Hi all
Built and tested a simple fan forced mirror cooling/ventilation system for my 10 inch scope. The fan was set to suck air out of the scope, not blow it in, as I found out thats what they do at the Anglo Australian Telescope, the theory is clean non turbulent air is drawn in, then washes over the mirror then out the back. Seeing wasnt that good anyway with a southerly wind and some cloud. Visually I couldnt see much difference with fan on, or off, but I took a single image of the moon with it on.
This is a newtonian focus image with the MPCC coma corrector, RAW, iso 100, 1/250th sec, mirror lockup on, with my 10 inch f5.6 scope. I was surprised to get the level of detail, considering the moon looked to be "boiling" when looking with an eyepiece due to the unsteady seeing.
I really decided to try some sort of forced ventilation after seeing Mike's telescope at Kulnura, and the fact that the mirror needs to be within a degree or 2 of ambient, warmer and convection near mirror surface runins the image quality. My mirror is full thickness and so takes a long time to cool down.
Ive also put in a full res. crop of the moon image, as well as a pic of the cooler. Very hi tech as you can see, a PC case fan stuck on some masonite in turn stuck to the scope with good ol duct tape lol.
I was worried virebration from the fan may cause issue but it wasnt noticable, wither in the image or visually.
Note, I used the masking technique on the moon to increase contrast a bit. Taken with my modded camera, no filters of any kind used.
Scott
Nice job Scott, it's a nice image. The widefield version is very sharp.
I too have my fan running while I image, and have not noticed any vibration at all. At times when the image on the laptop is boiling away I wonder if it's vibration, so I unplug the fan, find that the boiling is due to seeing and possible tube currents (not vibration), so I turn the fan back on.
I'm not sure about the sucking rather than blowing - bird may have some thoughts on that (mine is blowing air from outside onto the back of the mirror).
Yep i want to test the system on a night of good seeing. The reason Im trying pulling air from inside the scope is that the AAT used to blow air in from outside through a filter room, air from inside the dome exhausting out through the dome slit over the telescope. Then I heard that by reversing the fans and drawing air in from outside, clean air free of turbulence washes over the telescope. They do, however have separate fans blowing air over the surface of the mirror too I believe. Its mentioned at bottom of here http://www.aao.gov.au/local/www/cgt/...00000000000000
Also this
Quote:
"The dome is a steel structure, vented and insulated to reduce heating of the inside air during the day. An up-and-over shutter gives access to the sky and protection from wind gusts is provided by a rigid windscreen which allows the telescope to observe through a (6.5m x 5.3m) opening. This opening is automatically aligned with the telescope tube when observing under computer control.
The space between the outer steel skin of the dome and the inner insulated skin can be ventilated by fans. The same fans and their associated louvres can be switched to exhaust the dome air and replace it with air entering either through the open viewing aperture or injected by large ventilation fans on the second floor. These fans will be switched on by the night assistant when needed.
Interesting Scott, If your experiment works I might have to turn my fans the other way around. I was also thinking of putting some fans in to blow across the face of the mirror.
