Friday, March 29, 2013

There is always a little gem...

After few months of complete inactivity I finally got my imaging setup to work again. It has been a fairly long process to get all the cogs to turn the right way. I never realized how many little steps were involved from pulling the mount out of the shed to press the button to start an imaging session. In the past I used to perform the ritual quite often to the point that the entire sequence including polar alignment, collimating the optics, hopping to target, focusing, starting the autoguider and so on would take only about 30 minutes. Well if in the mix you throw a complete rebuilding of your beloved laptop you need to redo most of the work that was done years ago and now clearly forgotten. Finally yesterday I got everything to work again as expected. As a test run I decided to slew the scope to Capella and image the region around that star for about one hour. Why Capella? Well it is very bright so hopping to it was not an issue, it was reasonably well placed in the sky at the start of the imaging session (53 degrees above the Western horizon) and it would have been very easy to see in the autoguider (I have an off-axis setup on my imaging telescope and wanted to make sure the guiding scope was roughly aligned with the primary instrument).

All I cared was to be able to collect a bunch of subframes with nice, round stars to prove that the rig was again in good shape. I set the timer to 35 seconds subframe exposure for a total of 100 subframes. I set the delay in between subframes to 2 seconds to allow the scope to settle down in case the shutter would introduce some vibrations. Other than that I did not expect anything out of this test shot.

Here is the stacked image after being quickly processed in Photoshop (click on the image to see the original size version):

The dark ring about Capella is a processing artifact: very bright objects are notoriously difficult to handle properly, but I wasn't looking for a pretty picture so I didn't care fixing it.

Under close inspection I was quite satisfied with the result: stars appear reasonably rounded across the entire frame and for a 1 hour total exposure under a light polluted sky the amount of faint stars looked good to me.

I was ready to shelve the image when I remembered that Capella is actually a multiple star system. The main star (the super-bright one in the image) consists of a pair of giants: a G8 star (Capella Aa), about 3 times the mass of the Sun, 12 times its diameter and 80 times more luminous, accompanied by a G1 star (Capella Ab), about 2.5 times the mass of the Sun, 10 times its diameter and 75 times more luminous. The two stars are separated only by two thirds the distance Earth-Sun and orbit each other in 104 days. Their separation is so small that at a distance of about 42 light years it is impossible to visually detect them as a double. Only spectroscopy and interferometry are able to unveil the binary nature of Capella. However this is not the end of the story: Capella Aab has a much fainter companion, Capella C orbiting at about 0.17 light years away and so much farther out. Capella C is a red dwarf of spectral class M1. It is only about one third of the mass of the Sun and half the diameter and only a couple of percent the Sun's luminosity. It turns out that Capella C has a companion, Capella D, which is even smaller: a M5 red dwarf, about one tenth the mass of the Sun, one third of its diameter and less than one percent the Sun's luminosity. Capella CD is a much wider binary than Capella Aab: the two components orbit each other at a distance of about 48 Astronomical Units (about the distance of Pluto from the Sun at aphelion) making them separated by 6 arcseconds as seen from Earth. Of course being so small, Capella C and D cannot be very bright: the apparent magnitude of Capella C is about +10 and Capella D is about +14.

I know that with my 10" Newtonian and my Canon XSi I can reach magnitude +16/+17 from my backyard and I can separate stars as close as 5"/6" if they are of similar magnitude and faint. I then decided to check if Capella CD was visible in my image.

Googling for the celestial coordinates of Capella C and D proved to be a bit complicated. For reasons I am not going to discuss here, the two stars are also identified as Capella H and L. Capella C (or H) is also identified as G 96-29. Regardless, the coordinates I found through Google were off, sometimes by several arcminutes. That made impossible for me to pinpoint them in SkyCharts. To get down to stars of magnitude +16 I installed the UCAC4 catalogue now available for download from the Cartes du Ciel website. Eventually I turned to a more systematic search through SIMBAD.  Through the provided CDS Simplay utility, I was able to display several arcminutes of sky around Capella CD. Here is the image:

Capella C and D are the two close stars in yellow in the centre of the image. Luckily the other two stars in yellow on each side of Capella C and D are listed in the Tycho 2 catalogue which is included in SkyCharts. Using that information I was able to pin down the exact position of Capella C and D in SkyCharts. That way I could see where they were located in my image:

An enlargement of the region around Capella C and D is show here:

Capella CD is the orange star in the centre. It is interesting to notice that there is a distinct "appendage" below Capella CD. The same feature does not appear in any of the other stars in the image so it cannot be an artifact caused by poor tracking or poor frame alignment during stacking. Processing steps in Photoshop are hardly the culprit: in the case of this image each step (like level stretching and curves or colour saturation) was applied to the entire frame so getting only one star out of shape because of poor processing seems highly unlikely. I then believe that the irregular shape of the star is due to its binary nature. In other words, the resolution of the image is not quite enough to show the two stars as separate, but it is sufficient to show that there is more than one star.

That result made me quite happy. I finally decided to check what was the magnitude of the faintest star I could see in the original image. I was able to track down a +17.2 star. Not too shabby, considering the amount of light pollution present in the sky. Here is a single subframe added to show what the unprocessed data looked like:

Look carefully at your images: even those that are not supposed to contain anything interesting are always full on stories and surprises.


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