Solar Spectra, Filters & Slewing

       

Subject: Solar Spectra and Filters  

From: Doc G

Solar Spectra and Filters

The issue of use of filters to view the sun came up recently on this site and there followed a discussion of how these special solar filters worked and why they enabled viewing of solar phenomena is such beautiful detail. In order to understand how the filters do their job, it is necessary to have a basic understanding of the nature of the light emitted by the sun. I have attempted to meet the challenge posted by some Mapuggers. This has been a difficult assignment. I hope this post will be useful.

The details of how the sun works are obviously too complex to describe here. Even the brief but excellent descriptions given in the Encyclopedia Britannica take many pages. So what follows is a super brief description of the basic processes by which photons are emitted and absorbed and what we see as a result.

The body of the sun is a mass of hot gasses under great gravitational force because of its mass and with great internal pressure because of the nuclear reactions that take place deep within the sun. The internal pressure and temperature are large enough to cause nuclear fusion of hydrogen atoms into helium atoms. (Some 15,000,000 K) This process generates photons which migrate outward toward the surface of the sun. The photon pressure from the inside is balanced by the gravitational forces. The rather well defined ball we see as the sun is cooler at the surface. It radiates from its surface at a temperature of about 5800 K. The surface we see is called the photosphere. The radiation from the photosphere is characteristic of that of a black body radiating at 5800 K according to Planck's Law. This is the familiar yellowish color we see . We may compare this to earthly temperatures, a common incandescent bulb radiates at about 2800 K. Thus the light bulb also has a continuous spectrum but is much more yellow/red in color compared to the sun. The important thing to realize about the sun is that its basic emission is a continuous spectrum because it is due to thermal excitation.

Immediately above the photosphere is a layer called the chromosphere. The chromosphere has an emission spectrum of spectral lines which correspond to those of the gases of which it is constituted. In this case, the emission is in discrete lines because the gas is relatively rarified. Still further out is a region called the corona which consists of very rarified gasses. These gasses eventually become the solar wind. During a total eclipse, the corona can be seen as a whitish halo extending several degrees out from the occulted solar disc.

Basic to the understanding of the observed solar spectrums from the various parts of the layers about the sun is the concept of absorption and radiation of photons from atomic gasses. At low temperatures atoms are in their rest states and do not radiate significantly. But when atoms in a gas are heated thermally they absorb energy, electrons move to higher (excited) states and they can then re-radiate this energy in the form of photons. Each atom with its array of electrons radiates photons with characteristic energies. The line spectra emitted can be used to identify the elements involved. Atoms can also be excited by other photons, go to excited states and re-radiate their characteristic photons. We see these photon emissions in many lights such as sodium vapor and mercury vapor lights as well as neon signs and the like. These emissions are not continuous spectra but are line spectra. Helium was discovered first on the sun because of its characteristic radiation spectrum. One of the effects we see in sunlight is that the continuous Planck Law radiation is interrupted with dark lines. This is caused by cooler gases at the outer surface of the sun absorbing photons of certain energies, thus diminishing the spectrum in what are called absorption lines. These dark lines tell use which elements are doing the absorption in the cooler gases.

The surface of the sun is in a constant state of agitation like a pot of boiling water. Thus there are hotter and cooler regions on the surface. Because of the strong emission of radiation from hotter regions and absorption of radiation in the cooler regions, the surface of the sun has a granular or reticulated look. Very large boiling regions are seen as sun spots. Sun spots are so large that they can be seen with a very simple filter that does no more than attenuate the total light from the sun by a factor of 100,00 or so. Very fine filters for observing sun spots are available from several sources. One of the best is Thousand Oaks Optical (800 996 9111). I personally use their full aperture Type 2 plus filters on my LX200s. In addition to the constant boiling of the solar surface there are much greater spurts of material that are ejected from the surface of the sun which return in gigantic arcs. These eruptions are called prominences. The are visible during a solar eclipse as arcs of flame projecting well beyond the dark disc of the occulting moon. The granularity, the chromosphere and the solar prominences are so dim compared to the photosphere that they cannot be observed except during a solar eclipse or with very special telescopes and filters. While these features absorb and emit photons in many parts of the spectrum, one special line is of particular interest. It is the Hydrogen alpha line at 6562.8 Angstroms. This line is emitted strongly by the features we want to observe. The scheme required to form images is to use this spectral line while suppressing all other light from the sun. This is done with special interference filters.

