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Posts Made By: Dick Jacobson

July 29, 2003 01:32 PM Forum: Telescope Making

Spider Diffusion Spikes?

Posted By Dick Jacobson

To de-fuse terminology confusion and avoid mental distraction, let's change the subject name to its correct form, Spider Diffraction. Diffraction is a well-known optical phenomenon that has nothing to do with diffusion. Pardon me if I'm a nitpicker, but I cringe every time President Bush says "nucular".

August 5, 2003 06:46 AM Forum: Equipment Talk

Transportability (durability) of DOBS

Posted By Dick Jacobson

I once tripped and fell while carrying a 10" Orion dob tube (built by Discovery) on my shoulder. No damage was done (this was on sandy ground, if it had been concrete I'm sure there would have been damage.) I've been transporting 10 and 14 inch Sonotube Dobs for several years; they are very rugged and only need occasional collimation.

August 7, 2003 07:59 AM Forum: Telescope Making


Posted By Dick Jacobson

This really hits one of my "hot buttons". I think the finders on the market are garbage. A few years ago I tore apart an old pair of 7x35 wide angle binoculars and came up with a finder that functions much better than anything on the market. Its 11 degree, erect and correct image makes finding objects much easier. The main problem is that binoculars do not have crosshairs. To mark the center of the field, I drilled a 1/16 inch hole at the focal plane and inserted a tiny (#0 I believe) machine screw, filed down to a point at the tip. Since it is thicker than crosshairs, the screw is visible under dark skies without illumination.

I wanted more light gathering power, so have just ordered a 10x50 binocular from Orion (7.6 degree field, 18mm eye relief) and will tear it apart and get two finders out of it.

Nobody would bother with 1x finders if conventional finders weren't so bad.

August 8, 2003 08:06 AM Forum: Equipment Talk

Planetary scopes

Posted By Dick Jacobson

A perfect subject for controversy! My main point is that atmospheric conditions are overwhelmingly important. I've been using my 6" Mak-Newt and 14" Discovery Newtonian (both excellent scopes) side-by-side for viewing Mars. On nights of average seeing the 6" usually provides sharper views. When the seeing is unusually good the 14" beats the pants off the 6". I wouldn't want to use anything smaller than 6 inches because you can't get enough magnification to see all the detail permitted by the atmosphere. If you go beyond 25-30x per inch the image starts to look "grainy" due to eye defects, ruining the ability to see faint contrast. I know some people claim that a 4" refractor is perfectly matched to the atmosphere; I've never owned one so maybe they are right or maybe I just have bad eyes.

August 13, 2003 08:35 AM Forum: Equipment Talk

Finder scope question

Posted By Dick Jacobson

For several years I've used one half of a 7x35 binocular as a finder and greatly prefer it over either a conventional or 1x finder. The erect, correct, straight thru viewing makes it easy to relate the finder view with the naked-eye view in your other eye, or with the star pattern in the atlas. The field of view is 11 degrees which is much wider than other finders. The main problem is that most binoculars don't have crosshairs. My solution was to drill a 1/16 inch hole at the focal plane and insert a tiny (#0 I think) machine screw to mark the center of the field. The screw is visible without illumination under dark skies since it is much thicker than crosshairs.

I wanted more brightness so have just bought some 10x50 binoculars which I will use for a bigger finder.

If you don't mind some tinkering, this gives you a really nice finder. I wish some manufacturer would see the light.

September 9, 2003 11:19 AM Forum: Equipment Talk

Most disappointing telescope purchase.

Posted By Dick Jacobson

Here's some ancient history. My most disappointing telescope was my first, a Criterion refractor bought in the 1950s. Even by the standards of today's department-store telescopes, it was miserable. It had an objective a little over one inch in diameter, with a long cardboard tube. There were no eyepieces as such, just a few unmounted glass lenses that you could arrange in different combinations in the cardboard drawtube. Our dog ate the finder scope, which was no loss since it was worthless. The tube was mounted on a little ball-and-socket camera mount, on top of a very shaky wooden tripod. The scope was advertised in Popular Mechanics as having magnification up to 135x, plus extra lenses to give 400x "for experiments". Needless to say, I never saw anything other than a fuzzy blob of light at 400x; the only useful magnification was 35x.

