Photometry of Science images necessitates the identification of the target variable star, along with one or more reference stars, one check star, and potentially zero or more comparison stars. Throughout this chapter, we will commonly refer to the reference, check, and comparison stars as calibration stars. For each selected calibration star, it is essential to provide information about their magnitudes in various color bands, among other details.


To streamline this process, Phoranso employs an intelligent Variable Star Database, allowing you to input information about your target star and calibration stars just once. Phoranso memorizes this information for future use in what we term an ID Card. This functionality eliminates the need for repetitive data entry, ensuring consistency in your comparison stars even over extended periods of time. The Variable Star Database contains a dedicated ID Card for every Target star you intend to measure in your Science images. 


In this section we will explain how to create a new ID Card, starting from a DSS image, an AAVSO chart or one of your FITS images.


Target star identification


Choosing New from the Variable Card menu marks the beginning of the ID Card creation process. First, enter the name of the Target (variable) star for which you wish to generate an ID Card. As an example, suppose you have gathered Science images of the cataclysmic variable star V0844 Her. Input V0844 Her into the Name field and proceed by clicking the AAVSO Lookup button. 


The chart FOV (Field Of View) box will be used by Phoranso to retrieve AAVSO comparison stars from the AAVSO database. In the below example, Phoranso will retrieve AAVSO comparison stars within an area of 20' centered around FS CMi. 


 


Phoranso will establish a connection with the AAVSO VSX database to retrieve pertinent details about V0844 Her. This information encompasses the J2000.0 coordinates, the unique AAVSO designation (if available) for this variable star and the ID of the corresponding AAVSO chart (if available). 


Note that, before clicking the AAVSO Lookup button, you have the option to manually enter the AAVSO chart ID. This instructs Phoranso to retrieve that specific chart from the AAVSO server. When left empty, Phoranso will automatically select an appropriate AAVSO chart. This option is particularly relevant if in the next step you decide to base your ID Card on an AAVSO chart instead of a FITS image.



You have the option to completely bypass the AAVSO Lookup and manually enter the name of the target star along with its Right Ascension and Declination values in the respective fields. After completing these fields, you will be presented with the below image. 




Reference, Check and Comparison stars identification from a FITS reference image


To proceed, we need to select the calibration stars for use in the Photometry process. To achieve this, load either a DSS image, a FITS image or an AAVSO chart. In this section, we'll explain the process for a FITS image. 


Click the FITS image button to choose a FITS image containing V844 Her. This action expands the ID Card window to include the selected FITS image.


In the background, Phoranso will solve the image to retrieve the coordinates of as many stars as possible, extracting magnitude information from a photometric catalog. The solver, astrometric and photometric catalogs used in this step are specified in the Settings box of the File menu. Parameters related to this step can be configured in the Variable ID Card Settings box. 


Additionally, Phoranso will connect to AAVSO repositories to gather information about AAVSO comparison stars present in the image. When located, the magnitudes of the AAVSO comparison stars will appear to the right of the corresponding stars. Please note that AAVSO comparison stars may not be available for all Target stars. 



In our V0844 Her example, Phoranso successfully identified the Target star, distinguished by a purple Ringset. The rings size is inconsequential at this stage and will be configured during the actual Photometry process. Additionally, we observe the presence of multiple advised calibration stars. 


Continuing with the identification process, we now focus on selecting a suitable Reference star, considering various selection criteria. Hovering the mouse over the FITS image reveals changes in the mouse cursor shape: 


    • The default cursor (4 black lines) indicates that no star (with relevant information) has been found.

    • The cursor transforms into a small open circle surrounded by 4 lines when a photometric star is located, with colors designating its suitability:
      • Green for an advised comparison star,
      • Red for a star deemed unsuitable as a comparison star (high B-V color index)


In our V0844 Her example we choose the advised comparison star with an AAVSO magnitude label of 135 as the Reference star. To make this selection, right-click while the cursor is positioned over the star, and from the popup menu choose Select as Ref star. Your selection becomes visible in the Target - Reference - Check - Comparison stars section of the ID Card window (the AAVSO ID of the selected star is 000-BBX-448), and a red Ringset appears around the star.


Opting in this example for a single Reference star only (no ensemble photometry), we proceed with the selection of a Check star following the same procedure. We choose the star with label 141 as the Check star, and a light blue Ringset is drawn around the star, while its AAVSO ID (000-BBX-449) is added.


Finally, we may choose to select zero or more Comparison stars. Optionally, a comment text can be entered in the Comment section of the ID Card window. This comment will be added to the saved ID Card. Also, note the Solver results section in the ID Card window, providing information identical to what was explained here.


When applying second order Extinction corrections, Phoranso requires the B-V value of the target star. Enter that value in the B-V box of the Target star color section. Leave to 0 if no second order Extinction correction is needed. 


With the ID Card definition completed, press the Save ID Card button to store the ID Card of V0844 Her in the Variable Star Database. Along with the ID Card contents, Phoranso stores a screenshot of the ID Card in the form of a PNG image in the Variable Star Database. This PNG image is conveniently displayed when opening an ID Card using the Open command in the Variable ID Card menu.



Next to the names of the Target, Reference, Check and Comparison stars, you will see one or more icons. Hover your mouse over an icon to see its name. Note that those icons are not present if you opt to work with an AAVSO chart instead of a DSS or FITS image.



  • Click the blink icon to look up the selected star on your image. The corresponding ring set will blink for a few seconds to reveal its position.

