[AZ-Observing] Re: Getting started in photometry

The "standard" photomultiplier tube for astronomy is the 1P21, which is
electrically similar to the 931 tube -- but the 1P21 is MUCH quiter
(selected for very low noise).  The characteristics of the 1P21 are very
well documented, and the necessary circuitry is well known.  The
similarity in tube model no. has little relation to performance in this
case.  The 1P28 no doubt has a completely different color response than
the 1P21, and most likely is not even comparable as to gain, noise,
etc.  It would be useful for training on how to work with PMTs, but that
is probably about all. I believe the 1P28 is probably suited for use as
a position sensor and other similar uses, but probably not precision
photometry.  Also, you should appreciate that IF a photomultiplier tube
is exposed to even moderate amounts of, say, room lighting or sunlight,
the gain and noise may be permanently degraded, sometimes making a
particular tube useless for astronomy.  So you need to know something
about how the specific tube has been treated, or be prepared to go
through a large number of tubes to find the best one(s).  Vacuum tubes
are like that.  I know, as I used to work on vacuum tube radar systems,
in which we had to "burn in" and hand select sets of tubes in order to
achieve the necessary frequency response and gain matching.  Surplus
tubes are usually always "pull outs", replaced for some reason or
other.  This is not going to produce a useful PMT system for faint
object, astronomical photometry.  A new, selected PMT tube will cost
several hundred dollars from a supplier such as Hamamatsu.  (RCA, the
inventor of the 1P21, is now out of the PMT business.)

The main issue is NOT just wiring up a system that detects faint stuff,
but also having a system that will give predictable results, comparable
to other results from other workers.  Thus, color response, filter sets,
noise levels, etc. are very important in astronomical photometry.  This
is treated at length in any of the standard texts on the subject, such
as "Astonomical Photometry" by Henden and Kaitchuck.

I would recommend that anyone trying to learn electronics at the same
time as building a photometer system, steer away from high-voltage PMTs
and either try a PIN diode system, OR more reasonably, build a Cookbook
CCD camera.  The series of how-to books by Richard Berry (from
Willmann-Bell) and the parts kits available from University Optics will
almost completely ensure a successful outcome with the Cookbook Camera,
which will also be useful for imaging.  Not to mention not having to
worry about the "ZAP!" factor inherent with high-voltage PMT circuitry,
vs. simpler, low-voltage CCD circuits.  There is much software available
for CCD photometry and imaging to assist in getting good quality
results, without having to "reinvent the wheel" so to speak.

You CAN definitely do useful photometry with a Cookbook CCD camera!  I
will be happy to share a paper written by Gary Frey (who operates twin
20 inch scopes at the former Lines Observatory in Mayer, AZ) on
successful asteroid (white light) photometry, as an example of what can
be done.

Please contact me off-line for more web resources and information on CCD
imaging and photometry, I don't care to continue this somewhat off-topic
thread here.

Gene Lucas
(17250)
Joe Macke wrote:
> 
> For anyone interested in building their own PMT photometer, the Surplus Shed
> (http://www.surplusshed.com) has a supply of IP28 photomultiplier tubes
> (http://www.surplusshed.com/pages/item/r1435.html), $10.  I bet they'd even
> cut you a deal if you bought a dozen.  It seem to remember that this tube is
> similarly numbered to one in Jeff's book, although I don't have the book
> handy to check.  Of course, if you know about tubes, you know that similar
> numbers don't mean similar performance, so check your handy PMT data book.
> 
> I have purchased from Surplus Shed a couple of times and am satisfied with
> their service.
> 
> Joe
> 
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