I was lucky enough to be able to "play" with one of the first SunDancer II H-alpha filters and see what it is capable of. Even though I am not one of the most experienced H-alpha observers, I have been able to observe the sun with the two H-alpha telescopes of the Observatory in Heilbronn/Neckar again and again for more than 20 years. These are a 20/20 H-alpha filter by Wolfgang Lille with 0.8Å on the 150/2250 refractor (which complements a classical prominence filter), and a Lunt LS-60 telescope. I freely admit that I find the concept of the Lunt convincing, especially for public observatories: a complete telescope with which nothing can go wrong. It's foolproof, which is especially important in an club where many people work with the instruments.
I had also mentioned this to Mr Baader when, during a visit to Mammendorf, I happened to have the opportunity to look through a SolarSpectrum set-up that was being tested before it went on sale: Very nice image, but with an energy protection filter in front of the lens and a power connection for heating rather uninteresting for me.
Perhaps I owe the chance to take a look at the new SunDancer II to this remark I made. A Danaer gift, if I may say so - H-alpha is already addictive...
Using the SunDancer II
The SunDancer II has thoroughly cleared up my reservations about heated filters. It does need electricity, but that is no problem in practice: Since I work with tracking when observing the sun, electricity is available anyway, and my PowerTank has enough amps to run the heating of the H-alpha filter in addition to the telescope. So the effort for the heated filter is limited to interconnecting a Y-cable and connecting the power cables. The control box does the rest: It automatically heats the filter up to the operating temperature; the H-alpha line does not have to be set via a rotary wheel as with the other devices. This is especially interesting during longer observing sessions, when temperature fluctuations can shift the filter's transmission window. Especially during a public show, you don't look through the telescope yourself after each guest to check if the sun is still centred and optimally displayed; there it is worth its weight in gold if you don't have to worry about anything else like the settings of the filter.
The SunDancer II consists of two parts connected with the usual T-2 thread, which can always remain together in one piece: One is the 3x telecentric with integrated block filter, the other is the Etalon (i.e. the actual H-alpha filter) with its "oven". In addition, there is the small electronics box that regulates the temperature, and the power supply unit or an optional battery pack.
Briefly some words about the function of H-alpha filters: The Etalon is an interference filter that transmits the H-alpha line at 656nm, as well as multiples of this wavelength. The exact wavelength that is transmitted depends on the filter thickness, and the heating regulates the filter thickness - as we all know, heat expands. With the old filter from Wolfgang Lille, the transmission line is regulated by tilting the filter: When the filter is tilted, the light travels a longer way through the filter and other wavelengths are cancelled out by interference. This also explains the high demands on surface accuracy and the high price.
For the interference filter to work, the light beam must necessarily consist of parallel light rays and ideally produce a focal ratio of about f/30 - this is ensured by the telecentric system, which – unlike a barlow – not only changes the focal ratio by shifting the focal point, but also ensures a parallel beam path. (That's why I can't use the filter without a telecentric on my 125/3500 f/28 Schiefspiegler - the focal ratio would fit, but the light is still focused in one focal point instead of being parallel. Besides, I would still need a block filter and would have to think about where to place an energy protection filter, too).
The hard-coated block filter of the SunDancer II sits in front of the Etalon and even before the telecentric. It does not only block the unwanted wavelengths, but also almost everything else that reaches the H-alpha filter in terms of solar radiation. This makes it look like a mirror and protects the sensitive Etalon from the sun's radiation which would otherwise simply vaporise the built-in polarisers or the oil filling. Up to 80mm aperture this block filter is sufficient as protection for the SunDancer II, for larger instruments an additional energy protection filter in front of the telescope is absolutely necessary. The only disadvantage of working without a block filter on a small telescope is that you must not look into the telescope from the front, as the bundled solar radiation reflected by the block filter is reflected back through the lens. You have to pay attention to this if the telescope and the sun are quite low and you are doing public relations work, for example, where someone can always look curiously into the telescope. On a high mount or when looking from a balcony, this does not matter. Of course, an energy protection filter can also be used with smaller telescopes.
With my ED80/600 I therefore have it as easy as can be: I just have to connect the SunDancer II to the power supply, put it in the mirror star diagonal and insert an eyepiece. After maybe two minutes, the display shows no more temperature change and the filter is fully operational.
To fine-tune the H-alpha line, you can tilt the filter slightly via a micrometer screw to compensate for the Doppler effect: When a cloud of matter on the sun moves in our direction, the H-alpha line shifts into the blue wing of the spectrum. With the micrometer screw, you can quickly check whether there are any fast events that require a shift in the H-alpha line. In this way, the transmission line can be shifted quickly and easily if the preset value does not fit.
The H-alpha line can also be shifted via the temperature control of the control box. This avoids filter tilt, which may be interesting for the highest demands, and allows you to fine-tune the filter to your own system if the factory setting of the temperature is not optimal.