  • The Enable/disable for photometry check box : enabling a star means it will be included in photometry calculations, while disabling it will exclude it from these calculations. Disabled stars still remain part of the ID Card. This feature is especially useful when working with variable stars with large amplitudes. It allows you to create an ID Card with reference and comparison stars that suit the maximum brightness of the variable star, as well as stars that suit its minimum brightness (see example). Depending on the current brightness state of the variable star, you can then enable the most appropriate stars and disable the others without deleting them. In ID Cards created prior to version 1.0.2.4, all reference and comparison stars by default will be enabled for photometry.

  • The edit icon is discussed in more detail in this section.

  • Click the trash icon to remove the selected star from the ID Card. 



Reference, Check and Comparison stars identification from an AAVSO chart


Instead of using a FITS reference image for identifying Reference, Check and Comparison stars, an alternative is to select an AAVSO chart. To do this, click the AAVSO chart button in the box shown in the Target star identification section above.

This action expands the ID Card window to include the automatically downloaded AAVSO chart. 



Click the Select AAVSO compstars button to initiate the identification of the calibration stars. This opens a tabular window displaying all AAVSO comparison stars present in the field of V0844 Her:



We begin by selecting the Reference star, clicking on the row with Label 135, which turns red to indicate the selection. Next, we choose the Check star from the selection list at the top of the window, and click on the row with label 141, coloring it light blue. Note that for ensemble photometry, multiple Reference stars can be selected, and you can also opt for zero or more Comparison stars.



Click Apply when your selection is complete. This closes the AAVSO comparison stars window, and the selected Reference and Check star information is displayed in the Target - Reference - Check - Comparison stars fields. Click Save ID Card to store this V0844 Her ID Card in the Variable Star Database, along with a screenshot of the ID Card in the form of a PNG image.


The AAVSO comparison stars window contains two additional options:


    • IntelliFilter: when clicked, this limits the display of selectable AAVSO comparison stars to those with a favourable B-V color index and with small B-V error values

    • Hide error values: if unmarked, the AAVSO comparison stars window will display the magnitude error values near each magnitude value



Creating ID Cards for other variable stars in your field of view


If you opt to identify Reference, Check and Comparison stars from a FITS reference image, you'll observe that after saving your ID Card, a button Clone ID Card becomes active at the bottom of the ID Card window.  This button enables you to generate additional ID Cards for all variable stars situated in the same field of view as your Target variable. 


Consider the example of the ID Card for V1062 Tau below. Next to V1062 Tau, several additional variable stars are marked with white circles. Some of these might be of interest for inclusion in your photometry of V1062 Tau. Consequently, we will create ID Cards for them. Phoranso will detect the presence of these additional target stars during the processing of your Science images and automatically generates reports for them. This is a very powerful feature.



Click the Clone ID Card button to open the corresponding tabular window, displaying all known variable stars (from the AAVSO VSX database) present in your FITS reference image. For each star, name, J2000.0 coordinates, variability type, magnitude range, and period are provided. Use the mouse to select one or more target stars for which Phoranso should create ID Cards to be included in the Variable Star Database. In the example below, 5 target stars have been selected. Press Save ID Cards to close the tabular window and create 5 new ID Cards in the Variable Star Database. Each ID Card will share the same reference, check and comparison stars as V1062 Tau.


The FITS image in the ID Card window will be refreshed, and all target stars with an existing ID Card in the Variable Star Database will be highlighted in yellow. Target stars marked in white indicate the absence of an associated ID Card. 





It is entirely feasible to work with mixed sets of ID Cards within a single FITS image. For instance, consider a FITS image containing three bright target stars, each associated with an ID Card (defined and cloned) with bright reference, check, and comparison stars. Additionally, if there are five fainter target stars adjacent to the bright ones, each requiring fainter reference, check, and comparison stars, you can create and clone a corresponding ID Card for each of the five fainter stars. During the execution of the photometric reduction process, Phoranso dynamically selects the appropriate ID Card for each of the eight stars present in your FITS image.



Requesting AAVSO AUIDs for newly created ID Cards


The AAVSO AUID (AAVSO Unique IDentifier) is a unique identifier assigned by the AAVSO to individual variable stars. This identifier is used to uniquely and unambiguously identify a particular star in the AAVSO VSX database.


When submitting a report with photometric observations to the AAVSO International Database it is important that each variable star in the report has been assigned a unique AAVSO AUID. Phoranso can assist in obtaining AUIDs for newly created ID Cards, in case there's no corresponding AUID yet for the target star.


Select ID Cards matching FOV from the Variable ID Card menu. This reveals a tabular window displaying all ID Cards of target stars within the field of view (FOV) of your FITS reference image. The table includes a column AUID, where the presence of two dashes "--" indicates the absence of an AUID. 



To request AUIDs from the AAVSO, ensure you are logged in to the AAVSO website (refer to the AAVSO website for information on obtaining a login code). 


Once logged in, select the stars for which you want to request an AUID and click the Request AUID button. Phoranso will initiate this request through your web browser, creating a new tab for each requested AUID. Check your browser for completion status. If AUID assignment is successful, close the tabular window.


The Activate ID Card and Deactivate ID Card buttons enable the activation and deactivation of ID Cards. When deactivated, an ID Card is excluded from photometry calculations, but it remains an entry in the Variable Star Database. It remains deactivated till you activate it again. Click Delete ID Card to permanently remove an ID Card from the Variable Star Database.


The Export ID Cards button provides two options for exporting selected ID Cards: either to a folder or as a zip file. Refer to the Export ID Cards command for more details.