Astronomical Engines, Geeky Astronomy

Rooting Out New Celestial Objects

The Zeiss blink comparator at the Lowell observatory

Greetings, everyone.

Have you ever wondered how astronomers find new asteroids? Or spot new comets? Or even how they can spot a new supernova? Even looking through a big telescope, with all the billions of tiny little light specks visible, how in the world can you tell if something has moved or changed brightness?

The answer is a device called a blink comparator. These devices allow two photographic plates of the same region of the sky, but taken at a time interval apart, to be alternately viewed back and forth, and anything that has changed positions or intensity will appear to blink. A great many astronomical discoveries have been made using this device, one of the most notable was the discovery of Pluto by Clyde Tombaugh in 1930.

The blink comparator was developed from a device called a stereo-comparator, invented by Carl Pulfrich in 1900. In 1904 Pulfrich devised a monocular microscope attachment (he had but one eye) and a mirror mechanism that diverted the line of sight alternately between the two plates. The Carl Zeiss company marketed the “blink-mikroscop” as an attachment for their stereo-comparator that converted it into a blink comparator. The picture at the top of this article is of a Zeiss device.

To use the device, a glass photographic plate is mounted on each of the two easels and illuminated from behind. The plates must next be aligned so that they are in register, and show the same sky position. This is done by handwheels  on each easel that move them vertically and horizontally, while the astronomer compares to two fields through the microscope. Once that has been done, the astronomer can then traverse the plates methodically looking through the microscope while a motor operates the blinker mirror. There is also a manual control for the mirror.

Checking plates with the comparator must be an exercise in patience for an astronomer. In the top photo of the Lowell observatory’s blink comparator, the dirty looking piece of glass on the left is an actual photographic plate of the type used by the observatory. In fact, it is a copy of one of the plates used by Tombaugh to find Pluto. Here is a picture of one of the original plates:

Original Pluto discovery photographic plate in the Smithsonian Air & Space Museum

Here is a magnified view of a plate:

Not the Pluto Discovery plate. Click to enlarge.

The plates appear to be about 14”x17” (according to my calibrated eyeball), and the images on the plate look like dust specks. Very small dust specks mostly. Your job is to carefully scan the plates by viewing the flickering image through the microscope eyepiece, moving it across the plates using handwheels. And when you’re done with this set, there is at least a dozen more sets to be checked. The human eye is very good at spotting motion, but the human brain gets bored and forgets what it’s supposed to be doing.

You are obviously a graduate student or a low ranking assistant, as the head astronomer isn’t going to have the time or the desire, most likely, for that sort of analysis. Also, it’s not a good job for someone with photosensitive epilepsy.

Here are positive images of the portion of the plates showing Pluto:

Magnified portion of plates showing Pluto’s image.

The blink comparator was used at least into the 1990s. I know this because I saw a documentary about the comet Shoemaker-Levy 9, and Carolyn Shoemaker demonstrated how she spotted it using the Palomar comparator and plates from the 48 inch Schmidt camera.

As more observatories started switching to digital photography and charge-coupled devices, computers and software replaced the analog blink comparators and operators to search for objects in captured images. I doubt if any of the old devices are still in use, but you never know. It’s still a damned good use of graduate students.


Notes on the History of the Blink Comparator by L.F. Drummeter, Jr.…23.1347D

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  • mdharrell

    It's not quite the same, but if it's any comfort, a friend of mine has been involved in running the Summer Science Program for the last few years. They still make the students work with film in determining the orbit of an asteroid:

    Even there, however, much of the project now comes down to CCDs and digital processing.

    • The Professor

      That's a good looking program. Having the students observe and calculate the asteroid's position and orbit the 'old' way will give them a better understanding of what they're actually doing, rather than just relying on making inputs to a 'black box' setup. Getting them used to drudge labor is also an excellent idea. Prepare them properly for the real world and graduate classes.
      Nice amateur equipment that they're using, the lucky little sods.

  • Maxichamp

    When I was a kid, I was a huge astronomy buff. My step-dad took me to meet Clyde Tombaugh. By then, he was a tiny shrunken man with a cool bolo tie. I still have a picture of the two of us, both with grins on our faces.

    • The Professor


  • BlackIce_GTS

    This makes me wonder what other interesting scientific equipment was use before the answer to "Have you ever wondered how ____s ___ _____?" was "with computers, obviously."
    How did they model protein folding? Pipe cleaners?

    • mdharrell

      I think in the case of protein folding the answer is they didn't model it without computers, except in a very qualitative way for some relatively simple cases. I took my first x-ray crystallography course from a protein crystallographer who was amused by the charming little unit cells of inorganic crystals (such as minerals). Even the relatively simple task of cranking through the determination of the static structure of a typical denatured, crystallized protein takes a healthy amount of calculation. Folding is much worse.

      For a lot of other pursuits, however, the answer is that they used some quite clever equipment that now mostly sits and gathers dust, assuming it wasn't scrapped long ago. Heartbreaking, really.

      • The Professor

        It sounds like your instructor was a bit of a tit. I take a rather dim view of people that look down upon your chosen field simply because it isn't as ludicrously complicated as their is.

        • mdharrell

          I agree with the sentiment, but in this case he wasn't so bad. It cut both ways, anyway, because the handful of geologists in the course always got a kick out of his attempts to include us via the invocation of half-remembered minerals as examples. He tended to inadvertently mangle the names by making them sound more chemistry-ish, so "orthopyroxene" and "clinopyroxene" became "ortho-oxy-pyrene" and "clino-oxy-pyrene." Good times.

          Well, maybe you had to be there.

    • The Professor

      I don't know about protein folding, but a lot of molecular visualizations were done with those ball and stick models that you see in some chemistry labs. Think of those big models of DNA molecules that you see in publicity photos, that sort of thing.
      I don't know how you would go about modelling proteins without a computer. They're mind-bendingly complicated structures.

  • Mason

    2/22/2017-CNN)Astronomers have found at least seven Earth-sized planets orbiting the same star 40 light-years away, according to a study published Wednesday in the journal Nature. The findings were also announced at a news conference at NASA Headquarters in Washington.
    discovery outside of our solar system is rare because the planets have
    the winning combination of being similar in size to Earth and being all
    temperate, meaning they could have water on their surfaces and