Re: [CFDS] filters

[Jan Hollan <jhollan@amper....muni.cz>]

> It used to be possible to get a glass Didydium (?) filter that I have
> misplaced since moving to Shetland from Liverpool.

> Chris

I was wrong in writing that no suitable filter exists. Inspired by Chris,
I googled intensively. ``Didymium glasses'' would work somehow.

Didymium is a name for a non-element, a mixture of neodymium and
praseodymium. Perhaps one of this elements as a pure additive would be
even better for our purpose.

Standard didymium glasses are used to filter out sodium light for seeing
hot glass well, from the sixties already. With their 3.2 mm thickness they
let just some 63 per cent light through -- in another parlance, stars get
just 0.5 mag fainter.

This might be a reasonable sacrifice in those cases, where the sky
luminance gets lowered thrice or four times more, by an equivalent of say
2 mag. In the same time, glare from LPS would be almost nullified.

However, even thinner glasses could do well, with better transmission,
with still sufficient absorption of D line, see

http://www.hellma-worldwide.de/doc_pool/en/e_F7_Didymium_Filter_Glass_and_Spectra.pdf

I could not find any thin glasses on the net, I'd be glad if anybody
could say me a source.

The cost of the usual thick glasses is about 33 USD. It is not entirely
prohibitive, could be a nice Christmas gift for LPS sites. Even as a toy:
the lamps disappear miraculously, whereas a CFL or incandescent-lit scene
does not.

I should stress this is NO filtering for usual research purposes.
However, for enjoying the beauty of sky it might help.


There is another very interesting use of the didymium glass, namely a
calibration of wavelengths when taking a spectrum with any digital camera
(using a simple CD spectroscope).  I used solar spectrum for this purpose,
but it's not so easy to get a set of well recorded lines. With these
glasses it should be a trifle. Sunlight could be still used for
calibration at various airmasses, when the glasses would be removed; the
wavelengths would remain at the same positions as through glasses. (I can
fw solar spectra for various AM's).

(really, a productive question...)

jenik





------------------------ Yahoo! Groups Sponsor --------------------~--> 
$4.98 domain names from Yahoo!. Register anything.
http://us.click.yahoo.com/Q7_YsB/neXJAA/yQLSAA/1.XolB/TM
--------------------------------------------------------------------~-> 

<sb>Campaign for Dark Skies Mailing List
Please don't post HTML and don't crosspost to the list.
When replying to list messages trim ALL unnecessary material including THIS footer and any egroup advertisements.
 
Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/CFDS/

<*> To unsubscribe from this group, send an email to:
    CFDS-unsubscribe@yahoogroups...

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/