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I would like to use an 80mm refractor for public outreach for white light solar observation.
The telescope will track the Sun, potentially for several hours. In that time hundreds of people would observe the Sun. This includes large groups of school children. It will be supervised, of course. Though excited 9 year olds are quite lively!
Note that my primary concern is safety. I am not worried about small differences in the quality of the views provided.
I have posed the question to the Solar WhatsApp group which method is safest in this use situation. Among the helpful replies, three options have arisen:
1. Using a full aperture Baader film filter. This was my initial plan. But I am concerned about it being knocked off or pinholes appearing (the device would fit over the dew cap, held in place by screws, but not rigidly attached – taping in place each day is probably not conducive to long term use).
2. Using a Herschel wedge. This strikes me as a safer option and better from an engineering perspective as the wedge can be firmly attached to the telescope. My residual concern is that after several hours, the housing might get hot, presenting a burn hazard or be damaged in some way. Does anyone know whether these concerns are borne out in reality?
3. Using a projection box. This is probably safest of all, though my experience is that the image is often too dim for casual observers. It also doesn’t give the experience of observing THROUGH a telescope (similarly, Smart Scopes or cameras on the back of the telescope are not options under consideration).
Any views based on experience? Any risks assessments that can be shared?
Modern Herschel wedges use a heat sink dissipate the unwanted energy. I have used one for public demonstration and mine (1¼” fitting) doesn’t get appreciably warm even after serveral hours use, although my aperture (63mm) was somewhat smaller then your 80mm. Is your Herschel wedge an 1¼”or 2″ fitting? 1¼” wedges are OK for apertures up to 100mm, whereas 2″ ones are OK up to 150mm aperture. Perhaps a 2″ one would be better for you as it will have a bigger capacity heat sink.
I find the image produced by Herschel wedges too bright for comfort so always use a post wedge filter to give me the image I want.
There are a couple of other options for solar viewing.
My favourite is a long focus mirror fed by a driven flat mirror. My version had a 4 inch mirror of 47 feet focal length which gave a solar image 5 inches across…i.e. bigger than the mirror! So although the sun if focussed the surface brightness is less than that of direct sunlight. Sorry I don’t have any images but there was a short piece in Astronomy Now, November 2008 page 75. Not an original idea by me…Hale made a 100 foot solar telescope which was described in detail in volume 1 of the Amateur Telescope Making books.
The other option is to have a heliostat feeding light through a single lens (no need to be achromatic) with the image viewed through an H alpha filter as usual. A nice example is shown in Sky & Telescope, August 1980, page 162. The 24 foot focal length lens gives very detailed views of prominences etc.
These set-ups are obviously much more involved than conventional methods but would make nice projects for astronomy clubs. They have the advantage of having all the optical elements on show so people can see how they work.
Mike.Mike, both of these sound interesting. You should write them up for the solar sections newsletter or the journal. I’d be interested to see pictures too.
James,
A couple of photos of my “50 foot” solar telescope. One looking out at the 4 inch, very long focus mirror(!), and the other looking back towards the house where the solar image falls on the wall in a darkened room for ease of viewing.
Mike.
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