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Guy's Math & Astro Blog

Guy's Math & Astro Blog

Monthly Archives: November 2018

First Light on an 8” Scope and 1/4 Lambda on a 6” Mirror — All in One Night!

17 Saturday Nov 2018

Posted by gfbrandenburg in Uncategorized

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Tonight, Jim K essentially completed his 8″ telescope by putting in the primary mirror, positioning it correctly in the tube by focusing it on the Moon, and achieved FIRST LIGHT!

He also put on the Telrad finderscope and used it to aim the scope accurately on the star Capella with no difficulty at all. I did a brief star-test on that star and found that the scope passed with flying colors! Jim started grinding the mirrors back in the 1970s, put it aside, and brought it to us for help in doing the final polishing, figuring, aluminizing, and designing and constructing the telescope. It looks great and works well, too!

In addition, Pratik T may have finished figuring his 6″ f/8 mirror that he’s been working on. Using the Foucault/Couder knife-edge test measurements I made, the program FigureXP declared it to be 1/4 lambda error on the wavefront. This may be good enough, but more testing would be a good idea, later on.

We are closed all of next week for the holiday.

Some Progress – AT LAST! – With Figuring the 16.5″ f/4.5 Thin Mirror That Headlines This Blog

10 Saturday Nov 2018

Posted by gfbrandenburg in astronomy, Hopewell Observatorry, Optics, Safety, Telescope Making

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Tags

Bob Bolster, George Ritchey, Grinding, Hopewell Observatory, matching Ronchi, Mel Bartels, Polishing, Ronchi, ronchigram, Telescope Making, testing

I have been wrestling with this mirror for YEARS. It’s not been easy at all. The blank is only about twice the diameter of an 8″ mirror, but the project is easily 10 times as hard as doing an 8-incher. (Yes, it’s the one in the photo heading this blog!)

Recently I’ve been trying to figure it using a polishing/grinding machine fabricated by the late Bob Bolster (who modeled his after the machine that George Ritchey invented for the celebrated 60″ mirror at Mount Wilson over a century ago). That’s been a learning exercise, as I had to learn by trial and error what the machine can and cannot do, and what strokes produce what effects. The texts and videos I have seen on figuring such a large mirror with a machine have not really been very helpful, so it’s mostly been trial and error.

My best results right now seem to come from using an 8″ pitch tool on a metal backing, with a 15 pound lead weight, employing long, somewhat-oval strokes approximately tangential to the 50% zone. The edge of the tool goes about 5 cm over the edge of the blank.

This little movie shows the best ronchigrams I have ever produced with this mirror, after nearly 6 hours of near-continuous work and testing. Take a look:

And compare that to how it used to look back in September:

 

Also compare that to the theoretically perfect computed ronchigrams from Mel Bartels’ website:

perfect theoretical ronchigrams for guy's 42 cm mirror

Part of the reason this mirror has taken so long is that after grinding and polishing by hand some years ago, I finally did a proper check for strain, and discovered that it had some pretty serious strain. I ended up shipping it out to someone in Taos, New Mexico who annealed it – but that changed the figure of the mirror so much that I had to go back to fine grinding (all the way back to 120 or 220 grit, I think), and then re-polishing, all by hand. I tried to do all of that, and figuring of the mirror, at one of the Delmarva Mirror Making Marathons. It was just too much for my back — along with digging drainage ditches at Hopewell Observatory, I ended up in a serious amount of pain and required serious physical therapy (but fortunately, no crutches), so this project went back into storage for a long, long time.

Recently I’ve tried more work by hand and by machine. Unfortunately, when I do work by hand, it seems that almost no matter how carefully I polish, I cause astigmatism (which I am defining as the mirror simply not being a figure of rotation) which I can see at the testing stand as Ronchi lines that are not symmetrical around a horizontal line of reflection. (If a Ronchi grating produces lines that look a bit line the capital letters N, S, o Z, you have astigmatism quite badly. If astigmatism is there, those dreaded curves show up best when your grating is very close to the center of curvature (or center of confusion) of the central zone.

Using this machine means controlling or guessing at a LOT of variables:

  1. length of the first crank;
  2. length (positive or negative) of the second crank;
  3. position of the slide;
  4. diameter of the pitch lap;
  5. composition of the pitch;
  6. shape into which the pitch lap has been carved;
  7. amount of time that the lap was pressed against the lap;
  8. whether that was a hot press or a warm press or a cold press;
  9. amount of weight pushing down on the lap;
  10. type of polishing agent being used;
  11. thickness or dilution of polishing agent;
  12. temperature and humidity of the room;
  13. whether the settings are all kept the same or are allowed to blend into one another (eg by moving the slide);
  14. time spent on any one setup with the previous eleven or more variables;

Here is a sketch of how this works

bolster's ritchey-like machine

Major Moving Day at Hopewell Observatory

04 Sunday Nov 2018

Posted by gfbrandenburg in Uncategorized

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Yesterday we moved a lot of heavy metal and glass to temporary quarters so that we can mount a modern, heavy-duty Astro-Physics 1600GTO mount on one of our piers.

One of our founders, Bob Bolster, had built with his own hands a very unusual 30-cm Wright-Newtonian telescope and an equatorial mount on a permanent pier. Unfortunately, the drive stopped working and he was unable to get it back into working order before he died. So yesterday we removed it from its mount – and it took five of us with a 2-ton chain hoist, lifting straps, and a custom-built cart to winch it out of the observatory and into our operations cabin.

We all had fun doing it, nobody got injured, nothing got damaged, and the night that followed was the clearest one I’ve seen in a long time! Great open clusters in Cygnus!

I attach some videos and photos of the move.

By the way, that large disk of optical glass you see in the last few photos is for sale. We aren’t sure what type of glass it is, but you can find details here at Cloudy Nights or Astromart. It is 55 cm across, 83 mm thick, and we measured its weight as 59 kg (130 lbs). We calculate its density as 2.99 g/cm^3, and the nearest match in the Schott catalog is N-SF64 which has that exact density, but N-KZFS4 and P-SK57 are close as well (3.00 and 3.01, respectively). It’s definitely way too dense for Borofloat, Zerodur, or any other borosilicate glasses. The glass known as BAK-4 has density 3.05, which I don’t think is close enough, since there are several others between 3.00 and 3.05.  For comparison, the relatively inexpensive optical glass known as BK-7 has a density of 2.5.

We are asking $950 for the blank. If you were to order a brand-new one from any optical glass company it would cost you much, much more!

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