Sunday, August 20, 2017

CCD Cameras and Frost

September 11, 2009 by  
Filed under Articles, Equipment

A couple of years ago after owning my SBIG ST2000-XCM camera for about 10 months, I ran into a situation that puzzled me.  I was at Anza and started imaging M42 so I decided to start with 5 minute subs.  The first shot looked pretty good.  No field of view rotation, guiding looked good, in focus…a perfect start to a nice night of imaging, right?  Wrong.

Each subsequent frame started to slowly degrade from the edges in.  Nebulosity started showing where it shouldn’t be and stars were starting to diffract every which way just like the unprocessed shot to the left.

Since this was something I had never seen before, I kept taking a couple more shots until the last shot clearly showed the culprit…FROST!  It was like looking through a window in the dead of winter with the center of the frame still fairly clear but with crystal like spider veins on the edge of the frame.

M42 Frost, -5C

M42 Frost, -5C

Well, for frost to form, two things need to be present.  A surface at or below freezing, and moisture.  Now, what could I do about the moisture?  Nothing at all.  I could do something with the temperature level of the CCD Chips though.  I was running at -5 degrees Celsius, so I decided to raise it to 0 degrees Celsius.  I know what you’re gonna ask, 0 degrees is still freezing so why not go to 5 degrees?  Well, the chip itself is at 0 degrees at the back surface but the front surface is just slightly warmer due to it being exposed to the air inside the camera body.

After letting it warm up for about 30 minutes, I started taking shots again and could clearly see a difference.  Most of the frost had melted and is evident in the stars at the left, bottom and right edges.  M42 itself is a bit sharper and the nebulosity isn’t artificially enhanced.  What is also evident is the amount of noise from the increase in temperature.  Eventually all of the frost disappeared and the rest of the shots I got that night were usable.

M42 Less Frost, 0C

M42 Less Frost, 0C

After I got home, I immediately read over the manual and followed the direction on how to recharge the built in desiccant.  Fortunately SBIG’s design of the ST2000-XCM desiccant plug is very simple.  It simply screws out of the body, replace it with a dummy plug to prevent anything from getting in the camera and recharge it following the recommended procedure.

This happened almost after a year of use and in the fall, right after our monsoon season which makes sense since it was so humid.  So here are a few tips to follow to minimize the chance of your CCD Chip frosting over.

  1. Store your camera in a cool, dry place
  2. Throw a pack of silica gel or other type of desiccant inside your camera case when not in use
  3. Recharge your desiccant plug (if applicable) on recommended intervals or before big imaging trips
  4. If frost forms during an imaging session, set the temperature regulation to 0 degrees C or above
SBIG Desiccant Plug

SBIG Desiccant Plug

Dummy Plug

Dummy Plug

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