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Posts Made By: Dick Jacobson

February 28, 2003 07:33 AM Forum: Telescope Making

Where to buy motors?

Posted By Dick Jacobson

Does anyone know of a good source for small electric motors? I'm looking for a simple DC motor geared down to 1 or 2 RPM. I've used a "high torque" motor from Edmund Scientific but it's not as accurate as I would like.

May 2, 2003 11:20 AM Forum: Deep Sky Observing

Tall, narrow star map idea

Posted By Dick Jacobson

I wish someone would publish star maps that correspond to the way that I (and I suspect many others) actually do observing. I usually concentrate on a few hours of right ascension, whatever is in a good position that evening, and look for objects from the north pole to the southern horizon. It would be very nice to have a map that covers a sector of sky from pole to pole, and covering 2 to 4 hours of right ascension. The map would be folded horizontally into 4 to 8 sections; you would just unfold the section you needed. No more flipping pages in an atlas. I've made maps similar to this by making copies from the Cambridge Star Atlas and taping them together, and found them to be very convenient. It would be nice to have a single continuous map like this. Six to 12 maps would cover the entire sky, with minimal distortion.

May 15, 2003 11:05 AM Forum: Deep Sky Observing

New lunar eclipse photo idea

Posted By Dick Jacobson

Many of you have probably seen some of those impressive multiple exposures of a lunar eclipse that show the Moon passing through the Earth's shadow. What I haven't seen yet is a continuous exposure through the eclipse, which would show the Moon as a continuous streak - really a photo of the Earth's shadow, not of the Moon. The camera would be tracking the stars. What you would need is an accurate tracking mount, a camera with a mechanical shutter that can remain open for hours without running down the battery, and a fairly dark neutral density filter. I don't have the necessary equipment. I know its rather late for tonight's eclipse, but it might be interesting for someone out there to try!

May 15, 2003 01:47 PM Forum: Deep Sky Observing

Camera on the Moon

Posted By Dick Jacobson

When will we have a camera on the Moon during a lunar eclipse? I'll bet the picture of the Earth, surrounded by an orange ring of sunlight refracted by the atmosphere, would be at least as spectacular as Earth-based solar eclipse pictures!

December 15, 2004 07:51 PM Forum: Telescope Making

Equatorial Cradle

Posted By Dick Jacobson

Here's a picture of an equatorial mount I built for my 14" Newtonian. It's sort of a Dobsonian cradle tipped over to point north. The cradle and base are separate pieces. The cradle's dimensions and weight are about the same as a Dobsonian, so the only weight/size penalty is the separate base. I recently installed a motor drive.

I sawed the tube in half at the middle; the two pieces snap together and are connected by a pair of aluminum bars. There is a rotating bearing just below the focuser, so the eyepiece can easily be moved around to a comfortable position. The finder is one half of a binocular.

The mount works very well. It's not as solid as a Dobsonian, taking about 3 to 5 seconds to settle down after giving the tube a "thump", but vibration is rarely a problem.

July 10, 2006 08:30 AM Forum: Telescope Making

Window shade shroud

Posted By Dick Jacobson

Has anyone used window shades to make a shroud? It seems to me that if you mounted four roller shades on the sides of your mirror box, you could just pull them up after assembling the truss - a quick and easy way to deploy a light barrier. If the wind was blowing your scope around, you could lower the shades part way to reduce wind resistance.

July 25, 2006 01:33 PM Forum: Telescope Making

Ship-mounted telescopes

Posted By Dick Jacobson

As long as we're on the topic of offbeat telescopes, let me bring up a subject I've often wondered about. Has anyone seriously studied the idea of mounting a large telescope on a ship?

There could be many advantages. You'd only need an altitude axis, the ocean is the azimuth axis. Ultra-dark skies. Excellent seeing due to laminar airflow. If it's cloudy, sail to where it isn't. You can see the entire sky by sailing north or south. Since shipyards routinely handle thousands of tons, it would be easy to handle massive components without the logistical nightmare of hauling them up a mountain. No problems with getting permission to build on "sacred" ground. No endangered species to worry about.

Sure, there are a few problems. Number one: waves. But in this age of adaptive optics, I've got to think that the problem of a slowly rolling ship could be counteracted. Pirates. Corrosive salt air. Low altitude.

A few years ago I conducted a small experiment in shipboard astronomy. I set up my scope on a pontoon boat. I was surprised by how steady the view was. The main problem was that the boat continually turned from side to side. But a boat is a very pleasant place from which to do naked-eye or binocular astronomy.

I've never been on an eclipse cruise. No doubt those who have can offer additional insights.

September 5, 2006 07:22 AM Forum: Telescope Making

Center dot - black or white?

Posted By Dick Jacobson

I always assumed that the center dot "doughnut" on a Newtonian primary mirror should be black to avoid scattered light. Recently I was at a presentation where the speaker said it should be white. He didn't give any reason. Maybe white is more visible during collimation. Any advice?

February 18, 2007 07:36 PM Forum: Equipment Talk

Should all telescopes be gold?

Posted By Dick Jacobson

I have often noticed while observing that any black object becomes extremely cold and accumulates dew or frost. I suppose this is due to blackbody radiation - black objects emit as well as absorb light very efficiently. This leads me to believe that black is the worst possible color for telescopes, finders, binoculars, eyepieces, and anything else that needs to remain free of dew. Since we're talking mainly about infrared light, and infrared mirrors are usually coated with gold, does this mean that all our equipment should be gold plated or covered with gold leaf?

Has anyone done an experiment measuring temperatures of different colored objects at night? You could take a few tin cans, paint one black, another white, another left naturally reflective. Put a thermometer under each one and see what happens.

November 2, 2008 04:34 PM Forum: Telescope Making

Passenger Carrying Telescopes

Posted By Dick Jacobson

Is anyone aware of any portable telescopes that have been built where the observer is carried on the telescope's structure? I have an idea for a Newtonian of about 30" aperture that would include an observer's chair, eliminating the need for a ladder. I believe that this could be done without introducing excessive vibration from the observer's body.