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!

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!

The crew
Fig. 10: The observers of Sun


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.

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