[Aavso-photometry] When to submit "Fainter Than" versus actual numbers.

[Jeff Hopkins]

Hello Gary,

Since I work out of my backyard observatory in the middle of Phoenix, 
Arizona, even though we have lots of clear skies, they skies are 
pretty bright. That's why I stick to bright star work. About the 
brightness star that I can work with is around 8th magnitude, mainly 
because I cannot see to find and center anything fainter in a small 
diaphragm. That would one more advantage of a CCD system as you do 
not need to point the telescope as precisely.

I have not experimented with how faint an 8" telescope with photon 
counting PMT can go. However, from Arne's book "Astronomical 
Photometry" he has some charts for cooled and uncooled PMT detectors 
for a 20 cm telescope. For unclooled the limiting (SNR= 1) is 
approaching 14th magnitude. For a cooled versions it's approaching 
16th magnitude. A lot depends on the sky too. I would never attempt 
16th magnitude with a PMT and a 20 cm telescope. If I wanted to work 
that faint, I would look for a much larger telescope and dark skies, 
preferable on a mountain top. I'm much more interested in producing 
accurate results than pushing the envelop. There are several 
lifetimes worth of bright objects so I will never get bored.

Regarding the error source for photon counting, assuming perfect sky 
conditions, dark counts and dead time are probably the biggest 
sources of error. They tend to negate each other as the dead time 
goes up with brighter stars (but can be adjusted for), the dark 
counts become insignificant and are subtracted out with the sky 
subtraction. Going in the other direction, with faint objects, dead 
time approaches zero and if you cool the PMT the dark counts can be 
reduced to zero and sensitivity increased. From my experience, the 
sky is always the main source of data spread. Equipment problems can 
be a source of error, but are usually easy to see and correct so they 
do not enter into the final data. Problems like High Voltage drift (I 
continually monitor the high voltage so that is never a problem), 
having the star drift too close to the edge of the diaphragm or 
getting dew on the corrector plate (very sneaky error so even in dry 
Arizona I always use a dew shield and heater).

Jeff

At 10:43 -0700 1/27/05, BailyHill at aol.com wrote:
Hello Jeff;

Thanks for the explanation of a photon counter.  I have no experience 
with them.  It sounds very good.  What would the biggest error source 
be with such a setup?

What is the lower limit of brightness that you can work to with a 
scope of a certain size?  How large would the error be if the Target 
were 16th mag?

Thanks
Gary


-- 
Jeff Hopkins
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