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So in order to see the hydrogen-alpha features you need to accurately filter its narrow line in the red portion of the spectrum, precisely at 6562.81 Å. At its half intensity point (at half bandwidth), the line is only 1.20 Å wide. Then you also need to reduce the heat load which broaden the bandpass, and stop UV light that will shorten the filter life span. EquipmentYou can look at the Ha 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. To
properly isolated the narrow Ha
line most manufacturers designed an interferential filter which
principle is based on phase opposition of undesirable light. A
typical Daystar interferential filter is actually a Fabry-Perot
Etalon, which is a pair of very smooth plane-parallel surfaces
protected with a multi-layers
dielectric coating separated with a solid spacer crystal. Another
solution is using a Lyot filter which uses optical activity in
crystals as well as sets of polarizers and Due to their design, all these filters work with a lens cover called "Energy Rejection Filter" (ERF) attached in front of the scope to reduce the sun heat and absorbe UV light. In order to reduce light scatter and increase contrast this ERF is not coated. Each optical assembly has to be configured such a way the free aperture reaches a nominal f/30 ratio at the prime focus. Therefore the largest ERF is currently 127 mm for a 16" SCT scope. So from the 'sun' side of the filter system, there is the Energy Rejection filter, an antireflection window, then a narrowband blocking filter, the first etalon window, the solid spacer crystal, the second etalon window, a broad band "trimming" filter, and then another antireflection window. Also, there is not much "carbon" involved in the Daystar filters, just glass, quartz, and various optical coating materials. Low cost solutions Buying a high quality interferential filter is a huge investment, similar to the one of a good small scope; e.g. Meade-Coronado AS1-140 offering a free aperture of 140mm and providing a half-bandwidth of 0.6Å cost about $12900 ! As no amateur can really buy such an accessory, manufacturers have built various cheaper models providing in spite of everything excellents results.
The cheapest solution to observe the Sun on H-alpha light is buying aLumicon (temporary off duty) or Thousand Oak 1.5 Å Ha filter which will show you prominences in all their splendor. That will cost you around $730 with the ERF. Other manufacturers like Tele Vue have dedicated a small "Solaris" refractor to that activity. It is sold by Coronado filters (a Meade subsidiary since 2004) that sells interferential filters too, like the GEMINI model 2.5 Å suited to many scopes. But like the Coronado SolarMax 40 its aperture is only 40 mm which limits the resolution of disk detail substantially. But if you are interested in this small size, you can build a solar H-alpha scope for less than $2000. If you prefer a larger aperture, Coronado offers a mid-range solution : the combination of the interferential filter Solarmax 90 (aperture 90 mm, HBW < 0.6 Å) to place on the front of your scope and the blocking filter BF 30 to place on the back side. The filter is threaded to attach directly to the front of Tele Vue refractors (101, 102 and Genesis) but can be adapted on any larger objective (i.e C8, etc) using an off-axis mask. However this solution is not really cheap and will cost you about 8600 euros to Lichtenknecker Optics.
Plus side, using an adjustable tilt in the filter stack the T-scanner is well suited to study Doppler-shifted features like eruptions and flare moving quickly through the bandpass over 45 km/s. The temperature regulation stays however an important factor using Daystar filters. If you buy a T-scanner filter not properly configured for your climate (cold or warm) it will never display fine images, mainly in the winter when temperature drops below 0°C for example. You could only use our filter no more than 20 minutes, sometimes less depending the cold intensity, then the bandpass will go off and you will start to loss detail, mostly surface. Therefore ask at your nearest Weather office for climatology data of your location so Daystar can modify the settings of the filter accordingly a more accurately defined mean temperature.
A resolution limited to 1" Usually a solar telescope like the Tele Vue Solaris or Pronto is a small refractor from 30 to 70 mm of aperture. The problem is that even using a free aperture of 2.5" (63 mm) for a 8" SCT trying to see finest detail in such conditions are a real challenge. Without speaking about the price of a larger instrument, we have to explain why manufacturers don't sale larger filter than 127 mm and we will try to know if other solutions can not be drawed to increase our resolution.
