Posts Made By: Dave Erickson

very nice!

Hi Phil,
The method of figure control you mentioned works and in combination with a drop gauge for measuring CT you can assure removal of subsurface damage from prior abrasives.

A lot of work was done by the high power laser folks and industry in the early fusion optics days. From this work most shops developed minimal material removal protocols that are typically based on some assumptions of fracture depth for the prior abrasive or machining operation. To confirm these subsurface damage depths an acid etch using HF or Ammonium Biflouride followed by an examination with a surface profiler was used and-or the optic was sectioned and examined with an SEM. There is another method using TIR form a right angle prism hypotenuse that works as well… Very long story short there are minimums you can use to assure complete removal of subsurface damage from successive loose abrasive grinds. There are also equations you can apply for materials of varying fracture toughness.

Old style generators leave damaged surfaces akin to rough loose abrasive grinding we use for making our telescope optics. And in the early days those generators were the only thing available. Generating was followed with a 20 micron loose abrasive grind and removed ~0.005” of material. This was followed with a 9 micron loose abrasive grind where 0.003” was removed, a 5 micron grind was used and 0.002” were removed and the part was then polished. There are other nuances most high end shops consider trade secrets, but that’s the just of it. Softer materials are often scratched with 5 micron so we polished directly from 9 micron. It is very easy to pick up scratches from 5 micron and finer abrasives so most of the shops I am familiar with stopped at 5 micron. An operator running many spindles cannot give the attention needed for really fine abrasive grinding.

This protocol is likely over kill for your primary mirrors but is offered as a starting point. Good control of figure, using a method like you described, in concert with measured material removal will assure damage removal.

George Blahun said: “I’d just keep polishing and keep track of the quality with a laser pen on the front surface, there should be no spot on it when fully polished.” An excellent suggestion and exactly akin to what the better shops do when evaluating polished surfaces…

As a side note: Today’s precision generating machines don’t require any cleanup grinding if run correctly. Parts coming off a well run precision generator looked polished, and small lens surfaces can be final polished and figured using pads in a matter of minutes with figures better than ¼ wave.

Best regards,
Dave

and the Cad rendering

Hi Maciej,
Beautiful image thanks for sharing...

Regards,
Dave

Hi Floyd,
This is certainly the right place to go for an opinion. There are several spectrometers available for calibrating both the monitor and printer profiles. You might consider an i1Photo Pro 2 Color Management Kit available from B&H and others...

Floyd, another very nice image, looks good on any monitor!

Regards,
Dave

Hi Dave,
Very nice images and processing, looks from the performance like an excellent imaging system.
Regards,
Dave

Hi Chris,
Nice work on a tough object. The weather is a bit better here as well and I share your excitement for those summer skies...

Dave

Dave,
That's a very nice image...
Dave

Here is an image of the F/4 18" focus astrograph installed at Sierra Chaparral Observatory. Remote controlled and solar powered.

Doesn't seem to be much interest in this but I'll give it another try and show some glass

I set this project aside a few years ago to work on building Sierra Chaparral Observatory.

Attached is an image of the glass nearly ready for finishing. The 12.6" fused silica primary is complete. The fused silica secondary is generated. The Schmidt corrector is NBK7 and the lens elements are shop friendly crown and flint, nothing exotic here. All are ready for fine grinding.

With the observatory site nearly complete and more than a years site survey data a 12" F/7 seems a good first larger scope. I am reluctant to make such a slow instrument but want something for this time of year when the big hole in the Milky Way lets all that ancient light from those galaxies through, and image scale is important. I may opt for something else as more site survey data comes in, but this is the current project. If there is interest I'll post details of construction on my web site www.hbastro.com as the project matures...
Dave