Archive for November 22nd, 2010

Spectrometer: Quick and Dirty Camera Mount

This is a proof-of-concept lashup for a camera-mounted spectrometer; I wanted to find out if the image processing would work, but needed some images without devoting a lot of time to the hardware.

The general idea is that a direct-view spectrometer produces a focused-at-infinity image for your eye. Substitute a camera for your eye and you get an image with the spectral components laid out in a spatial array, suitable for measurement and calculation.

The trick is holding the spectrometer on the lens axis while blocking ambient light. I figured that I could mount the spectrometer in a disk that fit into the camera’s 58 mm filter threads, then hold it in place for the few pix I’d need to get started.

The end result was Good Enough for the purpose, although it’s definitely a kludge…

Spectrometer mounted on camera

Spectrometer mounted on camera

The (admittedly cheap) prism-based direct-view spectrometer has a slide-to-focus mechanism that substitutes heavy grease for mechanical precision. A guide screw in a slot prevents the focusing tube from rotating in the body tube, so I decided to replace that with a locking screw to clamp the tubes together. It’s a very fine thread, undoubtedly metric, screw, but a bit of rummaging in my teeny-screw drawer turned up a match (those are mm divisions on the scale):

Spectrometer screw vs standard thread

Spectrometer screw vs standard thread

I think the spectroscope makers filed down the head of an ordinary brass screw to fit the slot, rather than using an actual fillister screw. That’s a Torx T-6 head on the flat-head screw, which probably came from a scrapped hard drive. I eventually found a round-head crosspoint screw (requiring a P-1 bit) that worked better, with a brass washer underneath for neatness.

That got me to this stage:

Spectrometer with locking screw

Spectrometer with locking screw

Making the adapter disk involved, as usual, a bit of manual CNC to enlarge the center hole of a CD from 15 to 15.75 mm, then cut out a 57 mm cookie. A stack of CDs makes a perfectly good sacrificial work surface for this operation, with some fender washers clamping the pile to the tooling plate. Those homebrew clamps are smaller than the Official Sherline clamps and work better for large objects on the small table.

Milling outside diameter

Milling outside diameter

I briefly considered milling a thread into the OD, but came to my senses… I still have that pile of 10-32 taps, but now is not the time!

While in the Machine Tool Wing of the Basement Laboratory, I bored a short plastic bushing to a tight slip fit on the focusing tube to clamp the disk to the eyepiece, with the intent of keeping the eyepiece from whacking the camera lens. That’s the small white cylinder in the first picture.

As it turned out, I had to mount the whole affair on a sunshade that screwed into the camera filter mount, because the eyepiece protruded far enough to just barely kiss the lens.

A liberal covering of black electrical tape killed off all the stray light. Hand-holding all the pieces together and aiming it at the CFL tube over the Electronics Workbench produced this First Light image:

First light - warm-white CFL - no adjustments

First light - warm-white CFL - no adjustments

Believe it or not, that’s pretty much in focus. Much of the width in the red & green lines seems to come from the phosphors, as there’s a bar-sharp narrow blue line to the far right, beyond the obvious blue line.

Settings: manual focus at infinity, manual exposure 1/60 @ f/2.4, auto ISO = 100. Maybe 30 cm from the 27 W CFL tube: way more light than I’ll ever get through a liquid sample in a cuvette.

Now to fiddle with ImageMagick and Gnuplot…

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