Ben Krasnow’s SEM work involved two different projects: building a microscope from basic components, then adding digital image capture to a commercial JEOL JSM-T200. The first showed how a home-built instrument could generate and scan an electron beam; the second used a Tektronix oscilloscope to capture the JEOL’s analog video signal and reconstruct it as a digital image. The oscilloscope did not create the microscope image—it replaced the camera-based capture step.
Two projects, two different kinds of SEM hack
Krasnow’s homemade SEM and his JEOL retrofit are related, but they solve different problems. The homemade instrument is an electron-optics and vacuum project. The retrofit starts with a working commercial microscope and changes how its video output is recorded.
| Project | What it changed | Image path | What the reported result establishes |
|---|---|---|---|
| Homemade SEM, described by Krasnow in 2011 | Built the vacuum chamber, electron gun, beam-steering system and detector as a hobby project. | A scanned electron beam produces a signal through a secondary-electron detector. | An early test was estimated at about 50 µm resolution; roughly 1 µm was a future goal, not a demonstrated specification. |
| JEOL JSM-T200 digital capture, reported by Hackaday in 2014 | Added digital capture to a commercial SEM’s analog video output. | A Tektronix MDO3000-series oscilloscope captured a frame; GNU Octave rebuilt the raster from the exported waveform data. | The report describes a digital image of a fly eye without first metal-plating the fly. |
How the homemade SEM works
A scanning electron microscope forms an image by directing an electron beam across a specimen and measuring a signal that changes as the beam scans. In Krasnow’s design, those functions depended on a vacuum chamber, an electron source, beam optics, scan deflection and a detector.
Vacuum chamber and electron source
Krasnow described a chamber sealed with a bell jar and evacuated by a mechanical pump. The electron gun used a tungsten filament; he later replaced the pre-formed filament used in the project. The gun requires high voltage: Krasnow’s 2011 project response states, “The electron gun is biased at -5000V.” That is a reported project value, not a recommendation for a power supply or a complete operating specification.
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Focusing and scanning the beam
After electrons leave the gun, electron optics focus the beam and steer it across a conductive specimen. Krasnow compared the optical principles to those used in a CRT. Scan deflection moves the beam in a raster, so the detector’s changing output can be mapped to positions on the specimen.
Turning secondary electrons into a measurable signal
The project used a secondary-electron detector in the Everhart-Thornley arrangement. Secondary electrons strike a highly biased phosphor, which emits light; a photomultiplier measures that light. Krasnow explained, “The nature of the highly-biased phosphor screen allows even single electrons to create photons, and those photons can be counted by the photomultiplier tube.” The detector signal is what makes it possible to assign brightness to each scanned position.
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What the resolution figures mean
Krasnow estimated about 50 µm resolution in an early test and described about 1 µm as an eventual target. Both figures are project-stage estimates from 2011, not independently validated specifications or a guarantee of the resolution another builder can achieve. Vacuum quality, beam stability, detector performance and alignment all matter, but the project figures do not quantify those factors individually.
How the oscilloscope captured a JEOL SEM image
The 2014 Hackaday report describes a JEOL JSM-T200 that Krasnow acquired from Sweden. It had been dropped during shipping but reportedly suffered only a loose CRT neck plug. The retrofit was not a homemade microscope: it digitized the output of this commercial instrument.
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Why use the slow “writing to photo” scan
The JEOL’s normal image-storage path used a Polaroid camera mounted at the screen. In its slow “writing to photo” mode, a scan took about a minute. That slower pass gave the beam more time to emit and collect signal, producing a cleaner image than fast live-video mode. It also provided time to capture a complete frame from the analog video signal.
Capture and reconstruct the raster
- Connect to the video signal: Krasnow connected a Tektronix MDO3000-series oscilloscope to the SEM’s analog video output.
- Trigger on the vertical refresh: The scope used the vertical-refresh event to synchronize its capture with a complete image frame.
- Store and export the waveform: The oscilloscope captured the frame and exported waveform data to USB.
- Rebuild the image in GNU Octave: Software located the horizontal-refresh pulses in the exported data and used them to reconstruct the raster as a digital image.
The report says this method produced a digital image of a fly eye without first metal-plating the fly. That is a result from the reported capture, not evidence that every specimen can be imaged without preparation.
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What a similar project would—and would not—require
The two projects point to different levels of effort. Reproducing the digital capture method requires access to a compatible SEM signal and equipment able to capture and export waveform data; building an SEM from scratch also requires vacuum hardware, an electron source, electron optics, scan control and a sensitive detector. The available project descriptions do not specify a complete parts list, pump model, power-supply model, safe operating procedure or set of settings that would make a build reproducible.
- Vacuum: The homemade project used a bell-jar chamber and a mechanical pump. No pump model or required vacuum level is stated.
- Filament: The project used a pre-formed tungsten filament and later replaced it. No specific replacement part or sourcing specification is stated.
- High voltage: The reported gun bias was -5000 V. That single value does not establish what supply, current limit, insulation, interlocks or other protections a safe system needs.
- Digital capture: The reported method used a Tektronix MDO3000-series oscilloscope with USB waveform export and GNU Octave. The report does not establish that any particular current oscilloscope or software setup will work unchanged with another SEM.
These are not low-risk bench projects: the design combines high voltage with a pumped chamber. Do not treat a reported bias or component category as a build-ready electrical or vacuum specification. Anyone considering construction needs appropriate high-voltage and vacuum-system expertise, as well as equipment and safeguards selected for the actual design.
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Make reported a comparison in 2012 between a $75,000 entry-level commercial SEM and a hoped-for hobbyist build costing under $2,000. Those are historical figures from that article: the under-$2,000 figure was an aspiration, not a verified completed-build cost, and neither number is a current price estimate. The reported material does not establish the present cost of building, restoring or operating an SEM.
The practical distinction is that Krasnow’s commercial-instrument hack reused a microscope that already had the vacuum system, beam column and detector. His home-built project tackled those systems as well as image formation. The published descriptions support that difference in scope, but they do not establish a current performance or cost advantage for either route.
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