Space Situational Awareness (SSA) observations are sometimes performed through a spectral filter. The traditional filters used are those of the Johnson-Cousins photometric system (B, V, R, and I). The SSA community has been observing with these filters for decades and therefore has historical data spanning this duration. More recently, the astronomical community is replacing the Johnson-Cousins system with the Sloan photometric system as the primary system for optical observations. The most recent large astronomical surveys in the optical regime have used the Sloan filters: the Sloan Digital Sky Survey and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The Pan-STARRS 1 catalog sky coverage and its astrometric and photometric precision make it well suited for in-frame calibrations of satellite observations. Such in-frame calibrations would provide increased calibration cadence and the potential for improving accuracy by mitigating the effects of a changing atmosphere. Because a comparable catalog in the Johnson-Cousins photometric system that would allow in-frame calibrations does not exist, it makes sense for SSA observations to transition to the Sloan system. A consequence of transitioning from Johnson-Cousins to Sloan is the obsolescence of the historical Johnson-Cousins satellite photometry. To compare photometry between the Johnson-Cousins and Sloan systems, a transformation needs to be made to convert data from one photometric system to another.
A number of such transformations exist within the astronomical community for stellar objects. However, the Spectral Energy Distributions (SEDs) of stars are not the same as those of satellites. Reflection for spacecraft can be modeled based on diffuse and specular reflection components, where the diffuse components’ reflected spectrum may have spectral characteristics of the material off which it reflects, thereby altering the SED from that of the Sun. While the SEDs of stars are largely static, the SEDs of satellites are not. Specifically, their SED may change with phase angle (e.g., solar panel contributions are phase angle dependent and typically make the SED bluer). To investigate the transformation between Johnson-Cousins and Sloan for satellites, we performed the following analysis. We observed four satellites sequentially in Johnson-Cousins filters (B, V, R, and I) and Sloan filters (g, r, i, and z), covering a large range of phase angle. We then empirically derived transformations between Johnson-Cousins and Sloan for each satellite’s observed data and for all of the observed satellite data as a whole, and juxtaposed these with an astronomical transformation. We found mixed results for the transformation relations. The r – V as a function of V – R relation provides a great fit for all of the observed satellite data with low root mean square (RMS) error and is exactly the same as the astronomical transformation. The r – z as a function of R – I relation provides a great fit for all of the observed satellite data, but has large RMS scatter and is distinct from the astronomical transformation. Thus, we do not recommend transforming historical satellite photometry observed in Johnson-Cousins to Sloan to compare to observations of satellites taken in the Sloan filters. Since the transformations are dependent on the SED of the satellite, and the satellites’ SEDs are variable, transformations generally yielded poor results for the two photometric systems we studied here, i.e. Johnson-Cousins and Sloan. Moreover, our supposition is that such attempts with any two photometric systems may yield similarly poor results.Cousins to Sloan is the obsolescence of the historical Johnson-Cousins satellite photometry. To compare photometry between the Johnson-Cousins and Sloan systems, a transformation needs to be made to convert data from one photometric system to another.
A number of such transformations exist within the astronomical community for stellar objects. However, the Spectral Energy Distributions (SEDs) of stars are not the same as those of satellites. Reflection for spacecraft can be modeled based on diffuse and specular reflection components, where the diffuse components’ reflected spectrum may have spectral characteristics of the material off which it reflects, thereby altering the SED from that of the Sun. While the SEDs of stars are largely static, the SEDs of satellites are not. Specifically, their SED may change with phase angle (e.g., solar panel contributions are phase angle dependent and typically make the SED bluer). To investigate the transformation between Johnson-Cousins and Sloan for satellites, we performed the following analysis. We observed four satellites sequentially in Johnson-Cousins filters (B, V, R, and I) and Sloan filters (g, r, i, and z), covering a large range of phase angle. We then empirically derived transformations between Johnson-Cousins and Sloan for each satellite’s observed data and for all of the observed satellite data as a whole, and juxtaposed these with an astronomical transformation. We found mixed results for the transformation relations. The r – V as a function of V – R relation provides a great fit for all of the observed satellite data with low root mean square (RMS) error and is exactly the same as the astronomical transformation. The r – z as a function of R – I relation provides a great fit for all of the observed satellite data, but has large RMS scatter and is distinct from the astronomical transformation. Thus, we do not recommend transforming historical satellite photometry observed in Johnson-Cousins to Sloan to compare to observations of satellites taken in the Sloan filters. Since the transformations are dependent on the SED of the satellite, and the satellites’ SEDs are variable, transformations generally yielded poor results for the two photometric systems we studied here, i.e. Johnson-Cousins and Sloan. Moreover, our supposition is that such attempts with any two photometric systems may yield similarly poor results.

We report on the optical observations and analysis of the high-energy peaked BL Lac object (HBL), Mrk 501, at redshift z = 0.033. We can confirm microvariable behavior over the course of minutes on several occasions per night. As an alternative to the commonly understood dynamical model of random variations in intensity of the AGN, we develop a relativistic beaming model with a minimum of free parameters, which allows us to infer changes in the line of sight angles for the motion of the different relativistic components. We hope our methods can be used in future studies of beamed emission in other active microvariable sources, similar to the one we explored.

We report on long-term multiwavelength monitoring of blazar Mrk 421 by the GLAST-AGILE Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in the period 2007–2015, characterized by several extreme flares. The ratio between the optical, X-ray and γ-ray fluxes is very variable. The γ-ray flux variations show a fair correlation with the optical ones starting from 2012. We analyse spectropolarimetric data and find wavelength-dependence of the polarization degree (P), which is compatible with the presence of the host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA). Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of the EVPA. We build broad-band spectral energy distributions with simultaneous near-infrared and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite. They show strong variability in both flux and X-ray spectral shape and suggest a shift of the synchrotron peak up to a factor of ∼50 in frequency. The interpretation of the flux and spectral variability is compatible with jet models including at least two emitting regions that can change their orientation with respect to the line of sight.

A variety of interesting objects such as Wolf–Rayet stars, tight OB associations, planetary nebulae, X-ray binaries, etc., can be discovered as point or compact sources in Hα surveys. How these objects distribute through a galaxy sheds light on the galaxy star formation rate and history, mass distribution, and dynamics. The nearby galaxy M33 is an excellent place to study the distribution of Hα-bright point sources in a flocculant spiral galaxy. We have reprocessed an archived WIYN continuum-subtracted Hα image of the inner 6farcm5 × 6farcm5 of M33 and, employing both eye and machine searches, have tabulated sources with a flux greater than approximately 10−15 erg cm−2s−1. We have effectively recovered previously mapped H ii regions and have identified 152 unresolved point sources and 122 marginally resolved compact sources, of which 39 have not been previously identified in any archive. An additional 99 Hα sources were found to have sufficient archival flux values to generate a Spectral Energy Distribution. Using the SED, flux values, Hα flux value, and compactness, we classified 67 of these sources.