Do I have to/can I adjust the SunDancer?

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.

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