Frequently Asked Questions

Last update : March 2d 2017


Direct links top topics
Pixels in the annulus and spurious stars
How to cope with very faint Variable stars
Sky brightness calculations
Which data are considered as valid?
How the signal to noise ratio is calculated?
How LesvePhotometry is performing Ensemble photometry?
Which format to use for RA and Dec?
My images are clear/unfiltered.
In database, if I make the V (and other filters) magnitude zero....will LesvePhotometry
merely output differential magnitudes? Or will it blow up?

How accurate must the coordinates be?
I don't have Excel installed, only OpenOffice spread sheet
How Instrumental values are calculated in the AAVSO report?
Why to use Instrumental values in the AAVSO report?

Pixels in the annulus and spurious stars

The background level of the image is derived from the pixels laying in the annulus. This background level is subtracted from the values of the aperture. Pixels in the annulus are also used to calculate the Signal to Noise Ratio (SNR). As it is not always possible to have an annulus free of spurious stars, the following strategy is applied by LesvePhotometry to cope with the problem:

- Elimination of annulus pixels which are close to a spurious star (i.e. pixels at a distance less or equal to 1.5 the aperture radius). This elimination is based on star detection by astrometry routines of PinPoint. The background calculations will be based on this set of annulus pixels. Faint spurious stars not detected by PinPoint will not be eliminated by this method.
A special option exists in the "Photometry parameters" tab to allow or deny elimination of spurious stars in the annulus. By default the option is set to discard spurious stars.

- The background level is calculated from the median of pixel values contained in the annulus after elimination of spurious stars. The median is a way to get a reliable background level despite remaining faint spurious stars in the annulus.

- The standard deviation is also calculated from the same set of annulus pixels. Spurious stars in the annulus would normally increase the standard deviation of the annulus, but this effect is reduced as the spurious stars are eliminated from the annulus.

How to cope with very faint Variable stars

The first step in the photometry process is to run PinPoint (astrometry engine) on sky image. PinPoint provides a list of detected stars and LesvePhotometry identifies the variable and comparison stars in this list.
By default LesvePhotometry puts the photometry aperture on the centroid pixel from astrometry data. If a star is very faint PinPoint will not be able to detect it. When that occurs, for a Reference star (R) and for a check star (CK) the program generates an error, skips this image and will continue with the next one. If the variable star is not detected, in the reports the variable magnitude will be set to NaN (Not a Number).

But you have an option in the "Photometry parameters" tab to center the Variable aperture on the pixel corresponding the variable sky coordinates even if the star is not detected. If you use this option verify carefully the results and the aperture position.

Sky brightness calculations

The sky brightness is calculated as described in
"An Introduction to Astronomical Photometry Using CCDs" by W. Romnishin (University of Oklahoma)
Excerpt : Now, we have a star with a known magnitude and a measured number of counts, and a sky patch of a known angular area with a measured number of counts but an unknown magnitude. To calculate the magnitude of the sky patch, we use the fundamental equation relating magnitude differences to the brightness ratio of two sources, here the sky patch and star:
mstar - msky = -2.5 log10(Bstar/Bsky)
msky = mstar - 2.5 log10 (Bsky / Bstar)
Here the m's are the magnitudes of the star and of the one arcsec square patch of sky, and the B's are the brightnesses (or fluxes) of the star and sky patch. For the brightnesses, we simply use the counts. (The counts are not the true brightnesses, which are given in units of energy per unit time per unit area, but the counts are related by a constant multiplicative factor to the brightnesses. The constant is the same for star and sky, as the sky and star were observed with the same telescope, detector and exposure time, so it cancels in the brightness ratio.)
V sky brightness observed at Kitt Peak in Arizona is about 21.9 magnitudes per square arcsec.

