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Stellar Photometer

Building a stellar photometer clearly goes against the current trend. Almost all amateur photometrists use CCD cameras, and yet I have completed a complicated project to build a stellar photometer based on a photomultiplier. Why?. I could give two main reasons:

  • Using a photometer allows you to take photometric measurements that are different from the usual ones for a photometric amateurs, so you can go into less explored areas. An example might be high brightness variable stars, but as this photometer (based on a photomultiplier) is built, other very interesting areas such as high speed photometry arise.

  • A second reason is that doing photometry with a photometer based on a photomultiplier is literally "counting electrons" which allows you to approach photometry much closer to the underlying physics. That is, we can make the measurements more physically meaningful and learn a lot about them.

The stellar photometer is an instrument attached to a telescope that allows us to measure the luminosity of celestial objects, for example, stars. Before the use of CCD cameras for photometry, photometers were the most widely used professional instruments for measuring the brightness of stars.


There are basically two types of photometers: photoelectric ones in which a solid state detector based on the photoelectric effect is used, and those in which the detector is a photomultiplier tube.


Photometers are instruments rarely used by amateurs, which are mostly declined by the use of CCD cameras for photometry. As a photometric instrument it has some advantages over CCDs, for example in the measurement of bright stars, high-speed photometry, etc. The main drawback is that it is more difficult to measure weak stars and more complex to automate measurements.


At amateur level, it is possible to find second-hand photometers from the manufacturer Optec, specifically the SSP-3 model for a price of around 150€. This device is a manually operated photoelectric photometer. There is a superior model, called the second generation SSP-3a, in which measurements can be automatically recorded via a serial port connected to a computer. However, this equipment is not second-hand and new and has a price of about 1500€, or if we talk about the model that includes the automation of the filter about 1800€. Optec manufactures a version with photomultiplier tube, the SSP-5 model, in its initial version with manual operation, and currently available with data output and optionally with filter automation. This equipment is twice the price of the equivalent SSP-3 models.


My goal is to manufacture a photometer with the following features:


  • Based on photomultiplier, which allows measuring stars weaker than those based on photoelectric detectors.

  • With full automation:

    • Data output for computer connection

    • Centering system with integrated camera

    • Automated switching from star centering to measuting mode

    • Automated filter selection system

The following pdf describes in detail how this equipment was built and how it works.

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