Observing the Sun by Jamey L. Jenkins

Author:Jamey L. Jenkins
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

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Eyepiece focal length

Projection magnification

 12 mm

 15 mm

 20 mm

 25 mm

 30 mm

5X

 72 mm

 90 mm

120 mm

150 mm

180 mm

10X

132 mm

165 mm

220 mm

275 mm

330 mm

15X

192 mm

240 mm

320 mm

400 mm

480 mm

20X

252 mm

315 mm

420 mm

525 mm

630 mm

When projecting the Sun a telescope’s aperture is normally masked to 100 mm or less, so as to curtail excessive heat at the focal plane. Select an eyepiece constructed without optical cement that could possibly melt and destroy the lenses in the focused heat of the Sun. Typical projection eyepieces include the elementary Ramsden and Huygenian designs. Telescope owners having instruments of less than 76 mm aperture that generate a minimum of heat, occasionally use a higher quality eyepiece (i.e. orthoscopic or plössl) for sharper projection images. Regardless of the telescope/eyepiece combination a good rule-of-thumb is to periodically turn the instrument away from the Sun, which allows the optics to cool a bit before resuming observations.

An alternative to the reflecting screens illustrated in Fig. 3.2 is the adaptation of a translucent projection screen material creating a unique direct view. Again ambient light unless curtailed will reduce contrast, and the texture of the translucent screen must be considered regarding final image quality. Nevertheless interesting results can be achieved, making the technique worth investigating.

Fig. 3.2Homemade projection screens for casual sunspot observing. The box shaped example on the left was spliced together using discarded foam board, epoxy, and spare telescope parts, built by Raymond Rienks. The second illustration is a Hossfield Pyramid, named for AAVSO solar observer Casper Hossfield. Jamey Jenkins constructed this version from black foam board, masking tape, and white card stock

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