CCD-world: Removing anodisation

[Tim Abbott]

The following was posted to CCD-world:

Dear Michael,

        Thank you for your comments!

        A reasonably complete description of the instrument is
available at:

http://www.not.iac.es/new/technical/astronomical/instruments/notcam/

mac at astro.princeton.edu wrote:
>       I see a gear in the photo. If it is used in the system, I take 
> it that there is no problem with surface hardness regarding the teeth?

We have worm gears made of Vespel SP3 driving Al wheels, and also Vespel
gear sets.  The worm gears have mechanisms to take up slack, and are
unlikely to be affected by the loss of surface layers.

>       How is it you have determined outgassing of anodize as being
> the problem? 

By a process of elimination.  All bearings were thoroughly cleaned, 
wiring insulation is all PTFE, solder joints were thoroughly cleaned 
and the dewar pumped with various components present or removed.  Pumps 
are all dry.  Our only conclusion can be that the oil contamination 
we have experienced was absorbed onto the surface of the dewar and
components and has been released steadily during subsequent bake and
pump cycles.  Now that we have pushed down the oil contamination
to acceptable levels, we are still experiencing considerable water
vapour outgassing which seems reasonable to attribute to the anodisation.

Vacuum contaminants have been identified using an RGA.

The best vacuum we have yet acheived is 5*10^-5 mBar, after baking
and pumping for 2 days.

> How much "O" ring length is used, and the material?

The O-rings are a typical elastomer (I'm not sure of the specific 
material, but can find out).   The largest one provides the seal to
the lid, which is most of 5 sides of a 60cm cube, (i.e. the
seal is a 60cm square).

>       What kind of thermal black-body radiation shielding material
> is used? Mylar? Alum foil? Gold plated?

The experiences to date have all been without any such shielding
except an Aluminium box surrounding the cold components.  We have
just installed Mylar/nylon webbing superinsulation.

>       Are there any electronic components, plastic parts, or insulated
> wire in vacuum?

There is no plastic in the dewar, there are some electronic components,
but the situation is the same with them removed.

Cheers,

        Tim

-- 
Tim Abbott                                                    tabbott at not.iac.es
Support Astronomer                               http://www.not.iac.es/~tabbott/
                           Nordic Optical Telescope
       Apartado 474 E-38700, Santa Cruz de La Palma, Canary Islands, Spain
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