Angstrom T-Scanner + 3.5 inch ERF.

I'm afraid I agree which Fred; he is right on this, plus, again, I think you are being overly picky. Remember that both Fred (California) and I (Hong Kong) are in locations where you are more likely to need to cool rather than heat the filter. Temperature fluctuations matter, not to the point of undermining the effectiveness of filters, but just affecting the character of their performance. That’s all Fred was saying.


The temperature effects on the passband location are known, but Veio's statment about them by its mere presence in an article comparing filters to spectrohelioscopes implied that the filters couldn't do much about it. The DayStar filters control the temperature of the oven (filter temperature is kept well above ambient) to within 0.5 degrees C (ie: tracking to within 0.05 Angstroms of the H-alpha centerline). Once you get the filter on-band, little intervention is required unless you want to tune the filter by changing its operating temperature. In the case of the T-Scanner, you can adjust the tilt for different tempertures to keep the filter on-band. In short, the shifting of the passband due to temperature variations is not much of an issue, as these filters can compensate for it. I too live in a place where I occasionally have to cool my T-Scanner when the ambient temperature approaches its upper operating limit (35 degrees C.). However, if I used an oven-heated model, this would not be a problem. Again, if Veio wants to talk about spectrohelioscopes, he is more than welcome to do so, as they are a viable alternative (although with a somewhat lower spacial resolution than a filtered system). However, if Veio or another acting in his place mentions the narrow-band filter option, he should make certain to get the facts right before making such a comparison. Clear skies to you.

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Posted 16 June 2004 - 02:29 AM

Again, if Veio wants to talk about spectrohelioscopes, he is more than welcome to do so, as they are a viable alternative (although with a somewhat lower spacial resolution than a filtered system). However, if Veio or another acting in his place mentions the narrow-band filter option, he should make certain to get the facts right before making such a comparison.



Well, perhaps again you should direct your flames towards me, as I originally encouraged Fred to provide this comparison. Fred was aware that some people are sensitive about comparison of spectrohelioscopes and narrow-band filters. This is something I hadn't really anticipated, but after your post, I see that it is indeed an issue. Nonetheless, Fred is extremely knowledgable, and I'm glad he has shared his years of insight with us. And I do believe he has his facts straight. The additional input of a forum allows diverse views to be aired, to the benefit of all of us.

I might also add that whatever shortcomings you may encounter when comparing the spectrohelioscope head-to-head with an H-alpha filter are compensated for by the ability to image in many different bands of the solar spectrum. Spectrohelioscopes are truly marvelous devices which complement existing tools for solar observation.

#7 David Knisely

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Posted 17 June 2004 - 01:11 PM

Again, if Veio wants to talk about spectrohelioscopes, he is more than welcome to do so, as they are a viable alternative (although with a somewhat lower spacial resolution than a filtered system). However, if Veio or another acting in his place mentions the narrow-band filter option, he should make certain to get the facts right before making such a comparison.



Well, perhaps again you should direct your flames towards me, as I originally encouraged Fred to provide this comparison. Fred was aware that some people are sensitive about comparison of spectrohelioscopes and narrow-band filters. This is something I hadn't really anticipated, but after your post, I see that it is indeed an issue. Nonetheless, Fred is extremely knowledgable, and I'm glad he has shared his years of insight with us. And I do believe he has his facts straight. The additional input of a forum allows diverse views to be aired, to the benefit of all of us.

I might also add that whatever shortcomings you may encounter when comparing the spectrohelioscope head-to-head with an H-alpha filter are compensated for by the ability to image in many different bands of the solar spectrum. Spectrohelioscopes are truly marvelous devices which complement existing tools for solar observation.


Well, I won't necessarly flame you, as you did make an honest mistake in interpretation of what Woods told you about the filters. It is possible that prolonged exposure to the concentrated light from an ERF larger than 5 inches would lead to premature failure of either the blocking filter or some other part of the DayStar unit, although I have never heard of one shattering (even with apertures up to six inches). The blocking filters from the older models have shown a tendancy to go bad after about 12 to 16 years of heavy service (I had mine replaced at 14 years), although hopefully, Woods has succeeded in improving the blocking filter with the more current models.

