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Disclaimer: All the how-to directions below are for information only. They are not intended to be sold or used for unlawful purposes. The author takes no responsibility if they cause the reader bodily harm, loss of life, or cause the damage of any property, person, dog, cat, or any other animal. They are presented for information only and all liability is on the part of the reader.
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Building a simple Herschel Wedge
By Larry Alvarez
Purpose:
The purpose of this how-to is to demonstrate the building of a simple Herschel Wedge for solar imaging. There is a lot of controversy when it comes to wedge prisms used as solar viewers because they channel the mighty unfiltered light of the Sun. As such they channel not only the visible light spectrum but also the invisible IR and harmful UV light that you cannot see. There are also a lot of horror stories about them because the exit beam is focused sunlight and it usually ends with someone getting burned. No one wants to lose and eye or burn their leg off so if you are intimidated by that sort of thing then turn away. The truth is this is a dangerous world and everything used in an uncontrolled way or without preparation can be harmful or deadly. This project is one that is being presented for information only. A miscalculation or alteration of the process could result in permanent eye damage. Any harm done to you, your scope, your surroundings is strictly your fault. This how-to is for informational purposes only.
Understanding the basics:
A Herschel wedge works by channeling light through a wedge prism. The prism typically has a small angle to it around 10 to 20 degrees. 95% of the light exits out the other end of the prism. The other 5% gets reflected off the surface. The reflection is what we are interested in. 5% is still plenty to burn your eye out and leave you blind so it is necessary to add some filtering to the beam to bring it down to viewable and photographical range. A Herschel wedge provides a true whitelight image of the Sun. Because there are no filters at the front of the scope the image is more pure than ones that use Baader film or Thousand Oaks solar film. The image also does not contain the distortion inherent with the film type solar filters.
Parts List:
Wedge prism with 20 degree angle
ND 4 or ND 3 blocking filter
Oak Veneer
Small Oak panel 1/4" 4x2
Celestron 1.25" Star Diagonal
Small Mirror
Tools needed:
Razor blade cutter knife
Iron
Glass cutter and glass pliers to break glass
Jigsaw or bandsaw
Small drill bit
Brass nails
Stain
Wood Glue
How To: Below is a mock image of what I want the wedge to look like. I could not find any designs on the web so I made my own. They all show you the outside view by none show you how the wedge is oriented or how the thing actually works. I hope to clear it up so that you fully know and understand what's going on. From the image below we can see that I originally drew up the wedge to have an aluminum skin. This proved to be more trouble that it was worth. I changed it to Oak for the look and feel of the Old World from which the wedge was invented, that, and it was just plain easier to work with wood. I did not put the filters down that are included in the parts lists because they were not available at the time I build the wedge. One thing is certain. The ND 3.0 filter on the exit beam is a must if you are going to do visual work. I would also suggest a Baader UV/IR cut filter on the front of the wedge input and a variable polarizer on the eyepiece to dim the view.
Below is a schematic drawing of the parts for the wedge. I measured them out in millimeters. The front panel and partial bottom panel will be added near completion. They can be trimmed to size because they are made of thin oak veneer and do not need to be critically measured.
These panels will fit over a Celestron 1.25" or Orion 1.25" diagonal housing. The pieces should be cut out and set aside for now. The side will need to be cut out 2 times. One for the left side and one for the right.
We will next disassemble the Celestron Wedge for assimilation. Start by unscrewing the 4 bottom retaining screws. The back will come off and will allow you to remove the 90 degree prism or the mirror. In the first image below we have a mirror instead of a prism and in the image under that we have a prism diagonal. We could use the mirror/diagonal later so put it aside for now. If there is a prism inside it will be held by two tap screws on the side of the Celestron diagonal. Loosen these with an allen wrench and slide the prism out. The prism inside can be saved for a future project but is not need in this case.
Now its time to put the wedge housing together. Start by matching up the sides with the top. I like to place the side together and then sand each side until they are perfectly alike in size and angle. This is easily done with a belt sander or a block of sand paper. All you have to do is clamp the two sides together and sand them until the edge match. This provides better fitting pieces.
I used a small drill bit to drill starter hole in the top of the wedge housing. Then I cut out the top hole with a Dremel tool. You could also use a Rotozip or Jigsaw to cut it out. The Rotozip and Dremel work best because they work faster. It may be difficult to cut out the top with a jigsaw and not break the top piece at its smallest side.
After the parts are cut out and well sanded. Stack them together to check the fit. They should fit together well. The backing was purposely measured large so that I could sand the angle needed to match up the top to the back. I drilled starter holes for the brass nails on the top portion of the wedge housing and on the back. Be careful with the top holes near the back and the first set of back holes. One nail will be going vertical and the other will be going at an angle. Space these out so that they do not hit internally. The starter holes should be drilled about 80% the length of the nail. You want to be able to nail in the last 20%. This is necessary because the wood is only 1/4 thick and trying to nail through it without a starter hole will crack it. Before setting the nails in place, put a small bead of wood glue on the edges that touch. Nail the parts together and your housing should look like the one below.
