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
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
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.
A very nicely made almost cubic housing that contains
the Herschel prism, built of metal and beautifully crafted, sporting
a shroud opposite to the 2” outside diameter chromed plated
tube that connects to the telescope. At 90º from the connecting
tube you will find the 2” diameter eyepiece or accessory holder
that has 2 knurled screws situated at 90°. Inside the eyepiece
end of the housing, you can also find the ND-3 filter screwed to
it. (This filter is obligatory to use, except on special situations).
Four 2” optically flat to _ wave neutral density
filters, the obligatory ND-3, an ND-2, an ND-0.9 and the ND-0.6
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
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”
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
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
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
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)