I want to highlight some early results from high-resolution imaging and color photometry of the globular cluster Messier 13. The results have been presented as a talk at the yearly BAV meeting in 2012. Images have been taken using a 8" Cassegrain telescope and a modified Canon EOS 40D digital single lens reflex camera. Image processing has been done using an experimental linear image processing algorithm to restore sub-pixel resolution down to or beyond diffraction limited resolution of the telescope.

Using standard color calibration measured intensities of the images taken with my DSLR are transformed into Johnson B-V system. This is the first result of a color-magnitude diagram derived from ground-based high-resolution imaging using a series of short-exposure images taken with my 8-inch Cassegrain.

Figure 1: Calibrated B-V color-magnitude diagram (CMD) taken from the high-resolution image reconstruction. Line plot of the main sequence overlay was taken and reproduced from earlier work of Sandage (1970). Color intensities in RGB can be well transformed into standard Johnson BVR photometric systems, if the calibration task is carefully done. The gap in the horizontal branch of the CMD indicates the location of variable stars in the cluster. Varying intensities of these stars, can be well observed with high-resolution imaging. The variables stars are of types RR Lyr, SX Pho and BL Her.

Figure 2: Comparison of the raw detector resolution of a single raw image (left) and three different deconvolution results using different synthetic point-spread functions. Final resolution of the image reconstruction (right) is 0.3 arcsec/pix.

Figure 3: Animated comparison of the dimension of raw detector resolution at 1.4 arcsec/pix and reconstructed image at sub-pixel scale at 0.3 arcsec/pix.

Figure 4: Schematic illustration of the creation of a color-magnitude diagram from photometric star detection and measurements. Raw DSLR magnitudes are calibrated and transformed into Johnson BVR photometric system.

Figure 5: I found an interesting and pretty resolved image of M13 taken by the amateur Rob Gendler. From his result I tried to obtain a color-magnitude diagram for comparison (right) to be compared to my own result (left). Due to strong non-linear post-processing the high resolution image of Rob Gendler and the resulting JPEG image this did not result in any useful photometric results, however. Strong post-processing caused a very compressed intensity range. Not even the limiting magnitude can be determined well from the JPEG image.

Literature

Bauer, T., 2012: Projektbericht: Die Veränderlichen in Messier 13, BAV Mitgliederversammlung, 21.-23. September 2012, Jena, Germany [ Details ]

Observational data