The price of $18.95 ate up my life's savings at the time. In spite of all this, I had fun looking at the Moon with it; don't remember ever seeing anything else.

September 12, 2003 11:06 AM Forum: Equipment Talk

Eyepiece Startup

Posted By Dick Jacobson

I recommend building your eyepiece collection around four "magic" focal lengths that maximize different aspects of your telescope's performance. I call these eyepieces the Maximum Light, Maximum Field, Maximum Definition, and Maximum Resolution eyepieces.

The Maximum Light eyepiece is the unique focal length where both the total amount of light in the eyepiece, and the brightness of individual objects, are at their maximum possible value. This is the focal length where the exit pupil exactly matches the size of your eye's pupil. You can get the exit pupil by dividing the focal length of the eyepiece by the focal ratio of the telescope. For young people the eye pupil size is around 7 millimeters but usually decreases to around 5mm by middle age.

The Maximum Field eyepiece is a low-power eyepiece that has a field stop almost as large as your telescope's drawtube. Since most manufacturers don't specify the field stop, you can get an approximation by multiplying the focal length by the apparent field of view. For 2-inch eyepieces, to get a very large field stop this number should be in the area of 2500; for 1.25-inch eyepieces, around 1700.

The Maximum Definition eyepiece is a high-power eyepiece that yields a very "smooth", sharp image for planetary viewing. I find that about 25x per inch works best for me. When the power gets above this the image starts to become "grainy" due to defects in the eye that become obvious when the exit pupil is less than a millimeter.

The Maximum Resolution eyepiece is the oft-quoted limit of 50x per inch, beyond which further magnification is usually useless.

You'll need at least one more focal length to fill the wide gap between the Maximum Light and Maximum Definition eyepieces. If these magnifications are 5x and 25x per inch respectively, I find that about 15x per inch is a good choice. I don't use magnifications slightly above Maximum Light (8-10x per inch) very much because the small gain in visibility doesn't really offset the disadvantage of a smaller field of view.

September 15, 2003 11:28 AM Forum: Deep Sky Observing

Astonished & Disappointed - Please Comment

Posted By Dick Jacobson

I can think of three sights that made me gasp with delight. (1) My first view of a globular cluster with a 10" Newtonian, when I could actually resolve some stars for the first time. (2) The Orion Nebula (M42) through the same Newtonian. (3) The Andromeda Galaxy (M31) through 25x100 binoculars.

Aside from these, it's true that most telescopic sights are not what most people would consider spectacular. Some of the enjoyment comes from knowing what it is you are looking at, as others have commented. There is also a sense of satisfaction in finally being able to see a dim object that you've spent a lot of time hunting down.

Looking through a telescope is the nearest most of us will ever get to spaceflight. In looking at a galaxy through a telescope, you see almost exactly what you would see on an intergalactic space flight. There is something to be said for first-hand experience. Why do people travel thousands of miles to see the Grand Canyon instead of just looking at pictures or videos? Hubble photos are wonderful, but there's something better about looking at an object with your own eye(s), aided by the optics, and seeing what it REALLY looks like.

Thank you for your honest comments; I'm sure they reflect what many others feel.

September 24, 2003 11:13 AM Forum: Equipment Talk

Biggest Dob without a ladder

Posted By Dick Jacobson

If you have unlimited funds, check out the NTT-25 from Jims Mobile ( Folded 25" f/5 Newtonian, 72" zenith eyepiece height. I've always though this would be an ideal telescope for a wealthy benefactor to donate for public viewing.

December 26, 2004 10:59 AM Forum: Telescope Making

Merry Christmas (to me! )

Posted By Dick Jacobson

Looks great! I'm curious, what did you use for the light baffle in the secondary cage, wood veneer? It doesn't look like the usual black Kydex.

In case you haven't noticed, I took a little different direction with my 14" Discovery Dob, see the "Equatorial Cradle" thread.