If the Bsky value is negative, logarithm evaluation will generate an error and the Sky Brightness will be reported as NaN (Not a Number)
The sky brightness calculations will be wrong if the keyword PEDESTAL of the FITS header is not evaluated correctly during the Calibration (bias, dark, flat).
This PEDESTAL value can be overridden, the "Photometry parameters" tab has a check-box "Override PEDESTAL keyword with". If checked, the value entered in the associated textbox will be added to background level for Sky Brightness evaluation.
The "Limiting Magnitude" (i.e. the magnitude of a star which would have a Signal to Noise SNR = 3) is not affected by the value of keyword PEDESTAL.

Which data are considered as valid?

AAVSO and CBA reports are intended to be published and there contents will be limited to valid data.
But in the Excel and csv reports, all the data points are reported even if they do not seem valid.
These reports contain a column named "ValidData", which values are OK or NOK.

A data point is considered as valid i.e. OK and will be included in the AAVSO and CBA reports, if all the following conditions are met :
- the variable star and the check star have been detected and their magnitudes evaluated
- the SNR is greater than the minimum value specified in the Photometry tab
- the maximum pixel value in the aperture is lower than the specified Saturation value.
- the star Flatness value is lower than the specified value.
If you have no data in AAVSO or CBA reports, check if your validity constraints are not too strict.

How the signal to noise ratio is calculated?

The signal to noise ration takes account of the Poisson and background noises and the CCD gain. The calculation is perform in accordance to the relation (12) of the Newberry's article.
"Signal-to-Noise consideration for sky-subtracted CCD data"
Newberry M. 1991 PASP 103, 122

How LesvePhotometry is performing Ensemble photometry?

Ensemble photometric solution uses multiple comparison stars to define the Reference magnitude.
In the "Usage" column of Variable star window, a R indicates that a star will be used as a Reference star. For "Ensemble photometry" you select more than one Reference star.
Two methods may be used for Ensemble photomety: "Master star" and "Mean Value" which are described in a paper written by Tim R. Crawford.

1 - Master star

The sum of fluxes of all the stars included in the Ensemble will create the Master star.

Untransformed Ensemble

- In the skyimage, the fluxes (intensities) of all stars of the Ensemble are summed. So all the photons of the "Master star" are counted.
- From the catalog magnitudes, the reference fluxes of all stars in the Ensemble are calculated as
flux = 10 ^(-M/2.5) where M is the catalog magnitude.
These reference fluxes are summed to obtain the reference magnitude of the "Master star" = -2.5 log(sum of fluxes).
Usual differential photometry (based on the Master star) is performed for target,check and other comparison stars.

Transformed Ensemble

The reference magnitudes of the "Master star" are calculated in each filter bands involved in the Transformation. The color indexes of the Master star are derived from these calculated reference magnitudes. With those information, target, check and other comparison stars are transformed based on color indexes and magnitudes of the Master star.

2 - Mean Value

The untransformed and transformed magnitudes of the variable star are individually calculated for each star of the ensemble and the mean value is calculated.

Which format to use for RA and Dec?

Right Ascension has to be entered in hours (sexagesimal or decimal format)
Declination has to be entered in degrees (sexagesimal or decimal format)

Sexagesimal format is very flexible, the ASCOM functions are used for the conversion :
The sexagesimal to real conversion methods such as this one are flexible enough to convert just about anything that resembles sexagesimal. Thee way they operate is to first separate the input string into numeric "tokens", strings consisting only of the numerals 0-9, plus and minus, and period. All other characters are considered separators. Once the input string is parsed into tokens they are converted to numeric. If there are more than three numeric tokens, only the first three are considered, the remainder are ignored. Left to right positionally, the tokens are assumed to represent units (degrees or hours), minutes, and seconds. If only two tokens are present, they are assumed to be units and minutes, and if only one token is present, it is assumed to be units. Any token can have a fractionsl part. Of course it would not be normal (for example) for both the minutes and seconds parts to have fractional parts, but it would be legal. So 00:30.5:30 would convert to 1.0 unit.
Note that plain units, for example 23.128734523 are acceptable to the method.

My images are clear/unfiltered.

