Space Telescope Science Institute   6.2.3 Vignetting and Flat Fields  6.2.5 Residual Light Subtraction

6.2.4 Coronagraphic Performance


Early in Cycle 11, coronagraphic performance verification images were taken of the V = 0 star Arcturus ( Figure 6.6 and Figure 6.7). This star has an angular diameter of 25 milliarcseconds and is thus unresolved by the coronagraph. The coronagraphic image of a star is quite unusual. Rather than appearing as a dark hole surrounded by residual light, as would be the case in an aberration-free coronagraph, the interior of the spot is filled with a diminished and somewhat distorted image of the central source. This is due to the M2 mirror correction of aberrated light from the star that is not blocked by the spot. The small spot is filled with light, while the large one is relatively dark. Broad, ring-like structures surround the spots, extending their apparent radii by about 0.5 arcseconds. These are due to diffraction in the wings of the aberrated PSF by the occulting spot itself. A consequence of these features is that stars may saturate the interior and edges of the spot within a short time. Within the small spot, the brightest pixels can become saturated in less than one second for a V = 0.0 star, while pixels at edge of the larger spot will saturate in about 14 seconds.

The measured radial surface brightness profiles ( Figure 6.8) show that the coronagraph is well aligned and operating as expected. The light diffracted by the HST obscurations is suppressed below the level of the scattered light - there are no prominent diffraction spikes, rings, or ghosts beyond the immediate proximity of the spots. At longer wavelengths (> 6000 Ĺ) the diffraction spikes appear about as bright as the residual scattered light (at longer wavelengths, the diffraction pattern is larger and therefore not as well suppressed by the coronagraph). The spikes are more prominent in images with the large spot than the small one. This can be explained by the fact that the Lyot stop is not located exactly in the pupil plane but is instead slightly ahead of it, so the beam can "walk" around the stop depending on the field angle of the object. Because the large spot is at the edge of the field, the beam is slightly shifted, allowing more diffracted light to pass around the mask edges.

The residual background is dominated by radial streaks that are caused primarily by scattering from zonal surface errors in the HST mirrors. This halo increases in brightness and decreases in size towards shorter wavelengths. One unexpected feature is a diagonal streak or "bar" seen in both direct and occulted star images. It is about 5 times brighter than the mean azimuthal surface brightness in the coronagraphic images. This structure was not seen in the ground-test images and is likely due to scattering introduced by the HST optics. There appears to be a corresponding feature in STIS as well.

Figure 6.6: Geometrically corrected (29 arcseconds across) image of Arcturus observed in F814W behind the 1.8 arcseconds spot.

 
This is a composite of short, medium, and long (280 seconds) exposures. The "bar" can be seen extending from the upper left to lower right. The shadows of the occulting finger and large spot can be seen against the scattered light background. Logarithmic intensity scale.
 
Figure 6.7: Regions around the occulting spots in different filters.

 
The occulting finger can be seen in the 3 arcseconds spot images. Logarithmic intensity scaled.
 
Figure 6.8: Surface brightness plots derived by computing the median value at each radius.

 
The brightness units are relative to the total flux of the star. The direct profile is predicted; the coronagraphic profiles are measured from on-orbit images of Arcturus. "Coronagraph-star" shows the absolute median residual level from the subtraction of images of the same star observed in separate visits.
 

 6.2.3 Vignetting and Flat Fields  6.2.5 Residual Light Subtraction
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