High-Resolution Observations of the Globular Cluster NGC 7099.
Abstract
The globular cluster NGC 7099 is a prototypical collapsed core cluster. Through a series of investigations --instrumental, observational, and theoretical, I have resolved its core structure using a ground based telescope. The core has a radius of 2.15 arcsec when imaged with a V band spatial resolution of 0.35 arcsec. Initial attempts at speckle imaging produced images of inadequate signal to noise and resolution. To explain these results, a new, fully general signal-to-noise model has been developed. It properly accounts for all sources of noise in a speckle observation, including aliasing of high spatial frequencies by inadequate sampling of the image plane. The model, called Full Speckle Noise (FSN), can be used to predict the outcome of any speckle imaging experiment. A new high resolution imaging technique called ACT (Atmospheric Correlation with a Template) was developed to create sharper astronomical images. ACT compensates for image motion due to atmospheric turbulence. ACT is similar to the Shift and Add algorithm, but uses apriori spatial knowledge about the image to further constrain the shifts. In this instance, the final images of NGC 7099 have resolutions of 0.35 arcsec from data taken in 1 arcsec seeing. The PAPA (Precision Analog Photon Address) camera was used to record data. It is subject to errors when imaging cluster cores in a large field of view. The origin of these errors is explained, and several ways to avoid them proposed. New software was created for the PAPA camera to properly flat field images taken in a large field of view. Absolute photometry measurements of NGC 7099 made with the PAPA camera are accurate to 0.1 magnitude. Luminosity sampling errors dominate surface brightness profiles of the central few arcsec in a collapsed core cluster. These errors set limits on the ultimate spatial accuracy of surface brightness profiles. The limits apply even to images of arbitrarily high resolution; even to a perfectly functioning Hubble Space Telescope, as long as bright stars are not subtracted from the image. For the cusp cluster NGC 7099, if the core radius is zero (a pure cusp), the limiting resolution is 0.38 arcsec, and the minimum detectable core radius is 1.45 arcsec.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT.........3S
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Astronomical Photometry;
- Astronomy;
- Cameras;
- Globular Clusters;
- High Resolution;
- Hubble Space Telescope;
- Image Motion Compensation;
- Imaging Techniques;
- Photons;
- Spatial Resolution;
- Algorithms;
- Atmospheric Turbulence;
- Brightness;
- Field Of View;
- Gravitational Collapse;
- High Frequencies;
- Luminosity;
- Radii;
- Sampling;
- Signal To Noise Ratios;
- Astronomy