The key to comparing a spectrohelioscope to the narrow-band filters is pretty simple. Just keep it short and state the basic facts:

FILTER ADVANTAGES:
Simple setup on many telescopes.
Smaller size and lighter weight.
Slightly higher spacial resolution than many spectroheloscopes.
Available commerically from at least 3 manufacturers.

FILTER DISADVANTAGES:
High cost.
Limited Availability (they take a while to produce).
Single Passband (usually, although one Coronado model once allowed two passbands of choice).
Cannot be easily constructed by ATMs.

SPECTROHELIOSCOPE ADVANTAGES:
Can be constructed by ATMs.
Usable at a variety of wavelengths.
Somewhat lower cost than some filters.

SPECTROHELIOSCOPE DISADVANTAGES:
Bulky (often needing a Heliostat)
Not as high a spacial resolution as a dedicated filter when used with moderate apertures.
Not commonly available commercially.

Stating this in the article would have avoided some of the comments which others have expressed concerning its accuracy when it comes to filters. I am not certain of a significant advantage in being able to view across the solar spectrum, as only two specific spectral lines are commonly cited for most amateur solar observers (H-alpha and the Calcium K line). From merely an amateur's observational standpoint, I feel that H-alpha is "where the action is". Clear skies to you.

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Posted 22 June 2004 - 11:58 AM

I might also add that whatever shortcomings you may encounter when comparing the spectrohelioscope head-to-head with an H-alpha filter are compensated for by the ability to image in many different bands of the solar spectrum.

I have to say the article didn't do a very good job of emphasizing or explaining this advantage, and I'm still not convinced. The ability to see both the H-alpha and Ca-K is definitely an advantage, but the article did not explain what other lines are there, and what new information can you get from them. I think the truth is that while there are hundreds of lines in the visible range, most of them have very similar formation heights and therefore produce very similar images. Even researchers usually stick to three bandpasses when making plain images - H-alpha, Ca-K and continuum. ("plain images" as opposed to doppler maps, magnetograms and other data products)

I also strongly object to the statement that most interesting features on the sun are at 5-arcsecond level. This is a classic case of the instrument dictating what "interesting features" are. Whatever instrument you use, the most interesting features are always near the limit of that instrument, and even professional researchers sometimes fool ourselves into thinking there is no need for higher resolution. In reality there is an enormous amount of detail at 1-arcsecond level, and considerably more at 0.1-arcsecond level already achieved in many research telescopes (though not consistently).

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Posted 22 June 2004 - 11:10 PM

Post deleted by westland

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Posted 23 June 2004 - 01:45 AM

Your comments seem to me to be more an attempt to challenge the utility of a spectrohelioscope, and to denigrate Fred?s competence as a solar observer rather than being what I would call constructive criticism.

No, my comments are an attempt to challenge the advantages stated in the article. I think many of the stated advantages are rather forced, and the true advantages were not emphasized enough.

But, again, if you go through the various articles at http://spectrohelioscope.net you will find descriptions of events and phenomena that are visible in the various lines, as well as explanations of how to use a spectrohelioscope for exploring the Doppler shift in the H-alpha line, and other phenomena that are best explored with a spectrohelioscope.

But the article in question was lacking in this very important information. Also I did spend some time on that site and I didn't find any images apart from the three common bandpasses.

I think that Fred was clear that there are limits to the resolution easily achievable with amateur equipment, because of problems caused by ?seeing.?

That may be true, and that may be what Fred meant. But it's difficult to infer that from this passage:

All the main solar disk details of major interest are about 5 arc/sec detail therefore the resolution of an SHS is perfectly adequate for any of the small and large flares that can easily be studied. The same can be said for other events and surface details on the Sun. You do not need one or two second or arc detail; it?s nice to see, but not mandatory.

I'm sorry if you took my comments to be negative. I'm simply frustrated because the article was confusing and didn't make a clear, strong case for spectrohelioscopes.

