Posts Made By: Alan French

Ivan Gastaldo said:

A beautiful flower besides the road...

I noticed those too. I think it is an Ivy-leaved Morning Glory.

Clear skies, Alan

Darryl Simmons said:

Hi All,

Last night while attempting to view Mars I noticed the image flares out one way when short-focused and 90 deg rotated when long-focused. Ah-ha! I thought, astigmatism. Then I wanted to find out where it was coming from. So I shuffled my feet and turned my head until I was looking through the eyepiece 90 deg rotated and refocused. Same problem (did not rotate with my eye). Next I carefully rotated the eyepiece 90 deg and tried again. Same problem (did not rotate with my eyepiece). I have a 10” dob made from an old Mead Starfinder OTA. Magnification was 120X (10mm Plossl) Any suggestions? (I have tried very hard to collimate using a barlowed laser, and I think it’s pretty close). TIA

Darryl

I suggest checking for astigmatism with a star. The behavior you describe sounds typical of a Newtonian that is still cooling down.

Check a star image just inside and outside of focus, with about three rings visible. Is the Fresnel pattern round on both sides of focus? The closer to focus you are, the more sensitive the test.

Clear skies, Alan

"Darkness at Night (A Riddle of the Universe)," by Edward Harrison. Well worth reading, and probably available at a local library.

Clear skies, Alan

Rex Cox said:

Interesting story...

http://news.yahoo.com/s/ap/20080319/ap_on_fe_st/odd_meteorite_theft

Is this the one that was stolen at WSP?

No. As the story says, the recovered meteorite was stolen from a museum.

Clear skies, Alan

Gary Lohmeyer said:

I have a pair of Fujinon 8X40 BFL binocs that need a new set of rubber fold down eye cups. I have checked all over but have not been able to locate the Fujinon replacements. Does anyone know where to purchase these?

Gary,

I have the same binoculars, and also needed replacement eye cups. I called Fujinon and they sent me a pair at no charge.

Clear skies, Alan

That's not much to go on, but there is a good chance you are talking about Aristarchus. It is very bright and noticeable.

Clear skies, Alan

As others have said, "ED" stands for "Extra-low Dispersion." (Although the common usage of "ED" in television commercials may imply otherwise. )

It is important to keep in mind that it is not the extra-low dispersion of the glass that allows it to be used for improved color correction. If you want to design a lens that can be made in a reasonable focal length, is free of aberrations, and is agreeable to a production environment, you need to pick two glasses that have markedly different dispersions. For an achromat, this usually means a Crown like BK7 and a flint like F2.

A flint glass spreads out or disperses light more than a crown glass. If that was the only difference, we'd be in good shape. Unfortunately, the difference between the dispersions varies with color. In other words, there is no single number that would describe the difference between the two across the entire spectrum.

With ED material, the variation between the dispersion and the dispersion of some crowns as you move across the spectrum is much smaller. You could almost say at every point of the spectrum the crown spreads out the light X percent more. When a proper crown is matched with an ED material (fluor-crown) you can get a large reduction in secondary spectrum.

I hope that was understandable.

Clear skies, Alan



Clear skies, Alan

Often we seem to run with choices based on politics and special interests, and never bother asking "Is this really worth doing?" or "What are the benefits, and what is the downside."

For a civilization that has accomplished so much, we often seem pretty dumb. The fact that most politicians, a large percentage of our population, and almost all people in the media don't understand science doesn't help.

I am reading an interesting book right now - and may have mentioned it before here. It is "Don't Believe Everything You Think (The 6 Basic Mistakes We Make in Thinking," by Thomas Kida.

Clear skies, Alan

I have an 8" triplet. Thomas Back suggested dispensing with the dew shield entirely, which I was quite happy to do (improved balance and reduced wind load). Without a dew shield, the lens cools more quickly and there are no dew shield related thermal issues. I use a dew heater, and turn it on after a while. If it is windy, I generally leave it off.

My aluminum tube is unpainted, and the inside is baffled and painted, with just a little flocking at the lens end, so I don't know how a fully flocked tube will work. Mine seems to behave very well.

Clear skies, Alan

Peter Glus said:

Holy Cow...all of a sudden there seems to be a whole bunch of VERY big refractors....8" Brant...6" D&G...6" Mogey...5" AP in the span of like 10 ads. Is this the new trend (have we completed the infatuation with small imported rich-fields!)?

I thought the subject was "huge" refractors? An 8" is almost there, but five and six inch refractors are kind of dinky. 8)

Take a look what the reflector folks are often carrying these days.

Clear skies, Alan