Eyepiece - expanding the limits of the Raspberry Pi

Make no mistake, the Raspberry Pi is an excellent piece of equipment.

Like any device, it has its limitations, and like any device, there are ways to expand those limitations with clever use of hardware and software.

One of the limitations is the number of communication pins available to the user without using an GPIO expander.

I want to perform a large number of sensor readings and position actuator operations using the Pi. I want to simplify the software that runs those actuators and sensors. Essentially, I want a much more powerful system controlling my microscope.

I can do this in two ways - one is to scrap the idea of using the Raspberry Pi, and to install a big, custom-made control system. The other is to farm out part of the computational load to other, small computers.

The first option sounds expensive, and will require everything to be written as a monolithic system - one that will require a lot of re-writing every time I want to make a change.

The second option, however ...

Enter the Arduino nano. This is a "single chip" solution - it is actually a tiny PCB populated with an AT Mega microcontroller, ancillary circuitry and a set of pins that turns the surface mounted device into a DIP package with a couple of connectors on top.

Programmed in a dedicated version of C, the device is all about sensors and control actuation. It is fast, it is cheap and, above all, can keep track of several stepper-motors, travel limit switches and indicator lamps (LEDs).

Thus, each cluster of stepper motors in this project will be overseen by an Arduino, keeping the workload of the Pi neatly centred on the task of running the microscope.

Of course, this means learning yet another programming language, but at least it is based on one that I have used before, if briefly.

Eyepiece - Back to the camera.

This afternoon I reached the point of being able to test the Raspberry Pi camera module against one of the eyepieces from the microscope.

In order to do this, I kludged together an illuminator (using four white LEDs) and a printed image that sits in the graticule tray - an object that is in focus at the same time as the image from the microscope objective.

By holding the objective to the camera, I was able to estimate the separation between camera lens and eyepiece required for the best possible image - as well as to determine whether any intermediate lenses would be required.

The test was unexpectedly an immediate success. With the camera sat against the built in eye-cup on the eyepiece (a Vickers x10 Complan), the (circular) test image almost filled the height of the rectangular image - meaning that with a little tweaking and with the creation of a suitable adaptor, the camera is essentially able to be used as-is.

The graticule image - this was scaled down to a 19.5mm diameter.

In the graticule image above, the edge of the graticule mount covers the edge of the image - the fine, black line representing the expected limit of the vignette.

First Test - captured image through the eyepiece

As you can see, the test image is rather grainy - this is because the image is of a tiny picture printed out using an ink jet printer. There is also some distortion, but this is mainly due to the graticule being somewhat misshapen.

A blurred artifact (from the 3 o'clock to the 6 o'clock position) is the visual effect of some de-lamination in this eyepiece's optics.

The image is not properly centred because the test system was simply being held together in my hand. The pale mark in the lower left of the image is a reflection off of the inside of the eye-cup.

Conclusion:
This test was more successful than anticipated, and will warrant a proper adaptor being made in the near future.

Next steps:
A paper and card prototype adaptor will be used to perform more sophisticated testing and adjustment of the optical system. An acrylic design will follow, which will be capable of supporting the camera and associated computer while attached to the eyepiece.

Eyepiece - more on the Vickers M55 microscope

I spent time yesterday building a chassis for the stepper mechanism. Because the microscope has been modified to accept a bracket for a video camera (one of the big, old, heavy ones - which I do not have), I was able to use the mounting for this to attach the base plate. This plate is 2.5mm aluminium salvaged from an old piece of equipment. It will need a coat of the appropriate colour paint before it is finished.

The fine focus knob

 - with the old video-camera mounting base attached

The stepper chassis attached with mounted stepper motor and drive belt.

Some drilling and fiddling, and I have a removable base plate with a stepper mounted on it. Since I'm still waiting for the electronic parts to arrive, that is as far as I can go.

Once the chassis was built, I turned my thoughts to the macro lenses. Swapping the micro optics for macro optics is a five-minute exercise. I tried the incident illumination, and found that it is useless for looking at rocks (no surprises there), so I got out the oblique illumination attachment (a mirror on a simple swivel mount).

This is the first time I have attempted to use oblique illumination using the macro lenses, and discovered that the adjustment screw was sheared off.

An hour later, I had removed the broken piece of screw and had to find a 6BA screw to replace it with. Luckily, I have a big pile of old clock-repair parts, including a selection of BA size screws.

(The pictures show the oblique illumination mirror with the new part - which needs a coat of paint)

Some work with a countersink and emery-paper on an old bronze motor bush produced a respectable head of the correct size to turn a long countersunk screw into a short screw with a smooth knob for a head.A spot of steel epoxy permanently fixed the screw in the knob (with an exposed slot head to enable release with a screwdriver, if necessary).





Now, the way the micro-focus mechanism operates is by moving the objective lens up and down - which has a coarse focus knob and a fine focus knob (which now has a stepper motor attached.

The macro mechanism, on the other hand, moves the specimen stage up and down, using a large knob and a friction brake. This is also the knob that is used to move the stage out of the way in order to swap the optics.

This knob is big and rather stiff (it operates a physically heavy piece of equipment, after all), and if I want to do image stacking using the macro system, it means another stepper to operate this knob. I also need to be able to lock the position as the stage is heavy enough that the mechanism drifts downward quite rapidly when released with the locking knob released.

The range of motion is enormous (15mm between micro examination and x5 macro) - and this means that I will need to be able to adjust and lock the stage elevation automatically. Since I am planning on using the macro optics a lot, then this is a change critical to the project.

I have managed to find a higher torque stepper in my parts bin (I may have something even heavier duty knocking around - somewhere), and so I will try that for size.