![]() ![]() As the accuracy in these catalogs were refined over time, the pointing accuracy of HST has also improved. The coordinates populated in the FITS headers of HST observations retrieved from DADS (the HST Data Archiving and Distribution Service) were derived based on the guide star coordinates in use at the time of the observation. Total Error (including SI to FGS alignment)įirst version publishedfor the user communityĤ.5.2 Absolute Astrometry Improvements in MAST Table 4.1: Key Guide Star Catalog releases and associated errors A summary of the GSC catalogs and associated errors over the HST lifetime is provided in Table 4.1. Thus, after including uncertainties in the positions of the science Instruments (SIs) in the alignment of the focal plane to the Fine Guidance Sensors (FGS), the total error in HST absolute astrometry is ~1 arcsec for observations made with GSC 1.1, ~0.3 arcsec for those made with GSC 2.3.2, and ~0.1 arcsec when using the new GSC 2.4.0. An updated version of the catalog (GSC 2.4.0) was released in October 2017, improving the celestial coordinates with the positions from Gaia DR1 and reducing errors to < 30mas over the entire sky. ![]() This accuracy improved substantially in October 2005 (during Cycle 15) with the introduction of GSC 2.3.2, where rms errors per coordinate were reduced to ~ 0.3 arcsec over the whole sky. GSC 1.1 had nominal rms errors of ~0.5 arcsec per coordinate, with errors as large as ~1‐3 arcsec reported near the plate edges. I did read about absolute, apparent, and Johnson V magnitudes, but I still don't understand where the parameter G comes from, or the relationship between spectral type and magnitude as it relates to my simulation.Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars as specified in the Guide Star Catalog. I'm trying to relate these to spectral types, but when I researched it I wasn't able to find an explanation of G mag anywhere. The spectroscopic uncertainty depends on Johnson V magnitude, but the astrometric uncertainty varies with a different description of magnitude, a parameter $G$, e.g. I came across this text and the associated paper by de Bruijne: Science performance of Gaia, ESA’s space-astrometry mission, which includes exactly what I'm looking for: the varying astrometric and spectroscopic uncertainties for stars of different magnitudes. One of the telescopes whose uncertainties I want to use to simulate observed data is GAIA. My goal is, using the randomly generated spectral type (from a certain distribution) for each binary pair, to calculate the magnitude of the stars in order to assign the correct uncertainty in angular position and radial velocity. As I understand it, spectral type, in general, is related to the magnitude of the star. ![]() In order to do this, I will generate a random spectral type for each orbit, and from there determine the mass of the primary, the mass ratio, and so on. Makes sense.Ĭertain parameters of binary orbits such as the mass ratio distribution of the components depend on the spectral type of the primary, and I would like to include this in my simulation as well. The fainter the star, the lower the precision with which we can measure its position, velocity, and so on. In my simulation, I'm including the fact that the values of the uncertainties for said observables depend on the magnitude (brightness) of the star. ![]() I'm using astrometric and spectroscopic uncertainties from different telescopes to model what the data would look like if it were observed by different experiments. I'm adding uncertainties to my simulated observables such as angular position and radial (line of sight) velocity. I'm simulating a population of binary stars for a summer research project. ![]()
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