Fotometria d'apertura Nuovo !!

ConcettoFotometria d'apertura⚓

La fotometria d'apertura è una tecnica che riguarda la misurazione del flusso di un oggetto astronomico.

Nel nostro caso, lo strumento estrarrà la magnitudine grezza della stella che potrebbe essere comparata con la magnitudine reale memorizzata nei cataloghi.

La tecnica consiste nel sommare tutti i pixel non saturati e non nulli all'interno di un'apertura centrata sull'oggetto.

Nota

Il raggio d'apertura è definito dalla fwhm^[1] dell'oggetto.

Vengono quindi sottratti il contributo medio del cielo circostante e il numero di pixel all'interno dell'apertura.

Per stimare lo sfondo locale si può utilizzare un'altra apertura a forma di anello.

In una situazione ideale questo anello di cielo dovrebbe contenere solo pixel del fondo cielo, ma un algoritmo robusto, lo stesso usato nel software C-Munipack software , è in grado di scartare i valori anomali.

L'anello è definito usando i raggi di due cerchi.

Nota

I valori predefiniti sono di 20 pixel per il raggio interno e di 30 pixel per quello esterno.

Suggerimento

È possibile modificare questi valori nel riquadro delle impostazioni.

Nota

L'altro parametro è il parametro di conversione da elettroni ad ADU^[2] , per avere le ADU reali.

Se non è disponibile, 2.3 dovrebbe essere sufficiente.

Suggerimento

Per le nostre immagini di esopianeti questo coefficiente è memorizzato nell'header e il suo valore è 5.0.

Questo ti permette di elaborare le tue sequenze di stelle variabili o di transiti planetari.

EsempioTransiti planetari.⚓

EsempioStella variabile⚓

Vedi qui

Riferimenti

FWHM
Full-Width Half-MaximumxgenDocChm
The technical term Full-Width Half-Maximum FWHM , is used to describe a measurement of the width of a star
It is a well-defined number which can be used to compare the quality of images obtained under different observing conditions. In an astronomical image, the FWHM is measured for a selection of stars in the frame and the "seeing" or image quality is reported as the mean value. Solower value for FWHM is better.
With Siril it's possible to obtain the FWHM parameters in arcseconds units. This requires you fill all fields corresponding to your camera and lens/telescope focal in the setting parameter window. If standard FITS keywords FOCALLEN, XPIXSZ, YPIXSZ, XBINNING and YBINNING are read in the FITS HDU, the PSF will also compute the image scale in arcseconds per pixel.
ADU
Analog Digital UnitxgenDocChmBinary NumberxgenDocChmDigital CountxgenDocChm
There are several concepts and issues going on here that are getting mixed together.
First the ADC
An ADC (Analog To Digital Convertor) takes signal values in a certain range and converts then to a digital number. Let's run some numbers:
A N bit ADC will be able to represent 2^N states. So a 4 bit ADC can represent 16 distinct states, 8 bits => 256 States and a 12 bit convertor will give you 4096 states. By itself that tells you nothing, you need to know that going from a digital representation of say 533 -> 534 means that the input signal changed by certain amount. To keep the example simple I'm going to say that I have an input range from 0 Volts to 4.095 Volts on a 12 bit convertor, that means that when I have a digital number of 0 then I must have had 0 on the input. Let's also assume everything is ideal for now. With my sneaky choice of input range you can see that for every change in the ADC output digital number by one count then the input must have changed by 1 mV.
Why did I pick 4.095? Well a 12 bit convertor will have 4096 digital values that range from 0 to 4095 (2^N-1 when all the bits are set to "1"). So 4.095 Volts/4095 = 1 mv/count. That means that if you have a 3.567 Volt value on the output and it changes to 3.568 Volts ( 1 mV change on the input) then the digital number on the output should change from 3567 to 3568 digital number on the output from 110111101111 to = 110111110000 in raw binary.
Each of these binary steps are called an ADU (Analog Digital Unit) or Binary Number or Digital Count etc. etc. to keep it separate from the number of bits that the ADC has. This is to ensure that the relationship is in a linear range. If you start using "bits" then it is in a logarithmic range (2^N). The main reason these other names/units are used is that LSB's used to be used. What is a LSB? - it stands for Least Significant Bit, i.e. the zeroth bit in the digital number, which changes precisely in step with the ADU but using LSB (and the implied 'bit") is confusing. Imagine saying, " the ADC changed by 30 LSB's therefore the light flux must have changed by 3000 photons ". 30 LSB's really doesn't make sense, it is a mixing of terminology, so people are moving away from that. But that has meant that many different terms being used instead confusing again...
So you have a conversion from voltage into a digital number.
But you are collecting photons, where does voltage come from?
Next the Voltage conversion
All image sensors at some point in the signal chain will convert the collected signal charge into voltage to interact with the outside world. This is always done using a capacitance and a buffer amplifier. This capacitor and the gain of the amplifier set the conversion from electrons into voltage as A/C_sense which is derived from the Q= CV equation. With C_sense = the sense node capacitance and A = gain of the buffer amplifier.
Next the photon conversion
When a Photon comes rattling into a pixel it has to make it into the photosensitive substrate, this interface is controlled by the fresnel equations and dictates how much light goes into the substrate based upon the indices of refraction and the various materials and AR coatings present (AR = Anti-reflection). This is known as the external QE (Quantum efficiency). Once in the substrate the photons are absorbed through it is depth and generate electron/hole pairs through the photo electron effect. Electric fields sweep the carriers into storage structures. The absorption depth and the depth of influence of the electric fields (these two depths may not be co-incident) are what determines the Internal QE. NOt every converted photon's carriers get collected.
These collected carriers (either electrons or holes) are brought to the sense node and converted into voltages. Where this conversion happens (carrier to voltage) is determined by the type of sensor, a CCD or CMOS image sensor.
The total conversion process;
(Photon flux) * (Integration period) * QE_external *QE_Internal * A/C_sense * ADC_conversion Will give you units of Photons/ADU. (1)