#11 David Knisely

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Posted 23 June 2004 - 03:48 AM

Ken and David:

I’m actually a bit surprised with both of you. Your comments seem to me to be more an attempt to challenge the utility of a spectrohelioscope, and to denigrate Fred’s competence as a solar observer rather than being what I would call constructive criticism. This article was intended to provide an introduction to a little understood, and seldom encountered solar instrument—the spectrohelioscope. I’ve personally found many amateur astronomers curious about these instruments. Lacking access to one, they are usually unable to judge for themselves whether they actually want to spend the effort to build one (nor is there much accessible information on how to go about building one were they to decide to). The article clearly was not of the “Spectrohelioscopes are better than H-alpha filters …. nyah, nyah, nyah” ilk as you both seem to imply. I use a Daystar T-scanner in my own observation, and love it.


I implied nothing of the sort. However, the article "Comparison of DayStar and Coronado H-a Solar Filters with Spectrohelioscopes" did produce misleading or incorrect statements about *filters* when making an attempt at the "comparison" which is implied in the title. This comparison attempt was not all that successful for a variety of reasons. The analysis I provided in my last posting could have been a somewhat better way to treat the issue of filters vs. spectrohelioscopes. If the original article had not stated things in the way it did there would have been little to comment on, other than the obvious usefulness of the spectrohelioscope.

There were a few other additional things in the article which I did not comment on, but which also remain somewhat questionable, and had I wanted to, I could have *really* taken things up another level as far as criticism is concerned. However, the ones I cited were the most significant ones. Again, the specifics of the article were somewhat flawed, although it was nice to see at least some information on the spectrohelioscope. I would have prefered that Fred concentrate on the aspects of the use of the spectrohelioscope and not drag in the Coronado and DayStar filters in the title.

David, the points you raise about portability of spectrohelioscopes are addressed in detail in various articles at http://spectrohelioscope.net. There are many photos of spectrohelioscopes posted there, so I think it should be fairly obvious to most readers that spectrohelioscopes are big. No argument there.


Well, I'm afraid that wasn't the point of my response. If a comparison is made, it should be done fairly and completely with the proper facts. The article just had a few of them incorrect or stated things in a manner which could be misleading. This made any comparison less useful.

The misstatement on overheating of the etalon at large apertures was purely an editorial mistake, and you yourself were able to figure out how the article should have read. I can’t see the point of whinging, unless your primary goal is to demonstrate how ‘smart’ you are; or perhaps how ‘dumb’ I am. Big deal!


This is not a personal attack or denegration, and if you are interpreting it as such, then I apologize if the wording has made it seem that way. My response isn't to demonstrate anything except for some of the problems with the way the original article was worded. As for the Etalon heating, that wasn't the problem, as the original article stated that it was the ERF which would crack. Again, while it is within the realm of possiblity that an Etalon might overheat and have problems, I have yet to hear of any reports that any near-focus H-alpha etalons have actually cracked significantly under excessive heating when used with the proper ERF. Certainly, the oven-heated filters work at temperatures well above ambient, so they are designed to "run hot".

Overall, I’m surprised at the extent of picky and unconstructive commentary from both of you. I’m not really sure what general points you are trying to make. Is it that amateurs should not be encouraged to observe with anything except commercial H-alpha filters? Or do you have something else in mind?


Again, you are misinterpreting what is being said. The article got some things rather tangled up. That much is certain. There are advantages and disadvantages to the use of the spectrohelioscope and the dedicated narrow-band filter. If I were to do a comparison, it would read something like a previous posting of mine where I give a fairly accurate advantages/disadvantages analysis.

However, as for the opinions of some amateurs who have actually built them, I recall a certain incident at the Astronomical League National Convention in Kansas City, where one attendee brought his spectrohelioscope (mounted in a nice big 10 inch aluminum tube). Some of us were also set up with H-alpha filters but the clouds wouldn't move out of the way, so we just stood around and talked. I *really* wanted to look through that instrument, but since the clouds were killing that chance, I asked the guy who built the spectrohelioscope how well he liked its performance. Surprisingly, he immediately replied, "Keep using your filter". His comments indicated that he wasn't quite as satisfied with his spectrohelioscope as some people apparently are with theirs. Does this mean that spectrohelioscopes are useless and should not be used? Not at all. We don't know if this guy got things quite right when he built it, or whether he just liked filters better. I would still like a good long look through a good one. However, there are limits to the resolution of the spectrohelioscope, and it can be potentially less than the seeing will occasionally allow. Clear skies to you.

