[ATM] Spectroscopy and fiber optic info needed

[dlwebb at canit.se (Dominic-Luc Webb)]

This is definitely up my alley and definitely at a bad time
because I have much writing to do at this time. As you already
eluded to, spectroscopy opens up a giant can of worms. I am
in the first place not even certain you want a newt for this,
but maybe. I had a nice monochromator in California, which
ended up a charity donation that would work great for this.
It used two mirrors (parabolic?) and a difraction grating
between. The wavelength was tuned into by turning the grating
via a vernier. A cable was fitted on one side and PMTs on the
other. It was a great instrument and I would really like to
have another someday. The cable is critical, so be weary of
what you get. I have been considering bundled hollow core
nanofibers. Intensity is used to determine of the target is
in position and focused. This part is tricky. I suggest having
more than one bundle so you can also fetch signals from nearby
standard candles. Detector depends greatly on your interests.
Along these lines I have some ideas, but a set of 2 PMTs
immediately comes to mind (one blue optimized, one red optimized).

Especially if you go the monochromator or PMT routes above, I
would really like to evolve this posting. There are many others
here with ample experience with CCDs, so I'll wait and see what
other come up before getting into this.

Also, where are you? This might gave some hint of some pros
you could contact or even visit.

If you have some clues as to types of objects, wavelengths,
magnitudes, that would be helpful...

Dominic-Luc Webb



On Mon, 19 Jan 2004 jsgrmfg@gracemfg.com wrote:

> Hi All,
>
>     I am deep in the design and planning process for my "retirement 'scope"
> ,
> an equatorially mounted, computer controlled,  20" Newt. in a permanent
> observatory that will get a lot of use after I retire and can, at last,
> stay up
> all night without hating myself in the morning.
>
>     My observing program for this 'scope includes spectroscopy, a new area
> of astronomy for me.  In particular, I would like to be able to measure
> radial
> velocities and red shifts with reasonable accuracy.   As I understand it,
> such
> measurements require high resolution and dispersion and heavy insulation
> to prevent the line blurring and shifting that can result from temperature
> changes during an exposure.  The spectrograph also must not shift or flex,
> either internally or relative to the telescope optics.  A spectrograph that
> meets
> these requirements will be heavy and bulky and mounting it solidly over
> the focuser of a Newt. could be a real challenge.
>
>     It seems to me that the neatest solution to these problems is to put
> the
> spectrograph in an insulated box on the observatory floor and feed it
> light through a fiber optic cable.  Alas, I know next to nothing about
> fiber
> optics.  A few hours of Googling the subject has taught me that optical
> fibers
> can be made from several different materials and in many diameters,
> that cables may be single or multiple stranded, that multiple fibers may be
> arranged in various ways, including a disk at one end and a straight line
> at
> the other, and that cable length should be minimized to avoid excessive
> light loss.
>
>     What I have not yet found is fiber optic information that addresses the
> special needs of the astronomical spectroscopist.  Do any of the Wizards
> of the ATM list have knowledge and experience in this area that they can
> share with me?
>
>     For design purposes I need to know such things as:  The best fiber
> material(s)?
> Single or multiple fiber cable?  Best fiber diameter?  Does the light
> emerging
> from the cable have the same optical characteristics (e.g., focal ratio) as
> the
> entering image or is the light scrambled by the fiber?  Has someone
> invented a
> clever way to know when the image of a star or other object is well
> centered
> on the fiber or fibers?
>
>     Thanks,
>
> Jim Sturtevant
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>