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Comparison of DayStar and Coronado H-a Solar Filters with Spectrohelioscopes


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Amateur astronomers have shown a growing interest in solar observation ever since Coronado introduced their line of solar H-a filters to compete with the industry stalwart, Daystar, several years ago (other solar H-a filters are made by Hardin Optical and Baader Planetarium). However, amateur solar observers too often neglect another instrument – the spectrohelioscope. Spectrohelioscopes were invented by G.E. Hale in the 1920s and have steadily been improved and made more accessible to amateurs over the years. Fredrick N. Veio’s authoritative book “The Spectrohelioscope” established his reputation at the vanguard of amateur solar astronomy. This book, as well as myriad designs, photos, discussions, articles mentioned below and other information can be found at http://spectrohelioscope.net. I recently asked Fred to compare the performance of a spectrohelioscope to commercially available H-a filters. The remainder of this report presents (with minor edits and additional material on my part) Fred’s observations on the benefits and weaknesses of each type of instrument for solar observation (Chris Westland, 22 May 2004)

Observations on H-a solar filters versus Spectrohelioscopes

There are several H-a solar filters on the market, but only DayStar and Coronado filters will be discussed in this comparison. I have experience with a friend’s DayStar University model. 0.6A passband. The Coronado and DayStar filters come as complete, ready to use products – you only need mount them on the eyepiece end of your telescope. They are more expensive than an amateur spectrohelioscope (which can be made for US $500-1000) and the spectrohelioscope is more work – it must be constructed by the amateur.

In general, for any of these instruments, the H-alpha passband must be better than 0.8A for acceptable performance, assuming the telescope to which it is attached is performing optimally. Passbands of 0.6A or 0.5A are better, but considerably more expensive. 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. Thus 0.7A is a good, affordable compromise.

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.

Beyond passband, DayStar also differentiates filters as T-scanner (tuned by tilting the etalon) and their University and Amateur filters (tuned by heating the etalon). The University filter is their top of the line model, has no calcite flaws, and thus allows uniform photographic performance across its entire aperture.

The DayStar filter works at f/30 with the filter placed near prime focus. An Energy Rejection Filter (ERF) placed over the front aperture reduces the heat load, absorbs UV light and provides the telescope with a cool and optically undisturbed beam. Wave front error of the ERF filter is 1/8l maximum as measured at 6328Å. The ERF is a red filter placed in front of the objective lens that filters out most of the light except for a narrow band of reds containing the H-a line in which the DayStar and Coronado filters image the Sun. 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.

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.

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.

The H alpha line visually is 0.6A wide, with slight wings of 0.1A on red and on violet side. This is the reason that filter companies recommend 0.8A as the maximum compromise for acceptable contrast; 0.7A passband is a better compromise for good contrasty detail, and 0.6A passband for best contrast.

The Ca II violet H and K lines are broader at about 3.0A for K lines and about 2.0A for K line. The dark core of either is about 1.2A wide, and thus us the passband required for maximum contrast in viewing plages on the solar disk. For good contrast, about 3A passband filter is adequate. For acceptable contrast, about 5A passband is adequate. An 8A passband provides the minimum acceptable contrast, bearing in mind that solar detail also depends upon the size of the solar image. If the solar image is small, then plages will appear bright; if larger, plages will show less contrast.

Also note that a 0.7A passband filter passes more light than a 0.5A or a 0.2A passband, so the solar disk will be slightly darker visually (but, then, there is a lot of light to begun with, and the eye will automatically compensate.

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 pr.ominances 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.

Spectrohelioscopes

Spectrohelioscopes (SHS) employ an entirely different optical design than either the DayStar or Coronado filters. Thus one cannot compare equally and fairly the SHS with other filters. I published my initial design for a low cost, compact, portable SHS in Sky and Telescope many years ago (January of 1969 to be precise; this article is available for download on http://spectrohelioscope.net).