These filters are very complex, being made up of numerous thin interference layers and many reflecting optical elements. They are adjusted to cancel all wavelengths except that of the Hydrogen alpha line. These filters can be made to have passbands that are so narrow that they only pass from a few down to a fraction of an Angstrom. Such filters are often combined with other special optical attachments which are designed to obscure the image of the disc of the sun and so make prominences even more visible. While specialized filters and optical attachments such as those required to observe solar granularity and prominences are usually out of reach of the amateur astronomer they never the less are enticing. The observation of stars should surely include our very own star. Ellery Hale was, for example, fascinated by the sun all of his life and made a significant reputation through his studies of the sun. These filters and attachments are described in some detail in "Solar Astronomy Handbook" by Beck et al (Willmann-Bell) The design of these devices is for very serious amateurs only. Hydrogen alpha Filters are available from a number of sources.

A discussion of them is available at: <http://www.company7.com/daystar/>. These filters are very expensive and require additional prefiltering to eliminate the massive heat load from the sun first. The best of the filters are tuned by controlling the temperature of the filter with a small oven surrounding it. Additional special optical elements are used with the narrowest of these filters to insure that the filter is in a part of the optical path where the imaging rays are parallel.

An elegant description of the view seen with filters of various bandwidths is given in "The Manual of Advanced Celestial Photography" by Wallis and Provin (Cambridge University Press 1988). I quote:

"With a 1 Angstrom bandwidth the prominences lining the edge of the disk can be easily seen and solar flares can be viewed but the finer details over the dun's disc are not well displayed. With a 0.8 Angstrom bandwidth, the prominences are seen very well, with better contrast than is provided by a 1 Angstrom filter, and the details on the disc of the sun are readily seen in detail.

With a 0.6 Angstrom bandwidth , the details on the disk are seen with excellent contrast and clarity however, the prominences are now difficult to see."

Thus it is important to choose exactly the right filter for the visual effect desired. The Wallis and Provin book is, I believe, the most useful book on celestial photography I have found. I recommend it highly. I have appended to this note a list of other books on astronomical photography which may be of interest, with my notation about their suitability for various levels of expertise.

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Photography References - With particular attention to the Sun and Solar System

Personal comments about a few select books from my library which I have studied in some detail - Doc G (September, 1997)

A Manual of Advanced Celestial Photography; Brad D. Wallis & Robert W. Provin; Cambridge University Press 1988

The first among good books on astronomical photography. The book concentrates on photographic techniques rather than equipment. This is an extraordinary book and a must for anyone doing or wanting to do astronomical photography. Not for the novice but a required book after digesting one or two of the following books.

High Resolution Astrophotography; Jean Dragesco; Cambridge University Press 1995

An excellent book with emphasis on photography of the Sun, Moon and planets. Equipment is described in detail and numerous examples are shown. Not for the novice.

Solar Astronomy Handbook; Beck, Hilbrecht, Reinsch, Volker; SONNE (1982 German) (first English Edition 1995) Willmann-Bell

Great detail about equipment and many examples of photograph results. Lengthy discussion of observations and recording of all solar phenomena. Definitely for those with a deep interest in the Sun. Definitely not for the novice.

Astrophotography - An Introduction; H.J.P. Arnold; Sky and Telescope 1995

A nice, and simple, introduction mainly to photography of the Sun, Moon and Planets. A fine book for the novice and the serious amateur.

Astrophotography II - Featuring the Techniques of the European Amateur; Patrick

Martinez; (1983 French) (English Edition 1987) Willmann-Bell, Inc.;

A fine small book jammed with important information that everyone should know about setting up telescopes for photography. An excellent book for the serious amateur.

