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:
Here is a magnified view of a plate:
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:
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. http://adsabs.harvard.edu/full/1991BAAS…23.1347D
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