August 2015 - Picture of the Month
M92 revisited with some galactic extras
Images Courtesy of David Davies, Cambridge, UK. Please click on the images for the high resolution versions. For more images from David please visit his Flickr Photostream.
I first observed M92 visually several years ago and was struck by the beauty of this globular cluster that is often spoken of as the poor relation of the Great Cluster, M13 in Hercules. I understand that M92 is believed to contain stars that are nearly as old as the universe itself, since these stars contain just 0.5% of the metal elements that our sun contains. It is an intriguing object.
Therefore M92 has always been my list for imaging. Two years ago I succeeded in capturing a relatively low resolution image in black white with my ED80; clouds prevented capturing any colour data.
Friday 17 July produced a rare clear night with stable seeing and reasonable transparency. In planning the image capture I could see that there are many galaxies in the background of this area of sky and so offset the field of view of the image to also capture what I could of the background galaxies.
I elected to use my Equinox F/7.5 ED120 refractor rather than my faster Newtonian to get a wider field of view and as clean an image possible, without diffraction spikes. A test exposure time of five minutes showed that I would get sufficient signal but not saturate the brighter stars. To reduce the risk of not having a complete set of colour data, I captured a set of RGB images first and then captured the luminance data I could before the onset of dawn stopped work. Data capture started around midnight and I had to stop at 02:20 as the sky was brightening. I managed to capture two and half hours of data.
I've attached two images. The first is the complete LRGB image. The second image (NGC 6332) is a crop from the lower right part of the frame. This has been enlarged by a factor of two and is shown as a negative image. I have annotated it with the names of some of the galaxies that I've been able to identify from the NGC and LEDA catalogues.
The brightest galaxies are magnitude 13 to 14; the smaller galaxies still showing a distinct shape are around magnitude 16 to 17 and the faint, tiny smudges are around magnitude 18 to 19.
The faintest galaxy I've been able to identify is magnitude 19.2. I've also identified the double star HIP 84333 but my studies show that the apparent double visible in the image is an optical double; the true binary is only 0.4" from the primary and was discovered by the Hipparcos satellite.
The image scale of the galaxies image is 0.62 arc sec/pixel - half that of the main image image scale 1.24 arc sec per pixel.
- Telescope: Skywatcher Equinox ED120.
- Mount: Skywatcher EQ6.
- Camera: QSI 583 with SX Lodestar as off-axis guider Automatic focussing with Scopefocus.
Image Processing notes
The colour subs were plagued by satellite trails so I used median stacking in Deep Sky Stacker to eliminate them (mostly). Pixinsight w used to align, calibrate and integrate the colour frames. Photoshop was used to assemble and finish the LRGB image with a little saturation boost applied to the stars and a little high pass sharpening applied to the galaxies.
David Davies (21 July 2015).
So there are some challenges to those with large telescopes and other imagers. Can you provide us with some observations of these objects?
I realise that Hercules is slipping to the West, but the nights are getting darker... and there's are always your archives to scour.
And someone has taken a look! Dale Holt has pointed his 20 inch telescope and Watec 120N+ deep sky video camera in the direction of M92. Visit his blog to find out more and see his sketches.
James Whinfrey - Website Administrator