At this step you should check the fit of the diagonal after the glue dries. It should slide right in snuggly if your measurements were correct. Below is an image with the diagonal in the wedge housing with the prism in place. The prism is wrapped in optical tissue for now.
Its now time to put some finishing touches on the housing. We need to cut out the front face from the veneer and also the partial bottom. Since the veneer I got has some hot glue on the bottom we will iron these in place and then sand them to fit perfectly. I used a razor to cut out the front 1.25" hole in the veneer. I then put the piece on the wedge and marked the areas to cut it to make it match. I removed it and cut the excess off with a pair of scissors.
Next I used a hot iron to iron the pieces in place. Veneer is easy to work with and if it comes out wrong you can put an iron to it and remove it pretty easily. There are about 2 to 3 seconds after the iron is on the Veneer that you can move it around slightly before the glue sets. When the veneer is set and dried you can sand the housing to remove the rough edges. At this point you can also stain the housing with whatever color stain you like. In my case I like American Walnut stain. Its got an old world look about it and brings out the natural wood grain.
While the housing is drying we can cut the mirror. Start by measuring the slot in the back of the housing. Mine measured 80mm x 44mm. The mirror I got was enough to make 4 pieces. So I measured out 80mm x 43mm 4 times and drew them on the mirror.
I then moved the mirror to a flat surface and aligned a straight edge to the lines drawn on the mirror. These are only my 5th and 6th time to score glass and I am now being pretty good at it. I found that spraying the glass cutter with WD40 prior to cutting makes it much easier. After aligning the straight edge put the score tool at the very end of the line and put pressure on it. Drag it across the mirror with equal pressure until you reach the other side. You should hear the glass cracking underneath the scribe wheel. When its done take some glass pliers and snap the glass along the scribe line. The line should crack straight. Do this for the rest of the lines.
Check the mirror fit in the wedge after it is cut. If its too wide you can use some wet/dry automotive sandpaper and some water to sand the edge to size. Be careful of the glass edges, they are razor sharp.
More than likely the mirror will have finger prints and glass particles on it from the cut. Take the mirror and put a drop of hand soap on it and rinse it off under warm water. Dry the mirror with a paper towel and be careful not to touch the surface. The mirror should get reasonably clean. Set it aside in a paper towel to let it dry.
It is now time to cut the wedge base and glue the prism to it. Take the diagonal base and draw a line as seen in the image below.
The base should be sanded flat after the cut and then the small end of the prism should go toward the side of the prism base that has the top tube that allows the eyepiece to be inserted. I use 4 drops of hot glue to hold the prism in place and I used a large glob of silicon in between the two drops on each side to glue the prism in place. The prism should be aligned such that it does not stick up too far. If you good at scribing and cutting glass then the top excess part of the prism should be cut off. After the 4 drops of hot glue are on the prism check its fit in the housing. If it does not fit good remove the drops and try again.
After the silicon dries on the prism you should put a couple dabs of silicon rubber cement at the front of the wedge housing. It will be tough getting in there but its do able if you use a dab on your finger. Then put the diagonal with the wedge attached through the opening on the bottom of the housing. It should go in at an angle and then slide into place. The silicon will cure and hold it rock solid in place. The front of the diagonal and the top of the diagonal should be put on at this time.
One more step is to attach a small mirror to the back end of the wedge. This is critical because it is there to help exhaust the 95% beam from the housing. The mirror should be cut to size and fit in the back area of the wedge as seen below. I used an old makeup compact mirror that I found to make the mirror on my first wedge and I used a camping mirror to make the mirror for the second wedge I made. The mirror can be glued in place with silicon rubber.
Looking inside the wedge we can see the prism and mirror assembly.
Below is the first test of the Herschel Wedge on one of my scopes.
Safety: Below are some images that I took during the test. Better ones will be added as the weather cooperates. I found that using a wedge is an acquired taste, like fine wine and fish sticks. It takes some serious filtering to produce the images like the ones I have taken below. The wedge as it was created above cannot be used for visual use directly without a UV/IR filter in the optical train and an ND 5.0 filter prior to the eyepiece. The images below were shot using a UV/IR filter on the front of the wedge housing, and a Baader K-line stacked with a Violet 47 filter prior to the camera. The wedge described above also has an open bottom that is exhausting a 95% beam toward the ground. Anything under this beam will get burned if your not careful. The beam comes to a focus in mid air and not on the ground so there's not chance the grass will catch fire but you should be mindful if your using it in this configuration. The second part to this project will be focused on making the wedge safer for visual use and imaging use.
To be Continued.......................................
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