Astrophotography for The Amateur (revised 1991); Michael Covington; Cambridge University Press

A nice little book that covers the basics very clearly. The book tries to cover a bit too much ground for such a short book. Strictly for the novice.

The Cambridge Eclipse Photography Guide; Jay M. Pasachoff and Michael A. Covington;
Cambridge University Press 1993

A very nice book about eclipses with detailed tables covering eclipses through 1999. This is a timely book for the eclipse of 1998 in the Caribbean and 1999 in Europe. Excellent discussions of observing and photographing these events.

Subject: 1000 Oaks Solar Filter   

From: David W. Bonnell <Bonnellnist.gov>

JohnLX200 wrote:

>> 1000 Oaks Solar Filter + Meade ND96 gray filter (13%> >transmission).

>>1. These two combined should they be (SAFE) enough to view the sun visually?

>I won't go so far as to say it's OK, but I'd have no problem doing it

>personally, but I'd leave out the ND96. If you were using a big, fast scope

>I'd definitely use the 2+ but with slower scopes I'd think the legal beagles

>have made them design you into the safe zone despite the disclaimers.

This is particularly true using the ETX for visual work, relatively small aperture, and high f#. However, it is not clear exactly why the same filter is involved. If you are somehow balancing a big filter on the ETX - be REALLY careful - it only takes a split second to fry an eye.

>If you're going to do a lot of solar observing maybe it's better to be safe

>than sorry. But as David pointed out, the S&T tests don't show the factor

>of 10 difference between them that is claimed anyway. And if I recall

>correctly, even the 3+ offers more protection than the Questar

>full-aperture filter which they said was perfectly fine for visual, while

>trying to discourage people from using the 3+ for visual. Again, draw

>your own conclusions as to whether the recommendations are based upon

>physiology or fear of lawsuits.

And, pay attention to your visual comfort level. The blocking at all wavelengths seems to be sufficient that if the visual brightness is not uncomfortable, IR/UV problems should be no problems. However, do be aware that, as I remember, the retina has no pain sensors - sunburned eye is not desirable - thus, just like your skin, longer exposures mean more care need be taken.

>Two questions for you:

> what is the aperture of the filter?

> What powers will you be observing at?

>If you're using the ETX-diameter filter in an aperture mask on the 8",

>you're already above f/20 and again protected for the same reasons as the

> Questar filter is OK, namely high f ratio making up for slightly higher

>transmission.

>If you're observing at high power, the light is similarly spread out. If

>you're using a 55mm Plossl all the time and observing every day, I'd go for

>the 2+ for the added protection.

I agree, and maybe even consider a moderate transmission (e.g. polarizing) filter to cut back a little more. Note that, in daylight, your pupil is small, and you are quite likely to have trouble with the central obstruction, unless you work a little at being "dark adapted" anyway - that

is, sunglasses when not looking thru scope. Having the scope image a little darker works the same.

>When it comes to film photography, I must beg to differ with David. I

>would (and do) use the 3+ in order to get high shutter speeds. You'll get

>better results at 1/1000 to 1/8000 than you ever will using a longer

>exposure and the "hat trick" as a shutter. It will make you immune to

>wind, vibration, lack of tracking, or anything else. Plus when anything

>from a bird to the shuttle releasing a spy satellite transits the solar

>disk, you'll stop the action and see what it is.

I must bow to John's expertise and experimental confirmation, and tend to agree - the solar image is normally roiling pretty badly from stray thermals in any case, so one does NOT want really slow speeds (I wasn't trying to imply that was a good idea -- more that it could become an option. However, I am surprised that speeds of >>1/1000 are even better, since all the normal vibrations... are at a few Hz to 10's of Hz at worst (and by 1/125 - 1/500th, the mirror-slap recoil should be beaten) -- as for the slowest speeds, the scope is, after all, designed for long exposures. One problem with really fast shutter speeds, is that the shutter curtain opening becomes just a tiny slit, and the faster shutter rigs themselves cause some high speed vibration. Thus, any movement on that scale can appear to be a bit peculiar. However, the good news - in most camera designs, the shutter moves at full speed, regardless of shutter setting - the exposure is timed by the release of the second half of the curtain - so at medium speeds and below, there is just a tiny "pop" at the beginning of the exposure, and another at the end - tending to be self damping (at least, that is the design idea). Truth is, as long as aperture is not an issue (this for camera lenses where one trades shutter speed for a field stop, giving up depth of field for speed), higher shutter speeds generally are better!

