When I started observing the Sun on a regular basis, several decades ago, amateur astronomers were involved mainly in drawing and counting sunspots, in calculating the Wolf number and in following the evolution of solar cycles in white light. The “solar telescope” by definition was a small refractor with a diameter of 2 or 3 inches, equipped with a projection screen or a dangerous eyepiece filter. Equipment for observing in H-alpha light had a prohibitive cost and were in the hands of few lucky amateurs. Photographs of the Sun had to be made on films, notably the Technical Pan 2415, and resolution was generally poor unless through semi-professional dedicated telescopes.
But toward the end of the 90’s and the beginning of the present century, several revolutions suddenly occurred. David Lunt’s Coronado narrowband filters and telescopes made observing and photographing the Sun in H-alpha light accessible to a larger public at affordable prices; the CCD revolution opened a new world of possibilities in imaging the Sun and made films rapidly obsolete; and finally Baader Planetarium in Germany developed a solar filter, the noted Astrosolar™ film, that allowed to observe and image the Sun in WL at high resolution with any telescope at a very modest price.
Since then the market of solar observing products grew up immensely and now it offers to the amateur an incredible variety of devices for studying the Sun from the UV to radio waves, most of them for observing and imaging in the H-alpha line. Among these devices those allowing to image the chromosphere in the very near UV are also becoming popular, notably those for the H and K Fraunhofer lines of the ionized Calcium at 396.85 and 393.37 nm respectively. While the H-alpha domain lies in chromospheric layers from 1300 to 1650 km above the photosphere, in the Calcium domain light comes from a layer that can be up to 500 km higher as one can see from the following scheme
In this spectral region the Sun appears crossed by several features that – as the ones that are observed in WL and in H-alpha, corresponds to the interactions between the solar plasma and the magnetic fields that drive the solar activity. An excellent review of the characteristics of the Sun in the Calcium domain can be downloaded from this link and I very much recommend the interested reader to study it in detail.
Several devices exist for observing the Sun at these wavelengths, even an old model of Coronado PST that unfortunately is no longer produced. Below I will examine two filters for the spectral region between 390 and 400 nm, the Baader K-line filter and the Daystar Quark.
BAADER “K-LINE” FILTER: AN ALTERNATIVE TO NARROBAND DEVICES ?
The German brand Baader Planetarium offers a CCD filter 31.8 mm size, the so-called K-Line filter, with a bandpass of 8 nm that covers the range 390 to 398 nm. It is actually formed by stacking and tilting two filters, the tilting being necessary to avoid overlapping between the ghost images that form by internal reflections due to the stacking. The filter shall be threaded on the nose of the CCD but it lacks of a front thread, so any additional filter shall be inserted between the K-line and the CCD.
The bandpass of the K-line is relatively large so the filter doesn’t show chromospheric details with the same evidence of more selective and much more expensive filters (its current cost in 2017 is 295 euros only while a typical Calcium “module” easily goes beyond 1000 euros). Actually it works in the photospheric domain also, but as we will see this doesn’t make the filter useless.
I used the filter in many instances with reflectors and refractors, and as far as the chromospheric details are concerned it showed some limitations due to the large bandpass as one can see from the following images (color has been added in post-processing):
Chromospheric faculae are visibile, but the network and the smallest details that belong to the Calcium domain are much more difficult to discern and require a robust processing that however can’t replace a true narrow bandpass filter.
Although advertised for the chromospheric domain, the K filter is actually most useful for imaging the photosphere in the deep violet, and in fact is one of the best filters I know for this purpose. Details of granulation and spot penumbrae through the K-filter are much superior compared to the ones delivered by the traditional Baader Continuum filter. On the other hand the K filter requires a very steady air because seeing worsens significantly toward the violet end of the spectrum.
The filter requires a considerable reduction of the sunlight before it, so it must be used with a prefilter. This can be a front (objective) photographic filter as the 3.8 ND Astrosolar™ (the visual grade 5.0 would be too dark) and in fact an A4 sheet of this film is included in the package of the K-line. Even with the photographic grade of the Astrosolar™ I would recommend an aperture of at least 6 inches otherwise the resulting image would be quite dark and would require to boost the gain which may eventually result is a noisy image.