Q : My images are clear/unfiltered. (FITS header contains the filter information, and filter is named C ). Do I have to take special actions to properly process/analyze my unfiltered images?
A: No, LesvePhotometry will use the V database magnitudes. In the AAVSO report, the filter field will be CV. See the aavso-extended-file-format document.

In database, if I make the V (and other filters) magnitude zero....

Q : If I make the V (and other filters) magnitude zero....will LesvePhotometry merely output differential magnitudes? Or will it blow up?
A : If you enter 0 as a magnitude for a comparison star it will work as differential magnitude.
But if you enter nothing, the program will complain about lack of magnitude for the comparison star.

How accurate must the coordinates be?

Q : How accurate must the coordinates be? If I'm off by 0.5 arcsec in declination...will that cause PinPoint to fail to find/use that comp star?
A : LesvePhotometry puts a limit on distance to accept a star as a comparison star. This limit is 3 arcsec.

Q : Are the RA/Dec J2000?
A : Yes

I don't have Excel installed, only OpenOffice spread sheet

Disable the Excel report generation in the "General Settings" tab and check the "Csv report" option.

How Instrumental values are calculated in the AAVSO report?

Instrumental magnitude = - 2.5 * Log10(Flux)+ 30
where Flux is calculated as the sum of [ pixel values (ADU) inside the aperture - mean background pixel value]
A value of 30 is choosen as zeropoint to avoid negative instrumental values.

Why to use Instrumental values in the AAVSO report?

Here a copy of the interesting Arne's explanation

Hi Pierre (and Jim),
The AAVSO Extended Format was designed very carefully to contain the maximum amount of information in the fewest number of bytes. Specifically, both the comparison star magnitude and the check star magnitude are to be *instrumental* magnitudes. Instrumental magnitude in this sense is -2.5 log (count_sum) for each star, plus an arbitrary zeropoint. That zeropoint has to be the same for comparison and check, and if you are taking a time series, the zeropoint should be the same for all images in the series. However, how the instrumental magnitude is zeropointed is not important; you are not trying to match the standard system, but to provide information about the conditions when the observation were made and the quality of the observation.
The reasoning:
The standard magnitudes for both C and K are already defined, since you provide the VSP chartID which contains the magnitudes used for the sequence, and the CNAME/KNAME fields identify which stars of the sequence were used for the estimate. You don't need to repeat that information by using Cstd in the CMAG field.

(K-C) from the CMAG and KMAG fields gives a reliable error estimate for a time series, whether or not the software or user provides good uncertainty estimates for the target. The requirement, however, is that both C and K *have* to be instrumental magnitudes, or (K-C) will be undefined.

C (or K) from a time series, when they are in instrumental magnitude form, tell the researcher how stable the atmosphere was during the series. If C varies by a few hundredths, then conditions were stable; if you see big dips in the magnitude of the comparison, then you know that clouds were around and those target measures may be less reliable. The instrumental magnitude for C or K will also show you the trend of magnitude vs. airmass so that a researcher may be able to remove extinction effects.

For ensemble calculations, C and K get more difficult to use in the manner shown above. What we have come up with is that C remains undefined (though in hindsight, we might have asked for the instrumental magnitude of some arbitrary star in the field), and K is defined as a separate star from the target, but measured with the same ensemble. The reasoning here is so that a researcher can look at the measured K value, compare it with a new calibration of the standard value of K at a later date, and adjust your observation with an offset to match the latest calibration. This is required since we don't know what stars went into the ensemble and so cannot adjust the ensemble offset. We lose the uncertainty estimate given by (K-C), but assume that anyone doing ensemble photometry will get the uncertainty estimate from the ensemble and so their target uncertainties will be reliable.
I hope that helps. Whether or not you agree with the process, the requirement for the AAVSO Extended Format is that both comparison and check magnitudes *must* be instrumental. UNQUOTE

In the "NOTES" field of AAVSOreport, LesvePhotometry includes a comment giving the standard magnitude values for the comparison and check star.