Friday, May 30, 2014


140527 SBAU Telescope Workshop
Attendees:  TimC, TomW, JerryW, PaulW, MikeC, EdK, TomT

TomW made Ronchi pattern printout based on
his mirror's f5.9? final shape using macRonchi program.
More pics/vids at:
MikeC's pitch lap divot; white coating is embedded cerium
oxide on the pitch lap

MikeC w/ HP waveguide carriage
as a base for a possible mirror test setup
MikeC 10inch Ronchi not polished due to
pitch lap center divot?

EdK chordal polishing starting to smooth out outer ridge?

TimC donated 8 inch mirror 1st polish Ronchi result

PaulW showing TomT use of Registax5 program for bringing
out Jupiter moon shadows from Celestron NexImage5 frames

TomW getting TimC to use glycerine and detergent in polishing
compound to avoid stickage (worked for Sam grinding 10in)

Wednesday, May 21, 2014

140520 TimC comments

On 5/20/2014 11:21 PM, Tim Crawford wrote:

 Well, was that cool or not! We had 12 people including three students from UCSB tonight. I ground a little with my newly trimmed tool and freshly beveled Pyrex disc. The scratches and gouges are disappearing slowly. Paul mentioned one thing tonight very promising. Some of the lighter scratches and lesser gouges will disappear in the polishing stages. At that point we are smearing glass around at a molecular level. I think these were great comments that we can test in time. Thanks Paul!

Tom poured not one but TWO laps tonight. Thanks Tom!!!!!! We set up two different mirrors on the test stands. Both are on track to becoming great mirrors.
 Remember, we are working with wavelengths at this point- although the Ronchi patterns look sometimes different from ideal, they are very close to the end. It is just tweaking from here. There is a tendency to get overwhelmed by what you see through the tester. Do not be discouraged. You are very close! Stay in there. Don't rush- enjoy it. In the end, the absolute joy in looking through optics you have crafted is nothing short of spectacular.

At the near end of polishing, the laps Tom is pouring for us are just fabulous. In the past we poured pitch into dams created by using masking tape on the tools we were grinding with. Now, Tom and others are using molds they have purchased. But I'm the hands of a craftsman, these new laps can look flawless and clean! A true artist preserving an art form that would be easily lost in time if there were not people like you Tom.

My new disc that Christopher gave me, complete with a "king tooth" drawn on it is just great. I am pleased to say the Cerium Christopher purchased looks like it is going to be just fine. I was getting nervous with the deep red color of the Cerium. I thought it might be less than optical grade, but with just a little touch tonight, I realized it is really fine material and dispersed in water it looks a lot less red. It's good stuff. What a pleasure this class has become. Thank you all for your participation. We missed Jerry tonight. That would have made it an 11 on a scale of 1-10. Hope you are okay Jerry. We miss you. T

 [ChrisU small pitch lap tool not set to match his mirror, so contact surface shows spots of polishing compound in center of each square of pitch...possibly heat caused each square to sag?  tt]

[see more photos/vids: ]

Monday, May 19, 2014

150518 TimC mirror scratches continued

Greetings all,
We are scheduling a workshop for this week, Tuesday, May 20th. Bill, please have the East gate open for us? Thanks Bill. Please remember all, 2 cars only across the bridge. You may drop off heavier equipment at Broder and go park back in the lot. At the end of the evening  we can help transport your equipment back to your car. 

I had the incident happen again where something created a scratch in the mirror I am working on. I am saddened, frustrated and fascinated all at the same time. I would like your help with this. I've never had this happen before and I have decided to the right thing here and track down what is causing this to recur. Here is a pretty obscure image:
I did feel the edge grab this time. The brighter blip in the photo is a sub surface bubble. My first impression is that this is the tiles at the edge of my tool that are breaking off. There is no chip at the edge of the mirror although the bevel has gotten much smaller. Rather than grossly rubbing the edges with a sharpening stone, I will take these to work and use my dental hand piece with a diamond disc to soften the edges and create a new bevel. In our dental lab we deal with different alloys that have specs like Brinell Hardness or Vickers Hardness. In the workshop I think it will benefit our knowledge base if we can specify the hardness of Pyrex and the tools we use. In this case it is sheet tile purchased from the local tile shops. They come in sheets that are tied together with a webbing I think is rubber or nylon. Would it be better to have a softer tool material in case of fracturing or would that matter? Once a small chip of tile releases onto the surface of the mirror, it tumbles and my theory is it gouges the surface. I am not sure if a softer tile would leave the mirror untouched. Anyway, enough of this for now.

