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Overview of some accessories for your scope Colored and Interferential Filters (III) Too many observers continue to ignore colored and interferential filters during their observation sessions. However I have to convince you they are very interesting accessories to increase details in low density surfaces or panchromatic objects or to isolate a spectral line where your 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 from all good shops" is a very cheap item ($10-15) 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... It is good to know that dealers like Adorama Camera or Scopetronix sell a set of 3 or 4 filters for less than $35, Orion Telescope & Binoculars sell a set of four at $44.95, bargain ! Two recommendations : ask for a glass filter because some are in plastic and of very poor optical quality or a Schott or Kodak Wratten in 4x4 cm size.
The variable polarizing filter from Celestron or Orion Telescope & Binoculars ($39.95 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. As explained in the Research program pages, this apply to the study of 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 use to reduce light reflections looking at the sky, the sea surface or painting parts of metals. Note that some polarizers are so cumbersome they through out the light cone at a point where no focusing can be done.
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 Telescope & Binoculars 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 ($89-99). You will find all technical data about these filters and others in my filters report. The Sun is a peculiar object, the only near star we can observe as long as we like. First, forget the old green "eyepiece solar filter" that you can sometimes find in department stores. Even of high density, as it is well attached in its barrel, after a few minutes of observation the sun heat will explode it in several parts... Bad for your money and maybe worse for your eyes... So please forget it and buy a filter to fix in front of your objective and showing a density of 4 or 5 (transmission of 1/100000) !
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. The AstroSolar film provides however a better spectral distribution than the Mylar sheet or any solution made of polymer. It is also less sensitive to streaks, pinholes and other blisters than the other filters.
Hydrogen-alpha solar filter (Ha) What an unforgettable experience to see the sun prominences... What you need is a interferential solar filter. Any one wider than 0.5 Å will tend to show prominences on the solar limb. Such filters, called Ha filters use multi-layers dielectric coating on a glass surface and need to use a Lens Cover including an off-axis energy rejection filter (ERF) to reach a nominal f/30 ratio at prime focus. Only counterpoint except their price, multi-layers of such filters are sensible to the sun heat and UV light and cannot last more than 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 0.4 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 Ha 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 filters (now Meade) sell several interferential filters too, like the SolarMax Hydrogen Alpha (>$875) 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. They also sold an expensive model, AS1-140 of 0.6Å offering a free aperture of 140 mm ($12900) which can be adapted on many brands of telescopes. Coronado provides also telescopes dedicated to the Sun observation in H-alpha light. At last Tele Vue dedicated also its small Solaris refractor to the H-alpha light
As many observers, if you want to look at both the prominences and the disk details, a good compromise is selecting a 0.7Å Daystar filters, enterprise known for years for its expertise in that field. Their "low-cost" is a wireless model called T-scanner very well suited for the country ($1499) which oven temperature is manually adjustable. Add the mandatory Daystar lens cover (2.5" ERF for a 8" scope, $200) and you will be ready to observe one of the most marvelous phenomenon of Nature. Note however than going down 2.5" of clear aperture is difficult in regards of the low resolution. But Daystar does nomore provide ERF of 5" and larger. Your only representative is Baader in Germany, provider of Daystar ERFs. Read my report about using Daystar solar filters and foreign solutions for more detail and my gallery for other stunning images of the Sun in hydrogen-alpha. Last chapter |
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