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The Baader Herschel Prisma
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Some time ago when I did review the Baader Mark-IV Coronagraph, I also mentioned the Baader “Herschel”,a fantastic solar accessory that I think needs a sole review.
Since I bought the “Herschel Prisma” as Baader names it, one wonders what a “Prisma” or prism has to do with solar observing, well as a matter of fact, the Herschel is a peculiar type of prism that reflects about 4.6% of the light you pass through one of the prism faces that is flat to 1/10 of a wave, the rest that is 95.4% of light and heat goes into the prism and exits through the other face and out the backdoor of the housing, thus the excess light and heat is dispensed and not used for observing.
Ever wondered why this accessory is also called the “Herschel Wedge”, well it is because this prism has a wedge form, it is a very narrow angled prism around 23º, compared to the common 90º prisms used as Star diagonals, so that is why it is also known as a “Herschel Wedge”.
This particular “Prisma” or prism, is made with a proprietary glass and was made by Zeiss as described by Herschel, the reflecting face is as I said before, 1/10 of a wave flat, that means that it is a very precise optical surface and therefore it does not distort the image reflected from it. The exit face of the prism is antireflection coated so it does not reflect back any light that could produce ghosts in the eyepiece or heat inside the telescope tube.
Some people think that a Herschel Wedge is a very dangerous accessory to observe the Sun due to the UV and IR radiation, but I think they are wrong, because you have to remember that the deep UV radiation does not pass through a refractor lens because it is absorbed by the glass, but lets assume that a small percentage could get through, well that is taken care by the type of glass used in the prism and also by the eyepiece , the same happens with the IR radiation.
It is important to say, that the Herschel Wedges are not intended to be used with Schmidt Cassegrain or Newtonian telescopes, the reason is that those optical systems have primary mirrors with highly reflecting surfaces that reflect the UV and IR radiation, this last one can crack or unglue the secondary mirrors and even break the corrector plates in SCT systems due to the heat generated.
Maksutov designs on the other hand, have a thick Meniscus corrector lens that absorbs those radiations as an objective lens does too.
Aside of the almost null radiation concern when using a refractor telescope or a Maksutov, Herschel Wedges are known to be dangerous too because on the exhaust side, that is the backdoor of the housing, the light and heat comes out to a focus and yes, you guessed it, it can burn just like a magnifier lens when pointed at the Sun. As a matter of fact, I have burnt the cable of my focus control handpad several times, not too bad in my case because it just has been on the rubber jacket of the cable and the dark smoke warns you quickly, but if not taken care off, it can burn and could start a fire in the worst scenario.
Here Baader has come with an elegant and simple solution, no more light and heat out the backdoor, how? with a “Light trap” that Baader designed, it consists of a multilayered metal sheet made of dull blackened stainless steel, featuring thousands of tiny holes that dampens and dissipates the light and heat trapped in the layers. I have tried it and believe me no heat and no light reflected to harm or burn out of the housing. That finishes with the fire hazard and related problems that plague the Herschel wedge, you can even glimpse inside the exhaust port and you see nothing that can harm you.
With the hazard problem being resolved, now let me explain you what comes with the Baader Herschel Prisma kit.