Thursday, April 3, 2014

Impatient


I'm impatient.  I wanted to set up the scope in the backyard earlier today or go up to MBOSP to test the changes to the system that I'd done earlier.  However, I didn't because the CSC suggested clouds by midnight.

Currently the sky is clear.  Drat.  The weekend beckons with the promise of clear skies even with moon in the evening hours.  Moon sets at 1 am on Saturday night.



What's been done that I'm champing at the bit:

I pulled apart the camera and did a few updates, requiring nearly 2 hours of fiddling.  No pictures, again, no real time to take them.

I removed the cooling gear and tested the stacked devices with the larger finger as it appears in the previous work.  While I could feel the gradual change in temperature with the device running, I feel certain that trimming the size of the finger would better optimize the heat removed.  It would also provide a smooth surface for thermal transfer.  Condensation occurs on the finger within a few minutes, but I was hoping for faster performance.  Now I'm not sure if the devices I've stacked are well matched.  Maybe a future mod will be to replace the home made stack with a tuned stack from Tellurex.

I also found where the old thermistor failed as I was taking apart the old finger.  One of the fine wires was broken at the edge of the old finger.  It's possible that this breakage may have been shorted, too, causing the PWM controller to run continuously.

I put a new thermistor into place sandwiched between some thermal gel material and nearby the cold finger.  This is a more solid connection to the chassis of the camera and will provide a better estimate of the actual temperature in the camera.  I moved the exit point of this fine wire to a seam where I melted a channel in the plastic.  The thermistor wire was tacked in place in a few spots with hot melt glue to provide control and strain relief.

Also, the floating CCD sensor has been tacked into place with two spots of hot melt glue.  These spots can be removed easily with the point of a knife if needed.  For now, I can see that the sensor plate will not move.  It's unlikely that this will make a difference, but I think that the possibility of movement is too likely to be ignored.

Since I had fabricated a new copper finger, I had to replace all the foam tape around the peltier devices.  This allowed the chance to do a better job of sealing under the heat sink.  Many of the older bits of foam had thermal transfer paste on them and this is not a good idea.  I considered potting with silicone adhesive, but my tube of RTV was dried up.  Instead, I cut small strips of the foam tape and secured them in place.  I also made the effort to cover all exposed areas of the copper finger up to the inside of the camera.

The extra padding of the foam inside the camera made the challenge of closing the base of the camera a bit harder.  In the end, I did re-attach the dovetail clamp.  I had wanted to insulate the base of the camera a bit more but this added too much thickness.

I tested the cooling and am able to see the PWM controller respond to gradual temperature changes as detected by the sensor.  I was able to see a drop from about 21 C to 15 C in about 15 minutes.  Not super fast cooling, but if I start the device as I setup and do polar alignment, by the time everything is ready, the camera should be pretty cold.  I think that for the first times I start using it, I'll keep it set at 0C.  It may never get that cold but it's worth a try.

When I checked the camera on the desktop at 22C ambient temperatures, the sensor reported a quick drop to 15C and further to 10C.  However, as soon as I turned on the camera and started taking darks, the temperature started to climb and didn't stop.  Within 2 hours it was nearly 20C.  As I turned off the camera, the temperature dropped quickly again.  Not sure what this means except that maybe the stack of peltier devices doesn't have enough watt throughput to keep up with the heat generated by the camera.

Physically, everything about this configuration is significantly firmer than before.  The SSAG is now screwed into bigger drawtube elements, the DSLR is riding on a rigid dovetail again, the CCD sensor is on a fixed board, and the SV70ED is supported more rigidly.

All that remains at this point is to fix the alignment between the two scopes (again), focus the SSAG (again), and do some perfect drift alignment.  Maybe the weather will be good on Friday night.