Lasco C2-POISE
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The HAO POISE instrument observed the solar eclipse in pB on 26 February 1998 between 18:10 and 18:14 UT at Curacao, N.A. (12.29 N, 68.97 W). The POISE field-of-view was 1.0 to 3.2 Rs. LASCO-C2 took images at 16:45 and 20:20. On this page we compare the LASCO pB to the POISE pB between 2.2 and 3.2 Rs.

Plots below show the comparison. The POISE values needed to be multiplied by a constant radiometric factor of 3/2. The POISE images were binned by a factor of 4 in each dimension (making the images 508x508), thus reducing the std deviation of the noise by a factor of 4. The plate scale was determined to be 3.1 +- .02 arcsec/pixel as determined by the angular diameter of the moon (2021.56'' at that time as seen from Curacao). The top of the image is rotated clockwise by 20.9 from solar north.

In the plots below the diamonds are the POISE pB and the solid lines represent the LASCO-C2 pBs from before and after the event. The agreement between the two instruments is generally excellent, and it gives support the validity of the C2 pB's over the poles, although the north pole C2 pB values are greater than the south pole C2 pB values and the POISE pB values. The POISE pB are the same over the north and south poles. The excess in the C2 north pole was also noted in intercalibration paper that compared LASCO-C2 to the UVCS White Light Channel [Frazin, R.A., et al., ``White Light Inter-calibrations of UVCS, LASCO-C2, and Spartan 201/WLC", in The Radiometric Calibration of SOHO, ISSI Scientific Report SR-002, eds. A Pauluhn, M.C.E. Huber, & R. von Steiger (Noordwijk: ESA Publications) (2002)]. One should also note the diffraction rings from the C2 occulter are prominent in the less intense regions (i.e. poles) below about 2.4 Rs. In the bright regions the diffraction rings are negligible.

In addition, one can also see the evidence for a slight loss of polarized signal in the C2 images near position angles 45, 135, 225 and 325 deg. This is due to the fact that C2 has folding mirrors that can turn linear into circular polarization, which is not detected by the optical system. Tom Moran estimates this effect to be less than 20% at maximum, which happens that the above position angles. This is described in Moran et al. 2006, Solar Physics, 237, 211.

Plots were generated using Frazin's C2-POISE Code.

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