Monday, August 27, 2012

Shooting the moon

It's raining again - not much chance of getting anything useful on the sky tonight.  I wanted to do a bit more with the frame of the moon that I took Saturday evening (In memory of Neil Armstrong).  When the first mosaic CCD cameras were built in the mid-90s, it became common to show their field of view by imaging the moon. To astronomers who had traded in their large photographic plates for tiny 800 X 800 CCDs, this was a welcome advance.  However, the surface brightness of the moon is higher than the daytime sky, and so one used to have to play tricks like partially closing the mirror cover, or occulting part of the telescope beam with the dome.  This was in addition to putting in a filter that gave you the part of the spectrum where your CCDs were least sensitive so you could take a 1 second exposure, which was about the limit of reliability and uniformity for the shutters we used.

In an earlier post, I noted that because of the design of the Bonn shutter we have on pODI, we can take exposures that have a very short effective exposure time but are still relatively uniformly exposed.  This is because the shutter consists of two sliding blades that are controlled independently by stepper motors. A short exposure can be obtained by effectively moving a narrow slit across the focal plane, with the two edges of the blades close together.  We first used this to take twilight sky flats even before the sun had set.

It was still an hour before sunset when I decided to take an exposure of the moon.  I put in the z' filter (where the sky is pretty dark and the CCDs are the least sensitive).  I set the telescope focus to the value I had used the previous night.  I tried a 5 millisecond exposure and I got about 3000 counts per pixel.  Good enough.  Finding the moon was not trivial.  John Thorstensen was my savior.  His Skycalc program gave me RA and Dec for the moon right now, right here.  It was very close.  Center it up, and - snap - 5 milliseconds - perfect exposure. About 30,000 counts per pixel in the moon.  The image that I posted was almost raw. It had the overscan level subtracted, and the display script tries to adjust the stretch to match up the individual OTAs, but it doesn't do so well for something so extended with such large dynamic range.

So tonight I did a very simple reduction.  I subtracted a bias and divided by a dome flat.  Here is the result.  This is just the central 3 X 3 OTA part of the focal plane.  You can see the three bad cells. You can see that the image display doesn't make the detector to detector gaps quite big enough.  But overall, pretty amazing.  The moon in the daytime with a 3.5m telescope.



  1. Amazing! Thanks for sharing this, Todd! -ag

  2. I think Tranquility Base is in (or near) the cell two cells to the right of the middle bad cell.