Solar observation and H-alpha-filters for Schmidt Cassegrains

Again and again we receive requests for using H-alpha filters with Schmidt-Cassegrain or EgdeHD telescopes. In principle, this is possible, and a large aperture also provides much better resolution when observing the Sun than smaller instruments do. However, there are a few details to consider for telescopes with secondary mirrors; the following therefore applies in particular to Schmidt-Cassegrain and EdgeHD telescopes, but can also be applied to Cassegrains, Newtonians and other obstructed systems.

The problem: To protect the telescope from the solar energy, a filter must be placed in front of the telescope. For white-light observing, there is otherwise danger for the mount of the secondary mirror, which can melt so close to the focal point if the tracking does not work perfectly (after all, the primary mirror of a Schmidt-Cassegrain has about f/2), and for H-alpha, an energy protection filter is needed anyway.

We receive most enquiries about the C8, so for this article, we will look at this telescope in particular.

The ideal: A front filter for the whole aperture

Anyone who wants to use a large telescope for solar observation wants to use the higher resolution made possible by the large aperture. Ideally, a front filter should cover the entire aperture.

For white light observation, this is still easy: In the best case, with a self-made filter made of AstroSolar Photo Film OD 3.8 (various versions available) is a cheap and good solution. Even with an inexpensive sheet in A4 format (20x29cm) you will hardly lose any aperture through the filter mount on a C8. Better is a mounted ASTF: AstroSolar Telescope Filter OD 5.0 (80mm - 280mm) ASTF: AstroSolar Telescope Filter OD 5.0 (80mm - 280mm) (various versions available) , which not only uses the entire aperture, but also has a temperature compensating filter cell: The film is mounted tension-free and movably so that it does not become stressed when the filter cell heats up. A simple filter holder, on the other hand, can heat up and expand during longer observations, so that the film comes under tension. This causes the image quality to drop and the image to become blurry.

A solar filter made of glass would look more valuable, but does not usually offer better image quality: plano-optically polished, flat surfaces are more difficult to produce than curved surfaces, so most glass lens filters are either much more expensive than AstroSolar film, or much worse, or far too often both.

A Herschel wedge ( Baader 2" Cool-Ceramic Safety Herschel Prism (Visual / Photographic) (various versions available) ), which provides the best image, can‘t be used because of the secondary mirror on obstructed systems.

For H-alpha observation, an energy protection filter is needed that only allows light near the H-alpha line to pass. Since it is combined with expensive H-alpha filter systems, no compromises in optical quality may be made here. This is also why there is no ERF filter film, and energy filters made of glass are only available up to diameters of 180 cm. They are thus only large enough for a C5 or C6 - and at a price that reflects the high effort required to achieve the necessary quality. A polish on both sides to λ/10 is very expensive and not possible in this size for a few 100 euros - a high-quality 180mm E-ERF with mount g ( Baader D-ERF Energy Rejection Filter (75 - 180mm) Baader D-ERF Energy Rejection Filter (75 - 180mm) (various versions available) ) already costs considerably more than a complete C8.

The largest available D-ERF (red) with a diameter of 180mm is smaller than the Schmidt Corrector (blue) of a C8. The free aperture decreases while the secondary mirror mount (black) remains the same - the obstruction increases.

A C8 with the largest available front filter with a diameter of 180mm (minus the filter cell) is therefore forced to be stopped down. On a refractor this would still be tolerable, even an aperture of 180 mm still shows a lot of detail. However, as a catadioptric system, the C8 has an obstruction due to the 6.9cm secondary mirror. With an 18cm D-ERF, this obstruction would increase from 34% to about 40% - in practice even more, because of the filter cell (which also costs money). So the 8" quickly becomes a telescope with only 6 to 7" aperture and an oversized, contrast-reducing obstruction. From a photographic point of view, this may be more or less manageable with image processing, but visually the loss of contrast is more disturbing.

The solution

Instead of offering ERFs for large Schmidt-Cassegrains, which would cost considerably more than the telescope, we decided to take a completely different approach and equip the Schmidt corrector plates of selected Schmidt-Cassegrains with an energy protection filter coating. Thus, the Baader Triband-SCT - Schmidt-Cassegrain-based Multi-Purpose-Telescope, for Sun and Deep Sky Triband-SCT - Schmidt-Cassegrain-based Multi-Purpose-Telescope, for Sun and Deep Sky (various versions available) telescopes have been created which make use of the entire aperture of a C8, C925 or C11 and are nevertheless cheaper than a corresponding telescope with a suitable energy protection filter. In order not to limit the telescopes' range of application to the Sun, the telescopes are equipped with a complex triband coating that additionally makes deep-sky photography possible with narrowband filters, or white-light solar observation with a Herschel wedge – although the Sun then appears greenish instead of pure white.

