[Aavso-photometry] UBV Photon Counting

[Jeff Hopkins]

Hello Michael,

Intrinsically better? As Arne Henden says, it's "in the eyes of the beholder."

I truly like photon counting.  I find it very easy to do. Much more 
simple than CCD photometry, at least for me. No computer needed, no 
flat, bias or dark frames needed. Just point at the star and wait 10 
seconds for a reading. If there is a problem (e.g, contrail, thin 
clouds) it is immediately apparent.

The large dynamic range of photon counting (>1,000,000) makes it 
ideal for cases where program and comparison stars differ more than a 
couple magnitudes in brightness.

There is no gain calibration needed. No gain switching. A dead time 
is needed, but is a very simple one time determination.

Readout is direct on a counter, usually 10 MHz. Sky and bright star 
readings all without any gain adjustment.

Photon counting provides a digital readout (note: SSP photometers 
have a digital readout, but are not photon counting. They are analog 
and use a voltage-to-frequency converter to produce a count.

Very short integrations are possible. I did some lunar occultation 
work with Dr. Paul Schmidtke (ASU) using photon counting where we set 
integration times for 1 millisecond. For 10 seconds worth of data we 
ended up with 10,000 data points. Pretty interesting. For variable 
star work I use 10 second integrations, however.

The U band is well within the capability of a 1P21 PMT. I have many U 
band data plots of stars and some of these are most interesting. 
Things can be seen that do not show in the longer wavelengths.

Photon counting systems are relatively easy to make yourself and can 
be considerably cheaper than CCD systems, particularly when you have 
to add a laptop computer. The top end CCDs are way out of the 
financial range of many if not most amateur astronomers.

I also get a big kick out of the quantum link with the universe by 
watching the counts come in real time for a star system. I am indeed 
counting photons from the star system, not looking at a sum of counts 
that produce a current/voltage and then manipulated to produce a 
value. While it may be my age, this in itself is a BIG kick for me 
and makes long hours at my telescope in the middle of cold nights 
worthwhile.

I don't know how "little" your scope is, but here in Phoenix one is 
hard pressed to even see stars with a scope much fainter than V=7. 
The sky itself gets close to that. While CCDs are indeed very 
sensitive and have a higher QE, doing variable star photometry on 
very faint stars is difficult in itself. Just finding the stars, 
comparisons stars and any information on the stars is a major task. 
Producing accurate data is even harder. Personally I am happy to 
leave the fainter (<14 mag) stars to the larger professional 
observatories.

If indeed your CCD system saturates at V= 7, then trying to do UBV 
photometry with your system will be a big problem because of the 
CCD's lack of dynamic range. Also, just because a star is bright does 
not mean the U band will be bright. Some are very faint in the U 
band, while others are brighter in the U band than the V band.

BTW, CCD photometry is PEP. It is the photoelectric effect explained 
by Einstein that creates the current (electrons) in the cell wells of 
the CCD. A photomultiplier tube starts the same way except the single 
photon's initial current is multiplied by 9 stages and then produces 
a short (nanosecond range) pulse of current. One pulse - one photon. 
Note: For photon counting it is important that the PMT voltage 
divider be designed as a pulse forming network.

Jeff

At 13:04 -0700 1/19/05, Michael Koppelman wrote:
>I'm curious about this. PEP is a great thing and complimentary to 
>CCD work and just plain ol' valuable on its own. But your statement 
>implies that there is something intrinsically better about photon 
>counting? We are almost always interested in relative changes in 
>flux so I'm not sure what is so great about photon counting?
>
>For me and my little scope with a B filter about V=7 starts getting 
>too bright. A mag 3 star would be well within my capabilities with a 
>U filter (I would think). It might still be too bright. Kind of an 
>interesting idea, high time-resolution photometry in the U-band of 
>bright stars with a CCD.
>
>Cheers,
>Michael Koppelman
>http://www.lolife.com/astronomy/
>
>On Jan 19, 2005, at 1:32 PM, Jeff Hopkins wrote:
>
>>  but then the CCDs outputs are analog and cannot do photon counting.

-- 
Jeff Hopkins
HPO SOFT
http://www.hposoft.com/Astro/astro.html

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