[time-nuts] Re Danjon Astrolabe

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Sep 27 20:45:58 EDT 2006

Glenn wrote:
> Brooke Clarke wrote:
>> I'm interested in automatically measuring the earth's period by looking 
>> close to straight up with a fixed telescope.
> This sounds like an interesting project. I've been looking for something 
> to do with the Meade 4455D telescope I just got. (D=114m F=910, f/8) 
> Doing a fixed mount at a know location shouldn't be a problem. I've also 
> got an untested Hamamatsu E717-07 photomultiplier tube. Please let me 
> know how it goes.
> cheers,
> glenn
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The aperture of your Newtonian telescope is a little too small for this 
application especially in the daytime.
You need to use an IR filter to block most of the Rayleigh scattered 
You will also need to have good baffling and use a Lyot stop to block 
With an aluminium tube focus will not be stable as the temperature changes.
If you have a plate glass mirror then a steel tube will give a much 
better match to the mirrors focal length temperature coefficient.
Shading the telescope tube and mirror from direct sunlight is also 
essential to avoid heating the tube and creating large tube currents 
which will severely degrade the images.


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