Nearby A0V/G2V Stars

Locator for Nearby A0V and G2V Stars

This form can be used to locate A0V and/or G2V stars near (i.e., within a specified difference in airmass and angular separation from) a target object. Given the coordinates of the target object, the routine calculates the airmass (X) of the target, as well as: the airmass (X) and absolute airmass difference (dX), angular separation (Angle in Degrees), and Hour Angle (HA) of all A0V stars satisfying the following constraints: (1) between the observable dec limits of the telescope; (2) between HA of -5 h and +5 h; (3) within the specified difference limits in airmass and angular separation from the target object; (4) brighter than the specified limiting V magnitude. The A0V stars satisfying these constraints are then ordered by the absolute value of the airmass difference (dX).


  1. The A0V stars are taken from the SIMBAD database and the stellar properties listed in this database (object type - otyp, B mag, V mag, and spectral type) are also printed as output. (The description of the type codes listed in the otyp column can be found at the following URL:
  2. AVOID USING THE FOLLOWING TYPES OF STARS for either the telluric correction (due to uncertain spectral types) or photometric calibration (due to uncertain magnitudes):
  3. In general, in order to minimize the airmass difference between the Target Object and the telluric standard, stars with HA values LESS than that of the Target Object should be observed AFTER the Target Object (as they have larger RA values), while stars with HA values GREATER than that of the Target Object should be observed BEFORE the Target Object. For example, if the Target Object has HA = 0, then telluric standards with positive HA should be observed first, while standards with negative HA should be observed after the Target Object observations are finished.

This form and the associated code were written by Bill Vacca, originally for use with SpeX, as part of the telluric correction routines incorporated into the Spextool reduction package.

Input Observing Parameters

Choose a Spectral Type:

Select an Observatory:

Give Coordinates in Epoch/Equinox 2000
(Include Negative sign with FIRST NON-ZERO entry)
hrs/degs min sec
Right Ascension of the Target Object :

Declination of the Target Object :

Hour Angle of the Target Object :

Maximum allowable Difference in Airmass:

Maximum allowable Angular Separation (in Deg):

Minimum allowable V magnitude of standard:

Maximum allowable V magnitude of standard:

Back to SpeX Home Page

Back to IRTF Home Page