Whatever telescope is used it shall be corrected in the violet part of the spectrum otherwise images will be blurred by aberrations: achromatic refractors and some catadioptrics are no longer diffraction-limited at this extreme end of the visible spectrum, therefore semi-apochromats, apochromats and reflectors shall be preferred.
In case a refractor is used, instead of the front film a much better choice would be a Herschel wedge that passes more light, particularly useful for high resolution imaging (the K-filter shall of course be placed after the wedge). The usual ND 3.0 filter provided with solar wedges for visual observing would be too dark for imaging and should be replaced with something lighter. Even better than the wedge would be a solar newtonian, provided that only the primary mirror has been de-aluminized.
Although not related to solar observing, the K-Line filter proved to be very effective in imaging the UV markings of the Venus atmosphere with my Takahashi Mewlon 210
but it is a very dark filter for planetary imaging and would require an aperture of at least 8 inches.
MORE NARROBAND DEVICES: DAYSTAR CALCIUM H-LINE QUARK
Some time ago I was looking for a Calcium filter and I was undecided between a Lunt Ca-K module and the Calcium H version of the Daystar Quark. Both Lunt Solar Systems and Daystar are currently offering rear filters that can be fitted to almost any refractor (and not only refractors, as we will see). These devices are quite expensive and in some cases may cost much more than the telescope itself, but are worth the expense and repay the user with a windows on many interesting solar phenomena not accessible in white light.
Lunt modules are very popular and proved to be excellent performers able to deliver high contrast and detailed images of the chromosphere, but can be used only on refractors. I wanted more light, an easier view of prominences and the possibility to fit the device into my solar newtonian, so after some tests on sample units, eventually my choice fell on the Daystar Quark.
This filter is very similar to the Quark Combo unit currently sold for viewing in H-alpha and works in the same way apart the wavelength (the interested reader may refer to my test of the Combo published on this web site). It is relatively short (95 mm) and lightweight (200 grams). The etalon has 19 mm clear aperture and the bottom BF is 25 mm large. The device can show the full disc of the Sun with focal lengths up to about 1800 mm.
An external power source is required to heat the etalon cavity and put the filter on band; in the box together with the filter there is a 90-240 V AC adapter, however for this device and my Quark Chromosphere I use a smartphone charger pack able to deliver up to 2.4 A which allows for several hours of observing before a recharge is needed. The filter comes with a standard 1.25″ nosepiece threaded for filters, however a 1.25″ extension and a 2″ adapter (threaded for 1.25″) are also provided.
No additional filters are in the box but on its Quark page Daystar warns the user that a UV/IR cut filter shall be put in front of the Quark (i.e. before the Quark along the light path from the objective) for “safety”:
It is unclear to me what Daystar means with “safety”, whether it is referred only to the protection of the device from excessive heat/degradation (as it can be inferred from reading to the manual) or also to the protection of the observer’s eye, as this model of Quark is advertised for imaging and for visual observing. Eye sensitivity to the near UV is however poor: K/H lines are very close to the boundary between the true UV and the deep violet, just outside the range of human vision which is traditionally placed at 400nm. Young people and the elders that undergone cataract removal can have enough sensitivity for astronomical observing beyond 400nm, enough to perceive, for example, the clouds of Venus (this topic is discussed in Visual Astronomy in the Ultra-Violet, by H. Dall, JBAA 75, 1965) nevertheless in this region most people, including myself, would see only a ghost of the wealth of details that a CCD can easily reveal. Devices and filters for Ca-H or Ca-K lines shall therefore be intended mainly for imaging with monochrome CCD cameras.
That said, in the Quark manual I couldn’t find a clear statement that the device is absolutely safe for visual observing while I noted that Baader Planetarium doesn’t recommend the K-line filter (which operates in the same spectral region) for visual use as it may result in eye damage:
As I am using both filters only for imaging any safety concern doesn’t matter for me, but those interested in visual observing should investigate further. Of course the rejection filter shall be such to allow the light of 396.85 nm wavelength to pass through; I make use of the Astronomik UV/IR cut type 2C and it works fine with no loss of light or detail.
