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Daystar T-Scanner - 0.7 Angstrom Solar H-alpha Filter
The active face of our nearest star, the Sun, is interesting to watch with the proper white light solar filter. However, with a narrow band filter tuned to the wavelength of the Hydrogen-alpha light (6562.8 Angstroms), the Sun changes from a well-mannered star with an occasional case of acne, to a huge seething angry red ball of gas in almost constant turmoil. To view the fascinating spectacle of the Chromosphere requires a very narrow and fairly expensive filter to eliminate all but the Chromosphere's contribution to the image. One filter which does this fairly well without exactly breaking the bank account is the DayStar "T-Scanner", 0.7 Angstrom H-alpha filter. With it, the door to the wonderful world of H-alpha is opened a crack, allowing the amateur a peek at a vista which was once reserved for only professional solar astronomers.
The 0.7 Angstrom bandwidth T-SCANNER is a 2-component filter which is a variation on the Fabry-Perot Etalon design DayStar uses for its more costly oven-heated ATM and University models. It consists of a T-Scanner unit which goes in the focuser just in front of the eyepiece, and an "Energy Rejection Filter", or "ERF", which goes in front of the telescope objective. The T-Scanner uses somewhat lower quality optical materials than its more expensive brothers and operates from 32 to 104 degrees F (0 to 40 deg. C). It deals with the temperature control/passband drift problem by using an adjustable tilt in the filter stack to change the optical path length through the filter. Not only does this variable tilt allow the user to compensate for temperature variations which might push the passband off of H-alpha, but it also makes possible rapid tuning of the filter for viewing Doppler-shifted features. The lack of an oven also means no filter heater electrical power or warm-up time is needed, and reduces the cost of the filter to some degree. The T-Scanner requires at least an f/30 light cone to work, but this often can be achieved with auxiliary lenses, or by stopping down larger apertures. Observations of prominences can be done using the T-Scanner in an f/ratio as short as f/27, but most Chromospheric disk detail is weak or absent using that f/ratio.
Direct from DayStar, the T-Scanner comes in a 10" x 7" x 5" finely- finished wooden carrying box, with a rubber carrying handle and a push button front latch. Inside the box is a grey foam insert formed to fit the T-Scanner, as well as an open rectangular area for something else like a star diagonal or an adapter. The T-Scanner itself is about 5 inches long and weighs in at just under 0.5 lbs without an eyepiece. The T-Scanner's core is a small cylinder about 2.5 inches in diameter and just over 1.5 inches long, with an eyepiece holder fixed on one end, and a 1.25" OD tube on the other. The unit is solidly built, with the only vunerable looking thing being the small tuning knob on the side. It is this knob which allows the user to tilt the filter stack for tuning or temperature compensation. My version has the standard fittings for 1.25 inch eyepieces, but the eyepiece end is threaded to accept a 2 inch eyepiece holder if one is ordered. Recommended focal lengths for eypieces are 32mm and over, but I have used a 27mm Kellner with fairly good results. The T-Scanner also requires the specially-made deep-red ERF filter out in front of the telescope's objective. The T-Scanner will not work without the ERF, and can be damaged if the particular ERF from DayStar is not used.
Different telescopes require different-sized or configured ERFs, with DayStar making ERF's and mountings for most of the commerical SCT's, Maksutovs, and refractors. SCT's are usually stopped down to an equivalent f/30 with an off-axis stop containing the ERF, while refractors use on-axis stops. Shorter f/ratio Newtonians often must use rather small off-axis stop to get the effective f/ratio up to f/30. While Newtonians can indeed be configured to operate with DayStar filters at larger off-axis apertures, it often requires a little "playing around" optically to get things to work. This requires a firm understanding of exactly how to do it, as well as what the limitations for viewing will be when employing some of these optical techniques. A few DayStar retailers have in the past not sold DayStar filters to those who want to use them on a Newtonian, simply because many of these users fail to get it right, and end up returning the filters in disgust.
In my case, the f/30 requirement was a potential headache, since I wanted to use the T-Scanner on my ten inch f/5.6 Newtonian. I could stop my scope down to f/30 using a tiny 1.85 inch (47mm) off-axis stop, but I wanted more resolution than that limited aperture would provide. Using a larger 3.5 inch off/axis stop would get me to f/15.9, but to get to over f/30 would need a little optical trickery. I had heard that simply Barlowing the system to f/32 might not work well due to the amplification of the "field angles" of the light entering the filter. However, a friend of mine had successfully installed a Barlow in his T-Scanner for his ten inch f/5, and had gotten somewhat acceptable performance (see later comments), so I decided to try.
I had ordered a T-Scanner with a 3.75 inch ERF unmounted, and when it arrived several months later, I built a holder for the ERF out of a 12.25 inch diameter 1/8th inch thick plywood disk to cover the front of my scope. I cut a 3.5" diameter hole for the off-axis stop, making sure it was clear of the Newtonian's secondary mirror blockage, and mounted the ERF over the hole using two plywood rings of differing size. The ERF holder/stop then goes over the front end of my ten inch Newtonian's tube using 4 right-angle brackets in a friction fit to hold it in place. I had an old Meade 2-element Barlow, so I pulled it out and did a little machining to get the Barlow lens itself to sit inside the front end of the T-scanner.
