Arlo Landolt

The importance of calibration.

If our astronomer is a bit subdued these days, a little less boisterous and outgoing, it’s probably because in the last year he’s lost some friends.  Two losses especially are felt deeply by all astronomers.  But it’s highly unlikely you’ve heard of either one.

Arlo Landolt was known in particular for his photometric standard stars.  To understand their importance, we have to describe photometry.  In astronomy, it is the practice of measuring the brightness of something, a star or a planet or a galaxy or whatever.  One measurement of brightness can tell you a surprising amount about whatever you’re looking at.  Several measurements can tell you if it changes in brightness.  Many measurements can lead to a detailed physical understanding of the object.

But they have to be calibrated measurements of known accuracy.  And the number of photons you get on your detector can vary with the telescope you use, the details of your reduction pipeline, the state of the atmosphere, how long it’s been since the mirror was last cleaned; many things known and unknown, somewhat controllable as well as utterly out of your control.  You need something to compare your measurements to, a photometric standard, to correct for all these things.  In principle all astronomical brightness scales are based on Vega, a bright star in the northern hemisphere.  But it’s impractical to test you equipment on Vega.  It’s far too bright for most research telescopes; and it’s invisible from most of the southern hemisphere, and from the northern hemisphere most of the time.  You need something not much different in brightness from your target and not too far from it in the sky.

That’s where Arlo came in.  He marked out some hundred or so star fields around the celestial equator, and painstakingly (that’s almost too weak a word) measured the brightness of a number of stars in each, relating them to primary standards and eventually publishing all his data.  Our astronomer has a treasured, dogeared printout of the main part of his catalog, a document he carried to each observing run on each telescope he ever used.  In themselves, the Landolt standards didn’t actually do any science.  One could not conclude anything based on them.  But the accuracy they allowed to decades of astronomers led to many, many conclusions.  Although his work was not the sort to win Nobel prizes, our astronomer is convinced that it enabled some, if not directly then at one remove.  (We’re happy to say that astronomers were well aware of this, and gave Arlo a number of awards in appreciation.)

Arlo’s sort of astronomy is not done any more.  The setting up and observing of standards is automated, built into the software pipelines of the great, new observatories and space missions.  Individual efforts of his sort have been superseded by large collaborations.  And, of course, it’s good that such sustained manual effort isn’t necessary.  It does, however, mean that the way astronomy works is different.

It would be a cliche to say we won’t see his like again.  And an astronomer who wore a bow tie while observing all night, after having flown from Louisiana to Chile, would be nowadays be eccentric or affected, not being himself as Arlo was.  But we can hope that there’s still a place in the science for someone as gentle, kind, friendly and helpful as Arlo.