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Overview of some accessories for your scope
White light solar filter (III) The Sun light is warm with a solar constant of 800 W/m2 at ground level. If we can easily support this energy on a large surface, when this energy is concentrated on a point, the energy of photons transported by the light becomes burning. At the focus of simple magnifying glass of 10 cm of diameter, the temperature reaches 500°C. The ordinary glass showing a low thermal conductibility, if you expose a lens directly to the Sun without protection of the objective, on the other side, to the focus, the material wil not support such a heat nor the thermal shock and either will melt or break (see with this picture and this other one what happened to a DSLR darkroom which lens was directly to the Sun. Image what would happen if the lens would be a telescope). Highlight that when Venus (or the Moon) is within 10° of the Sun, you must shield the telescope from heat. Indeed, by aiming at Venus, the light beam returned by the primary mirror will place the image of the Sun right next to the optical center, which can burn any object near the secondary mirror, in particular on catadioptric and Newtonian telescopes. Without rejection filter or solar filter at the entrance of the telescope, use either an off-center mask or ideally add a long cardboard tube in front of the entrance of the telescope to block the light of the Sun and allow Venus (or the Moon) to be observed without risking an accident. Be very careful! At last, do never use the old green "eyepiece solar filter" that you can sometimes find in department stores or on the second hand market. Even of high density, as it is well attached in its barrel, after a few seconds of observation, the Sun heat will explode it in several parts... Bad for your money and maybe worse for your eyes... So forget these solutions ! Metallized sheet ang glass The best and safe solution to look at the Sun (and also for any optics directed to the Sun, including a simple visor) is to use an objective solar filter. It is made of a Mylar or a metallized sheet and comes in various sizes, usually in roll or Letter size sheet that cost a few tens of dollars or euros. You can cut it like an ordinary aluminium sheet to wrap your lens (scope, reflex, guider, etc). For a temporary use, the solar sheet can be fixed with a scotch tape or a rubber band. To
see : Celestron
EclipSmart Solar Filter Review, AstroBlender
However the best solution for a regular observation of the Sun is to buy a Baader Planetarium AstroSolar sheet made of alumined polyester ($25 for an A4 sheet, it is also available set in a barrel) that replaces the old PolySolar film (that gives also excellent resultats with a warm orange color as shows this picture of the transit of Mercury on 9 May 2016) or a metal-coated glass filter like the Thousand Oaks Optical 2+ solar filter (a 5" full aperture costs $99 and 105 € to Optique Unterlinden) or the similar model sold by Orion Telescopes & Binoculars. According to a test report published in "Sky & Telescope", the Mylar is one of the best product in its category, offering both an excellent optical quality and a pleasant orange image of the Sun disk, like show very well the three above images taken by Pedro Ré. However, the AstroSolar film provides a better spectral distribution than the Mylar sheet or any solution made of polymer (made of ethylene polyterephtalate or EPT). It is also less sensitive to streaks, pinholes and other blisters than the other filters.
The Hershel prism The alternative, although much more expensive, is to use a Herschel prism (or Herschel wedge) equipped with a neutral filter like the Baader Planetarium "Cool-Ceramic Safety Herschel Prism" elected "Hot Product 2006" by "Sky & Telescope". More recently, Altair Astro, APM and Antlia (via Teleskop Express) have also released a Herschel prism. Depending on the offer, the Baader prism is delivered with several neutral filters which density is ranging between ND 0.6, 0.9, 1.8, 3 et 3.8, offering a transmission from 1/400th to 1/6300th for visual (V model) or photographic (P model) use. Although its price is deterrent (between 362 and 520 € for a 2" model), advanced amateurs like Jean-Pierre Brahic, Thierry Legault, Pedro Ré or Salvato Giampaolo who use instruments of large aperture (350-450 mm) have prefered the Herschel wedge to the metallized sheet because besides the excellente quality of this optics, the system accepts neutral filters, continuum (offering a 10 nm bandwidth around 540 nm) and polarizing.
Hydrogen-alpha interferential solar filter (Hα) What an unforgettable experience to see the sun prominences ! What you need is a interferential Hα solar filter. Any one wider than 0.5 Å will tend to show prominences on the solar limb. An interferential filter use multi-layers dielectric coating on a glass surface and need to use a Lens Cover to reach a nominal f/30 ratio at prime focus and includies an Energy Rejection Filter (ERF). Only counterpoint except their price, multi-layers of such filters are sensible to the Sun heat (IR) and UV light and cannot work at their nominal performance more than 10-15 years at best (some low-ends even work only a couple of years). You can look at the H-alpha Sun on "two ways" : using a narrow or a broadband interferential filter. The first, limited at an half-bandwidth of 0.3 to 0.7 Å will show you the finest detail on the Sun disk, flares, filaments, plages but you will lose detail in prominences that evolve around the Sun's limb. On the contrary the broadband filter, due to its larger half-bandwidth from 0.7 to 2 Å will show you first and with a much better contrast solar prominences but only few detail on the chromospheric surface.