The Etalon works optimally from f/30, the built-in telecentric has a factor of three. It is therefore perfect on telescopes with f/10; with faster telescopes, a temperature adjustment can be useful for fine-tuning. My ED80/600 with f/7.5 is actually too fast (the filter works like this at f/22.5), but the image was still excellent - this is clearly an advantage of the beam of light being parallelised by the telecentric system. Baader has a 4x telecentric in preparation, but currently no one can say when it will finally be available. Thanks to Corona, it is difficult enough to obtain already developed products ...
Finally, important for handling: The focus position with the 2" sleeve hardly differs from that of a usual eyepiece when observing the sun in white light with astrosolar film. It should therefore come into focus (either with a 1.25" or 2" sleeve, both are available) on practically any telescope, even on Newtons.
The SunDancer II is a complete H-alpha filter with a width of about 0.6 Å, hardly bigger than an eyepiece and no more complicated to use: No energy protection filter is needed on telescopes up to 80mm, so it fits my ED80/600 without any effort. As a star diagonal I use an older dielectric Baader MaxBright 2" mirror diagonal.
However, I have a small problem: due to the telecentrics I work at 1800mm focal length, and my standard eyepiece park makes a jump from the 36mm 2" Hyperion Aspheric to the 9mm Morpheus – so in this case from 50x to 200x. Of course, the latter is too much magnification for an 80mm lens. But I still have a set of Baader Classic Orthos in the cupboard; of these I can use the 32mm Plössl (56x) and the 18 Ortho (100x) with the SunDancer II.
Very nice: I can see the whole sun with 600mm telescope focal length! And what a view that is: With 0.6Å it shows prominences and disk at the same time with nicely balanced contrast. My first impression: The SunDancer II blows the two H-alpha filters of our local observatory away. But without problems. Very, very nice.
The H-alpha line was hit right away, I haven't readjusted anything yet. Very important when observing the sun: Stay in the shadow! Lateral stray light ruins the contrast, even a good eyecup brings more contrast than a better eyepiece. I achieve the best contrast increase when I am under the Astrogarten observation cloth that I bought some time ago. This is probably the most important accessory for H-alpha observation if you can't sit in the shade. (In principle, a coat would also work - but it gets very stuffy very quickly underneath a coat).
After the first wow!-effect, of course, a few things have to be tried out to see what all works. Can I use my 36mm Hyperion 2" eyepiece with the SunDancer? Yes - either with a 2"/T-2 eyepiece clamp (which I don't have), or the Baader Expanding Ring T-2f / M48m (T-2 part #29) (#2458110, € 20,-) that I had left from some other project. With this I can screw the eyepiece directly onto the H-alpha filter: cool! The field of view even seems to be a bit larger than in the 32 Classic Plössl, and the view is definitely more relaxed than in the 1.25" eyepiece. The combination has now become my standard combination on the SunDancer II - with a T-2 quick release I can switch between the 36 Hyperion and the 1.25" eyepiece clamp which was included with the SunDancer II and save myself the trouble of screwing or using a 2" eyepiece clamp.
A regular solar filter has also become a constant companion: If the sun is not perfectly centred, it is hard to find it even with a simple sun finder. So I first take a quick look at the sun in white light before replacing the Herschel wedge with the SunDancer.
And then I make a big mistake: I connect a bino - the large field bino from my local observatory club, with a set of Eudiascopic eyepieces. It really knocks my socks off. I have never seen the sun so impressively, not even in the small Lunt, on which I had already used the bino a few times. Focusing is no problem either: the telecentrics mean that the beam of light is already perfectly parallel, and all focusing problems suddenly disappear into thin air. Even the glass path corrector can be dispensed with, and I still only have to get about three centimetres closer to the telescope than without the bino-viewer. This is much more fun than on the Lunt with a 1.6x glass path corrector.
And why was it a mistake to connect the bino? Because that was the moment when it was clear that the SunDancer II had to stay with me. Fortunately, almost all the holidays last year had to be cancelled and there was still some money in my wallet. But that also means that I have to get a bino as soon as the MaxBright II is available again. And suitable eyepieces. This loan really became a Danaer gift - but as I always say: as long as it only hurts once - when paying - and not afterwards, it's worth it. And thanks to being a freelance worker in the home office, a quick look at the sun is no problem for me, so it will be used even more often. It's not a cheap experience, but a great one (and the filter is supposed to be age-resistant, so over the years it pays off).
Some time later, I was able to make a few comparisons at our observatory. I actually use it on a telescope that is much too fast, so I stopped down my ED80/600 to 60mm. The effect was negligible: At first glance there is not much difference, probably the loss of resolution compensates for the gain in contrast - the jump from 80 to 60mm is already noticeable when observing deep-sky, after all the rule of thumb says that you should improve by at least half the aperture to notice the difference between two telescopes. In any case, stopping down has not given me a noticeable advantage.
