Dave Groski's Solar Page

david dot m dot groski at usa dot dupont dot com

(see also http://www.spectrohelioscope.net)
DISCLAIMER : NOTE! Viewing the sun can be extremely dangerous! The information provided here is meant only as a description of what one or two people have done. The reader accepts all responsibility and liability associated with the use of any information provided here, as it is possible that important precautionary information may be left out. Neither Dave nor Matt nor anyone associated with them is responsible for damage resulting from using the information and ideas herein!
A quick note on units and usage ... Keep in mind that the promscope needs a filter with a bandwidth of 10 Angstroms or 1 nanometer. A nanometer (abbreviated "nm") is 1x10^-9 meters. An Angstrom (abbreviated "A") is 1x10^-10 meters. So an Angstrom is a 1/10th of a nanometer, i.e. quite a bit smaller. The point is that one cannot use a 10nm (100 Angstrom) filter sold by a number of suppliers. You need one with a bandwidth of 30 A (3 nm) or less to see the prominences.

Also note the difference between the 0.8 system (H-alpha) and the promscope (coronagraph). The promscope will block the disk of the Sun with an occulting disk (which allows prominences to be viewed along the limb) while the 0.8A system allows the disk of the Sun *and* the area around the Sun to be viewed. The 0.8 scope shows H-alpha surface features, like filaments, and flares, along with prominences along the limb.


H-Alpha 'Scope
Stellafane, 2004

H-alpha scope parts list :

My 0.8 angstrom band width H-alpha solar telescope, gives images just like any telescope that is equipped with a narrow band Daystar or Coronado filter. That is the full disk of the Sun is visible in H-alpha light with prominences visible along the limb and H-alpha features such as filaments and flares visible on the surface. The difference is that my unit uses two inexpensive interference filter that are tuned by tipping and tilting them to narrow the band width to approximately 0.8 Angstroms. The filters were purchased from Robert Johnson, President of Omega Optical. These were New Old Stock (NOS) which he was selling on Ebay. Contact Mr Johnson for prices and availability. There is a link to his Ebay store on this page. The filters are nominal 1.5 A bandwidth, with a center wavelength (CWL) of 656.3nm. This is slightly on the red side of the H-alpha line which is 656.28nm. This allows the filters to be tuned by slightly tilting them to bring the CWL to be centered at 656.28nm. Two of them are used in series to reduce the bandwidth. One filter is tuned so the CWL is slightly on the red side ( longer wavelength) of 656.28 and the other on the blue side ( shorter wavelength) The transmission curves of each filter overlaps. The overlapping section has a narrower band width then the two filters by themselves. (see drawing ). To keep the bandwidth as narrow as possible the filters must be in a light cone that is at least F/30 or slower. I choose to place the filters in parallel light for the smallest possible band width. This is done by using two 50mm x 170mm achromats. These are surplus binocular objects. The first lens is placed at it's focal length away from the main image plane of the telescope. This turns the light back into parallel. The light passes through both filters and then to the second lens, which refocuses it. A standard eyepiece is placed at the focus of the second lens to form the image of the Sun.The magnification of the system can be changed by changing eyepieces just as with a standard telescope.

I choose the binocular objectives because they fit nicely inside 2" PVC pipe. This makes mounting them very easy. They are held in place with Nylon set screws placed at 120 degrees centers, around the outside of the tube. The image of the Sun, passing through the lens, is much smaller then the 2" diameter of the lens. Only a small section of the center of the lens is used, were the optical figure should be at it's best. There is nothing critical about the focal lengths of the lens, just about any focal length will do. I would just avoid lens with fast F-ratios because they may not be as well corrected as a slower F-ratio one. Companies like Surplus Shed have an excellent selection of surplus lens that would work well. The main objective is a 60mm x 1000mm, air spaced archromat purchased from Apogee Inc. Since the seeing in the daytime can be poor I choose to use the 60mm aperture. I felt that it would give me a good balance between the resolution that the seeing would allow vs the heat load on the system and also the cost. I've used the same lens with my Promscope with excellent results. The lens is mounted in a metal cells that screws into a baffled metal tube. A metal dew cap screws onto the outside of the cell. Placed over the dew cap is an Energy Rejection Filter made from a 5 3/4" diameter Wratten #25A photographic filter. This a war surplus unit made by Eastman Kodak. One could also use Schott filter glass RG630 or RG645. I mounted the filter in a short section of 6" OD cardboard tube, with plywood disks mounted to the inside. This assembly then slips over the dewcap of 60mm lens. This purpose of the filter is to greatly reduce the heat load on the system. Both of the Omega units are fully blocked to Optical Density of 5, which reduces or removes the IR and UV to a very safe level. So in conjunction with the ERF, and the use of two interference units, the system is safe.

