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POSSII IMAGES ASTROMÉTRIC REDUCTIONPowerPoint Presentation

POSSII IMAGES ASTROMÉTRIC REDUCTION

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### POSSII IMAGES ASTROMÉTRIC REDUCTION

Ignacio Novalbos

O.A.N.L. Barcelona

Reduction methods and their influence on Theta and Rho measures

ORIGIN AND OBJECTIVE

O.A.N.L. Barcelona 2010

ORIGIN

OAGCommon Proper Motion Wide Pairs Survey

Relative astrometry of new pairs discovered on POSSII plates

O.A.N.L. Barcelona 2010

OBJECTIVE

Knowing which astrometric reduction method / software get the most accurate, when we measured on images from the POSSII-J, which usually present saturation for stars with magnitude <12.

O.A.N.L. Barcelona 2010

O.A.N.L. Barcelona 2010

METHODOLOGY

- Selection of pairs from the Catalog of Rectilinear Elements
- Getting POSS II images of each pair
- Get the time of each plate from the FITS header
- From the linear elements and using a linear regression we calculate the values for
- "x" and "y"
- From the rectangular coordinates we calculate polar coordinates (theta and rho),
- correcting the theta value for the quadrant
- We evaluate the accuracy of calculations by plotting “x / y “ vs epoch and " theta /
- rho” vs epoch, adjusting the values to a linear fit and analyzing the R-squared value- We confirm the accuracy of the estimates for theta and rho with the residuals
- obtained from the ephemerids calculated and those of the CRE
- We calculate the calibration constants for each of the plates from Astrométrica +USNO
- A2.
- Relative astrometry is obtained for each pair with different astrometric reduction methods
- We evaluate and compare the results

O.A.N.L. Barcelona 2010

O.A.N.L. Barcelona 2010

SELECTION CRITERIA

- Pairs with Mag V>11 for both components. (61 Pairs = 5,2%)

- Pairs with DEC between -20º y +20º. (15 Pairs = 1,3%)

- Pairs with residuals <1º PA and < 0,5” Sep. (7 Pairs = 0,6%)

- Pairs with rho >10” . (6 Pairs)

- Pairs with historical measurements N >5 . (6 Pairs)

O.A.N.L. Barcelona 2010

OBJECT PAIRS

O.A.N.L. Barcelona 2010

O.A.N.L. Barcelona 2010

POSS II-J

O.A.N.L. Barcelona 2010

PLATES

O.A.N.L. Barcelona 2010

IMAGES ÉPOCH

O.A.N.L. Barcelona 2010

EPHEMERIDSCALCULATION

O.A.N.L. Barcelona 2010

LINEAR ELEMENTS

O.A.N.L. Barcelona 2010

COORD. TRANSFORMATION

Starting from the linear elements:

(x0,y0,t0) (rho0,theta0,t0)

We calculate the rectangular coordinates replacing in:

x = xa * (t-t0) + x0

y = ya * (t-t0) + y0

From “x y” we obtain the polar coordinates replacing in:

rho = Raiz(x^2 + y^2)

theta = arctg (-y/x)

O.A.N.L. Barcelona 2010

LINEAR REGRESSION

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LINEAR FIT “x”

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LINEAR FIT “y”

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LINEAR FIT “theta”

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LINEAR FIT “rho”

O.A.N.L. Barcelona 2010

ESTIMATED EPHEMERIDS

O.A.N.L. Barcelona 2010

O.A.N.L. Barcelona 2010

CALIBRATION CONSTANTS

18:43:33 - USNO-A2.0: 154 Records read (22.8' x 22.8')

Center Coordinates: RA = 00h 38m 24.00s, De = +01° 30' 00.0"

18:43:34 - Object List for Image 1 (BAL 947 STScI_POSS2UKSTU_Red_00-38-24.00_+01-30-00.0~1.fits):

753 Detections (75 Stars, 75 Ref. Stars, 0 Movers)

18:43:34 - Astrometry of Image 1 (BAL 947 STScI_POSS2UKSTU_Red_00-38-24.00_+01-30-00.0~1.fits):

75 of 75 Reference Stars used: dRA = 0.36", dDe = 0.41"

X = +3.035090985E-6 +4.942446179E-6*x' +1.451099745E-8*y'

Y = -3.718054588E-6 +1.501028067E-8*x' -4.946326400E-6*y'

Origin: x0 = 441.0, y0 = 441.0

Center Coordinates: RA = 00h 38m 23.96s, De = +01° 29' 59.2"

Focal Length = 3033.7mm, Rotation = 0.17°

Pixel Size: 1.02" x 1.02", Field of View: 15.0' x 15.0'

18:43:34 - Photometry of Image 1 (BAL 947 STScI_POSS2UKSTU_Red_00-38-24.00_+01-30-00.0~1.fits):

70 of 75 Reference Stars used: dmag = 0.41mag

O.A.N.L. Barcelona 2010

CALIBRATION CONSTANTS

O.A.N.L. Barcelona 2010

O.A.N.L. Barcelona 2010

RecToPol

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RecToPol

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fv FITS VIEWER v 4.1.4

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fv FITS VIEWER v 4.1.4

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ALADíN + “Dist”

O.A.N.L. Barcelona 2010

ALADíN + “Dist”

O.A.N.L. Barcelona 2010

REDUC & SURFACE

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REDUC

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SURFACE

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O.A.N.L. Barcelona 2010

O-C THETA

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O-C THETA

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O-C THETA (2)

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O-C RHO

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O-C RHO

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O-C RHO (2)

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STANDARD DEVIATIONO-C THETA/RHO

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CONCLUSIONS

- The largest deviations correspond to the results obtained with "RecToPol" which almost certainly is due to the low precision of Astrometrica in estimating the centroid for star-saturated images.
- Residuals derived from measurements made with the software "fv" are 10 times better than those obtained from the absolute astrometry and we can accept them within the error margins normally allowed.
- The measurements made with "Surface" improve even those obtained with "fv" and show theta and rho residuals in the order of tenths.
- The two softwares / methods that presents the lowest residuals between observed and calculated ephemerids (and therefore the greatest accuracy available) are “Reduc" and the tool "Dist" from Aladin.

O.A.N.L. Barcelona 2010

THE REASON?

We conclude that eye and brain working in concert, we always give more accurate results than any other synthetic method based on mathematical calculations. The high precision of “Reduc", similar to that achieved manually with "Dist", we are pretty sure that is due to the "human component " that we put into the software when we mark the photocentric star manually.

O.A.N.L. Barcelona 2010

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