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Posts Made By: Doug Peterson

August 7, 2007 08:05 PM Forum: Equipment Talk

I guess these companies don't want my money.

Posted By Doug Peterson

Funny how when you have money burning a hole in your pocket, all it would take is the answer to a question from a dealer or manufacturer and that money would end up in their pockets. Some are asleep at the switch though, and while the vast majority are responsive, I have failed to elicit a response through these guys' websites recently: Skywatcher, Burgess.

How about you, who doesn't want your money?

August 13, 2007 08:43 AM Forum: Equipment Talk

Annual Spellcheck reminder

Posted By Doug Peterson

It is spelled fluorite, not baking powder
It is spelled aperture, not temperature
It is spelled Crayford, not Broderick Crawford
It is spelled Meade, not mead which is a fermented-honey alchoholic beverage.

This will be on the exam.

Any others?

August 14, 2007 06:23 AM Forum: Equipment Talk

Criterion bell-shaped pier, where are they?

Posted By Doug Peterson

Every vintage Criterion newtonian I have seen has been separated from its pier, with only the wood tripod-- which actually fit inside the pier on the smaller 6-8" models--still surviving. All those piers must be somewhere. Perhaps they are where the other half of pairs of socks end up. Depending on which Criterion literature you believe, they weighed only 45 or perhaps 70 lbs., not nearly as heavy as they look, much less reason to leave them behind.

August 14, 2007 10:39 AM Forum: Equipment Talk

Strehl best way to describe optical wavefront ?

Posted By Doug Peterson

I think so.

Most people do not relate to waves peak-to-valley (PV) or waves RMS (much more meaningful, but numerically 3-5 times smaller).

The Strehl on the other hand--the actual peak intensity of a star (or point spread function if you want to impress people) as a fraction of the theoretical maximum--is something you can see at the eyepiece.

And numerically it is easy to relate to:

80% or 0.8 of 1.0 is nominally diffraction limited. Remember when 80-89% got you a B in school?

90-100% got you an A.

And 95% and above got you a A+.

Your thoughts on this most pressing matter?

August 19, 2007 10:03 AM Forum: Equipment Talk

Porter garden telescope reborn--pricey?

Posted By Doug Peterson

Cool, but at 59 thousand I wonder if even one will be sold. Anyone seen one in person? Or a review?

For the bux you could buy the garden and an observatory housing a real telescope.

August 20, 2007 05:46 AM Forum: AstroMart FAQ

Low-ball offers

Posted By Doug Peterson

The vast majority of transactions on AM are smooth with people who are serious about buying or selling.

Every once in a while I get a strange low-ball offer. Such as a lot of vintage oculars, many orthos, that I expected to get maybe 40 or 50$ each--a guy offered 12 dollars and 50 cents. A 500 dollar scope, an offeror who said he only had 150. Of couple of people asked for time payments, people I didn't know with zero feedback. A friend advertised a restored Cave 8"--a fellow came out, looked it over and asked him if he would take 75 bucks--he was run off with a pitchfork.

The low opening bids in an auction I understand. You want to get in line in case there are no further bids. But insulting cash offers make you want to reach for the feedback button.

My question is, do these unbelievable offers really ever work?

August 31, 2007 10:50 PM Forum: Refractors

How much better are apochromats than achromats?

Posted By Doug Peterson

I know this will stir up a lot of people, and any list is necessarily subjective, but I perceive that many inexperienced amateurs don't really know how much better APOs are, perhaps believing marketing hype and assuming they are all perfectly corrected.

Nevertheless here is my stab at a listing of relative color correction. Perhaps the group mind can add to--or rearrange--the list. Based on a combination of impressions over the years, ray tracing of some designs, and the occasional A-B comparison. Some of the subjective element I believe is to personal preference, for instance I find red error to be far more annoying than blue, whereas photographers may disagree.

The chromacor is not included because the correction depends on aperture and F#, can vary from essentially perfect down to 2.5x on very large achromats like my 8"F12.

1x achromat
1x semi-ED (meaningless)
2x semi-APO (meaningful, Zeiss and others)
3x Pentax EDUF
4x Meade ED/APO doublets
5x Pentax SDUF, original TV Genesis, TMB EDStar, 66mm F6 doublets, Nikon FieldScope ED.
6x Vernonscope/AP 94mm, TV Oracle, TV60
7x Vixen ED doublets
8x Fluorite doublets, Takahashi, Vixen, etc.
10x FSQ
12x TEC 110, 140 oiled triplet
16x TMB Russian triplets, TV NP101, Takahashi TSA, Astro.Tech.90EDT, WO 132 triplet

I haven't seen recent APs or the Tak TOA, but I expect they are at the top in correction, ie, bottom of my list.

September 2, 2007 09:25 AM Forum: AstroMart FAQ

Why do some sellers delete ads right after sale?

Posted By Doug Peterson

As a buyer, I would like to see the For Sale ad exist up to when I receive the item, so there are no misunderstandings. The immediate deletion makes me immediately suspicious.

I don't delete my own ads for this reason, and also as a convenient history of transactions.

September 11, 2007 05:15 AM Forum: Politics

This guy and his friends still want to kill you ..

Posted By Doug Peterson

Interesting that the new "kinder, gentler" Bin Hidin' is giving us Noam Chomsky suggested reading lists, scolding the Democrats for not doing his bidding after congratulating them on their big win, and in general attempting to be some sort of intellectual elder statesman. I am sure the above are asking themselves, with enemies like this, who needs friends?

Once a mad man, always. History will remember people who build something new rather than destroy the work of others.

September 13, 2007 07:33 AM Forum: Equipment Talk

Why you have to be careful with the star test

Posted By Doug Peterson

In the three sets of Aberrator-generated star tests below, note the top set is 5 rings inside and outside of focus of a perfect telescope.

The middle set is with 1/8 wave of normal (3rd order) spherical abberation.

The bottom set is 1/10 wave of what is called 5th order spherical abberation, what is left over after a fast lens has already been corrected for the normal spherical. The good news is it is typically not a problem. The bad news is that the small amount of 5th order upsets the star test quite significantly, causing people to worry that their objective lenses are bad.

What really counts is the Strehl, the ratio of the height of the actual airy disk intensity to its ideal. An ideal lens of course can be 1.00. The 1/8 wave of SA3 drops the Strehl to .95 which frankly is excellent. But the 1/10 wave of SA5 has a Strehl of .99, virtually perfect, yet the star test looks worse.

Another reason why the Strehl makes more sense than uninterpreted wavefront numbers.