Craig Posted July 27, 2018 Share Posted July 27, 2018 It makes a mess... Bottletopburly, Tweedledee and +Graham 3 Link to comment Share on other sites More sharing options...
Tweedledee Posted July 27, 2018 Share Posted July 27, 2018 Oh dear! A solar furnace! ? Link to comment Share on other sites More sharing options...
+Graham Posted July 28, 2018 Share Posted July 28, 2018 That mirror is going to take some cleaning ? Link to comment Share on other sites More sharing options...
Craig Posted July 28, 2018 Author Share Posted July 28, 2018 Just now, Graham said: That mirror is going to take some cleaning ? I reckon you'd be up to the task. ? +Graham 1 Link to comment Share on other sites More sharing options...
Brantuk Posted July 28, 2018 Share Posted July 28, 2018 Is there a link or a picture in the first post? I can't see anything other than text....... Link to comment Share on other sites More sharing options...
Craig Posted July 28, 2018 Author Share Posted July 28, 2018 30 minutes ago, Brantuk said: Is there a link or a picture in the first post? I can't see anything other than text....... It's a link to a public Facebook post. What are you viewing this on? It's working on my PC and iPhone. Link to comment Share on other sites More sharing options...
Brantuk Posted July 28, 2018 Share Posted July 28, 2018 (edited) Laptop running Windows 7....... (Oh and I'm running the forum "dark" theme) Edited July 28, 2018 by Brantuk Link to comment Share on other sites More sharing options...
Craig Posted July 28, 2018 Author Share Posted July 28, 2018 10 minutes ago, Brantuk said: Laptop running Windows 7....... I'm not sure why it wouldn't be working. I don't have a Windows 7 machine to test it on either. It's a link to this: https://www.facebook.com/EastMidlandsStargazers/posts/1910282572344340 Link to comment Share on other sites More sharing options...
Brantuk Posted July 28, 2018 Share Posted July 28, 2018 Ah that works - I can see it and click on it and it goes to the facebook page fine - thanks Craig ? Craig 1 Link to comment Share on other sites More sharing options...
Smithysteve Posted July 28, 2018 Share Posted July 28, 2018 I had the same problem Kim, ?, ok now! It is a good thing the car did not go up in flames, the mirror might have been damaged! That is quite a mirror too! Link to comment Share on other sites More sharing options...
Smithysteve Posted July 28, 2018 Share Posted July 28, 2018 (edited) I wonder what the focal length was! ? Edited July 28, 2018 by Smithysteve Link to comment Share on other sites More sharing options...
+Graham Posted July 28, 2018 Share Posted July 28, 2018 37 minutes ago, Smithysteve said: I wonder what the focal length was! ? Thankfully a lot longer than the distance between the mirror and the roof Smithysteve 1 Link to comment Share on other sites More sharing options...
Tweedledee Posted July 28, 2018 Share Posted July 28, 2018 (edited) 57 minutes ago, Smithysteve said: I wonder what the focal length was! ? 17 minutes ago, Graham said: Thankfully a lot longer than the distance between the mirror and the roof Probably an f4 20", but might have been f3. If we said f4 ie 2 metre focal length and the height of the car boot was 1 metre, the light from the sun would be concentrated to half the diameter, area wise that is a 4x concentration of the suns rays. As you know, leaning your bare arm on the plastic car window ledge exposed to direct summer sun can burn your arm. So that temperature might be say 90c. Times 4 could make it 360c! That is easily going to produce that sort of melting of the plastic trim. I dare say if you set up a 20" mirror accurately as a solar furnace, you might easily get 1000c at the focal plane. Just another reminder not to point a scope at the sun without a proper full aperture solar filter to attenuate the light several thousand times before it even enters the OTA. Some scopes have plastic parts around secondary mirror or focuser that could melt in seconds never mind the grave damage to your eye. Edited July 28, 2018 by Tweedledee +Ibbo 1 Link to comment Share on other sites More sharing options...
