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LBN 576 - The Garlic Nebula

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Posts Made By: Alan French

August 14, 2007 09:41 PM Forum: Equipment Talk

Strehl best way to describe optical wavefront ?

Posted By Alan French
Strehl is certainly a good, single number that can tell you how good (or bad) optics are. It is, unfortunately, often derived in entirely inappropriate ways.

A proper determination of Strehl (or RMS wavefront) requires testing the entire aperture, and testing at a large number of points. Generating Strehl from the limited number of Foucault readings is not appropriate.

Clear skies, Alan

August 15, 2007 08:18 PM Forum: Reflectors

Resolution: Reflectors vs. Refractors

Posted By Alan French
A minor quibble to start with. The in-focus diffraction image is an Airy disk surrounded by faint rings, and usually only the first ring is visible. The out of focus star image is a Fresnel pattern.

I am not sure why folks talk about resolution and contrast as if they were distinct, separate concerns. As detail moves toward the limit imposed by the aperture and the wave nature of light, contrast decreases. When you reach a point where the detail exceeds the resolution limit of the telescope, the contrast drops to zero, and the detail is not visible.

I suggest reading William Zmek's "Rules of Thumb for Planetary Scopes" in the July and September, 1993, issues of Sky & Telescope.

Clear skies, Alan


August 22, 2007 11:35 PM Forum: Birding Optics and Photos

IDENTIFICATION HELP NEEDED

Posted By Alan French
Looks like it may be a juvenile Mourning Dove.

Clear skies, Alan

September 7, 2007 03:27 AM Forum: Eyepieces

Ethos 13 mm 100 ° as magic as Leitz 30 mm 88°

Posted By Alan French
I tried the Leitz 30mm 88 degree eyepiece at WSP one year, and was not very impressed. I thought the images at the edge of the field were poor, and it was simply not worth the money. The Ethos, even with its wider field, does much better.

Clear skies, Alan

September 7, 2007 11:01 PM Forum: Deep Sky Observing

Largest DSO's ?

Posted By Alan French
IC 1396 is very nice under dark skies, and very large.

Clear skies, Alan

November 12, 2007 02:14 AM Forum: Beginning Astronomy?

Cold hands and cold feet

Posted By Alan French
I'd also recommend adding more insulation to your head. I found a second knit cap over the first made my hands and feet a lot warmer. Ditto for keeping the upper body warm. The best insulated boots and insulation on the ground won't help much if your brain determines your head or upper torso need more heat.

Clear skies, Alan

December 23, 2007 11:13 PM Forum: Birding Optics and Photos

Downy Woodpecker

Posted By Alan French
Chase,

That's very nice! Thanks for sharing.

Clear skies, Alan

January 12, 2008 05:15 PM Forum: SCI-FI

Larry Niven

Posted By Alan French
Scott Mortimer said:

Me too! What about "Lucifer's Hammer" with Jerry Pournelle. One my favorites, I've recommended it to a few friends who all loved it too.

-Scott

Alan French said:

One of my favorite science fiction authors is Larry Niven, especially his "Known Space" stories.

Clear skies, Alan

Yes, I liked "Lucifer's Hammer" a great deal. I was not quite as happy with "Footfall," but should really read it again. I'll have to bring it upstairs so I'll remember to give it another go.

Clear skies, Alan

January 14, 2008 10:08 PM Forum: SCI-FI

Whats the most realistic portrayal of an alien?

Posted By Alan French
Michael Jackson?

January 23, 2008 03:04 AM Forum: Birding Optics and Photos

Hairy Woodpecker

Posted By Alan French
Very nice. I've always enjoyed woodpeckers, and am pleased to have Hairy, Downy, and Red-bellieds visit the feeder on a regular basis.

Clear skies, Alan

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