by Raffaello Braga
Warning – Visually observing the Sun can be dangerous for the eyes and the equipment, solar filters must be used following manufacturer’s instructions. The author is not responsible for any damage deriving from improper use of the solar filters described below.
I have already had the opportunity to write here my impressions about Astrosolar, the filter material for white-light solar observations that Baader Planetarium has been selling for almost twenty years. The success obtained by this film consisting of a metallized polymeric support (not a mylar, as someone erroneously still writes today) to put in front of the telescope aperture, has been such as to revolutionize the approach of amateur astronomers to observing our star. Some still persist in producing low-cost glass filters, but the performance gap between them and the Astrosolar is such as to make the former to be preferred only for public observations for which the resistance and easy cleaning are important; in all other cases, especially for high resolution imaging, the Astrosolar is definitely preferable: in addition to the affordable cost, the high image quality and the possibility of being used to make your own filter yourself, this material returns a neutral, white sun, therefore preserving the whole spectrum from UV to IR.
Several companies produce metal cells for mounting the filter material in a more robust support than a self-made cardboard cell. An example is the EURO EMC cells, marketed by Teleskop Service in several diameters. I had the opportunity to use several of them for refractors, reflectors and reflectors and I found them very well made; I currently use the 146 mm cell for my achromatic 150 f/5 and the 96 mm cell for the Takahashi Sky-90 apochromat.
In these filters the film is clamped between two circular aluminum circles which, if necessary, can be separated to replace the Astrosolar should it become necessary. The cell is fixed to the front of the tube by means of four screws which are inserted into as many nuts covered with plastic cylinders and which protrude rearwards from the frame. The screws can be fixed both on the external surface of the tube (this is the case of reflectors and compund systems) and on the internal one (dewcap of refractors), it depends on the free diameter of the filter and on the diameter of the tube. The installation shall be done in such a way to place the center of filter aperture on the optical axis (unless the objective diameter is smaller than the aperture, in which case the filter can be mounted eccentric); therefore, it is necessary to adjust the position of the rubbers along the guides until they are all the same distance from the optical axis.
EURO EMC cells are available with free openings of 44, 54, 64, 76, 96, 120, 146, 178, 207, 253, 315, 363, 421 mm. Once the diameter of interest has been chosen, however, it is necessary to check the range of diameters to which they can be adapted, bearing in mind that the inside of the tube cannot be used in catadioptrix due to lack of space. Even with the reflectors, in my opinion, it is better to use a filter to be fixed outside. In larger diameters, the weight of the cell starts to be no longer negligible and could lead to a balancing of the tube different from the usual one.
Baader Planetarium also produces specific supports for the Astrosolar ™. According to the German brand, these cells compensate for the differential expansion between the film and its metal cell, keeping the former free from tensions that could degrade the quality of the image.
Baader cells are of three types. In one, called ASTF and designed specifically for astronomical telescopes, the cell is metallic but the filter material is fixed to a plastic ring which has an expansion coefficient similar to that of the film and absorbs differential dilation with the cell. In the type called ASSF, designed for spotting scopes but also for small telescopes, the construction is less sophisticated, the whole cell is in fact plastic and therefore the problems of differential expansion should be mitigated. Finally, there is a specific model with cells specially shaped for use on binoculars. It is also possible to purchase ASTF type filters equipped with photographic film (ND 3.8) instead of visual.
I tested an ASTF 80 and an ASSF 130 with an 80 mm f / 11.2 TMB achromatic refractor, a 130 mm newtonian and a 127 mm f / 15 maksutov. I will not described here the content of the boxes and the various assembly stages (the interested reader can download the manual from Baader website). I will therefore limit myself to just a few additional considerations. .
Unlike the EMC filters, the Baaders have reference graduations in correspondence with the guides along which the screws that connect to the rubberized tightening nuts must be positioned, and this helps a lot in fixing in axis with the tube. In each cell there are two series of guides where to place the anchors, according to the diameter of the tube to which the filter shall be attached. Guides that are not used must be capped using the black plastic inserts supplied as standard, to prevent sunlight from passing through them.