And, that dang boiling image can certainly use high shutter speeds - I was just trying to point out that, with modern high-speed film, you can use pretty high shutter speeds for the most part, even with the 2+ filter (I actually have a Solar Skreen filter, but find typical solar pics on Fuji 800 to be of the order of 1/250 - 1/1000 with reasonably printable density. However, I still have a ways to go to get what I consider really good photos - because of the seeing, it can be darn near as hard to focus on the sun as on nighttime objects!

>I did a number of tests before the February eclipse, and high shutter speed

>wins, with long exposures in second place if you're tracking and have no

>wind/vibration problems, followed by those nasty speeds from around 1/60

>to 2 secs where mirror slap can't be ignored. In those cases, lock up the

>mirror if you can.

This is, of course, great advice but as John notes:

>Few cameras let you do it anymore, but some do when you

>use the self timer. Another tactic which worked for me but everyone frowns

>upon is to hold the camera body firmly at those speeds, as if taking a

>snapshot, or place one finger under the camera body like a spring.

I will make two comments - even those with mirror lockup often find it requires force that tends to move the setup to actuate the feature, or tend to forget how the feature works - so in either case wind up not flipping up that mirror. John's advice above is good if you want to ignore the mirror problem.

So, it mostly comes down to what you do, and how much. Personally, I think either filter is OK for casual visual use as comfort level will clue you to problems. Adding an ND filter (or better, a polarizing filter) will give you the best of both worlds. And John - who has been frowning at you? -- all pro photographers know that the next best thing to something solid as a brace is tension -- in tough

low-light situations, you will see the pro working with his camera's strap tight as a drum. A little extra force does help damp medium frequency vibrations! This trick can be used successfully to damp a portion of the mirror vibrations, too. I have a lot of (non-astro) photos taken handheld at speeds like 1/4 sec and with moderate telephoto (105-200 mm) lenses that were sharp enough to do well in salon shows. Do it, as long as you can make it work for you, it just takes learning how to exert a steady force.

Most people frown, I suspect, because they confuse a little judicious useful tension for the quivering death grip many use on their point-and-shoots, where muscle tension is so high that it is the problem. A little force is a great calmer, for photography, and otherwise, as you show so ably when this list needs a touch of the strap!

Subject Slewing (classic LX200) to the Sun --part 1 of 2   

From: Bill Arnett <billnineplanets.org> Date: Oct., 2000
> The LX200 won't allow a GoTo an object that it regards as being too close to
> the Sun. Can this be overridden to enable automatic tracking of the Sun
> (rather than manual control) while solar observing or is there another way?

If you manually slew to the Sun (having taken care to cap the finder and put a proper filter in front of the corrector) your LX200 Classic will then track just fine.

You need some method to actually locate the Sun; however, this isn't very hard as there are several techniques. The simplest is to grab the Sun's RA and Dec from a planetarium program and GoTo them; the "too close to the Sun" check is turned off in that case. But I find it simpler to just try to minimize the shadow of the scope on the ground and then having gotten close that way I look into the eyepiece holder without an eyepiece and observe the glare off of the inside of the scope. That makes it easy to get the Sun in the field.

> While you are on the subject of solar observing. What is the difference from
> an observing standpoint between a full filter and a smaller off-axis filter?
> I bought the Baader material and want to make a solar filter.

A full aperture filter gives you the full resolving power of your scope. In very fine seeing that is a big advantage. It also gives you a larger exit pupil at any given magnification which may be important if you, like me, have bothersome floaters in your eye.

A smaller filter will be bothered less by the bad seeing that is commonplace in the daytime. In most cases you'll probably get a better image this way.

