[Aavso-photometry] CCD 'fainter-than' question

[Michael Newberry]

Richard,

I disagree with your assessment that the discussion thus far does not give a 
practical answer to his question because it has become "too theoretical". 
There's not much theory involved in the 2 suggestions I made. If he doesn't 
use Mira Pro then he can try to adapt the methods I described to the 
software he is using.

Michael

----- Original Message ----- 
From: "Richard Huziak" <huziak at sedsystems.ca>
Cc: <aavso-photometry at aavso.org>
Sent: Wednesday, May 28, 2008 3:57 PM
Subject: Re: [Aavso-photometry] CCD 'fainter-than' question


> Bob, et al,
>
> The discussion that supposedly answers Bob's straightforward question
> seems to have become quite theoretical and has not, IMHO, provided a
> very practical answer.  Although the theory is 'fascinating', it doesn't
> necessarily actually get you a measurement in a reasonable amount of
> time, and certainly doesn't keep a new(er)bie interested.
>
> So here is some practical advice.  First, with a faint image that you
> are struggling to see (and even with a bright image), a single
> 'detection' really is quite meaningless because it cannot be verified as
> representative.  This is the nature of any single datum anywhere.  To be
> certain, you want to repeat the measurement, and if you get the same
> answer within the 'expected' error (ie - mostly common sense), then you
> can begin to believe that you have a reliable measurement set.  We've
> heard recently about ghost images, noisy pixels, saturation, sigma
> detection, yada, yada, and all that is fine except with a single image
> you really don't know which of these effects has polluted your
> measurements, if any.  So...practical advice. If you want to shoot and
> get a good measurement, shoot a number of frames - 3 - 5 and then
> inspect them all and reduce them all.  If they all turn out to have
> values within a tight range, then you can believe the results. If they
> (or single points) are scattered, you can have ghosting, cosmic rays,
> poor skies, bad flats, hot pixels, etc - a host of things that cause
> errors.  But you can reject oddball points because you have something to
> compare them to.  At this point, you can either report all your
> individual measurements and errors (easiest) or stack your good frames
> and report a single averaged measurement which now has a better SNR but
> is smeared over time.  In this way, you can have a bunch of SNR=5
> high-error points or one SNR=50 low-error time-smeared point.  It all
> depends on what you want the data for.  If you need to measure a
> threshold (for the TOADs) and don't care about micro-variations then
> stack and measure for higher SNR.    However, a single hit point has so
> many possible errors at low SNR that I simply do not believe the results
> myself.
>
> As a practical example, we have to reject most single-point hits on the
> HMXB program because we cannot 'understand' the errors in the
> measurement, though in a submitted cluster of points, we'd have a better
> chance.  And since we are hoping to see light curves varying
> inter-observer by <0.03 mag, it is not practical to have single hit
> points from multiple observers, since many are clearly /way/ off the
> zeropoint for unknown reasons.  Same as with the recent request to
> monitor RE J1255+266.  I could have submitted a single image showing the
> star at 19.302 +/- 2.315 mags, but I chose to stack 10 or 12 frames for
> a better detection and much smaller error or around 0.150 (ficticious
> numbers - representive from memory). SNR was around 4 for any one image,
> but you can either work harder and produce a measurement, or cheap out
> and provide a less than.  Less than's result because your equipment is
> generally incapable (simply working beyond your practical limit) or
> simply from poor sky conditions that you judge to be giving you
> unreliable data.
>
> So - expose a few frames, if data good over a few frames - submit the
> data.  If shaky - stack and submit with stdev as the error,or however
> you want to calculate it.  If you are not confident, then take the
> brightest of your cluster of images, and report that value as the less
> than value (not the mean, since you have a 50% change that the star is
> indeed brighter than what you reported).
>
> You really don't have to know much theory to do this.
>
> rick (HUZ)  :-)
>
>
> Bob Crumrine wrote:
>> For CCD observing, what constitutes a 'fainter-than' observation?  If I
>> can see the variable on the image but only 10:1 S/N (50 minimum needed
>> for observation, below), is that a 'fainter-than'?
>>
>> Thanks,
>> Bob Crumrine (CRR)
>> near Rochester, NY
>>
>>
>
> -- 
> * * * * * * * * * * * * *
> Richard Huziak
> Manufacturing Engineering
> SED Systems
> Saskatoon, SK, Canada
> tel. (306) 933-1676
> <huziak at SEDSystems.ca>
> * * * * * * * * * * * * *
>
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