My big dob is not a truss. It's a solid tube.
https://drive.google.com/file/d/1ZMP...ew?usp=sharing
It has 9 ventilation fans.
- 1 in the centre of the tailgate blowing into the back of the mirror ( I had to add a little baffle card as the trilateral symmetry of the ventilation left a visible triangular distortion on the star test out of focus disks),
- 4 which blow air into the corner spaces of the mirror box (the hole for one of these can be seen at the top near the rear of the mirror box),
- and 4 flat fans from scrapped laptops which blow the corner-space air across the front surface of the mirror to scrub any warm boundary layer, which otherwise makes a visible plume on the bottom (due to inverted image) of the star test image.
The mirror is about 35mm thick. Normally the scope is stored in the garage, which being stucco is fairly stable and not much warmer than ambient, so cooling requirements of the mirror are usually modest. If the mirror has been in the house (as it was during polishing and figuring) the cooling took a lot longer (16kg of glass needing to be cooled to within 1 degree or so of ambient). In addition to thermal air distubances to the optical path, more importantly, during cooling, the temperature difference between edge of the mirror and the centre meant that the mirror was very much overcorrected (more flattened than the correct parabolic curve - the edge of the glass was shrunk lower than it should be) and the image was very much degraded by spherical aberration.
As I said in my first post, my aim with installing space blanket was to eliminate radiative undercooling of the tube, preventing the resulting convection currents of cold air down the inside of the tube which disturbs the seeing. The system seems to be working well. No dew and less tube current. The bottom interior of the tube, near the mouth, of course still has sky view and will unavoidably radiate since it is painted black.
Hi Alex, I am in the process of finally building a tube for the lovely 10 inch parker mirror you sold me. I had a quick test look last week at my local "artificial star" (local radio tower). Airy rings looked nice, it appears to be an excellent mirror!
With respect to cork, the idea has been used plenty in the past and should be fine at preventing not the heat plume FROM the potentially warm mirror, but actually preventing the excessive UNDERCOOLING of the tube due to radiation to the (minus 50 or so radiation wise on a cool clear night) cold sky.
Re the space blanket, I was just sharing my experience of what worked for me, and is cheaper than cork, and as a bonus greatly reduces dew. We had a big star party for the opening of our new dark sky park last weekend. All scopes were soaked with dew at the end of the night except the big shiny dob. I think any tube telescope, open or closed, will benefit from reduction of radiation to the cold sky. See also
https://www.cloudynights.com/topic/6...d-a-telescope/
(I might also have the excellent powerpoint slides which go with that but are no longer on dropbox).
https://www.skyatnightmagazine.com/a...our-telescope/
https://skyandtelescope.org/astronom...ng-the-seeing/
https://www.cloudynights.com/topic/6...e-c14-edge-hd/
Quote:
Originally Posted by mental4astro
The heat retention method only applies to a closed tube situation, like an SCT or Mak & even a refractor, not an open tube.
Open tube OTA's, be it a solid tube Newt or a truss structure need movement of air to avoid heat plumes by distrupting their formation.
Lining an open tube with cork will not give any benefit. Only complicate things.
* in an unventilated OTA it will still trap heat plumes
* adds an organic component that will only deompose over time, bringing the complications associated with this process
If flocking is your purpose, there are easier and less detrimental ways of achieving this.
Please do not confuse the insulation method for Maks, SCT's and fracs as applicable to open tube systems, like Newts, classical cassegrains, DK's, RC'S and the such. These need ventilation to distrupt potential heat plumes. In a closed OTA system, you are able to control the temperature differential between the fast cooling tube and the slow cooling optics and baffle tube because the air inside the OTA is trapped. In any open tube system (solid tube or truss) there is no trapped air.
If you want to control heat plume formation in an open tube, ventilation is the option available. This needs to be applied in a careful and considered way or you will introduce heat differentials inside the optics, inducing strain on the glass and distort the shape of the precious figure.
All the clues are given by professional observatories. Thing is prof obs are not designed by the astronomers. The astronomers say what they want to achieve, teams of engineers, opticians and technicians design and build the scope & surrounding structures and systems as they have the thermodynamics & material science knowledge. We should be looking more at how they do things rather than ignore them.
Alex.
Edit - I've just seen Hamish's post. How he has used the space blanket is a different application in a truss scope and there are other factors at play. It is not the same as heat retention/insulation of a closed tube OTA. Very clever , but a different application. I like it.
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