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Astronomy/thermal expansion coefficient of optical substrates

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Expert: - 10/29/2004


Question
Hi, it's me again. I have more questions! I've learned that the substrates of dichroic filters have to have low thermal expansion coefficients but I am not sure why. Is it because they don't want them to expand too much while they are coating them with the layers of dielectric compounds or because they don't want them to expand later on if they are being used to view something like the sun? Also, shouldn't the substrate have a thermal expansion coefficient very similar to that of the dielectric compounds that will be coating it so as they cool down they don't break apart? Does it matter what kind of thermal expansion coefficient a color filter has? Thank you for your answers!

Answer
Sorry Bridget,
I (and most astronomers) don't manufacture the filters,
we just use them.  (Just like flying, I flew the planes in Nam,
I didn't make them or even repair them).
In fact, I see you've done some research on them, so right
now, you probably know more about the characteristics
and materials used in filters, then I do.  
[But can you point a scope using an O-III nebular filter to the
Cat Eye Nebula (NGC-6543) in Draco, or the "Ghost of
Jupiter"  (NGC 3242) in Hydra,  and find those planetary nebula's  in less than 10 seconds?  I can.]

So you need to find a chemist, engineer in optics, or physicist
that is involved in the manufacturing of those filters,
as I doubt that there aren't too many astronomers out there
that actually get involved in the production of visual
filters, unless you get very, very lucky.
Clear Skies,
Tom Whiting
(astronomy expert, not manufacturing expert) ;-)
Erie, PA

PS...kind of like me asking you....hey, you know how to
use a fluorescent light, but do you know the materials
that go into making it, how its manufactured, and the principle that it operates on?  ;-)

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Tom Whiting

Expertise

Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.

Experience

Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at:http://www.velocity.net/~bwhiting and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

Education/Credentials
BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

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