In the standard configuration with the 1.25″ or 2″ nosepiece, the filter requires 61 to 67 mm backfocus that most refractors will be able to accommodate. Daystar recommends a refractor not faster than f/7, in case a shorter instrument is used a Barlow in front of the Quark is required. It may be unnecessary to use of the whole length of the Barlow, if the lens barrel is removable from the Barlow’s tube and has a standard male filter thread, it can be attached directly to the nose of the Quark making the assembly more manageable:
The Calcium version of the Quark seems less sensitive to the aperture of the light cone compared with the H-alpha versions for which a light cone as slow as f/15 is recommended as a minimum (actually if one is interested in H-alpha viewing of prominences this requirement can be somewhat relaxed). Nor it seems to strictly require a telecentric amplifier, and in fact I tried different focal extenders without noting any appreciable difference between them once put in front of the Quark.
The tuning of the Quark is made by rotating a knob that changes the temperature in the etalon assembly. A LED light turns from orange to green when tuning is stable. Tuning can be varied from -0.5 Å to + 0.5Å with respect to the center line, but actual changes in the Sun’s image are much more subtle than with H-alpha Quarks. I found that leaving the knob in the “0” position is satisfactory in most cases in the average climate of Milan, but I still have to test the device during winter.
As its H-alpha counterparts, Quark Ca-H filter is advertised for refractors; while different optical configurations can also be used with all Quarks if the objective is provided with a front-mounted ERF (Energy Rejection Filter), one shall consider that front ERFs for Calcium lines are more difficult to find than those for H-alpha, as far as I know they can be available on special order from very few manufacturers.
As for the K-line filter above, Quark Ca-H requires a refractor well corrected in the violet part of the spectrum as a good apochromat. However I noted that even a good achromat may work fine provided it is of a long focus design or it is stopped down in order to reduce aberrations in the violet: for example my 150 mm f/5 achromat can be used successfully if stopped down to 75 mm (f/10), at least for full disc imaging.
When I purchased the filter I wanted to use it with my solar newtonian too, but I did not know if there was enough light for imaging as I removed the reflective coating from the main mirror. To my delight (and surprise) the filter proved to work very well with my modified reflector.
UNDER THE SUN
The following images give an idea of what can be obtained by the Quark Ca-H and an ordinary telescope. As reported above I had to stop down my large achromat in order to make it acceptable for imaging in the violet while the newtonian (which has a de-aluminized primary mirror) has been used at full aperture. Particularly striking is the difference between the full disc image below and the one above taken through the less selective K filter. All images have been obtained by using of a Chameleon mono CCD camera.
Together with the usual surface chromospheric details, I have been able to image prominences as well thanks to the relatively higher light output of the Quark, although they are better seen and imaged in H-alpha:
The Baader K-line filter is one of my favorite tools for imaging the Sun and I much recommend it to anyone interested in increasing contrast in granulation and in sunspot details. In this respect it replaced my Baader Continuum filter that now I use only when the seeing is too bad for imaging in the deep violet.
As far as the Daystar filter is concerned some remarks about the user manual are necessary. The booklet is clear and informative but it contains some oddities already reported in the instructions for the other Quarks. For example on page 6 it is stated that “The optical filter life expectancy is extended up to 2-3 times by climate controlled storage”, but 2-3 times of what ? nothing is said about the estimated life of the device in normal use, nor it is specified what shall be intended for “climate controlled environment”. The above mentioned statement would also imply that some parts of the device are subject to natural degradation or ageing, but which parts can be affected and, in case, replaced is not reported.
The paragraph on the suggested eyepieces for observing in Ca-H is somehow misleading because the device works best for imaging the Sun, it is not a visual filter in the same sense of its H-alpha counterparts. Putting my head under a black cloth and by using of a 112 mm aperture, I have been able to clearly see the structure of AR12673 & 12674 sunspot groups and the bright plages around them, but the image contrast was very low compared to what can be seen on my CCD images (see the full disc above), the network was invisibile and the Sun limb not well defined. Personally I would not recommend to purchase any eyepiece to be used specifically with Quark Calcium-H filter.
Notwithstanding the above, I am very happy of the purchase (made through the Italian dealer Skypoint) and I recommend it very much for amateur chromospheric study of the Sun in the near UV.