Baader SunDancer II H-alpha Filter

Unfortunately, we cannot predict the availability in general, as we usually already have pre-orders from private buyers and dealers, which will be delivered first as soon as we have new Sundancers available. Also, the etalons are grown crystals - i.e. natural products who can unexpectedly develop a high failure rate. For this reason we can never forecast how good the yield of excellent etalons will be that can be finished to produce a filter without compromizes.

The focus position of the SunDancer is pretty much the same as that of a 2" eyepiece if you use the 2" nose-piece cut into the TZ-unit. So you will usually not need extension tubes, but can simply plug the eyepiece into a 1 ¼“ or 2“ star diagonal. It is important that the telescope itself does not contain any plastic parts, but must be suitable for solar projection.

If you use a bino-viewer, the focuser must be moved inward a little further, but usually only about 1-2cm.

For photographic use, you only need extension tubes if you are working without a star diagonal. This straight configuration has the advantage that the image is not additionally mirrored and that the SunDancer is always protected from direct sunlight in the shade instead of being heated up on one side by the sun. Possible quality losses due to a star diagonal are thus also eliminated.

Please note that the quality of the star diagonal also plays an important role for the contrast in H-alpha. In our tests we have found that our hard-silver-coated BBHS mirrors provide the best H-alpha contrast. Compared to prisms, mirrors have the advantage that the light does not pass through a glass body which it can heat up.

All SunDancer II filters are subjected to final testing before they are released for sale. As delivered, the micrometer screw of the tilting device can therefore be used to get finetune a high-contrast image. For best results, the tilting filter (aka: the etalon) should be absolutely perpendicular to the optical path. In this optimal position, the micrometer screw is then at or near its inner stop, or at a value mentioned in the documentation. By unscrewing the micrometer screw, this etalon can then be tilted very quickly into the "blue wing" of the H-alpha line, for example, in order to quickly get a better impression of prominences without having to wait for the heater to readjust, for example.

However, it is important to know that first of all the exact orientation of the tiltable etalon in a 90°-angle to the optical axis is necessary for best contrast performance! This is the only way to really judge the quality of the filter. So you start each observation session by waiting until the target temperature "0.00" is reached and doesn’t change any more. Only then you can check whether the surface contrast on the sun appears satisfactory and move the micrometer screw forwards (if possible) and backwards on a trial basis - it rarely needs to be moved more than one up to three strokes increments of the index. This is done by carefully observing whether a change in contrast can be detected in the surface details when turning the micrometer screw.

Only when the best contrast has been achieved - even if the position of the micrometer screw now deviates significantly from the zero position or the value specified at the factory - should you try other working temperatures if necessary. Since every lens behaves slightly differently (this is true even with identical aperture ratios due to different lens designs), the best position of the micrometer screw for ideal contrast may be slightly different on your unit than on our test unit.

However, it would be completely misleading for testing the filter quality (= judging the half-band-width =FWHM) to immediately vary the temperature setting and then subsequently adjust the tilt angle to get a good image again. This wrong working sequence can cause a filter with 0.6 Ang half-band-width to show only prominences at the edge of the sun, but almost no contrast on the sun’s surface. It is essential that the tilt angle of the etalon is always adjusted first so that it is in a 90°-angle to the optical axis as described above (best contrast) and only then the temperature is changed in very small steps, for example to tune the efficiency of the strip-heater surrounding the etalon to summer or winter temperatures.

The adjustability of the etalon’s tilting angle for rapid movement into the wings of the H-alpha line is a luxury that allows much faster adjustments than only by temperature changes. However, many suppliers of competing filter concepts deliberately do without this because it also represents a large potential for error. It is important to realise that the light in the tiltable etalon is reflected back and forth approx. 1000 times in order to finally "filter out" the H-alpha wavelength. Every deviation from the perfect etalon perpendicularity and every reduction in the parallelism of the beam path determined by the telescope is consequently amplified a thousandfold. Using this complex filter technique carefully and knowingly is therefore absolutely necessary - and, as already mentioned, it is important that a sufficiently parallel beam of light (i.e.- a long f-no) is available at the eyepiece end of the telescope.

H-alpha filters with integrated 3x telecentrics are initially designed for use on telescopes with f/10 focal ratio to convert into an f/30 beam. With even "slower" telescopes, these heated filters can be used without any loss of quality. On telescope optics with ever faster focal ratios the increasingly conical beam path formed by the objective lens, there would result in be a considerable widening of the filter’s half-band-width. As a result, an etalon that was manufactured with a half-band-width of 0.6 Å, for example, would very quickly only have an apparently larger half-band-width of 0.8 or 0.9 Å and thus provide considerably less contrast.

As a rule, you will be able to use the SunDancer without much further adjustment. Nevertheless, once you have familiarised yourself with the system and its intricacies, you should occasionally check whether you can achieve even better results with a minimally different operating temperature on your particular telescope. You will find more about this in the operating manual. Please also note that very slow telescopes can in principle provide a better H-alpha contrast, since the light beam leaving the telescope is much more parallel – but at the same time, however, you will quickly reach very (or too) high magnifications with 1.25" eyepieces due to the built-in 3x telecentrics. For this reason, on the one hand long focal length eyepieces are advantageous, and on the other hand, even a telescope aperture ratio of f/ 6 or f/7 can produce astonishingly good contrast performance on the solar surface. It is worth experimenting calmly and carefully - in the order mentioned above.