#12 asaint

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Posted 23 June 2004 - 06:25 AM

Hi all,

A good discussion is occurring here on a posted article. I'm happy to see the merits and technical details being bantered around by some experienced folks.

Please keep in mind that in any discussion it's extremely easy to misinterpret a comment as a potential personal attack. Take the time to PM the person and inquire what they meant if you feel this might be happening. 99 times out a 100 on the CN boards, it's a simple misunderstanding.

Thanks
Allister
webmaster@cloudynights.com

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Posted 23 June 2004 - 08:00 PM

Allister, I think you are the one who has misinterpreted, as I mentioned in my PM to you earlier. Let me reiterate that this article was intended to provide an introduction to a little understood, and seldom encountered solar instrument—the spectrohelioscope. I’ve personally found many amateur astronomers curious about these instruments. Lacking access to one, they are usually unable to judge for themselves whether they actually want to spend the effort to build one (nor is there much accessible information on how to go about building one were they to decide to). The intent was to inform, not to promote one instrument over the other. I also find it strange that you should delete my post, while allowing it to be quoted in its entirety in the subsequent post. I'm certainly happy with your decision. Thank you. :smirk: :smirk:

#14 asaint

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Posted 24 June 2004 - 03:37 AM

Westland,

The reason your post was deleted was it contained name calling. I did not nor will I judge the merits of the technical discussion that is occuring here. All we ask is that the discussion tone remain civil and no name calling occur.

In retrospect I should have edited your original reply and simply removed the name calling. My apologies for not taking this simple step instead of removing the entire post. Please feel free to post your original reply but without the name calling.

Thanks for your patience
Allister

#15 Guest_**DONOTDELETE**_*

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Posted 29 June 2004 - 08:51 PM

I have no idea what you are talking about

#16 Chris Lord

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Posted 26 September 2004 - 03:45 AM

spectrohelioscopes


Although I'm new to this discussion group, I'm not new to observing the Sun with both H-alpha and Calcium filters, or Spectrohelioscopes (SHSs), and I would like to comment on the debate about the pros and cons of etalon notch filters and SHS's.

I first viewed the Sun in both an H-alpha and a CaK line filter at Del Woods factory unit in Pamona on Memorial weekend 1979. The H-alpha filter was a University Series Å0.6, and the CaK Å4 (both oven tuned). Each was on a horizontally mounted telescope fed by a Coelostat (as I recollect). The image contrast in both passbands was excellent. The sky was however smoggy, with a considerable amount of scattered light.

I later began using a Å0.7 ATM series Daystar filter on an 8-inch f/15 Cooke refractor, between 1980 and 1982. Views of Prominences were excellent, but disc detail was awkward if the seeing was not good (Antoniadi II or better). This unit failed after 8 years so I was informed by the observatory director.

The first time I looked through a Spectrohelioscope was in June 1974. This was Henry Hatfield's Littrow style SHS with the Seller's oscillating slit synthesiser. He replaced the grating subsequently and I last used it in late June 1989. On that occasion the passband was adjusted to only Å0.2 in H-alpha, and even through fairly dense Cirrus, the image contrast and brightness of disc detail was astonishing, far better than in the Daystar filters I had looked through up to that time.

I have subsequently observed the Sun using a SolarScope60 fitted with a tilt tuned Å0.7 H-alpha filter and several of the current range of Coronado units, from the PST40 thru' the MAX Scope90. The latter was at EuroAstroFest in February 2004, when the sky had cleared in the mid-morning after an overnight thunderstorm. The filter, I was informed by the dealer, was Å0.45. Disc detail was very contrasty (not quite as contrasty as I recollect seeing through Henry Hatfield's SHS), but the Prominences were very faint and difficult to see without covering one's head with a cloth.

Hatfield's SHS was fitted with a line shifter. It was a simple matter to follow the various Doppler shifted components of eruptive Prominences. Only a tilt tuned filter could match an SHS equipped with a line filter.

This summer I purchased a Å0.9 passband H-alpha filter from Thousand Oaks together with a 3.5-inch ERF. I have fitted this to my TEC140APO, and have observed Prominences with it to some considerable success. Although it has too wide a passband to show disc detail clearly, I have discovered that by precisely adjusting the image brightness with a variable polariser, I can easily make out the Chromospheric network, bright Plages, and dark filaments.

