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

Wed May 28 13:22:44 EDT 2008

You are using Mira Pro. So suppose that the readout noise, exposure time, 
and gain values are present so that Error(T) can be computed. Then can't you 
just pick a star for which Error(T) is 0.36 magnitudes (which corresponds to 
S/N=3, or whatever other criterion you want). That tells you what magnitude 
to use for the "fainter than" value. But what star would you use? There are 
2 ways to do this:

1) From the photometry toolbar, switch to "move" mode and move the marker 
from point to point on faint stars and find one that gives Error(T)=0.36.

2) Measure a bunch of stars in the field. You might find 2 who's errors 
bracket the value 0.36 mag, and then interpolate the magnitude between them. 
Alternatively, you could make a scatter plot of Error(T) vs Magnitude (or 
vice versa) from the table.

Do those short descriptions make sense?

Michael

----- Original Message ----- 
From: "Wolfgang Renz" <w_renz at onlinehome.de>
To: "Michael Newberry" <mnewberry at mirametrics.com>
Cc: "AAVSO-PHOTOMETRY" <aavso-photometry at mira.aavso.org>
Sent: Wednesday, May 28, 2008 10:09 AM
Subject: Re: [Aavso-photometry] CCD 'fainter-than' question

>
>>> If its a less than a 3 sigma detection, it should be probably reported
>>> as a 'fainter than' with the value of the background + 3 sigma as mag
>>> But this mag value is unfortunatly nothing that is available with usual
>>> photometry software. So maybe someone of the experts can recom-
>>> mend how it can be fast and easily determined.
>>
>> I am not sure what you mean.
>> Can;t you get that number out of _any_ software that does photometry?
>
> Not necessarily, some simple ones hide everything except the mag or
> delta mag values.
>
> But I meant the mag value to use for the 'fainter as' to be reported
>
> I've just made a Mira photometry with some measurements below SNR
> 3 down to SNR 0.2 (which is of course just noise):
> #,Image,Obj,Magn,Std?,Error,Error(T),X,Y,Col,Row,Bagkgr,S/N,Resid.,Net 
> Count,Filter,Date,Time,JD - 2453000,Airmass,Exp Time,Weight,Object,Object
> 1,HV_Vir.fts,1,14.1000,*,0.0133,0.0134,609.615,654.057,609.615,654.057,108,81.1,0.0000,15067.4,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 2,HV_Vir.fts,22,18.3602, 
> ,0.5877,0.5944,732.713,723.969,732.713,723.969,107,1.8,0,297.827,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 3,HV_Vir.fts,23,20.8089, 
> ,5.6841,5.6727,964.941,433.693,964.941,433.693,108,0.2,0,31.2236,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 4,HV_Vir.fts,24,19.0179, 
> ,1.1263,1.0913,858.000,128.000,858.000,128.000,108,1.0,0,162.512,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 5,HV_Vir.fts,26,18.2622, 
> ,0.6146,0.5450,992.553,141.438,992.553,141.438,108,2.0,0,325.948,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 6,HV_Vir.fts,27,18.1889, 
> ,0.5030,0.5079,546.208,821.348,546.208,821.348,107,2.1,0,348.735,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 7,HV_Vir.fts,33,18.8217, 
> ,0.9170,0.9112,324.052,752.881,324.052,752.881,108,1.2,0,194.689,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 8,HV_Vir.fts,35,19.4107, 
> ,1.5484,1.5663,301.009,425.941,301.009,425.941,108,0.7,0,113.175,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> 9,HV_Vir.fts,39,18.4497, 
> ,0.6835,0.6474,448.693,62.319,448.693,62.319,108,1.7,0,274.263,V,2008-05-24,04:21:01,1610.681262,1.1673,90.000,1.000,HV 
> Vir,HV Vir
> Aperture Photometry Calibration Results
> Zero Point Calculation for 'HV_Vir.fts'
> Calibration Summary:
> Zero point: 20.356
> Rms Error : 0.000 mag
> Standards : 1
> Aperture Summary:
> R1=10.00, R2=20.00, R3=30.00, E=0.000, Theta=313.00
>
> How would you report them as < x mag ?
> How would you determine x ?
>
> Clear skies
> Wolfgang
>
> -- 
> Wolfgang Renz, Karlsruhe, Germany
> Rz.BAV = WRe.vsnet = RWG.AAVSO
>
>
>
> ----- Original Message ----- 
> From: "Michael Newberry" <mnewberry at mirametrics.com>
> To: "Wolfgang Renz" <w_renz at onlinehome.de>; "Bob Crumrine" 
> <rcrumrin at rochester.rr.com>
> Cc: "AAVSO-PHOTOMETRY" <aavso-photometry at mira.aavso.org>
> Sent: Wednesday, May 28, 2008 5:37 PM
> Subject: Re: [Aavso-photometry] CCD 'fainter-than' question
>
>
>> [...]
>>
>>> I think that its generally accepted that a 3 sigma detection above the
>>> noise is sufficient certain to report such a measurment with its high
>>> error.
>>
>>    yes.
>>
>>>
>>
>> Michael
>>
>> [...]
>>
>>
>>
>
>
> 