The main problem observing the Sun is the HEAT. As we know, the Sun causes thermal convection in our atmosphere, heating the ground and buildings around us initiating local convection. So when observing the Sun the air all around us becomes turbulent, creating temperature fluctuations much larger than 0.1 K. This effect affects the refractive index of the air and generates wave front aberrations which limit the practical resolution to around 1", using a 3.5" or a 16" scope. Exceptionnally at Pic-du-Midi using a 600 mm refractor, in late '50s B.Lyot and J.Rosch got a photographical resolution of 0.4" and recently telescopes of 2m of aperture got a resolution of 0.15". Therefore in ordinary conditions apertures greater than 100 mm are not so useful for any solar work unless you are at high altitude. Read the page written by Jean Dragesco on this website for more details about high resolution astrophotography (in French). The ERF that covers your objective, on or off-axis, serves many functions, one of which being to reduce the light entering the filter by 50-75% only out of other bandpass to increase the Ha line. Its second utility is to prevent the UV light from damaging the front coatings (which has since been resolved) and at last to avoid infrared light to broad the bandpass of a few tens of angstroms. This last effect must be regulated with accuracy when doing CCD imagery or studying filtergrams (Ha prints). If we know that each f/ stop lost reduces the accuracy of the filter of about 5 to 10% of the bandpass, it is mandatory to create an optimized optical system working at f/30. However, in any case we can work without ERF. The small Daystar ERF is probably not the only one solution although Daystar insists to each customer that his methods are the only one way a Ha filter will work. Until recently there we were no methods that could easily use a larger ERF and nobody could ask Daystar for explanation. But time running, here are some. New proposals If you want to use a full aperture ERF on a small scope or an ERF larger than 127 mm, I suggest you to experiment one of the next solutions. Some of them were experimented on H-alpha, others never but have already generated a great deal of theoretical discussions on Internet's forums. So, I would like to thank Jen from ICSTARS Astronomy, David W. Knisely from Prairie Astronomy Club, Anthony Seal, Jack and Gordon for their rigorous arguments in favor of these hybrid solutions. Here are the ideas One approach to enlarge your optical system aperture is to find a solution similar to the Herschel wedge. The San Francisco Sidewalk Astronomers built a Solar dobsonian using a partially aluminized optical cover plate that rejected 95% of the solar energy. They drew up a diagram and described this instrument in the September 1971 issue of Scientific American. Of course at that time this instrument was not set up for hydrogen alpha. But this article might be worth a check.
The last solution is the most interesting. We have seen the barlowed image is degraded. Excepting the vignetting, the Barlow is a negative lenses system which amplify the "field angles" of the incoming light, resulting in the "ring effect" and a limited passband field of view. So, to insure a decent image without vignetting we can use a telecentric optic to get intrinsic long f/ratio, specially to get the mandatory f/30 ratio. Recently Astro-Physics and Baader introduced the telecentric optic allowing large ERF to be used with a shorter optical system. The solution should be to ask Daystar to design a 4" (100 mm) ERF suited to a NexStar 5" f/10 for example and using a Tele-Vue Powermate 2.5x which is a telecentric and converging system. The resulting beam is f/31, nominal for the Daystar but using a 100 mm aperture. I bet this time you will get a finer solar image, sharper than using any Barlow with the advantage to give you a view of the entire solar chromosphere at once instead of just a ring-shaped patch of detail. I suggest you to test this configuration and to post me your appreciation with some pictures. Several amateurs tested solutions described above successfully. Enyo brothers for example use a 0.7 Å Daystar with a Tele Vue 4x Powermate fixed on a Tele Vue 101 refractor while John uses the AP Telecentric unit in place of the Powermate. Both work well. John also bought a 77 mm ERF from Lumicon - which quality is really similar to two photographic filters - for his AP 130mm. David Knisely uses his 10" f/5.6 Newtonian stopped down to 3.5 inches (f/15.9), which then becomes about f/39.6 with a 2.5x Powermate. The detail is extremely good (when seeing supports it) all through the field of view, and he has used the T-scanner setup at powers up to 220x, although most of the time, he stays around 117x. Baader solutions At last, if you prefer to invest your money in a smaller interferential solar filter but taking advantage of the full aperture of your scope (70-180 mm), know that Baader Planetarium in Germany provide two interesting solutions : - Interferential filters to use with a ERF filter - A small coronographic accessory called a prominence viewer Interferential filters Baader sell several different H-alpha filters : a 650 and 450 Å half-bandwidth passfilter (94 €) suited to 2" eyepieces and 6 Å and 1.5 Å half-bandwidth models suited to 1.25" eyepieces (~ 685 €). Both filters are equipped with parallel glasses polished at l/4. These two latter are only suitable to study prominences with a prominence viewer like a coronograph. These filters request nothing else than an energy rejection filter in front of your scope. Baader manufacture a series of 450 nm wide front energy rejection filters (C-ERF- filters) polished at l/10 wave and ranging in size from 70 up to 160 mm in diameter (485 € for a 110 mm model). These filters are being used as pre-filters to cover the entrance aperture of the telescope and shield off any excess of harmfull energy. Baader use these "cool" ERF in combination with all their H-alpha filters . Good news, you have not to worry about the f/ratio of your
optical system or any other consideration as in using a Daystar or a
Coronado filter.
