Hi Guys, always interesting reading what you're all up to...
Ken, there's been a lot of discussion about truss-style vs tube-style reflectors, it's not as simple as we might all hope...A truss doesn't have any ventilation problems, but on the other hand it lets air flow past the optics in any old way, and it's not unusual for air to be at different temps just a few feet off the gound vs close to the ground. Also if you're close by then you'll be heating the air a bit and it can be getting into the optical path.
Tubes keep all that out, and you get a column of air that sits above the mirror without moving very much - especially if you close off the mirror end of the tube...
My feeling is that for planetary stuff you want a closed tube + lots of spooky cooling, but for deep sky where you need a big mirror then a truss is more practical.
That makes a lot of sense Bird! The slightest breezes would blow right across the front of the mirror at varied speed and temps. I have seen some with the curtaining around them but defeats the purpose Davo is after.
When I first installed the fan inside the tube and right next to the mirror, the mirror cooled at an average rate 1.743 degrees per hour with the ambient falling at an average of 2.00 degrees an hour. This was very linear in nature and there was no levelling off in this time frame of 4 hours.
With the newest setup, the ambient again was dropping a very linear rate of 1.855 degrees per hour and the mirror was cooling at a very impressive 2.073 degrees per hour.
The actual fan was turned over half way thru the test and went from sucking to blowing cooler air into the tube. No noticeable change in the rate of cooling was observed.
So at this stage it looks to be a winner, but i would prefer to observe normal cooling now that a hole is in the side of the tube near the mirror.
Tonight, i will cool for an hour and then turn on the fan.
It is very hot at the moment, so hopefully there is a large temp difference to run the test under. And then a nice levelling off to see how the fan lags the ambient temp
Dave
Here is a a graph of the data that was captured through hyper terminal over a 6 hour period by capturing to a text file the run through excel.
At time of this post unit was still logging data once it finishes it's programed mission I will post the total data loged.
The data used was from the testing I am doing, the unit is yet to be fitted to the scope and wont be till I am sure it is all running correctly. All the lables are embeded in the microcontroller so that is why they come up as mirror temp, ambient temp and tube temp.
Testing was done using a "freezer block" as the "mirror" just so I could make sure the sensors all read a different temp.
Ving, I've had it happen a few times where the mirror was too cold, and I've noticed that the mirror can warm up pretty quickly, but it's the cooling down part where it has trouble.
I found that to be a bit counter-intuitive, I'd have thought that a cold mirror would have trouble warming up, just as a hot mirror has trouble cooling down, but it seems that mirrors can be good at absorbing heat and bad at losing it.