As for the EMC, also for the Baaders various free diameters are available each with its own clamping range, on the inside or outside of the tube, which must be checked carefully before purchase. The free diameters available are:
80, 100, 120, 140, 160, 180, 200, 240, 280 mm for ASTF series
50, 65, 80, 100, 115, 130, 150 for ASSF series
there is therefore a lot of choice, but strangely among the ASTF the diameter of 130 mm – that of the very popular 5-inch Maksutov, is missing: it is therefore necessary to use the 120 mm sacrificing an inch of aperture or, better, fall back on the ASSF model that works just as well. Furthermore, the largest diameters are missing, from 300 mm upwards.
The tightening system of the Baaders is similar to that of the EMC but there are only three rubbered nuts for clamping with the telescopic tube instead of four; however, the filters are also supplied with three strips of fabric which are fixed on one side to the cell screws and on the other to the tube by means of small pieces of Velcro and that should constitute a “safety” anchorage in case a gust of wind could overcome the friction between the rubbers and the tube exposing the observer to direct sunlight. Even if the idea is good, its realization leaves something to be desired. My experience, in fact, is that after a bit of assembly and disassembly of the filter, the Velcro counter-strips no longer remain fixed to the tube because the adhesive gives way, leaving behind an unpleasant yellow residue which shall then be removed with a solvent.
However, my experience is that if one adjusts the three rubbered screws so that they grip firmly on the tube, there is no need for further anchoring, as is also done with EMC filters.
Overall, Baader filters are better finished than EMC. For example, the cell is painted white, to limit the heating, the external and internal circumferences have raised edges that serve to better protect the film, the details of the various parts are more carefully realized.
As the filtering material is already well known and is the same for all the described cells, the test consisted simply in exposing them to the environment during the observation to highlight any differences in behavior between the Baaders and the EMCs, what I am going to illustrate by means of the photographs that I made.
The first test of the Baader ASTF 80 did not go too well despite promises. This is what the filter looks like after a quarter of an hour of observation:
As one can see, the filter material is stretched like in a drum and the eyepiece the Sun showed a noticeable loss of sharpness. After dismantling the filter to replace the Astrosolar and taking care to reassemble it a little more loosely, the problem eased considerably, however based on the declared characteristics this intervention should not have been necessary.
The ASSF 130 model, which is constructively less sophisticated than the ASTF, performed much better, I would say very well indeed. For comparison, I exposed this filter and the 146 mm EMC, both initially in this state:
that is, with the film free of tension. After exposure to the summer sun this was the appearance of the 130 ASSF:
that is with the film in optimal conditions while the EMC showed a more stretched film although still acceptable:
I conducted the tests in summer, with ambient temperatures from 15 °C (in the morning in the mountains) to 34 °C (in the afternoon in the city).
All the filters described here can be easily disassembled (in the case of Baaders the instructions shall be followed) to replace, if necessary, the filtering material, for example the visual grade (ND 5) with the photographic grade (ND 3.8) .
While in the EMC the film is simply sandwiched between the two rings, in the Baaders it is attached to the plastic frame by means of a double-sided tape, therefore during the replacement of the Astrosolar it is better to have a spare in order to be able to replace what comes away together with the film itself.
The question the reader will ask at this point is whether or not it’s worth getting one of these filters when the Astrosolar is so well suited to make the filter simply by using of cardboard and adhesive tape. In my opinion the answer is in the frequency with which the Sun is observed and photographed. In my case it is a daily activity – weather permitting – to which I dedicate a lot of my time as an amateur astronomer and with all the telescopes I have; for years I have used self-made filters with glue and cardboard but given the frequency of use I had to continually restore them, and on the other hand having little manual skills I do not feel like dealing with more robust materials than cardboard such as wood. More rigid cells like the ones I have described allow instead to insert and remove the filter as many times as you want, to transport it without damaging it and repaying the expense with greater resistance and durability and with the ease with which you can replace the filter material. An occasional observer, on the other hand, can certainly save money by building a filter by following the instructions of the German company.
Among the products examined here, my preferences go to Baader ASSF, light, well-made and with an excellent performance / cost ratio. On medium and large diameters, however, this type is not available and in this case the most convenient choice is the EMC, but you have to deal with the weight and when it is hot it may be necessary to loosen the film a bit to prevent it from becoming tense. Those who make high-resolution imaging of the Sun should be careful not to sacrifice the aperture of the telescope – especially if it already has a central obstruction – and take a filter equal to the diameter of the objective or slightly larger.