Quote:So in terms of making an absolute measurement of a star, how can you calibrate your system accurately if the standards themselves have a fairly large variation?
Quote:If a nova appears somewhere and everyone chooses reference stars in the field and provides magnitudes everyone accepts - then the measurement from everyone will all be fairly consistent. But in absolute terms it will depend on the system used to define those reference stars.
I ache, therefore I am
Quote:Looking at this from the point of view of a spectroscopist where I and others have been trying to reconcile spectroscopic and photometric measurements, it seems to me that the photometric method has fundamental limitation which means that any absolute measurement is inevitably an approximation. This is because although the method describes the filter passband it does not define the spectral response of the detector or the rest of the optics. The results from different systems (with the inevitable differences in spectral response) are made to agree with each other by comparing the results of measurements of standards and making appropriate (colour dependent) corrections. This quality of the agreement though will depend on the specific detailed spectrum of the target. Not sure how much effect this has in practice but given this limitation a 2% absolute difference between measurements using different equipment perhaps does not seem unreasonable.
Quote:these are the relative magnitudes of a bunch of stars in the sky
Used acronyms: NEML=Naked Eye Magnitude Limit, SQM=Sky Quality Meter, TML=Telescope Magnitude Limit, CO=Central Obstruction, delta_m=difference in magnitudes between double star components, RoT=Rule of Thumb, pD_mm=proposed D_mm for resolving a binary (ident with earlier used pA=proposed Aperture), D_mm=Diameter (of scope) in mm, UCAC4=USNO CCD Astrograph Catalog 4th edition
Quote:The sdss and apass values don't include a magnitude (M) estimate, so I use the aperture based magnitude from UCAC4 as the reference for my clear filter values.