| View previous topic :: View next topic |
| Author |
Message |
GPjon
Joined: 05 Dec 2008
Posts: 3
|
 Posted: Sat Dec 06, 2008 12:02 am Post subject: Mirror Info..... |
|
|
I aquired an 8 inch mirror some years ago, with a view to build my own Dobsonian.....unfortunately I have no details on its focal length, etc.
How can I find this info??? White wall, sun, tape measure????
Is there anything else I would need to know about the mirror in order to start a build??
It also needs re-aluminising....can anyone recommend anywhere in South West England????
Cheers Jon......  |
|
| Back to top » |
|
 |
bigbry49
Joined: 09 Jan 2008
Posts: 17
Location: Worcester
|
| Posted: Sat Dec 06, 2008 2:05 am Post subject: |
|
|
Hi John
Bet you didn't expect a response this quick!
Focal length is easy: tape measure, torch and a piece of white paper.
Stand the mirror upright against a wall; hold the torch and paper the same distance from it and shine the torch on the morror.
Shift it back and forwards until the reflected image of the torch on the paper is focussed. Measure this distance which is TWICE the focal length. Half of this distance is then the true focal length.
I can't help much on the aluminising other than to suggest you Google on the name 'galvoptics' who have a service on their website but they're in Essex if I recall.
p.s. DON'T EVEN THINK ABOUT THE SUN! YOU'LL BLIND OR MELT SOMEONE!!!
All the best
Bryan |
|
| Back to top » |
|
|
GPjon
Joined: 05 Dec 2008
Posts: 3
|
| Posted: Sat Dec 06, 2008 6:39 pm Post subject: |
|
|
Thanks for that. Certainly was quick!!!! ........I'll give it a go tomorrow!!!!........
Next thing is to track down some 'Sonotube' 
Catch you soon.....Jon.......... |
|
| Back to top » |
|
|
bigbry49
Joined: 09 Jan 2008
Posts: 17
Location: Worcester
|
| Posted: Sun Dec 07, 2008 3:29 am Post subject: |
|
|
Hi!
Sonotube is an American product. I don't think it's available here.
I got my tube from Essex Tube Windings Ltd. www.essextubes.com tel 01375 851613. It's just a plain tube with no lining so there's no messing about beforehand
The website's fancy but it's best to give them a ring - they're really helpful and down-to-earth.
I went and collected it as I didn't fancy it's chances on a carrier but perhaps that was a bit over the top.
Clear Skies!
Bryan |
|
| Back to top » |
|
|
GPjon
Joined: 05 Dec 2008
Posts: 3
|
| Posted: Fri Jan 16, 2009 6:11 pm Post subject: |
|
|
Just found some tube......cheap!
Trouble is, it's 300mm dia. Ideally I wanted 250mm....
Would it be a prob, other than the obvious ones, ie bit more weight, larger size, moving it around....I suppose its only 25mm extra per side..
But you know the difference an inch or two can make!!!!!!!
Regards Jon.......... |
|
| Back to top » |
|
|
bigbry49
Joined: 09 Jan 2008
Posts: 17
Location: Worcester
|
| Posted: Sat Jan 17, 2009 7:11 am Post subject: |
|
|
Hi there!
Glad to hear you're making progress.
I can't see a problem myself - if you've got it, flaunt it!
At least you won't be pushed for space. With only an inch a side things can get a bit tight so you'll benefit from the extra room although you'll need to take account of this in the mirror cell design.
You might like to consider baffling the front end but then this might introduce problems with diffraction, I don't know.
If you look at pictures of truss Dobs, the mirror box is often large in comparison to the mirror itself and the secondary/focusser housing likewise so I'm inclined to think there won't be a problem.
Best of luck
Bryan |
|
| Back to top » |
|
|
|