This SHS design contains three sections with at least seven components:

(1) a telescope that brings the solar image to a prime focus. Because this telescope is dedicated to observing only a single star – the Sun – size is described in terms of focal length. A 9 foot f.l. telescope gives an image of the Sun that is 1” in diameter; an 18 foot telescope gives a 2” image; you get the idea. Rather than the entire telescope rotating (as with a stellar instrument), in solar telescopes, a rotating flat mirror called a heliostat directs the sun’s image to a fixed objective – often a large lens – which then keeps the solar image stationary on the entrance slit. This change accommodates heavier, more complex instrumentation that otherwise might shift and vibrate if it were in motion,

(2) a spectroscope which has three components:

(3) a pair of ‘synthesizers’ (one for the incoming slit and one for the outgoing slit) that wiggle the solar image back and forth at a rate of 24 frames per second (the same as cinema film) to generate what the eye sees as a 2-dimensional image of the Sun in a single pure color (this can be H-a, or any other color of interest; different colors show details in different layers of the Sun’s atmosphere).

Here are the technical specifications on the low cost amateur spectrohelioscope (SHS) I designed back in 1969. I figured the two BK 7 lenses for the spectroscope myself. It had a telescope of 2.7 meter f.l. (nine feet) and a spectroscope of 1.9 meters f.l. (six feet). The grating was 32x30mm area on a 50mm blank, 1200 gr/mm, 90% theoretical resolution, 5000A blazed wavelength. Eyepiece power for the solar disk was 25X, and for the solar spectrum 50X. The heliostat (a rotating flat mirror that keeps the Sun’s image stationary on the imaging lens) was a 76x100mm mirror good to 1/8 wave. The synthesizer (i.e. the part of the SHS that ‘synthesizes’ an image of the sun from a sequence of line images, much like a television screen) is a rotating glass disk with etched slits (called the Veio synthesizer, you can find out how to make one of these at http://spectrohelioscope.net), was 100mm diameter (four inches), 24 slits cut in the paint, slits were 125 microns wide (.005 inch) for 0.5A passband. Costs to make the same design now will be about $600 if you make the optics yourself. Some laser optics can be bought (a comprehensive list of suppliers is provided at http://spectrohelioscope.net).

The DayStar and the Coronado filters permit only one wavelength, usually selecting the H-a line (Hydrogen-a is located at 6562.81Å in the red portion of the solar spectrum. At its half intensity point, the line is only 0.6Å wide. Providing optical filtration in this order of dimension is very demanding). A SHS can use all wavelengths of the solar spectrum by tilting the grating the proper angle. Also the SHS can use the solar spectrum in order to see various solar events, such as flares in emission, zig-zag shift of the H-a line due to emerging magnetic flux tubes, Zeeman effect (widening of the lines) in the umbra with some sensitive photospheric lines, the solar spectrum itself in very fine detail, being almost equal to photography by large professional telescopes. And with the slit tangentially placed on the solar limb, emission of the H-a line and the yellow helium line are easy to witness. Also, placing the slit over a hot plage (a flare is not needed) will show the yellow helium line in absorption, sometimes in emission, and likewise for a flare if present. Surge filaments and prominences will show a Doppler shift with distortions of the H-a line.

There are different kinds of detail on the solar disk and at the solar limb. Some details are medium bright to bright, such as plage, flares and prominences. Other details, such as filaments, are medium dark to very dark. There are also faint to very faint details, some being fairly easy to see or others difficult to detect (or not at all depending upon seeing conditions quality of the telescope optics, passband of the filters and the observers themselves).

To give a fair comparison with various solar instruments is not easy. The size of the details is important. Small faint detail is not as easy to see as large faint detail. Hopefully the following will give you a basis for comparing the views between an SHS and a narrow-band H-a filter (of course the best way is to see for yourself if have the opportunity to try one or the other).
Visibility of Solar Disk Detail

Passband Bright high contrast detail Faint, low contrast detail 10"arc Faint low contrast detail 5" arc

0.7A yes yes yes
0.8A yes maybe yes maybe yes
0.9A maybe yes no no

Now the limit of resolution, the Dawes limit, for two 6th magnitude stars is (See the Amateur Astronomers Handbook by J. B. Sidgwick)

Dawes limit= 4.56/aperture in inches (25mm) barely resolved

But the Dawes’ limit must be modified for sunspot detail.

