[Aavso-photometry] Screen for "dome" flat?

[Donn Starkey]

Jim:

If you use ordinary white "house paint" for your dome, it will reflect much
more of the red light than the blue.  The major pigment used for white paint
is titanium dioxide [TiO2].  The reflective spectral response for the rutile
grade of this pigment has very little reflection in the blue, which
unfortunately is where your CCD is also least responsive.  To overcome this
TiO2 reflectivity problem, you need to use a reflective coating for your
flat screen that has a nearly flat spectral response.  

There is an excellent paper written Wu et al. back in 1972 discussing just
this topic (Wu, 1972).  He found that a coating using barium sulfate [BaSO4]
as a pigment has a much flatter response than TiO2.  In his preparation, he
used a precipitated version of BaSO4 called blanc fixe.  This product is
much whiter than the raw barium sulfate and is by nature sub-micron in
particle size.  Wu's coating formula was based on a solvent/resin system and
had poor adhesion to most substrates.  

I have found that if you use a common water-based clear exterior urethane
varnish, found at most hardware or paint supply stores, and disperse blanc
fixe into the varnish at ~ 50 - 70% solids, the resultant coating makes an
excellent reflective source for our purposes.  I have a plywood screen that
I coated with this material in my observatory over 8 years ago and it looks
a good as the day I painted it.  Make sure that the urethane is an exterior
grade because the interior grades contain an aromatic isocyanate in the
prepolymer backbone and will yellow over time upon exposure to UV light.

I wrote a short paper on CCD flat fielding for a class in 2005 that detailed
some of the problems with reflective surfaces and light sources.  Anyone
that wants a copy of the paper can contact me.

Donn


Notes:
Wu, C., Anderson, C., et al., "The Design of a Spot Sensitometer for
Astronomical Use".
American Astronomical Society, Photo-Bulletin. (1972) No 7.

==============================================
Donn Starkey
starkey73 at mchsi.com
http://www.starkey.ws
AAVSO Observer SDB  -  CBA Indiana 
VSNET Observing Team  -  MPC Code H63
============================================== 


> Date: Sun, 11 May 2008 09:41:37 -0500
> From: Jim Roe <jroe at jamesroe.com>
> Subject: [Aavso-photometry] Screen for "dome" flat?
> To: Aavso-Photometry <aavso-photometry at mira.aavso.org>
> Message-ID: <482705A1.3040708 at jamesroe.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Thinking about installing a screen of some sort for taking "dome" flats
> ("dome" being VERY generic in this case.  I have a roll-off shed that is
> not very domish.).  What are suggestions for material, paint(?),
> illumination, etc.  Right now I am using C, B, V, and I filters.
> 
> Jim Roe [ROE]