Equipment for observing the Sun (Part II: narrowband)
2020 March 30
WARNING: Never look at the Sun with the naked eye or any optical instrument unless you are familiar with safe solar observing methods.
In the last issue, I covered the equipment used for broadband or ‘white light’ observations of the Sun. I will now turn my attention to narrowband observing, that is, to equipment that isolates very narrow wavebands of light to observe features in various layers of the Sun’s structure that are not apparent in broader wavebands.
By far the most popular of these bands is hydrogen-alpha, in the far red of the spectrum. This is followed, a long way behind, by calcium-K in the violet. Filters are also available to isolate the sodium-D, calcium-H, hydrogen-beta, helium-D3 and magnesium-I lines, but these are only in the possession of a few amateur solar aficionados.
The hydrogen-alpha and calcium-K bands are those that show the most excitingly distinct features compared to broadband observing. Hydrogen-alpha light shows the middle to upper chromospheric features, including filaments on the disc and the highly dynamic prominences visible at the limb. It also gives a different perspective on sunspots and reveals the magnetic active areas that do not show spots. Calcium-K light reveals a lower, cooler layer of the chromosphere, showing the bright chromospheric network, plage, and the magnetic regions strongly.
Etalon filters: exploiting interference
Amateurs have been engaged in narrowband solar studies for a long time, but up until about 30 years ago their equipment was usually home-built, cumbersome, and a labour of love: former BAA President Henry Hatfield famously built his house around his ‘spectroheliographoscope’. Amateurs today do not need to go to this trouble, for there are ready-made solutions available.
All solar narrowband filtration works using an etalon, which is essentially comprised of two flat plates of glass held parallel and very close together. Light waves reflected off two surfaces interfere with each other, giving constructive interference (amplification) for certain wavelengths, and destructive interference (cancellation) for others. A range of wavebands are passed by a simple etalon, and other filters are added to isolate the desired band.
The distance of the gap (or cavity) that the light traverses in the etalon is a function of the angle of incidence of the light, and for this reason, the etalon will only give reasonably consistent filtration across an image if the light is almost parallel. This leads to various design compromises in narrowband equipment.
Note that although they have similar names to filters used for deep sky imaging (e. g. hydrogen-alpha), those used to observe the Sun are very different and the two types are certainly not interchangeable. The bandpass (range of wavelengths transmitted) of a typical solar hydrogen-alpha filter is well under 0.1nm or 1 ångström, whereas the bandpass of a night-time hydrogen-alpha filter is quite a few nanometres. The solar version is therefore far more specific and transmits much less light.
Broadly, there are three approaches used for solar narrowband equipment. The first is to place the etalon over the objective lens of the telescope. This means that the light entering the etalon will be parallel, and the image should be uniformly filtered. Another filter assembly, known as a blocking filter, must be placed at the eye end of the telescope; this is usually but not necessarily built into a diagonal unit. Blocking filters are made with various apertures, increasing in price for the bigger ones. Using one that is too small for your system leads to a ‘looking through a keyhole’ effect.
The front etalon approach allows any refractor to be converted into a solar telescope, provided an adaptor is made to fit the screw-threaded etalon over the objective. These systems are made by Lunt and Solarscope. The results are very good, but the expense of making precisely parallel etalons rapidly increases with size. Hence either a lot of money has to be spent or a small etalon must be used – stopping down the aperture of the telescope, limiting the light and resolution.
The second approach is therefore to economise on the size of the etalon, making it sub-aperture and placed some way towards the focus of the objective, inside the telescope. Extra lenses are used to make the light parallel before it passes through the etalon, and to refocus it afterwards. These dedicated solar telescopes, including well-known models from Coronado and Lunt, are relatively economical. However, their optical systems result in a ‘hot spot’ or ‘hot band’ pattern of filtration, where maximum contrast is only experienced in a region of the solar image. Again, a diagonal blocking filter is commonly employed at the eye end.
The third approach again involves utilising a normal telescope – usually a small refractor – but this time a small etalon is placed near the eye end, beyond a Barlow lens that decreases the convergence of the light to the point where the etalon will work. Such systems are made by Thousand Oaks and DayStar, including the latter’s popular Quark range of filters. An extra energy rejection filter (ERF) over the objective may or may not be required. The advantages of this approach are the lower cost of the small etalon, plus quite a uniform filtering result. The disadvantage is the field of view; the long effective focal length required will generally mean viewing or imaging only part of the Sun at once.
All narrowband filters require tuning by the observer, as they are responsive to temperature and pressure. Slightly different tunings are needed for different solar features (because of the Doppler Effect). The most common method of tuning is tilting of the etalon, but Lunt also uses a pressure-tuning system. The DayStar filters have a heating element to tune.
At higher cost, double-stacking – that is, placing two etalons in series – is used to obtain very high wavelength selectivity and thus contrast. One etalon may be internal to the telescope, and one external.