In the March 1972 issue of Sky and Telescope, A.T. & L.G. Young discuss methods of near Sun observing revolving around keeping the Sun’s light off the primary optics. They cover in some detail the use of tube extensions and occulting disks to achieve this.
They cite the following examples:
- Using the dome to block the Sun, the 82″ reflector at McDonald observatory routinely observed at 12o elongations and down to 7o at reduced aperture.
- Using a sun shield extending 14′ beyond the dome of the 24″ reflector at Table Mountain Observatory, California the authors were able to get down to 4.5o at full aperture and 3o with acceptable aperture loss. This did need an assistant to monitor the shade’s position and adjust as appropriate.
The authors make the point that a solid tube reflector has in effect a built in sun shield.
Certainly it is desirable to keep the Sun off the main mirror for all sorts of reasons. I have a couple of times felt it prudent to curtail observations due to smoke emanating from the end of the telescope tube where an off-axis image of the Sun was burning into the woodwork! Although disconcerting there was no danger to eyesight, being off axis there was no way that the Sun’s image could enter the eyepiece on a Newtonian reflector. This happens quite some distance from the Sun and is a timely reminder that this is close enough. Personally, with my setup, I would consider even 10o too close.
Unless you really know what you are doing like Chris and have suitable equipment, again like Chris, don’t do it. You only have one set of eyes!