Charlie's Google Nexus 5X Ronchi and Foucault Tester

by brucesdad13, published

Charlie's Google Nexus 5X Ronchi and Foucault Tester by brucesdad13 Oct 27, 2016

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1798Views 95Downloads Found in Physics & Astronomy


Amateur Telescope Maker's Smartphone Tester

Based on Don Peckham's iPhone Ronchi Tester, this Thing is designed to exactly fit a Google LG Nexus 5X smartphone. The photos are of my cardboard and hot glue mock-up (functional but not aesthetically pleasing)!

Additional parts required:

  • Single edge razor blade (qty 1 @ $0.07 ea.)
  • Ronchi grating (print your own or buy commercially HOWTO
  • 35mm paper slide mounts (qty 2 @ $0.05 ea.)
    • Note: one for the razor blade and one for each Ronchi grating (for the various lines per in/mm; collect the whole set!)
  • 5mm Green LED (qty 1 @ $0.14 ea.)
  • Potentiometer (for variable brightness ~$0.76) or resistor (for fixed brightness $0.03)
  • XYZ linear translation stage with tilt ability... I'm planning to incorporate Musti's DIY Linear Translational Stage - Micrometer resolution
  • Small amount of wire to run from LED to potentiometer to power source e.g. 9V battery
  • DC power source
  • Dollar store red laser pointer for aligning the optics and tester

For more information on Ronchi and Foucault testing as pertains to amateur telescope making (ATM) see the Stellafane ATM testing guide. For the build thread on this tester [see my Cloudy Nights forum thread] (http://www.cloudynights.com/topic/553139-modified-dons-ronchi-tester-for-nexus-5x-smartphone/?hl=%2Bronchi#entry7483512).



  • 2016-10-30 1944 Eastern - Calling V5 good enough. It's possible there will be tweaks and mods over time. Considering a separate project which would be a parametric thin smart devices stand for Ronchi/Foucault testing. e.g. could fit all generations of iPhones, Androids, etc..
  • 2016-10-30 1102 Eastern - V5 printed well with supports generated by Slic3r. I ended up drilling the mounting holes to 1/4" to fit my Newport linear translation stage. Photos now show the gold version that works well along with its predecessors.
  • 2016-10-29 0951 Eastern - Back to the drawing board. Removed my attempted supports. Added a recess for the potentiometer alignment pin. Added voids along base to combat shrinking ABS and reduce plastic required. Added 1/4"/M6 mounting holes for XYZ stage. Added room for LED lip.
  • 2016-10-28 1846 Eastern - First printing ended with some spaghetti. Realized it was asking too much of the slicer to print without support. Decided to add in supports myself that can be cut out later so it'll be mostly clean in v4
  • 2016-10-27 1419 Eastern - Cleaned up solid and fixed errors with inside faces etc; added a knob to the potentiometer; other slight mods... getting close to first printable STL
  • 2016-10-27 1218 Eastern - Added slide holders, openings for fingerprint scanner and flashlight, chamfered and reduced amount of plastic, other tweaks...
  • 2016-10-27 0950 Eastern - Initial version with a few remaining items to implement including mounting holes for optical linear translation stage 1/4" or M6 type as well as possible screw for tilting knife edge (razor) into the returning light cone for Foucault testing.

Print Settings

Printer Brand:




Doesn't Matter








I tried ABS twice and wasted lots of material. If your printer is well calibrated it could work. Hatchbox PLA on unheated blue painters tape worked perfectly. Recommending PLA.


Mirror Testing

Position the tester near the radius of curvature of the mirror to be tested. In the past I used a laser pointer to align the optics. I had planned to add a laser mount into the design eventually. Then it occured that the builtin flashlight capability of the Nexus 5X should work. It's a high intensity double LED beam that should return back to focus at the radius of curvature!

Align the optics as best you can. Remove the slides. Lower the room lights and turn on the phone flashlight feature by pressing the power button, swiping down the menu, and selecting flashlight. If your rough alignment was fair you may see the return beam (bright white spot) on your shirt or the wall behind you. Adjust the mirror test stand until the return beam hits below the green LED on the slide holder. Exact positioning shown in the photo. This can be somewhat time consuming until you get the habit.

Reinsert the slides with either the knife edge or Ronchi covering the green LED. For Foucault testing the razor should block out 1/2 of the LED and camera lens in what's called a slitless tester. In software you will denote this as a "moving source" tester. The resulting image will show some vignetting (keyhole) so zoom in to the maximum. If you are a little outside or inside the radius of curvature (RoC) you'll see hopefully vertical straight lines for a perfectly spherical mirror.This worked well for my 8" mirror.

Excellent instructions on Ronchi and Foucault testing are available in various places. I recommend the Stellafane ATM testing guide.

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