I am so enthused by what I have accomplished so far, that I am now designing my own SHS, using a novel oscillating double sided mirror synthesiser, first suggested in 1998 by George Y. Haig, and converting my 10-inch f/10 Newtonian on its Calver German Equatorial to an Ebert-Fastie spectrograph.

I can understand why it is tempting to purchase a large narrow band H-alpha etalon filter. It requires no more investment than that of a deep wallet. But there are inevitable limitations by going down that route, attractive though the end result seems.

My first view of Prominences was in 1971 using a Å10 passband etalon mounted in a Prom'Scope fitted with a Lyot cone stop. I later, in 1980, used Horace Dall's similar setup (Å4 passband). Prominences in a relatively wide passband Prom 'Scope are quite bright, and it is easy to see a great deal of faint detail in them, far easier than in a narrow passband etalon. But obviously one cannot see disc detail because the Photosphere is blindingly bright.

So to see disc detail readily using an etalon filter, the passband has to be reduced to below A0.8, and ideally not wider than Å0.6. As the passband goes down the disc detail contrast goes up, but the image brightness decreases (for a given setup), and the Prominences get fainter. Too faint by the time you've got down to Å0.45 or less. (Well I find them too faint, lets put it that way).

Alhough an SHS is bulkier than a telescope fitted with an H-alpha unit, it does not need to be the enormous horizontally mounted contraption that most ATM's imagine. Yes the longer the focal length, the better the spectral resolution, but you only really need 5arcsec resolution to obtain a very satisfactory view, at low power (say x25), and it does have the advantage of enabling Doppler shifted parts of a Prominence to be followed at will, and you can also use not only the H=alpha line but H & K Calcium lines, the H-beta and the Mg b4 and He D3 lines.

An SHS can also be used in spectroscope mode (with the exit slit removed), and you can then increase the power to about x50 and observe the Solar spectrum at very high resolutions, not only in the first order, but in the second and maybe third orders.

I think that if your are seriously contemplating spending money on say a Coronado 90 MAX 'Scope (which at EuroAstroFest was £10,000 for the OTA -although I am aware it is not quite as expensive in the USA), then you really should take a step back and do some research into SHS designs, and consider building one. Even if you have to sub out some detail machining, and even if you have to build some sort of observatory to house it, it would cost less than the Coronado 90MAX 'Scope, and be far more versatile.

#17 Chris Lord

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Posted 26 June 2005 - 09:31 AM

Since my last post 26th Sept.'04 I have changed my SHS design to a Czerny-Turner, with 128-inch focal length collimators and a 4-inch 1200 groove/mm grating blazed at Å5500. The feed telescope will be a 6-inch f/21 Littrow OG, fed via a 12-inch Heliostat. I've also changed my mind about the Haig synthesiser. It introduced vignetting problems due to tilting of the the light path into the monochromator section, and across the entrnce pupil, so I opted instead for Anderson Prisms.

I have all the optical components now apart from a 120-inch focal length field lens which is on order from OptoSigma, and the OG which I intend buying from D&G Optical.

The instrument is going to occupy a fairly large space, so I intend building a dedicated solar laboratory 20' x 8' elevated 7' above the ground, with 7' head clearance.

If any on the group are curious about the design detials then e-mail me: <chrislord@brayebrook.demon.co.uk>

#18 David Knisely

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Posted 26 June 2005 - 01:53 PM

Chris Lord wrote:

So to see disc detail readily using an etalon filter, the passband has to be reduced to below A0.8, and ideally not wider than Å0.6. As the passband goes down the disc detail contrast goes up, but the image brightness decreases (for a given setup), and the Prominences get fainter. Too faint by the time you've got down to Å0.45 or less. (Well I find them too faint, lets put it that way).

Alhough an SHS is bulkier than a telescope fitted with an H-alpha unit, it does not need to be the enormous horizontally mounted contraption that most ATM's imagine. Yes the longer the focal length, the better the spectral resolution, but you only really need 5arcsec resolution to obtain a very satisfactory view, at low power (say x25), and it does have the advantage of enabling Doppler shifted parts of a Prominence to be followed at will, and you can also use not only the H=alpha line but H & K Calcium lines, the H-beta and the Mg b4 and He D3 lines.


