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

[Jeff Hopkins]

Hello Michael,

My point about the comparison stars is that is does not matter if 
they vary (due to the sky conditions) as it is the relative magnitude 
between the comparison star and program star that you are after. In 
fact some observes only report the delta between the comparison and 
program. I never liked that as more than one time a star got me 
interested. Looking further I found the magnitude was well beyond my 
equipment.

Since with CCD photometry you are normally imaging the comparison in 
the same frame as the program star, it does not matter what the 
comparison star is at the time of the exposure. This is actually 
better than single channel photometry because there is no time lag 
between measurements. Assuming you know the published values of 
magnitudes for the comparison star, you just find the difference 
between the comparison and program star and then adjust the 
comparison star to the published value and you have an accurate 
program star value (as accurate as the published value and observing 
permit).

One thing I wonder about with CCD photometry is the small dynamic 
range. Even 16 bit devices allow only 64K steps. With photon counting 
you have over a million steps and approaching 10 million. Does 
stacking somehow help the CCD data? If so, wouldn't that be 
essentially like just averaging the data?

As for non-optimal conditions, more than once I have gotten excellent 
data between clouds. In fact once summer I was pressed for data on a 
star and there were thunderstorms all around. I got he data through 
holes in the clouds. It was an interesting evening. The observatory 
was continually being lit from lightning flashes. I have found the 
non-photographic nights are when the very thin, sometimes not visible 
clouds start to play havoc with the readings. That's when it's time 
to pack up an head for bed.

I understand about  the short time scales. When doing PEP on Algol, I 
could not use three set of measurements because the star system was 
actually changing significantly in the time require to get those 
data. I settled for two sets of three comparison star reading 
bracketing three program star reading in each color. It was very 
interesting watching the averaged numbers change over a short period.

BTW, I am constantly amazed that folks with the CCD cameras are 
imaging such faint stars and getting any data at all.

Jeff

At 14:39 -0700 1/25/05, Michael Koppelman wrote:
>First of all, I have heard from other pros that single channel 
>photometry can be better than CCD. Seems you are proving that!
>
>A lot of us work on stars that vary on short timescales. The 
>standard deviation of the program star is not always an option. I 
>don't at all see how the comp star standard deviation is 
>"meaningless" -- these are stars that are not varying! The accuracy 
>of your program star will not be greater than the accuracy of these 
>constant stars. (Replace accuracy with precision if I have them 
>reversed.)
>
>CCDs allow useful work to be done in non-optimal conditions. If I 
>had to wait for photometric night I would forget photometry and take 
>up bowling.
>
>Yes, I agree that more photons means less noise, but I still feel 
>that the point is being lost: the 1/SNR type of error is assuming 
>that the measurement in question had no other sources of 
>uncertainty. That is almost never true. CCDers who report 1/SNR are 
>not reporting the entire uncertainty. Maybe your technique gets 
>around this somehow but mine doesn't.
>
>Cheers,
>Michael Koppelman
>http://www.lolife.com/astronomy/

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

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