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This post grew too long for one blog page so I have split it it into Pt.1 and Pt.2.
The open bottom of the Lacerta prism housing is not the usual open window of the classical Herschel solar prism. From which a terrifying beam of transmitted heat would exit. Enough to burn a track across the lawn beneath the telescope. Or to cause severe burns to any unfortunate skin passing below the open prism!
Well
worth bearing in mind if children circulate around a telescope used for
solar observation. Or a solar instrument merely standing outside in daylight for adjustment. A
classical Herschel prism would be a very poor choice for "public
outreach" use without very careful and continuous monitoring.No telescope should EVER be left unattended in daylight because the obvious risk of instant blindness! Even when it is overcast the sun could easily come out just long enough for an innocent to point it at the sun. With terrifying long term effects from instant blindness! The human retina may recognise brightness but has no pain receptors for invisible infra red and ultra violet. The blindingly bright, perceived brightness could be very short lived indeed for the unwary! Imagine the effects of a child or toddler looking upwards into the fiercely concentrated beam of a classical Herschel prism!
Fortunately
the Lacerta's base is only open for better air circulation. Though four
small screw holes are present in the base rectangle of the sturdy
construction. Presumably these could be used to keep inquisitive fingers
well away from the naked prism with the addition of a metal [?] mesh cover. A solid plate closure would probably be a bad idea. Possibly leading to overheating of the housing.The intense, and focused, solar heat from the Lacerta prism, is instead fed to the inside of the milled and cubed, heat sink on the back of the housing. Though there is no obvious sign of a specially absorbent surface just there.
Online user reports suggest that the Lacerta heat sink does not become as hot as the more expensive competition in other closed [heat sink] Herschel prisms. There are ample images online of owners putting their hands under the Lacerta's open base. Just to confirm the safety of Lacerta's solar prism design.
Their
2" prism size is recommended by Lacerta for larger refractors provided a
UV/IR blocking filter is also employed. No doubt anyone could fit a
small computer fan over the heat sink if it should become dangerously
hot after many hours of observing. Which could easily happen during a
transit for example.I shall report back on my experience with the heat sink. I have a remote thermal sensor "gun" to monitor the exact temperature. A device which is usually used to check home insulation [or more usually] a lack of it. The price of these laser guided sensors has dropped dramatically of late.
There was always a tendency to presume that Herschel prism owners were more serious solar observers than those who employ the much cheaper solar film filters. Perhaps it is rather like new hifi components bringing rapture to the owner. But most Herschel prism owners do seem to think they see more surface detail than with the Baader solar foil. Others are more doubtful of any real difference.
I took the solar prism housing outside and photographed it in the even light of today's overcast. This thing is huge
and very sturdily built, it will require considerable balancing on most
equatorially mounted refractors. Even my relatively large, 7" f/12 will
need some careful longitudinal balancing. With three, sliding, tube
balance weights mounted on rails already I should be able to cope. Some people imagine the rather weird exit angle of Brewster Angle prisms will mean aching necks. However, one would need a very lowly mounted refractor to cause such problems. At 55N the sun never exceeds about 57 degrees altitude and then only for while each day. So I can't imagine this being a great difficulty compared with the more normal 90 degree devices. Those further south will just need a lower seat or a taller pier. I haven't measured the exact angle in the image but it is nearer 90° on the [capped] telescope side. The eyepiece angle is still manageable IMHO.
Having read the warnings on the Lacerta web page I decided I'd like the safety of a UV/IR filter to cope with my 7" refractor. TS offer a cheaper filter in 2" than Baader and others. Which can be left in the prism housing instead of changing a 1.25" filter with each change of eyepiece or the addition of a camera. Cameras are more sensitive to UV and IR well beyond human vision. There is always the nagging doubt that long term IR/UV "cooking" of the MkI eyeball is going on unnoticed. Better safe than sorry.
Click on any image for an enlargement.
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