Well, I would have to disagree somewhat here. Generally to see disk detail requires a passband of less than a full angstrom, and again, the narrower the passband the higher the contrast. I can still see quite a bit of chromospheric disk detail in a 0.8 Angstrom T-Scanner although it does not have as much contrast as it does in my 0.7 Angstrom unit. As for the statement, "but you only really need 5arcsec resolution to obtain a very satisfactory view, at low power (say x25)" that would not necessarily be true. There are many times when you would want higher resolution, as there are fine details which would not be resolvable at 5.5 arc seconds (daytime seeing often allows down to 1 arc second resolution, although 2 to 3 arc seconds is more typical). Even the lowly PST has a resolution of around 3 arc seconds which would allow more detail to be seen and a greater range of powers to be employed. Still, there are times when I find even the PST inadequate, so I reach for the T-Scanner and my 3.5 inch aperture to watch the fine goings-on inside prominences or emerging flux regions. Clear skies to you.

#19 Chris Lord

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Posted 03 July 2005 - 04:16 AM

I'm referring to Prominences here. When you reduce the passband below Å0.6 to obtain better disc contrast, in general Prominences become very faint. To see Prominences well you need a passband wider than Å0.8. Disc detail in a Å0.8 passband is elusive at best. A Å0.7 passband is the threshold in my estimation for workable disc contrast. Wider and it becomes progressively more washed out, the image is too bright, and needs dimming down with either an nd filter or variable polariser (which is what I do with my Å0.9 Thousand Oaks tilt tuned Prom filter).

An SHS, even a compact one, has the advantage of adjustable passband and a line shifter to follow Doppler shifts, even extreme Doppler shifts associated with sprays, extremely fast, 20Å Doppler shifts and more. Sacrificing a few arcsecs detail is of no consequence. You cannot follow this detail at any resolution in an etalon filter, so it is a moot point. The same goes for surges with Å4+ Doppler shifts. What you see in a narrow passband filter are the lower velocity components. Even a tilt tuned etalon cannot be detuned more than about ±Å2. It isn't enough, and tuning is too hit and miss unless you have a unit with a wavelength offset readout. With a line shifter on an SHS you can tilt the plate progressively against a calibrated Doppler shift scale and follow the Doppler shifted components as you do so, both bluewards and redwards.

What you can also do is alter the passband and offset to observe different heights of the Chromosphere. At Å0.1 on the core centre you see the top of the Chromosphere, at ±Å0.3 from the core the middle of the Chromosphere. At ±Å0.5 near the bottom of the Chromosphere, and at ±Å1.0 the Photosphere is in view. You can do this with two simultaneous adjustemts, of the exit slit width and the line shifter tilt angle. You can make these adjustments to & fro progressively too see all the Chromosphere from top to bottom. It would be nigh impossible to photograph what you can see. You are able to build up in your mind a 3D image of the Chromosphere. You simply cannot do this with any etalon filter. Yes a very narrow passband, narrower than Å0.5 enables you see the 3D nature of the Chromosphere, but you are seeing only part of it, the middle to the top. And with an aperture larger than 4-inches, given good seeing, in marginally greater detail, but the SHS's versatility more than compensates. And if you build a full blown SHS with a focal length greater than 9 feet, you'll have that versatility plus arc sec or so resolution.

I yearn for clear blue skies, send me more.

#20 David Knisely

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Posted 03 July 2005 - 01:55 PM

Chris Lord posted:

I'm referring to Prominences here. When you reduce the passband below Å0.6 to obtain better disc contrast, in general Prominences become very faint. To see Prominences well you need a passband wider than Å0.8. Disc detail in a Å0.8 passband is elusive at best. A Å0.7 passband is the threshold in my estimation for workable disc contrast. Wider and it becomes progressively more washed out, the image is too bright, and needs dimming down with either an nd filter or variable polariser (which is what I do with my Å0.9 Thousand Oaks tilt tuned Prom filter).


Then you were somewhat unclear about what you stated. The prominences may be a tad less bright with a 0.6 Angstrom filter than they might be in a slightly wider filter, but they are still *very* easily visible along with rich disk detail. In fact, they are still visible in a 0.3 Angstrom filter. One of the better units to view prominences in is the PST, which has a passband somewhere around 0.7 Angstroms. Its views were vastly better than the 1.5 Angstrom Lumicon prominence filter I used at a Astronomical League convention one year. As for brightness, I do not find the brightness in my 0.7 Angstrom T-Scanner all that annoying, especially since I usually start my viewing at over 100x.

