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Posts Made By: Danny R Lunsford

January 21, 2009 06:31 AM Forum: Equipment Talk

Sales fees

Posted By Danny R Lunsford

Someone over on CN was hot and bothered about a fee charged here for sales. No one seems to have mentioned what said fee is, and the threads are locked (they're a might sensitive over yonder, as Uncle Rod would say).

So what is this fee and when does it start?

Thanks in advance.

-drl

July 3, 2009 02:47 AM Forum: Equipment Talk

Spacing in Fraunhofer acrhomats

Posted By Danny R Lunsford

Hello all,

How does spacing affect the spherical correction of a Fraunhofer achromat? I have one which had a 1mm plastic spacer ring which got broken and could not be repaired, so I used several layers of aluminum foil to achieve spacing. The spacing went from about 1mm to a fraction of a mm (just enough to forestall the appearance of Newton rings).

The resulting objective performs fine for its purpose (terrestrial and guide scope use) but the star test reveals some very slight spherical aberration. I never bothered to test it when it was spaced at 1mm so I'm wondering if restoring the original spacing will correct the spherical error.

Thanks in advance.

-drl

January 23, 2009 07:36 PM Forum: Global Warming - REAL or NOT

I don't much care...

Posted By Danny R Lunsford

Point taken but we should stop talking about the planet. The planet is a ball of silica and iron with a molten core and a thin crust of rock. On top of that is a coat of dirt from which we get most of our food. Over that is a tiny wisp of gas without which there is no life here. This isn't about the planet, or the environment, it's about the atmosphere. It's very easy to screw that up. It's also very easy to fix it.

-drl

Christine Canelos Welsh said:

I happen to believe that evidence supports that human activity is contributing to global warming. I also happen to appreciate the skeptical questioning by those who believe otherwise. We need critical thinking on all matters before us, that’s what we hold most dear in science.

However, I find that I don’t much care whether it is or it isn’t human caused. We should be changing the way we live on the planet, not just because of this threat, but because intrinsic value of having a clean environment is that it is good for us, both economically and for our health.

If the threat of global warming, human caused or not, scares people enough to change their ways, frankly, I’m fine with that. We do a lot of things to prevent the “what ifs” in our lives, why not this one? The statistical likelihood of my house burning down is small, for instance, but I still keep smoke detectors and fire extinguishers handy just in case.

If the threat, even the possibility of human-caused global warming causes people to embrace alternative, renewable energy technology and to conserve the amount of energy they use in the first place, I'm fine with that too.

This is a darn small planet overall, by not limiting the emission of man-made greenhouse gases, even if we are not contributing to global warming, we are contributing to the degradation of air quality, water quality, and mucking up a great deal of plant and animal systems, with which we ourselves are symbiotically connected. If the United States doesn’t take the lead on this issue, emerging industrial nations never will. If we ignore this, we are effectively peeing in our own well. Bottoms up, folks.

July 3, 2009 05:46 AM Forum: Equipment Talk

New Explore Scientific large Achromats.

Posted By Danny R Lunsford

I found the CA of an f/8 6" too much to bear. Also they cannot be profitably used with a radical violet blocker like the Baader SemiAPO because the best focus on stars results in a rosy colored fringe on them, even at low power, which is worse than the unfiltered CA! So much blue is defocused that planets take on a sickly yellowish color. The minimum acceptable range for a standard crown/flint achromat is 5" f/9 and 6" f/12. Now if they are using lower dispersion glass, they may be able to get away with faster scopes. Still, in that case, one is better off with a premium 6" f/4 or f/5 mirror.

For a 6" f/8 doublet one is more or less forced to go with an ED achromat at a minimum, I think, and then one has the issue of super-critical collimation. No free lunch!

I have a 5" f/9.3 Fraunhofer and it gives outstanding performance. In direct comparison with the 6" f/8 from the same optical shop, the 5" won in every way other than light gathering power, and weighs 10 lbs less.

-drl

September 20, 2009 01:48 PM Forum: Equipment Talk

Bad C11 optics? HELP!

Posted By Danny R Lunsford

Your description of the diffraction pattern is classic severe spherical aberration, as in a scope with a fast spherical mirror = an SCT without the corrector geometry. I would seriously investigate the corrector plate for authenticity.

-drl