Re: shields for lens

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

  Tom wrote:

> Saturday I opened up the TOM2 lenses and dried them out.  To solve the
> problem for the future I used some of the 3M heat shrinkable film that is
> used to make temporary storm windows.  This made a very uniform and
> transparent shield for the lens.  
 ...

> When I ran the telescope, the results were awful.  Where usually I can see
> a 5 mil focus effect, 25 mils made little difference.  The stars were just
> big fuzzy blobs with a FWHM of 7 pixels or so.  

  Laird Thompson did experiments using thin Mylay films as 
storm windows.  Read about it in PASP, available from ADS:

      http://adsabs.harvard.edu/cgi-bin/nph-bib_query?
           bibcode=1990PASP..102.1086T&db_key=AST&high=3f4f81b16f18057

> Does a haze cause more loss in Ic than in V?  Even so, should not the
> photometry step take care of this?  The Ic measurements are being compared
> to all the other Ic measurements??  So I should be able to put a neutral
> density filter in front of the Ic telescope and not change the photometry??

  Yes, the ordinary reductions employ differential photometry, so that
any clouds which dim ALL the stars by some amount should not lead
to any particular star looking fainter in a long-term light curve.

  If there were color-dependent extinction due to wierd clouds, and
blue stars were dimmed more than red ones, and most of the calibration
stars from Tycho were blue, but one particular star were red ....
then that star would appear _brighter than usual_ on the night
with wierd extinction.  

  Tom, I'm sorry that I haven't investigated this more carefully 
already.  If you can say that these anamolous dips occur only for
the redder (or bluer) stars in a field, that would test this
hypothesis.  

  If _all_ stars in the field are said to be dimmer than usual
on a given day, well, I can't explain that.

                                        Michael