Why do you need to cool your mirror for high resolution imaging?

 This post, part 2 of my mirror cooling series will give evidence that a closed tube is not efficient at keeping the mirror at ambient temperature of the air.  For a 24 hour period my scope sat with the dust cap screwed on and a tarp covering the whole OTA assembly.  During this time you can see how much the mirror temp slowly follows the air temp.

 

 

In the above example you can see the symmetry of each temperature, following a cooling and warming curve with temperature on the Vertical/Y-axis and time on the Horizontal/X-axis.  Looking at the blue (air) you will see it cool from the left at midnight until about 11am where it starts to warm again in my garage, where it is parked.  If you focus on the yellow (mirror) temp curve you will see the same shape, just slightly phase shifted away.  This means that the mirror is also cooling but is delayed, likely as it's a larger heat sink than the air.  After 11am the air heats more rapidly than mirror, which again is slower to keep up with the changes in temperature.

Let's zoom into the times between 0245 and 0545, a THREE hour time span where if you focus your attention to the vertical and horizontal guides, the big dashed PLUS in the middle of the graph.  Following the horizontal line you'll see it intersect with the TOP part of the blue curve on the LEFT and the BOTTOM part of the yellow curve on the right.  This horizontal line represents the temp ~75.7F and is the time delta between the air and mirror at the same temperature.  Not using active cooling and only letting the scope and mirror follow thermodynamics you can see that the mirror, capped and closed inside the OTA took about 3 hours to catch up to the air. 

 

I wanted a temp comparison, so I created a graph showing the mirror and air delta for a visualization in real-time and record of what the differences in the below graph.  The idea is that I can use a large temp difference to activate the cooling unit I described in part 1 - Akule's updated cooling fans  The cooling unit was NOT used in any of these tests but the idea is that with a significant enough temp delta I can trigger the cooling unit to run when not in use

 I hope you enjoyed seeing real evidence of why you need active cooling!  See you in the next part!

Mike