An SHS, even a compact one, has the advantage of adjustable passband and a line shifter to follow Doppler shifts, even extreme Doppler shifts associated with sprays, extremely fast, 20Å Doppler shifts and more. Sacrificing a few arcsecs detail is of no consequence. You cannot follow this detail at any resolution in an etalon filter, so it is a moot point. The same goes for surges with Å4+ Doppler shifts. What you see in a narrow passband filter are the lower velocity components. Even a tilt tuned etalon cannot be detuned more than about ±Å2. It isn't enough, and tuning is too hit and miss unless you have a unit with a wavelength offset readout. With a line shifter on an SHS you can tilt the plate progressively against a calibrated Doppler shift scale and follow the Doppler shifted components as you do so, both bluewards and redwards.


Assuming you are still taking about prominences here, extreme Doppler shifts are not necessarily present at the limb, as the radial velocity component for material on the limb is greatly reduced or nearly zero depending on the geometry of the event. Hence, the ability to tune to such lengths is not absolutely necessary. A 2 Angstrom Doppler shift (the equivalent of a radial velocity of 91 km/sec) is rather extreme even for disk detail, so something like the DayStar T-Scanner can follow most of what is going on as far as disk detail is concerned. Even with highly Doppler shifted solar flare detail, a tunability over +/-2 Angstroms is enough to follow much of the structure (my T-Scanner covers about this range and on really cold days, it can get to about +4 Angstroms from the centerline). For disk detail (and even for some limb detail) high resolution tends to provide a noticable increase in what is seen, making details like the Arch Filament System and Ellerman Bombs in Emerging Flux Regions easy to see. At 5 arc seconds, much of this detail is simply lost. The spectroheloscope has some advantages in tuning for things like the very rare explosive major flare coming straight at the observer, but other than this, its size and somewhat lower resolution (in the more compact designs) are drawbacks. Clear skies to you.

#21 Chris Lord

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Posted 04 July 2005 - 01:36 AM

David, have you ever used a spectrohelioscope?

#22 Chris Lord

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Posted 04 July 2005 - 01:37 AM

David,

Have you ever used a spectrohelioscope?

#23 David Knisely

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Posted 04 July 2005 - 02:52 AM

David, have you ever used a spectrohelioscope?


No, unfortunately, the one time I was in the presence of one at the Astronomical League's National Convention, it was cloudy and we couldn't use any H-alpha equipment. The owner, however, said a curious thing when I asked him about how well it performed. He said, "It works, but keep using your filter", indicating that he perferred the view in a narrow-band Etalon-based unit. I don't know whether the gentleman didn't get things quite right or what the reasons were for his feelings, but it was a little startling to hear this. I still hope someday to observe with one, but with the low cost and popularity of things like the PST, I suppose it will be a while before I see one again. Clear skies to you.

#24 Chris Lord

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Posted 04 July 2005 - 12:20 PM

David,


That was a pity really. In two ways; that you didn't get his particulars (it would be worthwhile following up on the spec.), and that his setup wasn't up to expectations.

I first observed through a huge SHS built in the 1930's by Ellison. In July 1974 it was in the hands of Henry Hatfield. I observed through it again in June 1989. It is an awesome machine.

The point I was making was that an etalon filter is a compromise between the sub-Ångstrom passband needed to see Chromospheric detail &c at the core of the H-alpha line, and a wide passband which is advantageous to viewing Prominences and detail in Prominences. I have used various Prom 'scopes and birefringent & etalon filters, with passbands less than Å0.5 up to Å10.

Prominences in a Prom 'scope with either a Å10 or Å4 passband and a Lyot stop appear many times brighter than in a narrow sub Ångstrom passband filter intended to enable disc detail to be seen. And it is far easier to see fine detail in Prominences using a Prom 'scope (what is now misleadingly referred to as a Coronograph).

At present I have a Å0.9 Thousand Oaks unit. I can see some disc detail at low contrast, but the Prominences are bright and easy to see, but they do not appear as bright as they do in a dedicated Prom 'scope, with a wider passband.