With everything now in place, I got the scope out and put the ERF mount on the front of the scope. Using an old 27mm Kellner eyepiece, I put the T-scanner into the focuser and started looking. At first, all I saw was a deep-red disk of the sun with white-light details (ie: sunspots, ect.), but I knew that the exact geometry of the tilting filter stack was critical. You first have to adjust the stack tilt knob while rotating the T-Scanner in the focuser until the on-band H-alpha details begin to appear. It takes a little playing with things, but eventually, I began to see areas of the disk begin to show the fine features of H-alpha. However, because of the Barlow lens amplification to achieve the over f/30 light cone, the entire field of view was not in the passband at any one time. This is the so-called "ring" effect, in which the area of the field which is in the passband is shaped like a thick annular ring. By rotating the T-Scanner itself in the focuser and adjusting the tilt, the area within the passband (the "ring"), could be moved over any area of interest on the disk for closer examination. The rest of the field just looked like a very reddish white light view. The inside edge of this annulus of detail is in the "blue-wing" of H-alpha on the shorter wavelength side of the centerline, while the outer edge is in the "red-wing". This made identifying Doppler-shifted detail somewhat easier.
At low ambient temperatures (near the low temperature operating limit of 32F), this ring contracts into a broad disk of H-alpha detail roughly 1/3rd of a solar disk diameter. At somewhat higher ambient temperatures, the disk expands into the thick ring-like annulus of H-alpha detail. At temperatures near the filter's upper operating limit, the ring expands into a broad low-contrast arc of detail. At all temperatures within the T-Scanner's operating range, prominences could be seen on the limb when the annulus of detail was put over the limb, but disk detail tended to lose contrast at temperatures approaching the upper operating limit (90 to 100 degrees F.). This is due to the fact that the filter's passband broadens signficantly when the filter stack is tilted, and more tilt is often needed at high temperatures to keep the filter tuned to the H-alpha center line's wavelength. When I moved the "ring" of detail over various active regions on the sun, the detail was enormous, with filaments, plage, the fine mottling, and the occasional solar flare being fairly easy to see. Prominences on the limb also showed an incredible amount of fine filamentary detail when seeing permitted. The large quiescent filaments which were on the disk were faint but still visible, again with the greatest contrast when the filter was cool. The overall contrast of the disk detail is not as high as in some of the black and white H-alpha images you see in books and on the Internet, but most features were still fairly easy to see. Major solar flares, however, are incredibly bright on the disk when seen in the T-Scanner, with more detail than I have seen many photographs show. The contrast and level of detail varied slightly from place to place on the filter, and was not nearly as high as I had once seen in a professional 0.5 Angstrom Lyot filter on Kitt Peak, but for $1800, I really had very little to complain about! Not long after I got the T-scanner, I managed to catch a truly spectacular surge prominence which looked like a gigantic pencil standing straight up on the limb and extending for nearly a solar radii into space! I also watched several gigantic quiescent prominences "lift-off" in majestic "disparition-brusque" eruptions lasting over an hour.
I got my T-Scanner in 1989, and put up with the "Ring" effect for 10 years, happily scanning the sun and watching all the action. However, it is possible to get away from the "Ring" effect and get full-field H-alpha detail with shorter f/ratio telescopes by employing special sets of lenses in front of and just behind the T-Scanner in a so-called "telecentric" system. This lets the light rays entering the filter go through it at nearly the same angle. I had heard that the Televue Powermate "Image Amplifier", was "nearly" telecentric, so I put in my 2.5x Powermate and 30mm Ultrascopic eyepiece to see if this was true. Sure enough, the entire field of view was now nicely in the passband, and the eyepiece was filled with spectacular H-alpha detail.
The best overall performance for my particular filter was on cool or cold days with the filter a little above its minimum operating temperature, but for other T-Scanners, the exact temperature for best performance varies a bit from unit to unit. I have what is known as a "cool" T-Scanner, which works best in cooler climates, having a center wavelength of 6564.5 Angstroms at 73 degrees F. with zero tilt angle. The passband moves approximately 1 Angstrom per 17 deg. F. of temperature change. To get near zero tilt and the narrowest passband, my filter has to be near 45 degrees F. However, even at temperatures up to 80 F. the filter still continued to show H-alpha detail when the tilt knob was properly adjusted. Much beyond 80 F, only the prominences showed up very well, with the disk detail being of very low contrast. I have put the T-Scanner in the refrigerator (cool it SLOWLY) for 10 to 15 minutes and then observed with it for a while, but in humid climates, this can cause serious fogging, and is not recommended. I also once attached ice-pouch Mug coolers to the sides of the T-Scanner, and it did prolong its usefulness on very hot days. One other possibility for cooling would be a Peltier cooling device, but again, fogging might be a real difficulty. On really cold days (below freezing), the T-Scanner had to be kept warm prior to or during its use, although solar heating tended to keep it on-band after a few minutes of use even in temperatures near 0 deg. F. If you can get DayStar to make you one, it might be best to try for a "warm" filter, as it is considerably easier to heat the T-Scanner than it is to cool it.
The DayStar T-Scanner is guarenteed for 5 years of normal use (ie: don't EVER forget to put on the ERF!), and I know of several that are still going after 15 years. After 10 years of use, I did have a problem with a failed blocking filter on my T-Scanner which meant a trip back to DayStar for repairs, but it only cost me about $384 to have that done (less than some repairs have cost me with my minivan!). For those of you who are considering a DayStar filter, remember that Del Woods is only one man, and it takes a lot of time and care to make the type of quality filter needed to do serious H-alpha. In summary, the DayStar 0.7 Angstrom T-Scanner is an impressive device for those who are interested in observing the real action on the sun.
David lives in southeastern Nebraska and has been observing the sun in H-alpha for nearly 15 years. He currently owns 4 telescopes and has compiled the stunning article "Observing The Sun in H-Alpha" at the website of the Prairie Astronomy Club: http://www.4w.com/pac