SearchCal Wiki Page

The new version of SearchCal contains more than 2 million calibrator candidates whose angular diameter has been estimated in a statistical fashion. Users may want some authorized advice on which star to choose in the possibly very long list of stars returned by SearchCal. They also may want to know how to use the quite precise diameters SearchCal provides, that are now precise to a few percent.

This page is open to the calibrator group experts to give such advice.

-- AlexisMatter: - 14 Sep 2022: Here are a few pieces of advice for selecting calibrators for MATISSE:

• General rules when using SearchCal and the various calibrator databases (JSDC and MDFC):
  • Choose objects with calFlag = 0 in JSDC and MDFC.
  • Choose objects without IR excess, i.e. even IR flags (0, 2, 4, 6) in MDFC.
• Specific rules
  • AT observations:
    • Low spectral resolution in LM and N band:
      • Small AT array: choose one calibrator for both bands with the following characteristics: Flux_L > ~ 15 Jy, Flux_N > ~ 15-20 Jy if N-band absolute visibility measurement or Flux_N > 8 Jy if N-band correlated flux measurements, diam < 5 mas, preferably K spectral type.
      • Medium and large AT arrays: Choose one calibrator for each band with the following characteristics: one L-band calibrator (Flux_L > ~ 15 Jy, diam < 3 mas, preferably K spectral type) + one N-band calibrator (Flux_N > 15-20 Jy if N-band absolute visibility measurements or Flux_N > 8 Jy if N-band correlated flux measurements, diam < ~ 5 mas, preferably K spectral type).
    • Higher spectral resolution in LM band:
      • Same advices but prefer a brother LM-band calibrator with Flux_L > ~ 30 Jy.
    • High spectral resolution in N band:
      • Same advices but prefer a brighter N-band calibrator with Flux_N > ~ 40 Jy.
  • UT observations:
    • Low spectral resolution in LM and N band:
      • Choose one calibrator for both bands with the following characteristics: Flux_L < ~ 70 Jy, Flux_N > ~ 5 Jy, diam < ~ 3 mas, preferably K spectral type.
    • Higher spectral resolution in LM band:
      • Same advices but constraint on the maximum L-band flux (detector saturation limit) is lifted.
    • High spectral resolution in N-band:
      • Same advices but prefer a brighter N-band calibrator with Flux_N > 10 Jy.

• if your science object has V<10, you probably can always find a calibrator in "Bright" mode, where the spectral type is known (safer, smaller errobars)
• if you observe redwards of J band, select K giants in priority, first in "Bright" mode, if not sufficient, in "faint" mode where the spectral type is guessed...
• ... Unless you need an earlier spectral type with few absorption lines, to calibrate properly high-resolution cooler stars. In this case, selecting a luminosity class I to III is recommended, they have less chance to be double than theur class V counterparts.
• on the bluer side (V, R) you probably have to remove filters that block class V early-type stars.
• warning, B and A dwarf stars that are equal-brigthness close doubles (and thus not suitable as calibrators) will fool SearchCal (until Gaia gives the distance).
• No calibrator appear --> try removing some filters (eg: enable lum. class V)
• There is a nice calibrator at a distance of 0 arc sec from my source: congrats, your source is a calibrator! Do not observe it (it's gonna be boring), just use SearchCal diameter.