[Aavso-photometry] Re: Getting started (Michael Koppelman)

[pdshankland at comcast.net]

Sure, no problem - 
S/N will benefit from adding photons, no doubt; S/N is the ratio of signal (amplitude of the flux) to the noise (the noise floor). I agree. But when you are collecting for a photometric study, you are "presuming" that the delta is two dimensional, but really, it's sort of an area under the light curve.... ie, its a combination of the amplitude minus the noise floor at every point along the flux curve... make sense? And so, if you narrow the observed flux (truncate the ends of the flux curve) with a filter, you have a "more efficient" combination of flux-minus-noise-floor measurements for a given curve. 

So, while the peak photons observed will be dimmer thru a filter, the noise floor will be pushed even further down than without the filter. Ergo, your eyeball doesn't like the dimmer view, but you have pushed the noise floor down with the filter per above...

I used metaphors here so please forgive me.... You still improve precision in the photometry this way, over filter-less-   Walt's points in the end made great sense-

You can see it in the integration of the flux curve here: http://spider.ipac.caltech.edu/staff/jarrett/2mass/3chan/noise/#sn, but that's alotta math... :-(

Paul

-------------- Original message -------------- 

> Sorry, maybe I've lost something in your reasoning, but I can not see 
> the point you raised. 
> 
> >From what I've learnt, more photons means better SNR. In the pure 
> "photonic" noise situation maths are clear. If the "photonic" noise is 
> not predominant then the improvement could be poor, but an improve. 
> 
> Could you please re-elaborate your reasoning in order to a non-expert to 
> be able to comprehend? 
> 
> Thank you very much in advance. 
> 
> Pedro Pastor 
> 
> 
> On Wed, 2004-12-01 at 03:17, Paul Shankland wrote: 
> > Well, I'd have to agree with V + LPR :-) - that's just cutting out photons 
> > with no narrowing - one or the other IMO... maybe I missed that one - hmmmm- 
> > 
> > Well, each has their opinion, and I've found that to work millimag precision 
> > you must only use filters; in fact if you expect to survive a peer review 
> > it's pretty much a requirement (do a search at 
> > http://www.journals.uchicago.edu/ApJ/search.html - let me know if you find 
> > any support for unfiltered differential photometry, it'd be interesting to 
> > read). More photons never equals better S/N, just pushes the amplitude 
> > higher up the response curve, and [assuming you remain on the linear portion 
> > of the ccd] you increase effectiveness, but not efficiency (height above 
> > noise floor). In work on my 16", I agree, it appears dimmer, more difficult 
> > to image (requiring longer exposures ergo more noise) etc etc, but I'd 
> > rather go up to the 24" at work then. But even with the other variables 
> > (like aperture) constant, you still do not add noise by narrowing the the 
> > bandpass. Even if the exposures are lengthened a bit (as was noted, 
> > ameliorated by bias/darks/flats), if you stay linear, you maintain improved 
> > precision, and stay cleanly above the noise floor. 
> > 
> > It may be harder to do filtered differential photometry, and less fun, 
> > dimmer, but a requirement at some point for millimag precision. I guess the 
> > real point is to match your target to an aperture which can handle filtering 
> > with sufficient photon collection capability. That is, assuming you can 
> > change apertures. That or pick easier (greater flux delta) targets- 
> > 
> > But more photons doesn't equal better S/N (it's a ratio, not just the flux 
> > amplitude), just seems thataway! :-) 
> > 
> > Clear skies, 
> > Paul 
> > 
> > 
> > ----- Original Message ----- 
> > From: "Michael Koppelman" 
> > To: "Paul Shankland" 
> > Cc: 
> > Sent: Tuesday, November 30, 2004 8:17 PM 
> > Subject: Re: [Aavso-photometry] Re: Getting started (Michael Koppelman) 
> > 
> > 
> > > Well, to clarify, I thought he was talking about using a V filter AND a 
> > > light pollution filter, which I don't think anyone would recommend. 
> > > Second, I agree with Walt that from my experience more photons equals 
> > > better S/N and all filters yield less photons. 
> > > 
> > > Cheers, 
> > > Michael Koppelman 
> > > http://www.lolife.com/astronomy/ 
> > > 
> > > 
> > > 
> > > On Nov 30, 2004, at 6:46 PM, Paul Shankland wrote: 
> > > 
> > >> Just remember, the neat thing about differential photometry is, that you 
> > >> can use any filter or any SNR improvement process, JUST SO LONG as you 
> > >> apply it across the whole image to be used.... Opposite from tainting 
> > >> data, in differential aperture photometry, it will be a plus... Really, 
> > >> how the Hutech handles color balance/transmission is immaterial to how 
> > >> differential photometry works- (other than if it adds noise by being a 
> > >> lousy filter) 
> > > 
> > 
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