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Posts Made By: Alan French

March 24, 2004 02:26 AM Forum: Eyepieces

A Measure of "Sharpness"

Posted By Alan French

Is there some sort of "sharpness" scale? How can "sharpness" be measured? I can't recall any optics book that deals with the subject.

To me, it seems like an extremely subjective term that gets bandied about with wild abandon, but really provides little useful information.

Clear skies, Alan


April 15, 2004 02:47 AM Forum: Equipment Talk

Re: Skunk Chaser?,

Posted By Alan French

We have lots of skunks, and they sometimes wander by while we are out with the scopes. It never entered my mind that they needed chasing.

Clear skies, Alan

July 24, 2004 01:52 AM Forum: Equipment Talk

Re: focal lenght or focal ratio

Posted By Alan French

Focal ratio always causes a lot of confusion, and it might be easier to think in terms of aperture and focal length. Aperture determines light grasp and potential resolution, while focal length determines image scale and maximum field of view.

Clear skies, Alan

August 9, 2004 09:59 PM Forum: Telescope Making

Re: Cooler than ambient?

Posted By Alan French

Dan,

Rick gave you some good information.

When you take a warm telescope outside on a cool night it immediately starts cooling. There are two basic mechanisms at work - convection and radiation. The tube warms the air next to it, which rises to be replaced by cooler air. The tube also radiates to the night sky. At first, convection cools more than radiation. As the tube approaches ambient air temperature, radiation takes the lead. Eventually, the tube will cool down below the ambient air temperature, and the warmer air will begin heating the cooler tube. At some point, you will get an equilibrium between warming by convection and radiational cooling, and the temperature will stop dropping. If this temperature is below the dew point, you will get dew on the tube.

Both cooling by convection and warming by convection are far more effective on a windy night, so dewing is far less likely if you have a breeze. The scope simply can not get far below the ambient air temperature.

I am surprised folks don't use fans to prevent dew. As Dennis di Cicco suggested long ago in a similar discussion, an umbrella would help keep amateur astronomers warm at night by cutting down on radiational cooling .

Clear skies, Alan

August 11, 2004 11:43 PM Forum: Star Parties

Re: Astrofest or Disneyland?

Posted By Alan French

From "The Weekly World News" -

Scientists at the University of Chicago have discovered that the use of a bouncy-bounce increases both visual acuity and the ability for the eye to adapt to low light conditions. Dr. Barely Quacks comments, "For the amateur astronomer, 15 minutes on the boucy bounce several hours prior to observing proves to be equivalent to adding about 25 percent to a telescope's aperture. Longer periods on the boucy-bounce result is only a slight gain over this." Several companies are considering marketing variations of the bouncy-bounce to amateur astronomers, according the the article, and one has been installed at Star Hill Inn, which caters to amateur astronomers.

So there is method to their madness, and the kids will have a great time too.

Clear skies, Alan

October 18, 2004 10:32 PM Forum: Equipment Talk

Feathertouch Question

Posted By Alan French

Since folks are talking about the Feathertouch focuser......

I have an old version, and the 1.25" adapter is held in by the same screw that serves to clamp the eyepiece in the adapter. Because of this there is only one correct orientation for the adapter, and positioning is aided by a small screw head that protrudes past the outside of the holder and fits a small depression in the focuser.

The works well and is little trouble in warm weather, but is disagreeable on those colder nights when gloves are required. At any rate, I just wondered if the current Feathertouch focusers have the same arrangement with the adapter.

Clear skies, Alan

January 29, 2005 10:17 PM Forum: Equipment Talk

Are You a Maximizer?

Posted By Alan French

The premier issue of Scientific American Mind has an interesting article "The Tyranny of Choice," by Barry Schwartz.

Maximizers are people who want to check out every option so they can select the elusive "best." In contrast, satisficers look for an item that meets their needs, and stop when they find it.

Maximizers are more likely to be less happy with their choice, and worry that there was something better they might have bought instead. (I think I've met a few of these folks searching for that perfect telescope over the years.)

Anyway, it is worth reading, and has some good advice for maximizers.

Clear skies, Alan

March 6, 2005 02:05 PM Forum: Reflectors

Re: Tom Scott Mirrors

Posted By Alan French

Craig,

Since you haven't had any replies...

I have heard the name, and his mirrors were said to be very good. I didn't post this before because I hoped someone could post personal experience with his optics.

Clear skies, Alan

March 19, 2005 05:05 PM Forum: Equipment Talk

Right Angle, Erect Image Finders

Posted By Alan French

Does anyone have and use a right angle, correct image finder that they would recommend? I am looking for one of about 50mm aperture. Thanks!

Clear skies, Alan

June 7, 2005 10:12 PM Forum: Equipment Talk

Comparing Telescopes of Different Apertures

Posted By Alan French

In his book "A Portfolio of Lunar Drawings," Harold Hill makes a comment I find interesting. He originally used a 6.5" reflector, then moved up to a 12", and finally observed with a 10". At one point, he had an opportunity to once again observe with his old 6.5", which was freshly recoated.

He writes that he felt "positively handicapped by the apparent dimness of the image in the smaller instrument although using magnifications strictly proportionate to those in my present telescope." He continues that he could simply not see the amount of detail in the 6.5" that he had seen when he had used it as his regular scope.

Mr. Hill opined that his older age might account for some of the difference, but wondered if time is simply required to get used to the brightness delivered by a particular aperture.

If this is the case, a person who regularly uses a particular aperture may not be the best judge of the performance of some other aperture, and only the investment of time would overcome this handicap.

Just something to think about and consider when you are reading about or doing scope comparisons. If you want to read Hill's comments in full they are on page xx.

Clear skies, Alan