The Baader Herschel Prisma

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.

I suggest buying the 2” to 1.25” Baader reducer that has a nice feature, it has an added T-2 thread at the eyepiece end, so you can adapt it for eyepiece projection, this thread is protected by a small ringlet that can be taken off and be used as a rotating T-2 locking ring.

The eyepiece end of the Herschel housing as I said, comes with 2 knurled screws situated 90° apart, you can insert an 2” eyepiece or any 2” accessory and it stays incredibly secure with no wobble at all, even the 2” to 1.25” adapter sports the same type of set screws, it is a delight to use.

Now lets see how the “Herschel Prisma” should be used.

1- Without pointing the scope at the Sun, connect the Herschel to the telescope making sure that the ND-3 is screwed in place.
2.- Insert your picked 2”eyepiece on the eyepiece holder, screwing on it the ND-2 or ND-0.9 or ND-0.6 filter, depending on the aperture and focal ratio of your telescope, or if using an 1.25” eyepiece you can screw the filter to the 2” to 1.25” adapter.
3.- Make sure that your regular finder and or guidescope have their covers on, and then point your telescope at the Sun using a safe solar finder
4.- Focus the Sun and enjoy the vista. If you find that the image is to dark, take off the ND-2 and use either the ND-0.9 or the ND-0.6
5.- Now, if you have at hand a single polarizing filter (cheap and easy to get in a camera store), insert it instead of the ND-2 or the ND-0.9 or the ND-0.
6.Why?, because you can benefit with another of the Herschel prism properties, it reflects polarized light, so if you rotate a single polarizer filter fitted to the eyepiece and or to the 2” to 1.25” adapter, you can dial the Sun brightness until the image suits your taste, really another incredible feature, just make sure before observing the Sun, that you rotate the polarizer up to the point where the Sun looks darker looking at the eyelens from a few inches back.

What I do when visual observing with my 6” f/12 A-P triplet refractor at full aperture, is to use the obligatory ND-3 filter, plus the polarizing filter and sometimes instead of the polarizer, either the ND-2 or the ND-0.9 depending on the contrast I want and also if imaging.

Now, if you want to use a Barlow or a Powermate or eyepiece projection, then you have to install them after the ND-3 filter, that is at the eyepiece end of the Herschel Wedge.

As for imaging, I use the obligatory ND-3 and an Edmunds 520nm (10nm) filter for granulation detail, the Sunspot shown was taken with this setup using a 4X Powermate.

Baader has comed up with a new redisigned foolproof “Herschel Wedge”, that aside of sporting the “Light Trap”as a standard feature, it also has a non removable ND-3 filter, so there can not be an accidental burnt Retina if you try to look at the eyepiece end with no filter on. (Remember that 4.6% of the light and heat of the Sun is reflected and that amount can burn a Retina)

By the way, I have never needed to take off the ND-3 filter, so the newer version of the Herschel is safer than the old one in this respect.

So as you can see, the Herschel Prisma by itself is not a hazardous accessory to use to observe the Sun in white light, nevertheless we always have to take in account and not forget that the Sun is a dangerous object to look at, even if baresighted, so if you take care to use the Herschel as directed and use just plain common sense to observe the Sun, you will not have a problem at all.

I can say that Baader has comed up with a design that makes the new Herschel Wedge the safest one available and not only that, because now the older type of Baader Herschels can be retrofited with this new light trap, to make them as safe as the new ones.

You may ask, why use a Herschel if I can use the Baader film or any other type of solar filter made of glass or Mylar film?, well, because if you are a serious solar observer or want to be treated to the best possible Solar image in white light, then the Herschel Wedge is the way to go.

Thanks for your attention.
Eric Roel. (México)
http://voltaire.csun.edu/roel/roel.html