I rarely employ powers in excess of x50. You must have much better daytime seeing than I do. A power of x80 is about the best the seeing here permits, and then only shortly after dawn before the ground heats up.

If you use a lower power that affords a whole disc view, I think you'll find the image in a Å0.8 passband filter is a bit too bright.

As for Doppler shifts. I'm afraid we are going to have to agree to differ here. I have occasionally observed eruptive Proms on the limb with sight line velocities well in excess of Å4 using an SHS. In my T-O Å0.9 filter which can be shifted ±Å2, you see some (not most) of these Doppler shifted components.

As for Doppler shifted disc detail. I maintain ±Å2 tuning range is inadequate. For instance on February 14th this year at 12h:00m - 12h:30mUT a narrow dark filament alongside a bi-polar sunspot group erupted and went into emission. It was easy to see even in my Å0.9 passband unit. The event was observed by a friend of mine using a Manning compact SHS. He measured Doppler shifts in excess of 400km/sec. My view would have had a resolution of about 2 arcsecs (the OG was stopped down to 2-inches), he would have had a resolution of 5 arcsecs. Yet he could see it quite clearly.

The lower resolution of a compact SHS is more than outweighed by its greater verstility.

A SHS with a focal length of 9' and a 3 thou slit will obtain 3 arcsec resolution. Increase the focal length to 18' and you halve this value, or reduce the slit width to 1.5 thou. However it is a mistake to assume all compact SHS's have lower resolution than that of a medium sized refractor and an etalon filter.

Camiel Severijns has built a compact SHS <http://home.hetnet.nl/~c.severijns> The entrance & exit slits may be adjusted between 8 & 1000 microns. At 8 microns the passband is Å0.1 and the resolution 1 arcsec. You'd be hard pressed to out perform this particular compact SHS with any etalon unit, stacked or multi-stage. What Camiel Severijns has achieved is not easy I'd be the first to concede, but I know through discussions with him that his SHS cost much less than a twin stacked Coronado 90 OTA, and this includes the home built dedicated GEM on which it is mounted.

Compare the cost with a Zeiss Å0.1 passband H-alpha unit, and there is no contest. The SHS wins hands down on all fronts. All it takes to build one is determination and commitment, and I suppose a certain degree of obsessiveness.

#25 David Knisely

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Posted 05 July 2005 - 01:07 AM

Chris Lord wrote:

[quote name="Chris Lord"]
I rarely employ powers in excess of x50. You must have much better daytime seeing than I do. A power of x80 is about the best the seeing here permits, and then only shortly after dawn before the ground heats up.
[/quote}

I find this somewhat puzzling, as I rarely have seeing that compromises the performance of the PST and its meager 40mm aperture (3.4 arc seconds resolution). Even during the heat of the day, I rarely see much of a significant disurbance in the image, although slight degredations do appear from time to time. At the "default" magnification of 32x, seeing variations are often subtle, so if seeing is even halfway decent, I kick things up to around 44x or even 62x. I have used the PST at 80x on occasion, but quite frankly, the detail looked a bit clearer at 62x or 44x. With the T-Scanner and the larger 3.5 inch aperture, I have gone as high as 176x, but usually, I run at a little over half this level. The best seeing I have ever had with the sun was during late morning hours when with my 3.5 inch off-axis aperture and the T-Scanner, the sun was nearly rock-steady at 130x (probably seeing was on the order of 1 to 1.5 arc seconds). However, daytime seeing is more typically between 2 and 3 arc seconds with occasional excursions to 4 arc seconds very late in the day.

[quote]
The lower resolution of a compact SHS is more than outweighed by its greater verstility.
[/quote]

But not its size or its commercial availability (which is virtually non-existant). Many people simply are unable to even consider constructing even the simplest SHS, so for them, the etalon-based system is often the better way to go. I certainly would not be happy with any SHS unless its resolution was at least on the order of two arc seconds (and possibly better). I also like something which I can take out on the patio in one hand and instantly be observing the sun for the few minutes I have prior to having to go to work in the afternoon. The PST makes a great companion to my T-Scanner and is easier to use, so of all my solar setups, it gets used the most. Clear skies to you.


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