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Observation of Sun directly by filter |
The direct observation of the sun by filter
Caution!
If you use foil filters then in the case of foil tearing, the sudden flash damages your eye irreparably.
Examine the foil attenuation at the light adjacent wavelength ranges infrared (IR) and ultraviolet (UV). Inquire if
necessary with the manufacturer for detailed information. Since our eyes do not possess direct receptors for this
radiation, this is particularly insidious!
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The transmission factor of the used Objective filters for solar radiation should be very small and the remainder
has to be reflected. The reducing factors for observation should be approximatly at 105, using unprotected eye and for
photography in the range of 104. Beside solid glass filters an economical kind of filters, very thin foils (10 - 15 µm
thickness), are available. They are coated by metal steam. Thermal problems do not arise, because the Ir-part of the solar
spectrum is reflected too. However it's not all gold, which shines. As base material you should select a plastic with a
high tear strength. Remind, that high reducing factors cannot be reached by simply vaporizing the foil. to reach the
attenuation some filter manufacturers combine 2 foils: The contrast of the illustration suffers. Reciprocally coated
foils should be uses instead!
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Advantages of the
filter method with foils:
- You only need the foil and you can manufacture the mounting plate of the
filter by your own.
- The self made filter is economical. This is particularly important for a beginners, which
do not want to spend much money fist.
- Compared with glass the foil filter is shatterproof.
- Nothing at
the telescope needs to be changed.
- Heat problems do not arise, since most of the filters reflect the infrared
radiation too.
Disadvantages of the filter method with foils:
- If you want to observe by photograph and by eye, you need 2 filter foils of different optical density
(OD5 for eye- and OD3.5 for the camera-use).
- Since the foils are very thin, the production of a wrinkle-free
filter requires to be skilful or handy.
- The foil filter can be damaged by sharp articles or may get abraded
at the surface.
My first attempts of the sun observation were carried out by the projection method. In order
to test the filter method, I purchased filter foils at Baader planetarium, Using pasteboard, I built the mounting plate
by my own. I manufactured two annuli with the inside diameter of the telescope aperture and glued the foil between.
A thin-walled hollow cylinder, which fit to the tube was pasted perpendicular.
You will find a guidance for that
mounting plate on the web pages:
Baader
Planetarium.
Tap: If you provide a second filter mounting for your the alignment of the telescope to the sun becomes
easy like a children's game.
Read on: Transit of Mercury
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