In this way, even a large Schmidt-Cassegrain with full aperture can be used for solar observation in white light or H-alpha - only the conversion of an existing Schmidt-Cassegrain is not possible without reducing the aperture and increasing the obstruction.

Smaller energy protection filters as an alternative?

Of course, it is understandable that not everyone wants to buy a new telescope if they already have a proven C8 or similar.

At first glance, it seems obvious to simply use a smaller energy filter - after all, very improve solar observations are already possible with 80mm, and during the day the seeing limits the possible magnification anyway if you don't have a very good location.

But again, while it is no problem to stop down a refractor and then perhaps even use a filter system such as the Baader SunDancer II H-alpha Filter (#1363056 , € 3545,-) , whose integrated buck filter is also sufficient as an ERF on telescopes up to 80mm aperture, this is not so easy with a Schmidt-Cassegrain. To protect the secondary mirror, an expensive ERF is needed anyway, and the price advantage is lost.

By the way: even with such a strongly stopped-down telescope, a telecentric is still necessary, because the etalon needs a parallel beam path.

So what are the options if you don't want a new telescope?

Large energy filters placed off-centre

When a large D-ERF is mounted sideways, the secondary mirror makes it act similar to a Scheiner mask and causes undesirable optical effects.

Theoretically, you could move a larger ERF towards the edge so that the obstruction of the secondary mirror decreases proportionally - or even distribute several small ERFs around the secondary mirror. In practice, however, this leads to many „interesting“ effects, to say the least. Instead of a filter system with reduced (but still present) obstruction, you would construct a complicated form of Scheiner mask which mercilessly reveals every misalignment and blur.

The image quality will therefore suffer, so we can only advise against this option.

A small, obstruction-free energy filter?

The cheapest solution that does not make any optical compromises would be an ERF that uses only the area between the secondary mirror cell and the Schmidt corrector plate mount - in the case of the C8, this is a ring only about 63cm wide. At least, the smallest and cheapest D-ERF with a diameter of 75mm would be sufficient. However, the price advantage is eaten up by the fact that the focuser has to be tilted by about 3°: Due to the oblique incidence of light, the beam path does not hit the centre of the secondary mirror, and in order for the light to hit the etalon straight on, the focuser has to be aligned with the new, smaller telescope aperture. This would be possible, for example, with a slanted extension plate or, better, a sufficiently stable tilter, which is also not a cheap solution. As soon as observing the night sky with full aperture, of course, this has to be dismantled again.

The second problem: A 200/2000 f/10 telescope like the C8 becomes a 63/2000 f/32. And even at f/32 there is no parallel beam path, which is necessary for an H-alpha etalon to work perfectly. With the Baader SunDancer II Telecentric System TZ-3S Baader SunDancer II Telecentric System TZ-3S (#1363070, € 365,-) , the stopped-down C8 becomes a 63/6000 f/96, which with a 40mm eyepiece has a minimum magnification of 150x at 0.4mm exit pupil - far beyond what even the best 63mm optics can deliver in terms of useful magnification. Of course, you could bring the focal length back to 2400mm with the Research Grade H-alpha 0.4x Telecompressor 2" for Solar Spectrum (#2459260 , € 366,-) but it is much more economical to instead equip a cheap 80mm refractor without D-ERF with the SunDancer II and, if necessary, retrofit a better focuser like the SteelTrack with optional SteelDrive motor focuser. For larger apertures than 80 mm or SolarSpectrum filters without integrated energy protection filters, a D-ERF is of course still necessary - but as long as you don't get into the price range of a TriBand, the price/performance ratio is still better than with a C8, which is cut down in its possibilities.

Dust cover of a C14 with integrated energy protection filter. The focuser has to be tilted additionally to be able to use the system.

With larger, observatory-class telescopes, this is more worthwhile: With a C14, about 12 cm of aperture can be used, and a 135mm ERF can be mounted off-centre. With this resolution, a telecompressor is worthwhile. With a C14, this solution is also interesting, since the compact telescopes are often used in observatories where there is no space for an additional 12cm refractor.

About the author: Alexander Kerste

Alex 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.