Posts Made By: Thomas Dey

I'll confess that I'm probably just lazy to try to learn the new format, and rather prejudiced vs Facebookish sites, which this now feels like. As long as I can see the Class Ads, I'm fine. But disappointed that older Ads (sold and not) and older Forum images have disappeared. On other sites (not talking CN here) archives are still readily/rapidly searchable, even from over ten years ago, including high-res images.  Tom

Originally Posted by Rod Kaufman

Thomas,
Your "doll-house" concept is quite innovative. The design is highly efficient and user-friendly.
If I recall correctly, I seem to remember an article I read years ago wherein a technician used a "hand held reflectometer" from Cal-Tech to measure reflectivity of a new recoating of the 200" mirror at Mt Palomar. The image in the article showed a tech in a clean suit taking the small gizmo out of his pocket and taking a reading on the mirror surface. I'd sure like to have one of those but I don't even know where you'd find one and I'm assuming they cost a lot, too...

Just remembered, the context of the Dollhouse was big, flat copier scan-mirrors that we were making for Xerox, tens of thousands. We would do the absolute cal once a day (pantry location) and then use the differential sliding motion platform for the rest of the shift, and one or two more shifts, depending on the work-load. The "hole in the floor" was small, so the two flat mirrors could be ergonomically slid L/R to get Ref/sample1/Ref/sample2/Ref/sample3... without the detector ever seeing broken beam or misalignment. It was a wonderfully-boring job that turned around the measurements as fast as they could go through the trays from coating.

PS: It was FUN having production responsibilities!  Design test-sets, breadboard, build, integrate... and on to the next problem. In a high-volume 24/7 shop, no excuses for procrastination, obfuscation or "not my job." I LOVED it!

Re' In-Situ R measurement. Here, I’ve set up measurement of a GIANT folding-flat. And yes, the light source is a humble laser-pointer. We found that some are stable, and some are terrible. Did the ref/sample1/ref/sample2… as usual, and it worked well. We had considered a little commercial puck thingy… but I looked at their literature and patents, and realized they had built a Lot of assumptions into their design, that would kill traceable BRDF (bi-directional reflectance distribution function) buy-off. So, as usual, we built our own. Nearly all of my concoctions could be used on the production floor, ambient factory lighting. Good design, conjugate baffling, bandpass filters… works wonders!  Tom

Originally Posted by Roger Johnson

That Cloudy Nights loads for you as quickly for me, yet for me, Astromart lags I find interesting, regardless of whether employing my RAM-jammed quad-core laptop or much older desktop when loading the two sites.  If internet speed were the sole factor, I am at a loss for the new Astromart page thumbnails taking so long to load, particularly since it has been a relatively recent development.  Cleanups have routinely been implemented, unfortunately.  Will continue to look for more hoops to jump through.

Hmmm... old forum image loads aren't working at all for me, and I just got my reasonably-new laptop professoinally cleaned up and upgraded to the most recent Win 10 a couple of weeks ago. Maybe I wasn't waiting long enough, but do get that NF404 message, when I try to load even fairly recent images, like from e.g. last feb. So, very recent images are right there, in the posts, historical images... apparently from before the New AMart... are ... gone? It is acting as if the Forum history images were purged from memory when the change occurred, or that the "open in new tab" is sending all requests to "can't find."  Tom

Thanks! I just revisted old forum histories, and notice the images are now appearing there. Whatever that took (reloading the data-base, whatever?) anyway... Thanks! I was afraid they had been lost forever.  Tom

Hi, Rod, et al.! For decades, most of my work responsibilities were Optical Testing. Our (very large) group was even called "Test and Technology Center", serving commercial and government programs.

Mostly interferometry, radiometry, alignments, scattered light, baffling, coatings, mirror supports, materials, and field support of instrument operations.

Right around 1/20-wave, the design, build and calibration of the Test Sets DOMINATED the difficulty of the task. Even factory-fresh interferometers (Zygo, WYKO, etc.) were certified to "only" 1/20-wave. They (very reasonably) assumed that the user would either use as-is (with that uncertainty affecting knowledge of the reported test wavefront), OR that he would calibrate the test cavity...

The Later, highly-desirable option, meant either having NBS-traceable references (typically concave spherical mirrors and flats), or doing absolute-cal in-house... an extraordinarily laborious procedure, developed by one of our own seasoned scientists (my boss!) So, you go to the NBS heritage... and they reference my boss!

Consequently, when I see "interferometrically tested" and superb numbers... I can't help but wonder about the traceability of the equipment. The computer will dutifully spit out all kinds of PV, RMS, Strehl, Zernikes, energy within a circle, impulse response, more. But that is always the combo of the tested article (e.g. telescope mirror), AND interferometer, nulling optics, mounting, etc. Even with laborious "back-outs" of the test-set, the residual uncertainties can be quite significant.

Yet, most houses seem to just USE the interferometer, it they're lucky enough to have one, and report the output. If asked, they proudly say they use a ~Brand Name~ interferometer.

Enter the challenge of parsing mirrors A, B, C... At small fractional-wave shoot-outs, the test environment dominates. Yet, many/most suppliers just flatly/proudly state 1/20-wave, ~diffraction-limited~ etc.; I can't help but wonder.

Even Hubble was delivered as... the most perfect telescope ever made.  Tom

Two and a half hour exposure?  Tom

I like Juan, but notice that he is extremely reluctant to stray too far from the Party Line, even when overwhelming evidence points contrarily. I guess most people are like that, even otherwise liberated, thoughtful ones, as we all like to self-identify. In his former life, Juan was so inculcated into the comforts of the Democrat plantation... he must still miss the simplicity of being... owned.  Tom

Very nice! That seems like a really good price for a 16-inch mirror. From beginning to end, receipt to delivery, several hours work for sure. And Al/SiO2 should be a very durable coating.  Tom

Very cool! On the interferometer, I Think I see what he is doing there? There's a Twyman Green configuration and (I believe) a Fizeau alternative. The Laser Pointer would be coherent enough to get good contrast fringes in the TWG, provided the arms are of approx equal length, and that is indeed adjustable. The Fizeau would be for testing powered mirrors, not sure how he would get coherence, could be hit and miss. There's a beat-frequency requirement that would need another level of adjustment, otherwise you might or might not get contrast, a very sensitive function of path length. BUT, nice and fundamental!

The Dob Build. I had just coincidentally, over the past couple of days, watched John Dobson's EXCELLENT YouTube tutorial on building a 16-inch Dob, from scratch, on the cheap. Runs 90-min. And takes one from finding a piece of glass thru grinding, polishing, figuring, testing, coating, building the entire mount, and Using it! Amazing.

It's been 45 years since I made my last mirror... and that got me to wanting to make another one, or TWO?! Like a 6-inch F/12 and then a 16-inch F/5. Hmmm... hmmm... ?!

55 years ago, I made everything CHEAP, because I honestly had no $. Later, I gradually got spoiled, and started buying things... really too much. Should return to basics!  Tom

Yes, I know! My 2nd mirror was a 6-inch F/12.5 that produced breathtaking images with a modest Orthoscopic eyepiece. Much later I had a (good!) 17.5 Coulter mirror in a scope that I made from... oil drums! And I DID have an off-axis 6.5-inch stop on that, which gave wonderful planetary images. I think I miss those scopes. My eyes were probably a lot better too.  Tom