Christopher cut a round of plywood for me to create a pitch lap on. Thank you Christopher (cool picture you drew on it). I covered this with epoxy. It turns out we can get the better epoxy from Home Improvement Center. I tried Home Depot and OSH. They both dropped carrying Devcon materials. They carry Loctite but in an 8-9 ounce package it is inferior. In fact at OSH I talked with an employee who agrees. Too bad customers and employees can't influence purchasing department heads. Too bad cost trumps quality these days.

We may pour a lap- I will check with Tom. We will continue with projects and I believe we will be looking at a Schmidt-Cassegrain this week and trying to clean or plan cleaning the optics. Please let us know if you will need testers and racks this week. Possibly much going on. You may come a little early if you want. I think Bill and the Museum is okay with that. Please, no sooner than 6:30 though, I think that is the time we okayed. I will put out an email if it is not. Thanks all! The workshop is going great. We had 10 last week. Let's keep going and please remember, let's leave Broder better than we found it.


140513 SBAU Telescope workshop
attendees: TimC, TomW, PaulW, JerryW, DmitriiZ, TomT, MikeC, BruceM, EdK, Matt

Paul waiting outside Broder building at the Santa Barbara Museum of Natural History Mission Creek redwood tree and boulder area.
 Bruce machined laser pen holders for 1.25" eyepiece slides.
 TimC marked mirror with felt pen to examine after grinding to see if contact with tile tool is even.
 Santa Barbara Astronomical Unit Telescope Workshop crowding around TimC doing all the work.
Bruce showing Matt his C8 SCT telescope cleaning methods that had been recommended by PaulW.

Saturday, May 3, 2014

140503 From the Workshop TimC

140503 From the Workshop Tim C
Starlight comes to our telescopes from afar – essentially from infinity. This means that light comes to our scopes as parallel light. So, if we finish with a spherical mirror, will this be good enough? It turns out that, since we need this parallel light to converge to a point, we need our finished mirror to have the shape of a parabola. In the figure below the solid line represents a sphere, and the dotted line, a parabola. Imagine that the parallel rays of light fall onto the surface of these two figures. What happens to them after reflection?  For the sphere, these rays do not come together at one point. But, in the case of the parabola, they do.  Recall that in a previous article, you were asked to stand at the center of a sphere, holding a candle and to watch what happens to the reflected light. Since this light emanates from a single point (the radius of curvature of the sphere), it returns to this point after reflection. Starlight doesn’t enter our telescopes from one point. So, ultimately, we have to change our mirror’s figure to that of a parabola. How do we do this in our optical shop? 

It turns out we achieve different shapes for our mirror’s surface by using different grinding patterns as we fashion our mirror. There are many resources you can go to that demonstrate these principles.  One great one is the Stellafane website at Let’s get back to business: we have our 8" flat blank and our grinding tool.  For simplicity let’s assume our tool is a piece of plate glass covered with tiles glued to the glass. Begin by placing the tool face-up on the work surface. Spray a little water on it and sprinkle its surface with 60-grade Silicon Carbide. To preferentially grind the middle of your mirror, start with the chordal stroke.  The mirror is always on top for this stroke. And the tool is always on the bottom. Much of the mirror “hangs over” the edge of the tool so that the center of your mirror will frequently pass over the outside of your tool. This will cause more wear on the center of your mirror and, likewise, more wear on the outer regions of your tool. Your mirror becomes concave as your tool becomes convex.  Walk around your work as you grind. As you move around your work in a clockwise fashion, turn your mirror counter-clockwise. This “organized randomness” is actually the secret to mirror grinding. To me, it is one of the processes that, although simple, makes this process so elegant.
[from May, 2014 SBAU newsletter]