Next telescope: an old Vixen 90/1000 with a Baader Clicklock star diagonal. Here, at f/33, the filter works at about the optimal aperture ratio, and the contrast is a good deal better. But with 90mm and no energy protection filter, I only do this for a short time, even though the energy load should hardly be greater than on the faster ED80, based on gut feeling. With only one centimetre more aperture, the better image is clearly due to the more optimal focal ratio.
What I notice negatively at 1000mm focal length is that the internal filter has a 19mm aperture, so at effectively 3m focal length the sun no longer fits completely into the image. 600mm is pretty much the sweet spot where the sun still fits all the way through the filter. This fits wonderfully with the many small, fine APOs that are currently on the market, and of course larger filters cost more money. But that's only a small downer: the increase in image quality more than makes up for the limited field of view. When I want to magnify higher to observe details, the whole sun no longer fits into the eyepiece anyway.
Due to the weather, I could only do one more test at the observatory: How was the image in the small Lunt? Admittedly, the complete solar disk did not appear that much worse, and the image was brighter. But you can't enlarge the image much: the 60mm of the Lunt do not allow for much magnification of details. The image is still nice, but - yes, only nice. Especially in a bino-viewer, it lacks the wow!-effect that knocked my socks off with the SunDancer II (apart from the fact that the Lunt needs a glass path corrector for this).
I have not yet had the chance to compare it with the Lille filter. But: Here, a similarly large filter with 0.8Å is used on a 150/2250 refractor stopped down to 100mm, at 4.5m focal length. So only a smaller section of the solar disk is visible, in a broader filter and due to focal length and eyepiece (1.25") at 140x minimum magnification - I can actually dispense with the test. It can't be that much fun.
Then, of course, there's the desire for the perfect photo... At 1800mm focal length, the sun would still fit on a full-frame camera; but the largest camera in my collection is only a DSLR with APS-C: the edge of the sun is missing. Just for fun, I try connecting a camera lens to the eyepiece: in principle, I get the entire sun on the sensor even with a Micro Fourthirds camera, but that's through an even larger stack of lenses and still only with a colour camera - thus it is a combination that is combines all the worst things for solar photography.
What works wonderfully, however, once the question of adaptation has been resolved: the SolarSpectrum Research Grade 0.4x Telecompressor 2" (#2459260, € 325,-) . It only has a field of view of 16mm because of the strong compression, but since the filter itself only has 19mm aperture, that's no problem, and I even get the entire solar disk on the sensor of my MFT camera. Cool. So the compressor has to stay with me too... My first snapshot with a little post-processing in Lightroom and Photoshop is far from what the filter can do, but I'm doing astrophotography for beginners, after all. And when I look at the YouTube tutorials that show almost an hour of image processing, I'll probably stick with it for now.
With the 0.4x telecompressor, the image is again small enough for an MFT camera.
As a black and white camera, I only have the small ZWO ASI Mini anyway, which I normally use as a guider. In principle it works wonderfully, but it shows the interference stripes that can be seen in many H-alpha setups and are probably due to the camera. At least that is the common explanation, especially because not every camera on the same set shows the effect. Possibly it can be eliminated by tilting the camera or filter - or with flat-frames, but try flat-frames for H-alpha.…
Unfortunately, in the case of my ASI Mini tilting is useless: The pattern is independent from tilting, neither form nor position change, when I tilt or rotate the filter in front of the camera or the camera itself – so it must be caused by the camera. But: If I hunt for prominences with a not-perfect tracking and stack the images, the patterns disappear (just like hot-pixels). I have to say that I‘m quite satisfied with my first image of a prominence – made of a 10 second capture, stacked and processed in Registax, taken with my ED80/600 and the integrated telecentric system at 1800mm effective focal length. Not bad for a first try:
The conclusion is simple: The filter has to stay, even though I didn't want it initially. It is simply too cool, to put it bluntly. For the money of a Lunt H-alpha telescope I only get the filter, but on a small 80 refractor it beats the familiar 60 Lunt from our club by far, and I already have a telescope anyway. Now its usage time has also multiplied - I wouldn't have thought that a filter with electronics would be so easy to use and so much fun. Especially binocular...
About the author
Alexander Kerste is a studied biologist and works as a freelancer as an author, consultant and translator. After his studies and the publication of the Kosmos Starchart-Set in 2004, he was a regular freelancer for Astronomie Heute and the yearbook Der Himmel for the Spektrum-Verlag in Heidelberg. He is in charge of the Beginner courses on www.Astronomie.de and is a voluntary active member in the Robert-Mayer-Observatory since 1993. Since then, he has published a number of books on Celestron-Telescopes as well as Digiscoping and Astrophotography. One of his books on Astronomy with binoculars is also freely available at freebook.fernglasastronomie.de. In addition he supervises the Northern lights and star tours from Hurtigrute – these were also published in a travel guide, further articles can also be found on his blog kerste.de.View all posts from