The 3x telecentric system allows you to achieve very high magnifications or very small exit pupils very easily. A good guideline for the maximum magnification for H-alpha is an exit pupil of about 0.7-1mm, or a magnification that is not very much higher than the telescope aperture in mm. Due to the limitation to only one wavelength, the rules of thumb for observations in white light no longer apply! We were able to use a SunDancer on an 80mm refractor up to about 100x, and a few minutes later on a triband C8 at more than 200x – so before air turbulence became the limiting factor, magnification was limited only by the available aperture and not by the filter itself.

With telescopes with long focal lengths (especially when stopped down, resulting in slower focal ratios), you can quickly achieve exit pupils of less than 1mm due to the 3x telecentrics. Under certain circumstances, a 20-30mm eyepiece may already provide the highest usable magnification.

Fortunately, you don't need special "H-alpha eyepieces" - our Hyperion eyepieces reproduce the H-alpha line (which is also important for gas nebulae/deep-sky-observations) well and are just as suitable for deep sky as for solar observation. The scope of delivery of the 31mm and 36mm Hyperion Aspheric 2" eyepieces also includes 1.25" nosepieces (originally intended for use with a bino-viewer). The Hyperion eyepieces are not only recommended by us for H-alpha observing (and of course also for deep sky - unlike special "H-alpha eyepieces").

For more on eyepiece and magnification selection, see e.g. these threads:

https://www.cloudynights.com/topic/629655-h-alpha-eyepiece/?p=8772056
https://www.cloudynights.com/topic/808040-best-eyepiece-type-for-halpha-solar-observing/?p=11651756
https://www.cloudynights.com/topic/765356-getting-started-in-h-alpha-solar-limited-options-questions/?p=11017690
https://www.cloudynights.com/topic/265312-eyepieces-for-lunt-ls60tha-b1200/?p=3371594

and in German:
https://forum.astronomie.de/threads/hilfe-zum-baader-sundancer-ii-gesucht.324786/

There are some H-alpha systems that also deliver impressive results at a much lower price. However, to make this possible, compromises are necessary in terms of image detail, resolution (maximum magnification), field size or durability, which we did not want to make with the SunDancer. As we all know, the last 10% of quality requires the highest effort... With the SunDancer, we want to maintain the same high quality standard for which the SolarSpectrum filters are known, so that our customers and dealers are do not have to cope with ageing ("rusting") filters or exchange orgies.

We are happy to explain the advantages of the SunDancer over competing models.

1. The Sundancer front blocker-filter has a hard, non-ageing coating. The Quark H-alpha filter, for example, uses an entrance blocker filter with only 8.5 mm free aperture. As of this date, it consists of so-called "soft coatings". Such a soft filter can age so much that the system needs a new blocking filter after only a few years. For some other filters, too, reports of ageing or "rusting" filters can be found in the relevant forums - partly depending on the year of production. The blocker filter (i.e. heat protection filter) used in the SunDancer II has 12 mm free aperture (40 % more aperture) compared to the front-blocker filter used in the Quark and has 5 times the production costs - but above all it is hard-coated and can withstand the heat at this point in the light path of telescopes (having up to 80mm aperture) without ageing or fading, even without an additional D-ERF-filter. Even though the SunDancer itself is a new product, our many years of experience with the (considerably more expensive) filters from SolarSpectrum went into its design. The longevity of H-alpha systems is always a topic in the relevant forums, discussions can be found here, for example:
   • https://solarchatforum.com/viewtopic.php?f=10&t=25951
2.  The 3x telecentric system of the SunDancer II likewise has larger lens diameters and an optically more sophisticated construction. If necessary, it can be unscrewed from the actual etalon and replaced by a 4x telecentric system. (As of 2022, this TZ-4 is still in preparation. The blocker filter built into the 3x telecentric must be placed into the 4x telecentric in the process). With the larger telecentric system and the larger blocker filter, larger telescope apertures can be used (of course only with additional D-ERF front filters or with SC-Triband-optics) to achieve much higher resolution. The Quark in comparizon is not nearly as flexible as it does not allow to exchange the TZ-3 into a TZ-4 configuration because that telecentric system is fixed onto the etalon.
3. Most H-alpha filters are only designed for telescope apertures up to 90 mm and therefore at best achieve ~ two arc seconds of resolution. The SunDancer can deliver 0.7 arc seconds of resolution on a 200 mm Triband SC-telescope – even more with a future TZ-4. See our customer reviews for triband optics
4. The etalon itself is also of larger size and of a higher quality, provides more contrast and is finer and quicker to adjust due to the additional tilting device - you can shift the transmission window a little (for observing blue-shifted structures moving towards us) without having to wait each time for the new temperature to adjust.

Please note that these filters are natural products, as each etalon is made from a grown crystal. We test all filters to ensure that they are within the specified range of 0.6 +/-0.1Å. For best results, you may want to adjust the operating temperature (more on this in the instructions) so that the filter is optimally adapted to your particular system.