Bottletopburly Posted July 28, 2018 Share Posted July 28, 2018 Assume this was his experiment a solar oven http://www.kossover.net/journal/2009/5/28/solar-oven.html Tweedledee 1 Link to comment Share on other sites More sharing options...
Tweedledee Posted July 28, 2018 Share Posted July 28, 2018 13 minutes ago, Bottletopburly said: Assume this was his experiment a solar oven http://www.kossover.net/journal/2009/5/28/solar-oven.html Good find. ? Looks like it's about f1, but probably not quite 1/10 wave optical quality. ? Bottletopburly 1 Link to comment Share on other sites More sharing options...
Guest Posted July 28, 2018 Share Posted July 28, 2018 I guess it would be the same if the mirror was spherical or hyperbolic. Link to comment Share on other sites More sharing options...
Tweedledee Posted July 28, 2018 Share Posted July 28, 2018 34 minutes ago, Sunny Phil said: I guess it would be the same if the mirror was spherical or hyperbolic. Or even just concave to a reasonable degree. ? Link to comment Share on other sites More sharing options...
+Bino-viewer Posted July 28, 2018 Share Posted July 28, 2018 (edited) A lesson for us all here...... About 20 years ago, my wife came close to burning down our house by leaving a mirror 'unatended' It wasn't an astro mirror, but a simple bathroom makeup mirror. Its one of those 2 sided ones where you get a more magnified view on one side, about 6 inch diameter. Hot summers day, mirror left on the dressing table. About 10 feet away was a dressing gown hanging up. You can guess the rest. At a certain point, the sun reflected off the mirror and on to the dressing gown. Luckily, VERY LUCKILY, i was off work (my wife was AT work) and was in the upstairs bathroom decorating. I smelt smoke, and went to investigate the back bedroom. It was always hot in there as it faced south, and i went in to see the dressing gown literally burst into flames, and really take hold. Thankfully a bath towel was nearby and i managed to smother the flames in the nick of time. Had i been out or downstairs even, i reckon the whole room and attic above it would have gone up in no time. That was a close shave and a lesson learned. Although we don't have one (but should) this is as good a reason as any for buying a fire extinguisher) We still have the mirror, but are now alot more careful with it. Edited July 28, 2018 by Bino-viewer Smithysteve 1 Link to comment Share on other sites More sharing options...
Tweedledee Posted July 28, 2018 Share Posted July 28, 2018 Wow, that was a very close shave Rob! Bottletopburly 1 Link to comment Share on other sites More sharing options...
+Bino-viewer Posted July 28, 2018 Share Posted July 28, 2018 They are probably responsible for more house fires than most other things. Link to comment Share on other sites More sharing options...
Smithysteve Posted July 29, 2018 Share Posted July 29, 2018 On 28/07/2018 at 11:34, Tweedledee said: Probably an f4 20", but might have been f3. If we said f4 ie 2 metre focal length and the height of the car boot was 1 metre, the light from the sun would be concentrated to half the diameter, area wise that is a 4x concentration of the suns rays. As you know, leaning your bare arm on the plastic car window ledge exposed to direct summer sun can burn your arm. So that temperature might be say 90c. Times 4 could make it 360c! That is easily going to produce that sort of melting of the plastic trim. I dare say if you set up a 20" mirror accurately as a solar furnace, you might easily get 1000c at the focal plane. Just another reminder not to point a scope at the sun without a proper full aperture solar filter to attenuate the light several thousand times before it even enters the OTA. You could put your 20” to a good second use Pete, instant barbecue! You would have to shield it from the sun whilst you turn the burgers and sausages over! Or you might barbecue yourself! ???? Tweedledee 1 Link to comment Share on other sites More sharing options...
Tweedledee Posted July 30, 2018 Share Posted July 30, 2018 18 hours ago, Smithysteve said: You could put your 20” to a good second use Pete, instant barbecue! You would have to shield it from the sun whilst you turn the burgers and sausages over! Or you might barbecue yourself! ???? Well, I do like my barbecue meat very well done, but I think it might be incinerated at F4 and I don't like the idea of fat and oil dripping onto my mirror. ? Link to comment Share on other sites More sharing options...
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