But, if you make a full aperture filter, you can always put a simple mask in front of it to avoid the problems of bad seeing.

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Subject: Slewing to the Sun --part 2

From: Marc Castel <castelrobosky.com>

If you use a software program to control the scope then you can slew to the Sun or any other object directly. (i.e. The Sky, ACP, etc.)

Subject: Using the LX200 with an H-alpha filter

From: Tom Mote <pytomtexas.net>

Leonard B. Akers wrote:
I use Baader filter material from Adirondack in a cell made from a white PVC plumbing fitting I got from Lowe's. Lined it with green self-adhesive felt from same place to make a tight fit so I didn't have to worry about it falling off. Works like a charm. 30 minutes of work.

One of the other messages in this thread suggested putting the H-alpha filter on a smaller piggybacked scope and saving the LX200 for a large diameter white light solar filter. I agree with his advice. I have a DayStar H-alpha filter and you need to know that they require an f-ratio of at least f/20 in order for the light pencil to be parallel. That means that you either have to have a very, very long focal length [and your LX200 does not] in order to use the full aperture or you have to stop it down to obtain the required f-ratio. The DayStar filter comes with a heat rejection filter that goes on the front of the telescope and, for an 8" f/10 Celestron or Meade, that heat rejection filter is only about 2" in diameter. Ergo, you are wasting most of the aperture of the main telescope. My filter set was for an f/10 C8 and worked fine but I have since moved it to my 4" Televue Genesis and added a Televue 4X PowerMate to obtain the required f-ratio. Works like a charm, is easier to move about and doesn't tie up the larger scope.

Subject: Locating the Sun in the Scope --part 1 of 3  

From: Jim Burrows <burrjawearthlink.net>

Bruce Gillespie wrote:
>1) Put on Solar Filter on OTA (Thousand Oaks)
>2) Keep the finder covered
>3) Put on left-over Eclipse Glasses
>4) Do a visual alignment along the top of the OTA (using the middle
> screws on the top and bottom rings)
>5) Some tweaking on RA and Dec knobs might be necessary

Here's a fun way to line up with the Sun with full-aperture solar filter on the LX200. Make sure the filter is on good and tight, all the way around (good idea, anyway). Punch a hole in a piece of cardboard (the box the filter comes in is handy), hold it up between filter and Sun. Looking at the back of the cardboard, line up the hole with its image reflected off the filter by tweaking the E-W, N-S buttons. Remove cardboard and observe sunspots, limb darkening, etc.

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Subject: Locating the Sun in the Scope --part 2

From: Ells Dutton <eduttoninfi.net>

I believe Galileo did use the projection method and credited an colleague for the idea, but then again, Galileo did go blind later in life. Add it up.

BTW, I've used the shadow method and finder filters and the filters work much better. Also works to add the sun as a user object (AutoStar) and update it from a Dec/RA source and then you can sync on it and then go to the planets and some stars, pretty neat. Not sure if you can sync on user defined coordinates on the classic hand box.

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Subject: Locating the Sun in the Scope --part 3 of 3  

From: Paul Goelz <pgoelzeaglequest.com>

You wrote:
>I've used the shadow method and finder filters and the filters work
>much better. Also works to add the sun as a user object (AutoStar) and
>update it from a Dec/RA source and then you can sync on it and then go
>to the planets and some stars, pretty neat. Not sure if you can sync on
>user defined coordinates on the classic hand box.

I find that in the field, using the shadow method gets me close. Then I just look into the OTA with filter in place and without any eyepiece. I can get a pretty good idea of where the sun is by the reflections on the side of the OTA, and when I am actually pointed at it, the corrector lights up. Put in an eyepiece and I'm done.

Subject: Solar Filter Source for Telrad Finder  

From: John Donovan <donovanspeakeasy.org>

Kendrick sells a "Sun Finder" that fits on top of the Telrad device, Item No. 2059:
<http://www.kendrickastro.com/astro/solarfilters.html>. I have been using it for about 3 years and find it to be very effective. Price is $30 US.