White Light Solar Newtonian


 By  David Groski




 Most solar observers start out by using a modern metallized plastic film

filter to fits over the front of their telescope. This method offers excellent

image quality, for little expense. When used properly the technique is safe. I

used this method for years and was very pleased with the results.  As with any

other type of specialized observing, having the equipment optimized will allow

the observer to obtain the best possible image. With this in mind I set out to

design and build a telescope for high-resolution white light observing. Since

solar observing can be very dangerous, it was imperative that the telescope be

inherently safe to use. I planned to use it at public star parties and I did

not want to worry about having a filter come loose or any other possible way that
 someone could damage their eyesight.




 I looked at a number of designs. The solar Dobsonian looked very appealing.

The system used a large aluminized window placed at 45°. The window acted has

both a filter and a diagonal mirror for the Newtonian configuration. Two things

concerned me about the design. The optical quality of the window needed to be

high and it would difficult and expensive to find such a piece of glass. The

window needed to aluminized and the coatings could be damaged.  The solution

was still to use the Newtonian configuration but to have all the optical surfaces
uncoated.  For safe viewing, the image of the Sun needs to be reduced by 100,000
to 1 in brightness across the full spectrum. This is an Optical Density of 5

( LOG(100000) = 5).  Bare glass reflects about 5% of the light striking it. This
translates into an OD of about 2. The two reflections in a Newtonian would

results in an OD of 3.5. Still too bright for comfortable viewing so a third

reflections is needed. I used a homemade Herschel wedge to achieve this. I also

took advantage of the fact that when light is reflected off of bare glass at an

angle it becomes polarized. The closer the angle of reflections gets to the Brewster
angle of 57 degrees, the more complete the polarization. With both the diagonal

mirror and Herschel Wedge placed at 45 degrees the light becomes crossed polarized

and the image brightness can be farther reduced.  Then Herschel Wedge is rotated

so the eyepiece is placed parallel to the main telescope tube, the image would be

at it’s brightest. When the Herschel Wedge is rotated so the eyepiece is at right

angles to the telescope tube the image will be at it’s dimmest.  By simply rotating

the Herschel Wedge I can tune the image brightest to fit the sky conditions,

magnification and color filters used. Note that none of the optical components have

concentrated Sunlight hinting them. This greatly reduces the chance of failure

caused by heating. In the unlikely chance that any one of them would fail, no direct

Sunlight could reach the observes eye.




 High quality observations require high quality optics.  The weakest link in the

optical train will most likely be the solar filter. While modern day plastic film

filters have good optical quality they are still below that of a well-made telescope.

One is not going to achieve the highest quality image be placing a filter with a ½ wave

of error, in front of the objective that has been corrected to 1/10 wave or better.

Most serious solar observes use a high quality refractor and either a high quality

metalized glass filter or a Herschel Wedge with farther filtration. A glass filter of

true high quality will cost a few hundred dollars. A Herschel Wedge can be less

expensive and still be of very high quality but when used with a refractor has a

safety issue in that an intense light beam exits the back of the wedge. One needs to

be aware of the present of this beam so one does not get burned. Also the wedge itself

is placed near the focal point of the telescope and sees direct, partially focused

Sunlight, of great intensity and heat. This may cause the optical surface to warp, and

also heat waves to be generated in the optical path. Wedges have also been known to

crack from the heat.  In my solar Newtonian design, none of the optical surface see

any concentrate light so there is no danger of them failing. Since the light has also

been greatly reduced in intensity, distortion of the optical surfaces by heating is

not a concern.



 I choose a 4.25” f/10 mirror for the main objective. These are commonly available

for around $50 or less. Most of them will have a spherical surface. At the focal ration

of f/10 and an aperture of 4.25”, if left spherical, the residue spherical aberration

is under ¼ wave.   The figure of the mirror can easily to be tested to confirm its

quality. To farther correct the surface does not require a large effort. A few minutes

of polishing are all that is needed to correct the surface to a parabola, which I did

with my mirror. Since I would be using a Hershel Wedge with the system I needed more

of the optical path to extend outside of the telescope tube then what is normally done

in a standard Newtonian configuration. Having an F-ratio of 10 or higher allows for

the additional distance while still keeping the size of the diagonal small. The aluminum

coatings on both the secondary and primary mirrors were removed using a Ferric Chloride

etching solution sold at Radio Shack for making copper circuit boards. I used a small

elliptical diagonal mirror with 0.75 minor axis. For the tube I used a standard 6” heavy

walled mailing tube which is commonly available and inexpensive. Mine was rescued before

it was discarded into the trash. Cardboard is an excellent insulator and at this size,

fairly light.  I wanted an oversized diameter tube, since any heat generated in the

optical path would tend to stay along the walls and out of the optical path. The mirror

cell for the primary holds the mirror only along the outer edge and is open in the

middle. This allows the light to pass through the mirror. The back of cell is a closed

design.  I wanted a light tight cell so no scattered light from the ground could enter

the optical path and reduce the contrast. In the center of the cell is mounted a fan

which pulls air down the tube and exhausts it out the back. A cover is placed over the

back of the fan and spaced about ¼ off of it. The cover allows the exhaust to escape but

stops any light from entering.  The secondary is mounted on a ¾ dowel. A large diameter

hole was bored into the center of the dowel and acts as a light trap as most of the light

that passes through the uncoated secondary. The Herschel Wedge system was made from

a 1 ½” PVC pipe tee. A short section of  1 ½” PVC pipe is attached to the main telescope

tube. One end of the tee slips over this section and allows the tee to rotate. Nylon set

screw allows the tee to be held in place. In the center outlet of the tee is mounted

a 1 ¼” female sink trap adapter. A standard 1 ¼” eyepiece fits nicely in the adapter and

allows for push/pull focusing of the eyepiece. On the outer outlet of the tee is short

section of 1 ½” is inserted and cover with 1 ½” PVC pipe cap. In this short section of

pipe I glued in another section of 1 ¼” PVC that I turned down to fit. The end of section

has been cut at 35° angle on it is mount the glass Herschel Wedge. The wedge angle is

10 degrees so when mounted on the 35° cut, the front surface is at 45°, and directs the

sunlight into the eyepiece. Since the wedge is mounted on a section of pipe the opening

again allows light to pass through and acts like a trap. The wedge is one that was once

sold by Edmund Scientific for many years (G30,265) and referenced in ‘All About Telescopes’

by Sam Brown. The wedge was 40mm x 55mm but it had some chipping along the bottom. I

ground it down into a 35° ellipse and which fits nicely on the end of the 1 ¼” section of

PVC. I also checked the front surface and found it to be around 1/10 wave flat. One could

also use a Newtonian diagonal in which the mirror coating is removed and grind a wedge

into it and them polish the back.




 The ‘scope has been a joy to use. It consistently shows me more detail and then

any of commercial white-light filters I have.  The Transit of Venus was a memorable

sight and received many very positive comments on the image quality. Solar granulation

is almost always visible. Sunspot at high magnifications shows a wealth of detail not

visible in other telescopes. I especially like the ability to tune the image brightness

by rotating the Herschel Wedge to adjust for the sky conditions, magnification and

auxiliary eyepiece filters used.