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Posts Made By: Dick Jacobson

March 25, 2007 01:32 AM Forum: Telescope Making

Multiple primary reflector

Posted By Dick Jacobson

I believe all of the multiple or segmented mirror telescopes employ control systems that continually adjust the mirror positions using computers and piezoelectric crystals. The problem is that the frames holding the mirrors are nowhere near stiff enough to keep them positioned within a fraction of a wavelength. My impression is that this would be a real nightmare to implement on an amateur sized telescope, but with a large enough capital investment and all the recent advances in optics and computers, maybe Meade or Celestron will take up the challenge.

May 5, 2007 07:15 PM Forum: Equipment Talk


Posted By Dick Jacobson

After adding a ring bearing to make a rotating eyepiece section, PLUS a binoviewer, PLUS a lead-acid battery on the opposite side from the binoviewer to balance it on the ring bearing, I needed a LOT of counterweight on the bottom end of my 14" scope (about 15 lbs). At the bottom end I added a cooling fan/battery assembly which also has some 5/8" bolts, nuts, and washers for precise balancing. I split the Sonotube into two pieces but even at that it's near the limit of what I can lift.

On my new 20" truss scope, I planned from the beginning to use a removable fan/battery/counterweight assembly behind the mirror. The mirror box is a few inches too short for proper balance by itself, but the fan/battery/counterweight balances it perfectly.

When not using the binoviewer, I substitute a light shield assembly that hangs on the eyepiece drawtube making the telescope balance the same as if the binoviewer were in place.

I have also experimented with a "clothesline counterweight" made from a loop of cord, a pair of pulleys, a pair of S hooks that hook on either end of the tube, a spring, a turnbuckle to adjust tension, and a small weight.

It's also possible to balance the scope using springs, but you have to design it carefully so the springs exert the right amount of force at all angles.

June 12, 2007 06:43 PM Forum: Telescope Making

Cast Aluminum Rotating Rings

Posted By Dick Jacobson

Here is what you want: . For more choices of large Lazy Susan bearings, look up and search for "Lazy Susan".

I have also made a large rotating bearing, about 24" ID, out of 3/4" x 1/8" aluminum angles. I built a ring roller using three polyurethane keel rollers (used on boat trailers). It works well but was quite a bit of work to build it.

August 1, 2007 07:33 PM Forum: Equipment Talk

Rich field scopes - nobody consider Newtonians?

Posted By Dick Jacobson

Several years ago I did a side-by-side comparison of 25x100 binoculars versus a 6" Mak-Newt at low power. I don't remember what eyepiece I used in the Mak-Newt. This was from a very dark sky site. My conclusion was that the binoculars offered better views. The view of M31 through the 25x100 binos and a very dark sky was one of the most unforgettable astronomical sights I've ever seen. I think giant binos are a great complement to a large telescope.

August 14, 2007 06:33 PM Forum: Reflectors

Dob Tube Sliding Weight

Posted By Dick Jacobson

Another solution, which I've tried, is a "clothesline counterweight". Hook a pair of pulleys and a piece of cord onto your tube using S hooks. Maybe include a spring and a turnbuckle to adjust tension. The beauty of this solution is that you can quickly adjust balance without moving away from the eyepiece.

An unexpected benefit is that the counterweight acts as a vibration damper. If you "thump" the tube, the counterweight starts oscillating, absorbing vibration from the tube.

August 22, 2007 06:48 PM Forum: Equipment Talk

A couple quick Feathertouch questions

Posted By Dick Jacobson

To answer your second question, yes, you can orient the focuser any way you want. I did exactly what you are proposing on my 20". It has an unusual rotating secondary cage and I wanted to be able to reach the focusing knobs from either direction, so I oriented the focuser so that the focusing knobs are on the bottom when the scope is pointed to the zenith. I used the flat mounting base since it is on a flat board.

As for the first question, I'm not sure, I'll have to look at it when I get home.

August 30, 2007 06:56 PM Forum: Equipment Talk

So what's the next "big thing"

Posted By Dick Jacobson

How about a "captioned eyepiece". When you saw something interesting in the eyepiece, you would press a button and see a description of the object, right in the eyepiece field. This could be done using input from digital setting circles and a computer. Sort of a Sky Scout at 500x.

November 13, 2007 09:25 PM Forum: Telescope Making

How did I live without this ? (Bandsaw)

Posted By Dick Jacobson

No doubt your bandsaw is a huge improvement over the other methods, but for heavy duty cutting you should have a horizontal bandsaw. These run at a very slow speed to avoid blade overheating and the bigger ones have a liquid cooling system. For example, see

Ideas for Xmas 2008!

November 29, 2007 06:41 PM Forum: Equipment Talk

Whats the worst scope you had?

Posted By Dick Jacobson

My first scope, purchased in the 1950s from Criterion for the princely (to me) sum of $18.95. The objective was about 1-1/8" diameter. It didn't have eyepieces but had some loose glass lenses that you could arrange in the cardboard drawtube for various magnifications up to 400x (!) Needless to say, I saw nothing but dim blobs of light at 400x, but at 35x I could see craters on the Moon and was thrilled. It had a tall shaky tripod (basically three broom handles) with a ball-and-socket camera mount at the top. It also had a little finder but the dog ate it.

December 31, 2007 03:16 AM Forum: Reflectors

Dob tracking

Posted By Dick Jacobson

On the 14" Discovery that I purchased in 2001, the altitude pads were very far apart (about 160 degrees) making the motion very stiff. I relocated them about 70 degrees apart (as recommended in Kriege & Berry's "The Dobsonian Telescope") which helped a lot.