At
left the Baader
C-ERF-filter of 90 mm of diameter. At center the 1.5 Å half-bandwidth Baader interferential solar
filter. At right a prominence recorded with this latter
using the Baader 80 mm prominence viewer (see below).
Documents from the manufacturer. Note
that Baader sell another 450 Å passband filter for 2" eyepieces
with a l/4-wave
polish but it is reserved for Deep Sky CCD-work together with SBIG interline and full frame chips. It is not intended for solar work
!
If
an interferential solar filter at wideband (1.5 Å wide and up)
prevents to see details on the solar surface, why not using a
coronograph ? In
this regards, Baader
sell for some years a 80 mm prominence viewer (1595 €).
This is an accessory constitued of a mask that comes inside the tube of
Vixen 80 mm refractors from 800 to 1600 mm of focal length. Its
external ends
is equipped with a star diagonal of 1.25" and can be equipped
with any reflex camera or CCD. It supports all Baader interferential
filters from 9 to 1.5 Å.
At
left the Baader prominence viewer adapted to Vixen 80 mm
refractors. At right some fine prominences recorded with
this system with an additional H-alpha filter of 9 Å. Documents from the manufacturer. Life
span of H-alpha filters A
typical Daystar T-scanner filter can last years without the least
problem. In some circumstances this kind of filter need to be recoated
every 7-10 years if you begin to see a loss of contrast. Apparently,
the early blocking filters (bough in 1980-1985) did not last more than
about 15 years of extensive use (a few times per week). The first
symptoms are a loss of contrast and light intensity in one side of the
field. Still, the repair cost much less than buying a new filter,
about $300, and once servicing the filter is still performing for a
new 15 years or so. This is thus a very good investment. Some
owners used their Daystar about 16 years before experimenting trouble
with their blocking filter. Today Daystar has changed the blocking
filter a bit, possibly at an attempt for a still longer lifespan. A
good pratice to avoid trouble is to never use the Daystar or similar
filters without the ERF. Note that Daystar itself specifically recommends to *not* use it
without ERF or heat and UV *will* hurt the filter. The focal ratio has
to be f/30 or longer for it to work properly. So in order to keep you
filter in good state, do respect its specifications. Being
available for a few years only, we cannot estimate yet the life span
of Coronado or Baader interferential filters. But according to their specs you should keep them as long as any colored filter, even
used one hour each weekend, thus if you take well care of them, you
can use them all your life. Further
readings and contacts Gallery
of masterpieces (the Sun in H-alpha, on this site)
Daystar corporation (USA)
Coronado filters (USA) Tele
Vue (USA) Manufacturers
and Clubs (via my 1001
links).
Astrophysics of the Sun, Harold Zirin, 1988, Cambridge University
Press Solar Astronomy Handbook, Beck/Hilbrecht et al., Willmann-Bell Solar Astrophysics, Peter V.Foukal, Wiley Interscience The Sun, Michael Stix, Astronomy & Astrophysics Library High Resolution photography, Jean Dragesco (see also his comments on
this
webpage, in
French) Amateur Telescope Making (Book III), A.Ingals, Willmann-Bell |
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