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The SDS, and Variations of the Solar Diameter: Flight 11 (October 16, 2009). Sabatino Sofia Yale University New Haven, CT, USA. The SDS is a balloon-borne metrologic instrument that measures the SOLAR DIAMETER AND ASPHERICITY

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The SDS, and Variations of the Solar Diameter: Flight 11 (October 16, 2009)

Sabatino Sofia

Yale University

New Haven, CT, USA


The SDS is a balloon-borne metrologic instrument

that measures the SOLAR DIAMETER AND ASPHERICITY

It has flown in Fall 1992, 1994, 1995,1996, and 2009 from Fort

Sumner, NM

Future flights, especially connected with PICARD, are being

considered.


RESEARCH TEAM

American University (U.J. Sofia)

CNRS/S.d’A (G. Thuillier, D. Djafer)

CSA (Stella Melo)

NASA/GSFC (W. Heaps, L. Twigg, E. Georgieva)

Yale University (S. Sofia)




Differences between existing diameter measurements

  • Ground-based vs. space based

  • Wavelength of observation

  • Analysis Method

  • Calibration

  • These issues are extensively described in a paper by

  • Djafer, Thuillier and Sofia, ApJ, 676, 651, 2008.


Ground-based measurements are affected by terrestrial atmosphere

Seeing is 1”-4”, and we need sensitivity of mas.

This cannot be simply solved by statistics, since

atmospheric turbulence is not random.


Once you go to space, there are 2 measurements: atmosphere

SoHO/MDI

SDS

SoHO/MDI is not a metrologic instrument. It has not

been calibrated before launch, and cannot be calibrated

in space.

The only metrologic instrument to measure the solar

diameter is the SDS.


The MDI results claim an accuracy of a few mas. over atmosphere

more than a decade

This is equivalent to knowing the effective focal length

to an accuracy of a few microns over this time period.

The instrument is frequently refocused.

Large corrections are made through “characterization”

i.e. on the basis of a “thermal model” not calibrated before

flight.

There are corrections for aging

Etc.


The previous corrections are made in addition to distance corrections that are well understood.

By contrast, the SDS can separate instrumental changes (regardless of its origin) from changes of the solar diameter.

PICARD can calibrate scale in two separate ways:

Stellar pairs

Wedges (similar to SDS)


Philosophical corrections that are well understood.difference:

SDS-PICARD: We determine the scale (arc sec/mm) and

its changes regardless of the cause.

MDI, RHESSI *, etc: They correct for each known

instrumental process, and assume the rest is solar change.

* RHESSI only claims accuracy for the asphericity determination.


Determining the solar diameter is a complex process corrections that are well understood.

When we look at the image of the Sun obtained in any

detector, we do not see the Sun, but an image obtained

through a typically complex optical system.

Besides the peculiar solar issues described earlier, we

also have general optical distortion effects that

we have long ago learned from stellar astrometry


ORIGIN OF PRINCIPAL OPTICAL DISTORTION corrections that are well understood.

TILT OF THE DETECTOR PLANE

COLOR

CHARGE TRANSFER EFFECT

CLASSICAL DISTORTION

COMA

POSSIBLE CROSS TERMS


“New” Yale SDS Flight Data Reduction Pipeline corrections that are well understood.

flight-cycle filename list

get next cycle filename

output relevant HK

read data (HK+CCD)

apply photometric coefs

odd/even offset & spike filter

preliminary edge detection

subtract background level

photom. coefs

subtract ghost images

PDS xy's of CCDs

final edge detection

distortion coefs

transform to focal-plane x,y

correct for distortion

fit direct & reflected circles

find minimum gap

output gap, sep, Rd, Rr, etc.


“New” Yale SDS Flight Data Reduction Pipeline corrections that are well understood.

(cont.)

Output from program #1 becomes input for program #2:

cycle#, gap, sep, Rd, Rr, etc.

cycle#, HK (time, rotat., etc)

correct for refraction

normalize to 1 AU

cull/average by rotation angle

output diameter & oblateness


When you need sensitivity of 1 part in 10 corrections that are well understood.6, and very complex analysis procedures, you run the risk that the

results reflect the method rather than the object measured.

The incompatible results claimed by different authors

regarding the solar diameter confirm this.

WE NEED TO PROCESS THE DATA TO THE POINT

OF PRODUCING THE DIAMETER, AND THEN PUBLISH IT AS SOON AS POSSIBLE FOR THE BROAD SCIENTIFIC COMMUNITY TO EXPLORE ITS

IMPLICATIONS


SDS/PICARD SYNERGY corrections that are well understood.

PICARD measurements are complex, and their analysis is

very demanding.

Analyzing the SDS data obtained before the PICARD launch

has assisted development and testing of the PICARD algorithms. Thishas accelerated PICARD science productivity.

The 2009 SDS flight, added to the earlier and the proposed

follow up flights during and after PICARD will enhance

the science value of the mission by:

Providing validation of measurements

Allowing normalization of past results

Allowing the extension of the PICARD mission beyond its

lifetime.


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