19,154 research outputs found
An Astrometric Approach to Measuring the Color of an Object
The color of a star is a critical feature to reflect its physical property
such as the temperature. The color index is usually obtained via absolute
photometry, which is demanding for weather conditions and instruments. In this
work, we present an astrometric method to measure the catalog-matched color
index of an object based on the effect of differential color refraction (DCR).
Specifically, we can observe an object using only one filter or alternately
using two different filters. Through the difference of the DCR effect compared
with reference stars, the catalog-matched color index of an object can be
conveniently derived. Hence, we can perform DCR calibration and obtain its
accurate and precise positions even if observed with Null filter during a large
range of zenith distances, by which the limiting magnitude and observational
efficiency of the telescope can be significantly improved. This method takes
advantage of the DCR effect and builds a link between astrometry and
photometry. In practice, we measure the color indices and positions of Himalia
(the sixth satellite of Jupiter) using 857 CCD frames over 8 nights by two
telescopes. Totally, the mean color index BP-RP (Gaia photometric system) of
Himalia is 0.750 \pm 0.004 magnitude. Through the rotational phased color index
analysis, we find two places with their color indices exceeding the mean \pm 3
\sigma.Comment: 10 pages, 5 figures, 4 table
Astrometry via Close Approach Events: Applications to Main-Belt Asteroid (702) Alauda
The release of Gaia catalog is revolutionary to the astronomy of solar system
objects. After some effects such as atmospheric refraction and CCD geometric
distortion have been taken into account, the astrometric precision for
ground-based telescopes can reach the level of tens of milli-arcseconds. If an
object approaches a reference star in a small relative angular distance (less
than 100 arcseconds), which is called close approach event in this work, the
relative positional precision between the object and reference star will be
further improved since the systematic effects of atmospheric turbulence and
local telescope optics can be reduced. To obtain the precise position of a
main-belt asteroid in an close approach event, a second-order angular velocity
model with time is supposed in the sky plane. By fitting the relationship
between the relative angular distance and observed time, we can derive the time
of maximum approximation and calculate the corresponding position of the
asteroid. In practice, 5 nights' CCD observations including 15 close approach
events of main-belt asteroid (702) Alauda are taken for testing by the 1m
telescope at Yunnan Observatory, China. Compared with conventional solutions,
our results show that the positional precision significantly improves, which
reaches better than 4 milli-arcseconds, and 1 milli-arcsecond in the best case
when referenced for JPL ephemeris in both right ascension and declination.Comment: 11 pages, 22 figure
A Convenient Solution to Geometric Distortion and Its Application to Phoebe's Observations
International audienceA simple but effective approach is proposed for measuring the geometric distortion of a CCD field of view of a ground-based telescope. For three open clusters (M35, M67, and NGC 2324), 425 CCD frames taken by a 1 m telescope at the Yunnan Observatory are used to test this approach. It is found that the geometric distortion pattern depends strongly on the corresponding filter used. The geometric distortion is then used to correct the pixel positions for Phoebe, the ninth satellite of Saturn, and its reference stars imaged in 220 CCD frames taken by the same telescope. The standard deviation of the (O - C; observed minus computed) residuals of Phoebe is significantly improved after correcting the geometric distortions
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