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One Of The Largest Spectrohelioscopes Around

I didn't know what a spectrohelioscope was, but I knew I wanted to view solar prominences, as well as details such as sunspots on the disk. I tried various avenues, and decided after doing research that the spectrohelioscope was the way to go. I had done several telescope restorations, and decided that although I do embrace new technology, I like the "old stuff" the best. Most of the literature I found was from the 30's or so, until I found a reference to Fred Veio's book. I sent off and got his first book, and spoke with him about my hopes to build one. He very graciously offered plans, details, and ideas. I wasn't sure what size or type I would make. Then I happened to get a most unusual item -- a 10" Gaertner coelostat from the 30's. It was missing several things, including the flats, but the basics were there. I knew then that my spectrohelioscope would be a permanent one, and a large one at that! I decided to make the Hale type, and my quest for parts was on. I started looking and found a Perkin-Elmer 6" f50 lens, parfocal to the “C” and “K” lines  then a pair of matched 6" f27.5 mirrors. A Bausch & Lomb 4" square grating on a 6" flat followed, and I was well on my way. I tried several methods of making slits, and ended up making several that didn't work very well, and a few that work fine. One of the problems I ran into was just how to set up the coelostat. It had originally been used on a roof, which meant that it sent the reflected sunlight downward, but it was not feasible for my facility, so I had to make some modifications to use it horizontally. We have since moved, and now the coelostat directs sunlight downward again. 

Details of operation:

The primary (10") flat of the coelostat reflects sunlight (parallel) light to the second (8-3/4") coelostat flat. This reflected (still parallel) light is sent downwards to an 11" flat, then reflected to the 6" F50 lens. The light then travels 25 feet to form an image on the first slit. Just before it hits the slit, though, it goes through a rotating square prism. One of the 6" F27.5 mirrors is focused on the back side of the first slit. It takes the image and turns it back into parallel light, reflecting it on the 4" grating. The grating reflects whatever wavelength you want to look at, which is accomplished by moving it left or right. The second mirror is focused on the grating, and takes the parallel light and turns it back into an image, reflecting it to the back of the second slit. On the front side of this slit is the second square rotating prism, then an eyepiece which is focused on the slit. I can control the motors for guiding, focus, and change speed of the rotating prisms all from the eyepiece. This took some extra wiring and switches, but makes it useable by one person now. I also have the two telescopes in the cage assembly which were originally mounted on the ceiling, but my building is not that tall, so I put them in the light path. You can rotate the whole 2-telescope assembly in or out of the light path easily. When out, the spectrohelioscope operates as described above. When in the light path, the one telescope provides a white light solar image on the wall or a screen. The image from the other telescope goes into a two-prism spectroscope and projects a "rainbow" image on the wall or screen.

What happens:

Sunlight is divided into many different wavelengths by the diffraction grating. By moving the grating, you can see whatever wavelength you want to view in the eyepiece. The rotating prisms simply build up an image, much like television camera makes a "picture" by scanning several hundred lines, one at a time, then projecting them in sequence on your screen. This happens so rapidly that your eyes make it seem like you are looking at one picture. The slits are only opened a thousandth of an inch or so, so you would see a very tiny slice of the sun at one time were it not for the rotating prisms. The sunlight hits the two prism assembly and is broken up into the various colors, kind of like when you look at a rainbow. 

Shed on rollers to house coelostat




One of the 6" F27.5 mirrors


Both 6" mirrors, showing adjustable base. Base moves both mirrors at once; motor on left moves only one mirror.


Rotating prisms


Prisms and 3" diagonal, along with eyepiece


Back side of cell holding grating showing micrometer adjustments


Slits. I used scraper razor blades, held in place with magnetic strips. They are easily adjustable. Also they can be rotated by loosening the thumbscrews on bottom.


Spectroscope assembly. The 2 prisms are visible.



12" flat in mounting. I motorized it so I could scan the surface of the sun.  


6" prism in light path to invert the image so it would be right side up. 


3" diagonal - goes between the rotating prism and the eyepiece.