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SYFY WIRE Bad Astronomy

Did a telescope start a house fire in Arizona?

By Phil Plait

Well now, here's a story you don't hear every day: a telescope in Carefree, AZ may have caused a fire that burned part of a house. KNXV TV in Phoenix carried the story. I have comments below.

There's are also articles at AZCentral and at Firegeezer with some more details.

My first reaction was, "No way." Then I looked into this more, and now I think the 'scope may have indeed been the culprit.

The telescope is a design called a Schmidt-Cassegrain, or just SCT. I have one myself! It's a tube with a big mirror (called the primary) in the back that's curved. It reflects light back up the tube. At the top of the tube is a flat piece of glass (called a corrector plate) with a smaller mirror embedded in the middle. This reflects the light again down toward the bottom of tube, where it passes through the hole in the big mirror and into an eyepiece (and from there into your eye or camera). The inset diagram here shows how this works; click to newtonianate.

I called the Carefree fire department, and talked to Colin Williams, their Press Information Officer. He was very helpful, and told me more of what happened. The place where the fire was most intense -- and therefore the likely origin -- was on the ceiling, a few feet to the side of being directly above the telescope. Also, a few days ago a neighbor had noticed that there was an intense spot of light on the ceiling coming from the telescope. At the time, the owner didn't think much of it.

I'm thinking that the telescope may have been pointed near the horizon. The setting Sun happened to pass into the line of sight of the 'scope -- perhaps he was setting up to see the Transit of Venus. Most people who use an SCT have a device called a diagonal which reflects light at a 90° angle, making it easier to use (you don't have to stoop over as far to see through the eyepiece). For a horizontal telescope, this means the light coming out the back end could've been projected up onto the ceiling.

Here's a shot of the burned out 'scope:

You're seeing the top of the telescope there on the left. The corrector plate is broken, most likely due to falling debris. The telescope is shown as it was found, but you can't say for sure that's the position it was in when the fire started; note the debris is essentially on top of it. The telescope could've started off more horizontal and been whacked by something falling, knocking it into the position we see it here.

It does seem unlikely that the 'scope happened to be pointing at the Sun, but weirder things have happened. It's also possible that the primary (big) mirror was misaligned, and sent focused light straight up and out of the tube at a slight angle, causing the fire. That seems even less likely to me, though. Had the mirror been misaligned it would've been impossible to use, and the geometry is all wrong for that.

So I think what we have here is a likely scenario -- the telescope dunnit! -- but the details are unclear.

The lesson here? Telescopes are powerful instruments. When you're done observing, put away all the accessories and stow it in a safe place and position. And put the protective cover on it! At the very least that keeps the corrector plate protected from dust and scratches. And in this case, it may have prevented a fire, too.

Image credit: Rural/Metro Fire Department. Tip o' the dew shield to BABloggee John Santucci.

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