Globular Cluster M5, in the constellation Serpens, contains approximately 100,000 stars, many of which are red giants.
The night of June 1, 2012 was not a good night for astrophotography, because the moon was bright, two nights before the full moon. Many subjects would be out of the question on such a night, but globular clusters, especially the brighter ones, have enough contrast to make them fair targets against a bright sky glow, whether caused by the moon or by light pollution.
M5 is a bright globular cluster, with an apparent magnitude of about 6, but it's position on this night about 20 degrees from the moon, combined with poor atmospheric transparency, still made it challenging. In order to capture enough light to make the image interesting, I had to reduce the camera's ISO setting significantly to use sufficiently long exposures.
This image is a combination of 6 four minute exposures at ISO 200 and 13 eight minute exposures at ISO 100, processed along with 16 flat field exposures, 16 dark-flats, and 17 dark frames acquired for four minutes each at ISO 200. The raw images were calibrated, registered and stacked with Iris software. Because the dark frames did not match the exposure values of most of the light frames, I used one of the dark optimization routines (OPT3) in Iris to match them. There was a very strong gradient in the sky background due to the subject's proximity to the bright moon, exacerbated by the poor sky transparency. I used the "Remove gradient (polynomial fit)" routine in Iris to correct the gradient. Finally, I used Photoshop CS5 to adjust the levels and curves, producing the final result.
The bright star near the lower left corner of the image is 5 Serpens - it is actually a double star, with the two components separated by about 11 arc-seconds. The brighter component is a spectral class F star, similar to our sun. The fainter component star is spectral class G, appearing slightly more yellow than the main star. If you view this image in it's "original" size (click "original" below), you should be able see the different colors of the two component stars.
| Imaging telescope: | | 8" f/4.9 newtonian reflector |
| Telescope mount: | | Atlas EQ-G |
| Mount control software: | | Cartes du Ciel, via EQMOD |
| Imaging camera: | | Canon EOS 350D |
| Guiding telescope: | | 80mm f/11 achromatic refractor |
| Guiding camera: | | Meade DSI-C |
| Guiding software: | | PHD Guiding |
| Raw image processing software: | | Iris |
| Image finishing software | | Photoshop CS5 |
