High Contrast Imaging

Transcription

1 High Contrast Imaging Suppressing diffraction (rings and other patterns) Doing this without losing light Suppressing scattered light Doing THIS without losing light

2 Diffraction rings arise from the abrupt cutoff in baselines (and hence spatial frequencies) at the edge of the telescope primary mirror. If we could modify the reflectivity of the mirror so it fell off toward the edge and there were fewer of those long baselines, we would suppress the diffraction rings. This process is called apodization; mathematically is is what we did to control the feet on the line profile in a FTS.

3 Small-minded astronomers do not like the thought of putting black paint on their telescope mirrors. Therefore, we do the apodization at a pupil. There is a mathematic form (a) that can suppress the diffraction to the level of However, it is hard to make the necessary graded absorption accurately. A simpler approach is to use a mask that works in only one dimension (lower left), producing the image to the lower right.

4 Nice, but we have lost a lot of light along with getting rid of diffraction. We can do better by manipulating the photon phases at the diffraction plane (we have seen this before when we talked about diffraction gratings as a series of slits). The diffraction artifacts are imaged at the diffraction plane.

5 An apodizing phase plate (APP) is put at the diffraction plane. It can modify the phases of the photons so all the diffraction rings are thrown to one side of the image (where they are twice as bright). Subtle features on the APP can control other diffraction artifacts.

6 Great! But this does nothing to keep light from entering the instrument and scattering to produce a bright background. Here is one approach. However, it has a few technical issues to solve.

7 Maybe some day those issues will be solved. Meanwhile we try to accomplish the same results within an instrument. The Lyot coronagraph is the classic approach. The starlight is blocked by an opaque spot. The diffraction from the spot is removed by a stop around the pupil. The cleaned image is focused onto a detector array.

8 A coronagraph only works well if the telescope is diffraction limited because of speckle pinning. Since seeing speckles are from the same photons that produce the other parts of the image, they interfere with the diffraction artifacts and produce a source of noise that cannot be suppressed in long integrations. It is an evil manifestation of the signal.

9 Lyot coronagraphs can achieve contrasts approaching However, they do this by throwing away a lot of light. Coronagraph development centers on preserving more of the light and reducing the inner working angle (IWA). The Phase-Induced Amplitude Apodization (PIAA) coronagraph overcomes these difficulties (in principle). The first set of optics maps the light from the telescope so it behaves as if the telescope were apodized (e.g., a Gaussian envelope). In doing so, the Abbe Sine Condition is ignored, so the images are very bad at the focal plane, but they are still good enough for an occulting mask to remove the light from the central source. The wavefront errors are then corrected in the second set of optics to give good images of the region surrounding the central source.

10 Computer-based image manipulation Contrast and resolution can be enhanced by computer after-the-fact Recall the telescope MTF Shows the effect of the telescope on the spatial frequencies In principle, dividing the Fourier Transform of the source image by that of a point source should correct the source image for the telescope attentuation of high spatial frequencies In practice, this does not work well because of noise amplification A number of alternatives reduce the noise Wiener filter Lucy-Richardson Maximum Entropy Method Pixons Still, only modest gains can be achieved In matching images with different resolution, it may also be necessary to degrade the resolution of some using a PSF kernel (e.g., inverse transform of the ratio of the FT of the large beam divided by the small one