*
12.50
The seeing conditions have been going downhill during the morning. So I
have just been experimenting while carefully monitoring the Barlowed, live view
on the SharpCap screen.First I stretched green, agricultural, shade netting over the dome slit. Right up to the maximum height below the telescope's light path. The image went very mushy! Removing the netting returned the image to normal. i.e. Thermally agitated, boiling and wobbling but the greatly magnified, surface structure is still clearly visible.
Then I tried the shiny, metal coated camping mattress. To shield the bare metal of the big mounting. The aluminium skin was soon warm to the touch! The bare aluminium of the mounting is colder at 18-20C. Nothing to be gained there!
I found some large, white, double, woven polythene[?] agricultural bags. Then tried hanging those in front of the lower slit. A marginal improvement in image stability at best but not very much. I was hoping it would shield the interior of the dome from solar heating. I measured 55C on bare 2x6 timber inside the dome where the sun was shining on it! The matt black, painted wall panels were only a little cooler.
The white, man-made material remained cool at 16-18C even in full sunshine. Useful information for future experimentation! This means it can act as a shade without contributing excess heat from solar absorption. Lightweight, white, woven tarpaulins are very similar in appearance. Though I'll have to check its temperature response just to be sure. This stuff is relatively cheap and could be draped and stapled over the shutters. Much cheaper than paint and both last about as long as each other out of doors.
14.30 62/55F In/out. I have had the door open, down on the ground floor, to aid cooling by the "chimney effect."The theory being that heat will rise and pass out of the top of the slit. Thereby keeping the dome cooler than if the observatory was sealed.
There are 20cm/8" deep gaps all around the observatory floor under the veranda. This provides a year round draught to push heat out of the lower building and possibly via the dome slit via the open pier.
I have now opened the westerly observatory doors out to the veranda. There is a cold draught from the NW wind!
Opening the obs. doors didn't help the seeing noticeably. Nor change the temperatures showing on the In/Out thermometer on the obs. wall. I had imagined the breeze would clear out any excess heat in the dome. Not noticeably, as far as the image on the SharpCap screen was concerned.
This makes me think that plans to fit large louvers in place of some obs. wall panels would be a complete waste of time.
14.56: 62/56F. The insides of the plywood, dome panels read up to 40C. Higher is hotter. Outside in full sun they measure up to 50C! Higher is hotter again. The sun is at about 42 local altitude, max today.
The reading is only 20C on the outside of the dome panels which are in the shade of the open shutters. Shade obviously matters. Provided the shade doesn't absorb heat and become a convection heater in itself. In this case the shutters are the main solar heaters and are also closest to the light path from the sun to the telescope objective. Shading the shutters would obviously be a very good idea. How best to do it while coping with wind and assorted weather?
I suppose I could paint only the shutters white on the outside. Then they'd remain cooler as well as shading the nearest panels when open. Which is quite a large area of the dome subject to direct sunshine. All the other panels receive only oblique sunshine and are much cooler.
If I want to avoid attracting attention I could turn the the dome shutters away from the road. Not to mention the distant neighbours who can see it though a gap between the house and neighbour's trees. Though I have to say I do like the dome entirely in sage green. It is reminiscent of traditional, verdigrised copper domes. Though I imagine copper domes must soar in temperature.
I'll have to check this with some scrap copper sheet. I know stainless steel gets very hot in sunshine. While bare aluminium remains very much cooler. I have some more measurements to make with thin, scrap, sheet aluminium to confirm actual temperatures. Cladding the dome in aluminium rather appeals. If only to cure the leaks and provide greater longevity.
Next day Fri. 10th at 15.11pm: 60/54F. I rehung the white woven sacks over the lower slit to block the sun. Using one clamp at the top of each, on each side, I obtained two separated curtains. This would allow free air circulation. Rather than blocking the air flow out through the slit.
The temperature immediately seemed to plummet in the dome. Purely subjective I assume. Thin high cloud is reducing temperatures slightly compared with yesterday's full sun form a blue sky. The black painted NE wall is now at 44-46C in sunshine. With the bare timber at 38-40C. The drop in light levels on the desk was well worth having to be able to see the monitor more clearly.
White, lightweight tarpaulins do remain at ambient in sunshine. I haven't checked the black rubber skirt's temperature yet. This might be producing convection currents which would pass upwards across the light path if it gets very hot. I am trying to pin down all the ways my dome could be spoiling the image.
There is a 3F difference between the top and bottom of the sloping OTA. Though still close to ambient. A thermal stratification within the dome? The objective cell is mat black and showed a higher temperature than the [shiny aluminium] D-ERF filter cell. It might be worth making a circular mask to shield the objective cell and the timber adapter ring just behind it.
*
No comments:
Post a Comment