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JR's Ronchi and Foucault Tester Page

I started making telescopes in 1999,but have never made my own mirror tester.   In the last year or two have I seriously questioned the quality of some of the mirrors I own.   For example, I recently came into posession of a small five inch f/8 Meade, and noticed it was unusually hard to focus.  I began to wonder if it was the mirror, or something else.  I also have recently acquired two 12.5" mirrors, one from R.F. Royce and another from an eBay store.  I want to see what a .86 Strehl looks like vs. a .96 Strehl.   One day, when I get the garage cleaned out enough to accommodate a mirror-grinding area, I will of course need a tester.  For all these reasons, and for the time to do it, between Christmas 2006 and New year's day 2007, I made and successfully operated a foucault tester for the first time.  Here are a few snapshots of that experience. Hopefully it will help someone else. I'm sure more experienced mirror testers could tell a lot more than I about the mirror from even the first focographs below.

I decided to use the Stellafane plans for building the tester. It called for materials I had laying around and knew how to work with.

First of all, here is my shop. As you can see I am totally organized.

I simultaneously have many project going on: A Mel Bartels telescope motorization project, a mineral cross-sectioning project, and several telescopes in various stages of construction. You have to be good to be this organized!

Here are the main parts of the tester, already cut but not assembled.

This is the LED I used. It is green but is not the jumbo one shown on Stellafane's page. So far it has worked just fine.:

Front of the tester. I did not add the "focusing aids" yet.


In this view you can see:

  • In the upper-left center, the viewing hole, LED with black rubber washer surrounding, and knife edge. The purplish area on the right of the viewing hole is the right side of the credit-card plastic frame that houses the knife edge.
  • Upper right: The vertical KE rotator knob and Stanley tape measure.
  • Lower left: vee-block of the stage resting on the base slider rod.
  • Lower center: dial gauge and forward-left corner of the supporting block.
  • To the right of the dial gauge, the stage tensioner spring.
  • Above the tensioner spring, the on-off pull switch. It was from a flourescent light fixture. when I installed it and it turned the LED on, it had the overall effect of a tiny neon sign. WAY cool!
  • Note the dial gauge and the OA adjuster press against the same credit-card plastic on the edge of the stage. It was actually a room key from the Venetian hotel in Las Vegas. This may be the my classiest tester ever!

Mirror side of the test head, showing the simple LED battery setup and the for-now duct-taped knife-edge. I used a utility knife with the ends snipped off by tinsnips in a credit-card frame.

Top View

Here you see the top edge of the head's face and side braces., with the wiring in between. The power source is two AA batteries duct-taped into a "pack". This tester was made exclusively out of scrap from my garage, except for the dial gauge and the small-gauge monofilament line used to make the Ronchi screen (not shown).

Side View

From this perspective we can see the base on the table, supporting the stage, supporting the test head.

This is the first mirror I tested. It was VERY dirty! I cleaned it before I tested it of course. I clean mine with Windex and damp paper towel. RF Royce recommends a certain brand of towel (I couldn't locate the link) for more sensitive care of expensive optics.

The first test setup

Despite the primitive conditions such as the evil remnants of a boston fern, a cat turd on the floor (from a recent holiday escapade involving a turkey and some other unmentionables), and almost no room to spare, my first setup worked well enough to test the tester, to teach me the value of making a good test stand, and to finally and directly open my eyes to the true nature of telescope mirrors. It was here that I first felt the rush of First Light from something besides a telescope.

First Light

The mirror is lined up with the tester and we are near the ROC. We can see the LED fill the entire mirror with diffuse green light.

First Image

Knife edge is near the OA and we are near the ROC.

Second Image

Picture is out of focus because I am holding the camera by hand - no mount! Nevertheless, some information is visible.

Third Image

Able to see an edge, and that we are outside the mirror's focus (shadow on the right).

Fourth Image

The left darkness is not the mirror but the knife edge. We can see the right half of the mirror and see some surface features - note the dark spot in the upper left, the edge features, and an apparent radial darkening towards the center. Still not quite there.

First Light

Here I first managed to capture an image showing some difference of focal points of the mirror's zones. As you can see I did not place my center spot exactly on the center of the mirror (I am about 5 mm off)

Second successful picture (Fuji E500 on automatic, no flash, held by hand) this time showing the entire surface more clearly.

The dual images are artifacts of the camera shake, not hte mirror. A stationary camera would have shown a single, bright edge image.


Next Steps
  • Build a test stand. The stack of magazines and business cards to alight the mirror simply won't do. On the other hand, how does one put a mirror on a test stand that is already glued in place to its cell? I suppose I will simply separate them and re-glue the mirror later.
  • Build a Ronchi screen from a credit card and the cool .004 monofilament line I got from Wal-Mart. Monofilament line: six dollars. Old credit card and hot glue: zero dollars. No substrate interference: priceless. Herding photons with your own stuff: priceless. A six-dollar ronchi screen performing better than a hundred-dollar ronchi screen: priceless.
  • Secure the head to the stage. I kept bumping it when I was getting started and taking pictures.
  • Make an Everest pin or Couder masks.
  • Take quantitative measurements.
  • Analyze the measurements and produce meaningful results.
  • Test more mirrors!

      Created: Jan 1 2007 hits since then: Hits on this page