Dawes limit = 2.2/aperture in inches barely resolved

Here is an example of how the Dawes limit needs to be used in solar observation. Consider a 60mm (2.4 inch) front mounted Coronado H alpha filter. It will barely resolve about one arc/sec detail, or easily about two arc/sec detail. A DayStar filter with a 60mm lens at F30 will perform similarly.

A visual SHS is designed differently for visual performance. The entrance slit of 0.005 inch (125 microns) and a 2.7 meter f.l. telescope will pass about 5 arc/sec detail, depending upon the shape of the detail itself. That said, some amateurs have built spectrohelioscopes of around 2.5 arc seconds resolution at a cost of around one thousand dollars (mainly due to higher quality optical components; see Brian Manning’s and Phil Rousselle’s instruments on http://spectrohelioscope.net). The spectroheliographs made by Brian Manning of England and Philippe Rousselle of France have recorded about 2.5 arc/sec detail with very compact optical designs (you can find out more at Philippe Rousselle’s site at http://csep10.phys.utk.edu/astr162/lect/sun/spectrum.html).

There are differences as well in the maximum eyepiece power possible in an SHS. With a good quality telescope – e.g., a standard amateur telescope such as a refractor or a Newtonian – of 1/8 wave at infinity focus, one can use about 50 power per inch of aperture on high contrast objects such as double stars or the Moon or about 25 power per inch for medium contrast targets such as the planets and the Sun.

In contrast to a telescope with only one main primary optical component, an SHS has many optical components in the whole assembly. For the solar disk, about 12 power per inch can be used. So a telescope of 60mm (2.4 inch) diameter will allow about 30 power as a maximum in the spectrohelioscope mode. With a longer telescope f.l., about 60 power can be realized. In the spectroscope mode, more power will work, namely about 50 power to examine the solar spectrum and the solar events in fine detail. (this topic is discussed at length in my book The Spectrohelioscope which can be downloaded in full at http://spectrohelioscope.net)

If you have the inclination and money, then the ideal instrument for solar observation should be a solar spectroscope and DayStar or Coronado H-a filter combination on a single mount. The telescope can be about one meter f.l. (40 inches) and used with a 2X or a 3X Barlow, producing a 25mm solar image (one inch); the spectroscope optics can be about one meter f.l., although about two meters f.l. would be better. A 1200 gr/mm grating will be the minimum, but an 1800 gr/mm will be better (you can find sources at http://spectrohelioscope.net). The solar spectrum will be about 0.5 to 1.0 meters long, depending upon optics used. Use the first and the second orders just by tilting the grating a bit.

From this comparison, one could easily be led to believe that the spectrohelioscope is only a poor cousin to the H-a filter. But this would exaggerate the differences. First, H-a filters can be built only up to about 4” aperture (beyond that the heat buildup is great enough that there is risk of the ERF shattering on the DayStar filters, and in Coronado’s case the cost of building the etalon becomes prohibitive), but the telescope of an SHS often has an 6” to 8” aperture, and similar scalability on the spectroscope end allows it to regain its edge on resolution. 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 can’t over emphasize how much of the rich and constantly varying solar detail you are missing if you only observe the solar disk in H alpha light. There are almost 4000 spectral lines from the violet to the red. About 1500 of these lines are medium strong and very easy to see (the rest are faint to very faint, barely seen). Each of these strong lines shows different detail of the Sun’s ever changing surface.

I have tried to make the above comparisons as fair and honest as possible. I can only state that my working with solar instruments in various configurations the past some 40 years has been most enthralling, and that you can never capture the full joy of solar observation without the tenability of the spectrohelioscope. If you interested in further reading, please feel free to download my book, The Spectrohelioscope (.PDF format, 119 pages) at http://spectrohelioscope.net.




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