The cheapest solution to look at the Sun in hydrogen-alpha is buying a Lumicon 1.5 Å H-alpha filter ($730 with ERF), the same as the one sold by Thousand Oaks Optical, which will show you prominences in all their splendor. Some other manufacturers like Coronado (Meade) and Baader sell several interferential filters too, like the SolarMax II Hydrogen Alpha (~$900) to Coronado suited for scopes between 50 and 90 mm of aperture. But using an aperture of only 40 mm the resolution of disk detail is limited substantially (2.9"). Coronado also sells an expensive model, AS1-140 of 0.6 Å offering a free aperture of 140 mm ($12900) compatible with many brands and models of telescopes. Coronado provides also telescopes dedicated to the Sun observation in H-alpha. At last, Tele Vue dedicated also its small "Solaris" refractor of 60 mm f/30 to the H-alpha light, which sales are today ensured by Coronado. As many observers, if you want to look at both the prominences and the disk details, a good compromise is selecting a 0.7 Å Daystar filter, a manufacturer known for decades for their expertise in that field. Their "low-cost" is the Ion model very well suited for the country which oven temperature is manually adjustable. Add the mandatory Daystar ERF (a 2.5" for a 8" scope, $200) and you will be ready to observe one of the most marvelous phenomenon of Nature. To read : The market of solar telescopes
Note however than going down 2.5" or ~60 mm of clear aperture is difficult in regards of the low resolution. Daystar filters and Baader in Germany till provide ERFs of 5" and 6". At last, some manufacturers (Daystar filters, Coronado, Lunt) sell small solar telescopes with a free aperture between 40 and 90, sometimes 127 mm of aperture. Most use a narrow-band interferential filter (0.3-0.7 Å). The price of such instruments is ranging between ~$700 and $3000 but some models using high-end filters are much more expensive (up to $14000 for a single interferential filter). Please consult the next article for more details. Solar Scintillation Monitor (SSM) The former French optical company AiryLab developped the Solar Scintillation Monitor (SSM), a small detector not taller than a pen to place in piggyback to your solar scope and linked to the Genika program running under Windows and optionally to a CCD camera. Since AiryLab ceased its activities, this accessory is sold by Miratlas. The system is able to estime in real time the atmospheric turbulence that it converts in arc-second to make the interpretation easier. As we see below, these data are displayed graphically on screen, and optionally the system can start the acquisition of CCD images when seeing conditions are at best and that turbulence falls below a specific user-defined value. The basic kit starts from 400 € including 70 € for the SSM sensor. Here is an accessory that would please to many amateurs.
Note that AiryLab has also created the first Celestron EdgeHD dedicated to the Sun observation at full aperture, the "HαT" model (we see a part of that C8 HαT above left). The modification includes an ERF filter centered on a 120-nm half-bandwidth around the Hα line, and a 2.7x telecenbtric lens that can be applied to Celestron EdgeHD C8, C9.25 and C11 models. Although this model does no more exist, for memory we will review it in the page dedicated to the market of solar telescopes. Have you ever noticed that using a scope offering a short f-ratio, your pictures of the sun disk where sometimes partially fuzzy. An the same if you picture prominences. The unfortunate experience has happened to many novices while they mastered perfectly their camera and their scope. How can we make such an error and how to prevent it ? 1. The focusing tolerance Exposed to the Sun, an optical tube (the tube and the mirror) will slowly expand. A Pyrex mirror, famous of its low expansion rate, will take over 2 hours to reach the equilibrium (you can download this calculator from Cruxis). The tube of the scope will also expand of a fraction of millimeter. So, from 10 to 25°C (50 to 77°F), an aluminium tube (with a CFE of 23x10-6 K) of 1000 mm long will extend of 0.345 mm or 345 microns. It is 100 times higher than a fiber carbon tube (with a CFE of 0.2x10-6 K, a high resistant fiber carbon tube will expand of only 0.003 mm or 3 microns). To check : Linear Thermal Expansion Calculator This expansion will have an impact on the focal length, and the focusing distance will be extended consequently. So, you need to check the tube expansion and to compare it to the focusing tolerance according the next relation : Focusing tolerance FT = ±8 * (F/D)2 * λ550 * Δλ with F/D, the focal ratio of the optical system, l550 the working wavelength and Dl the accuracy of mirror polishing at the working wavelenght. For a scope at F/D=6, λ550 = 0.00055 mm and Δλ = λ/10, FT = ±0.0158 mm or 16 microns. It means that exposed to the Sun, an aluminium tube will expand at least 20 times more than the focusing tolerance of the optics ! Note that the tolerance increases with the focal ratio (for the same scope at F/D=20, FT = ±0.176 mm) and thus pictures will be much less sensitive to defocusing in using a longer focal ratio (say F/D=15 or 20). 