The tipping and tilting of the filters is the critical part of the system. The filters need to be tipped and tilted in two axes that are 90 to each other. You can think of the motion of the filter mounts as the same as that of an Alt/Az telescope mounting. As I stated earlier, the filters are made so that their center wavelength is set to be slightly on the red side of the H-alpha line. In this way a slight tilting of the filter can be used to exacting tune the filter for maximum transmission at 656.28nm. In my case, I want them to be tuned slightly on either side of 656.28 nm.





The filters are mounted in plastic grommets that are slightly smaller then 2" in OD. The grommets are normally used in electrical circuit boxes to allow wiring to be passed into and out of them without scrapping the insulation. The hole in the center of the grommet is 3/4" in diameter which works well with the 1" diameter filters. Holes were drilled and tapped around the outside of the 3/4" hole for 4-40 screws. The screws hold the filters in place. On the outside of the grommet, two metal stand-offs were epoxied at 180 centers. The stand-offs are 1/4" long and internally threaded. They are commonly used to hold circuit boards. Bolts pass through the 2" PVC pipe and screw into the stand-offs. The bolts allow the filter holders to be rotated and moved side to side. To accomplish this, two set of holes at 180 apart were drilled into the PVC pipe. The holes are 1 1/2" apart. On oneside of the PVC pipe, the holes were elongated about 1/2" with the elongation running parallel to the length of the pipe. The holes on the opposite side were only slightly elongated. Through these holes a bolt with a knurled knob passes and screws into one side of the filter holder. Rotating the knob rotates the filter toward and always from the objective lens. This motion is the same as a telescope moving in altitude. To move the filters in a side to side motion like a telescope moving in azimuth, bolts passes through the 1/2" elongated holes. Two small right angle brackets are attached to the PVC pipe, and two of the same type of brackets are attached to the bolts that passes 1/2" elongated holes. The two sets of brackets are attached to each other with a bolt and a spring placed between them. A knurled nut is screws onto the bolt which moves the filters toward and away from each other. The space between the two 50mm lens were the filters are mounted is lined with black flocked paper. A 2" OD rubber O-ring holds the paper in place and also fills the slight gap between ID of the 2" PVC and the OD of the filter holders. The O-ring ensures that only light passing through the filters reaches the eye.

TThe system is tuned by first starting out with both filters parallel to each other in both planes and at 90 to the incoming light. The front filter is slightly tilted ( Alt like motion) and the rear unit rotated slowly through it's range of motion. Look for prominences to appear along the limb of the Sun. Once they are seen, play one filter against each other while observing the surface of the Sun. The filters are correctly tuned in the one axis when H-alpha surface details become visible. The surface will look 'hairy'. At this time only a section of the Sun with show H-alpha detail. Now using the other the set of adjustments that move the filters toward and away from each other ( Az. type motion). Make a slight movement of the front filter and then slowly move the adjustment of the back filter. Keep adjusting the front filter in small steps and then moving the rear filter through it's side to side range of motion. You will see that the area of the Sun that shows H-alpha features will expand and fill all, to almost all of the complete disk of the Sun. Once the system is tuned only a slight adjustment will be needed from observing sessions to observing sessions.

The following is a parts list for my telescope.

     1 ) Mounted  60mm x 1000mm  from Apogee Inc      $ 25
     2 ) Metal Tube for 60mm lens from Apogee Inc     $ 10
     3 ) Metal Dew Shield for 60mm lens from Apogee   $  2
     4 ) Two 50mm achromats Surplus Shed  ( 2 x $7)   $ 14
     5 ) 2' of 2" PVC pipe                            $  5
     6 ) 2" PVC to NPT female adapter                 $  1
     7 ) 2" PVC to NPT Male adapter                   $  1
     8 ) 5 3/4" Wratten #25A filter Surplus Shed      $ 10
     9 ) Two 1" dia 1.5A H-alpha filters B. Johnson   $200
     10) Hardware,paint etc...                        $ 20

$300

The telescope was displayed at Stellafane in 2004 were it won an award. I use it every chance I get. It continues to show me a wealth of detail and just how dynamic the Sun is.


Copyright 2004-2005 Dave Groski. All rights reserved.