Image of the day

Captured by
Gary Taylor

M27 - Dumbell Nebula

About Astromart

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: Alan French

June 17, 2009 03:34 PM Forum: Equipment Talk

Looking to get a White Light Solar Filter

Posted By Alan French
Brian Lee said:

I'm looking to get a White Light Solar filter for my meade ETX-70AT.
Does anyone have a suggestion?

Reg

I think the Baader material is superb, and does noticeably better than my old aluminized mylar filter.

I hear people say they are happy with glass filters, but the only glass filters I have seen that worked well cost several hundred dollars (and are not available today). We gave in and tried a highly recommended glass filter for one of smaller telescopes some time ago and wound up sending it back. The relatively inexpensive ones I've looked through at conventions have also been disappointing.

Clear skies, Alan

June 27, 2009 12:50 PM Forum: Eyepieces

First 1.25" ocular you purchased?

Posted By Alan French
Hi Darian,

I suspect it was the 40mm that came with my 60mm Unitron, although that was obviously a package deal. As far as individual eyepieces, it must have been something for my 8" f/8 Newtonian, but the specifics have long vanished from my memory banks.

The first serious eyepiece "upgrades" I made were some Vernonscope Brandons, which found new homes long ago. I bought the TeleVue Plossls when they first came out and still have and use them in a binoviewer.

Clear skies, Alan

July 17, 2009 01:34 PM Forum: Refractors

True APO ??

Posted By Alan French
I know it is a lost cause, but APO stands for Army Post Office or Adirondack Public Observatory. If apochromat requires an abbreviation, it would be apo or Apo.

Perhaps the usage of the bizarre "APO" is a reflection of the confusion over its precise definition. My take is that a lens that reveals no out of focus color when in focus and shows sharp, contrasty, high power views of the planets is good enough to be called an apochromat (perhaps it would be more prudent to simply say such a lens keeps me very happy). Imagers might be fussier since they have to deal with a wider portion of the spectrum than our eyes perceive.

It would be helpful if the manufacture or seller provided some details on the lens - the chromatic focal shift at the very least.

The "true" definition that gets quoted was actually written by Abbe with microscope objectives in mind.

Clear skies, Alan

July 30, 2009 12:40 PM Forum: Global Warming - REAL or NOT

Climate Change Shocks Summer 2009

Posted By Alan French
I note they've been a bit toasty in the northwest.

Clear skies, Alan

July 31, 2009 05:53 PM Forum: Refractors

PV What?

Posted By Alan French
William Green said:

With respect to the optical quality of a triplet-lens refractor, what does the phrase "certified as minimum 0.25PV" mean? Would "0.25PV" be good, bad, or mediocre? I think I know, but I'd rather have someone more knowledgeable explain this to me. smile

Thank you.

Bill

Bill,

I suspect it means 1/4 wave peak-to-valley (PTV). It simply means that the maximum deviation from perfection is 1/4 the wavelength of light. It is important to know what wavelength it applies to, and whether it is at the wavefront or at the optical surfaces (although I suspect the optical surface readings are usually done for single surface optics like Newtonians).

A 1/4 wavefront is usually considered the minimum to provide good performance, or be labeled "diffraction limited" with performance only impaired by the ill behavior of those pesky photons. Unfortunately, a single measure is not helpful. If the optic has a lot of rough smaller variations it will not perform as well as a smooth surface with only minor and gentle smaller variations. Often an RMS measure will be given to measure these problems. It should be a four or five fold improvement on the PTV measure.

Clear skies, Alan

August 3, 2009 04:50 PM Forum: Reflectors

Aperture Mask Question??

Posted By Alan French
With one hole you've created a smaller, unobstructed aperture. With four holes you've simply modified your full aperture, adding a large diffraction producing obstruction in the incoming light path. Essentially you have four very large and oddly shaped spider vanes. You've also eliminated you nice round entrance pupil.

Clear skies, Alan

August 6, 2009 10:27 PM Forum: Astro-Physics

I've been bested by an old coot

Posted By Alan French
Amazing. I think almost anyone with an interest in astronomy and any sort of public exposure winds up with strange calls from time to time. We often used to get thick envelopes in the mail once in a while. "Dear Sir, I have a theory about the formation of the universe. The pro astronomers don't want to look at it. Would you please look it over and let me know what you think."

My favorite two from over the years...
"Do you know anything about the secret UFO files at Dudley Observatory?" He was quite insistent we had some (I was and am on the board).
"Where can I get photographs of all the galaxies in the universe?" When I tried to explain the difficulty here, "Who could I pay to take photographs of all the galaxies in the universe?"

Clear skies, Alan

August 7, 2009 05:15 PM Forum: Astro-Physics

I've been bested by an old coot

Posted By Alan French
I've heard some pretty dumb things that started with "I'm an engineer...." For instance, I heard one claim that there was a momentum equivalent in thermodynamics and that was why hot water will freeze faster than cold water. "Hot water gets cooling off and just blows right through the heat of fusion."

Clear skies, Alan

August 8, 2009 02:54 PM Forum: Astro-Physics

Am I crazy?

Posted By Alan French
Luke Leege said:

I've been kicking round the idea of sending my Traveler to AP and have it painted white. Am I nuts? Would it hurt the value even is it is done at AP? Roland, is this even an option? I'm a sucker for AP White. 8O

Luke,

It's your scope. If you like AP white and AP is willing to do the job, why not? Perhaps they are simply too busy, but you'll probably get an answer to that question.

Hurt the value? If that is a concern, a "unique" white AP Traveler might actually have appeal to some folks. Of course I can't imagine anyone actually selling a Traveler, but that's just me. My wife says her first generation version is the last scope she will ever part with.

Clear skies, Alan


August 8, 2009 06:38 PM Forum: Astro-Physics

Am I crazy?

Posted By Alan French
I was very fond of the Cadet Blue that was used on some of the Starfires.

Clear skies, Alan

Funding Member

Sponsors


View all sponsors

Search