2. The size of the sharpness field In addition, remind that the size of the sharpness field in the optical axis at focal plane depends on the F/D ratio as follows: Field sharpness radius (°) = arctan (0.0109 * F2/D3) If parameters are quoted in millimeteres, the field radius is expressed in degrees. Note that on hand calculators, arctan is the key tan-1. You can also use this online calculator. So, for a 200 mm f/6 scope, the radius of the sharpness field is 0.112° or 6'44", less than half the Sun radius (its diameter is ranging from 31'27" to 32'32"). In focusing at center of the optical axis, the Sun foreground will be sharp but the limb where faculae are the most visible (and where prominences develop in H-alpha light) will be out of focused. So, either you focus on a specific sun area or you ensure that all the field will be sharp. To ensure a sharpness all over the field, you need to increase the focal lenght of this same scope to 2000 mm or F/D=10 (field radius = 0.312° or 18'44") and even a bit more if you observe very large prominences extending of a few arc minutes from the limb. About the image resolution At last, dealing with the Sun observation in various lights, noted that the resolving power (PS) decreases to the longer wavelenghts following the next relation : PS = 251643 * λ/D So, using an instrument 60 mm wide offering a resolution of 2" in white light, it becomes 2.75" in H-alpha (λ=0.000656 mm). It is not much, and probably not visible visually in a turbulent atmosphere, but a photography can record this difference, all the more in stacking multiple images. Colored and interferential filters Too many observers continue to ignore the market of filters, they are colored, selective or interferential. They are however very interesting accessories to increase details in low density surfaces or to isolate a spectral line where the object shines of all its fires. I think first about planetaries observations but also to enhance the contrast of deep sky objects (DSO) against the sky background.
A typical colored filter, "available to any good shop" is a very cheap item ($15-$25 for a 1.25" filter) that gives you many satisfactions looking at planets if it is well selected in order to increase the detail contrast. ALPO recommends to use one of the following filters : Violet W47, Yellow W12, Green W57, Orange W23A, Blue W80A, Magenta W30. If you know that a filter blocks or darken its complementary color, you know how to use them. Filters of high density like the Red W25A need at least a 8" scope to avoid losing too much light. Do not select a large 2" to fit in the "Visual Back" or you will have many difficulties to change it... Good new, Orion Telescopes & Binoculars sell a set of four Sky-Watcher colored filters of 1.25" for $49.99 or 56.58 € and AstroShop in Germany outbids with a kit of 6 Omegon colored filters of 1.25" for 59 €, bargain ! One recommendation : check always that these filters are in glass because some are in plastic and of very poor optical quality. Name also the Baader Fringe-Killer filter (cf. AstroShop) that only transmits light between 480 and 680 nm to remove all color aberrations from the blue/UV and red/IR spectral zones in achromat and semiachromat refractors. Knowing that this filter reduces the light intensity up to 30 %, sometimes some images can show a light greenish dominance (otherwise they are simply darker), the equivalent in photography to a light loss of 1/2-aperture. Baader also sell an IR-blocking filter. Both are dielectric filters.
Then, there is the variable polarizing filter from Celestron or Orion Telescopes & Binoculars ($46 for a 1.25" model) constituted of two rotating pieces (a vertical and a horizontal polarizer) fixed in a short assembly house can also be useful when looking at objects presenting several vibrations planes or components. You can eliminate one or another source of light vibration by rotating one piece on the other until extinction. The polarizer is for example useful to study the solar corona (true and false components), to find young dusty deposits on the Moon, SNR hidden a pulsar, quasars, or simpler looking at objets in which the light is slightly refracted. For example, the polarizing filter can successfully be used to reduce light reflections looking at the sky, the sea surface or painting parts of metals. Note that some polarizers made for eypieces are so cumbersome that they through out the light cone at a point where no focusing can be done. To read : Astronomy Filters, Which Telescope Choosing a Color/Planetary Filter, Agena Astro Planetary Astronomy, C.Pellier et al., Axilone, 2016
On DSO's there are others difficulties. By nature, these objects are by far less bright than planets, they are often of very low density and many of them are of very small size. Worst, in our urban or suburban towns DSO appear in front of a bright background sky due to the light pollution. But sometimes this background is affected by the presence of haze, moisture or dusts in the air, or simply because the DSO resides in a bright area of the sky, in full center of the Milky Way for example. All these parameters reduce the contrast between the DSO and the sky and do no help us to find them. The best think to do is thus to find a way to reduce all this "pollution". The best filters manufactured to reject the light pollution are the UHC from Lumicon and the UltraBlock from Orion Telescopes & Binoculars ($80 or 116 € for the 1.25" model), Optolong ($54) or Woodland Hills Camera ($84). Then you can also use an H-beta for low brightness nebula and the DeepSky or Minus-Violet to look at all reflection nebulae ($110-140). You will find all technical data about these filters and others in the short report dedicated to filters. Last chapter |
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