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Sirius A & B

Brian Shelton with his Royce 12.5" D-K


This year, the new telescope was the 12.5" f/20 Dall-Kirkham that Brian Shelton brought along. The intent was to image Saturn but, as is often the case, things did not work out exactly as planned. The weather was, again, rotten and we really only had Monday and early Tuesday evening to observe, and neither of those nights were all that good. The remainder of the week was impossibly windy and pretty much a bust. But, being resourceful we took a wild shot Tuesday early in the evening and went for an image of Sirius B, that illusive companion to the brightest star and the source of many exaggerated observing reports - but more of that later.

Becoming frustrated with Saturn, and able to make only a single middling quality image, I began looking at Sirius as a possibility. Suddenly the seeing seemed to improve unexpectedly and I asked to swing the scope up there for a possible look at the Pup. I had clearly seen it in 2008 with one of my 10" D-Ks, in 2009 with a 12.5" D-K and 2010 with a 16" D-K. These three telescope were brought down by Mark Beale of New Jersey who purchased them. The 16" D-K was here this year. Anyhow, Brian's 12.5" was pointed at Sirius and after a bit of fooling around the Pup showed up at 250X and better at 350X. After a little general visual observing I suggested that Brian use his planetary imaging camera on Sirius for a shot at the companion. It was obvious that bad weather was coming in and this might be the end of the good observing for this year; the predictions for the rest of the week were dismal, to say the least. So, we had to work rapidly.  We hooked everything together and Brian was placed under a blanket seated on a lawn chair. Before I knew what happened he had captured a streaming video of the great Dog Star and its illusive companion. Later that evening he processed this into the single image seen below.

And, please note - NO SPIKES! -  on the brightest of stars. It's round and the little guy shows through!

Best Quality Version

Click on picture to start streaming video. Unless you have a Mac, you will have to save it to disk and then run it with QuickTime. (To save the file hit the down arrow at the left after you click on the picture above.) The file size is 121,068 KB and will take a while to download but it's worth it. This format was used to retain the quality of the image. If you do not have QuickTime, go here and just click on the Download Now button. Once QuickTime is downloaded, start QuickTime and load the file from disk and hit play.

Next Best Quality Version

For a much simpler process go to Flickr. Not too bad.



This image was made about 15 minutes before the Sirius B image above. It can be taken as a reference as to the quality of the seeing. The image was taken at prime focus at f/20 (no Barlows used) and is raw and unfiltered.



After we had made the image shown above it was interesting to hear the number of observations of Sirius B emerge from the tiniest of telescopes. The companion last passed through periastron passage in 1993 and is now separated from A by about 7 arc seconds. Observing the Pup at WSP in recent years has been relatively easy if you have a telescope 8" or more in diameter and with excellent optics, but after I reported the image we had made, a flurry of visual reports began to emerge, some of which were, well ..., questionable, to say the least. All of a sudden 4" refractors could show it, if you had the very best ones and you looked real hard. By the time we left, someone with 20-20 vision who applied the correct squinting techniques could see it with the unaided eye. Human nature, such as it is, was well and at work.

Looking to an expert, I contacted my friend, Bill Zmek, a professional optical engineer and avid amateur astronomer. He gave his thoughts over in an e-mail and I offer them to you verbatim.

Sirius B is about 9th mag, or roughly 10,000x dimmer than Sirius A. This ratio will be true for the ‘surface’ brightness at the two peaks, or anywhere else at the same distance and direction from the respective peaks. The question is, what is the relative brightness of the diffraction rings of Sirius A at the location of Sirius B? (Assumed distance is 7 arc-seconds.)

For an absolutely perfect 4” telescope and evaluated at 0.00055 micron wavelength, a distance of 7 arc-seconds puts Sirius B straddling the fifth and sixth diffraction rings. The fifth ring peaks at about 1/2300 the peak brightness, and the sixth, at about 1/3700. Note that the peak of the fifth ring is over 4 times the brightness relative to Sirius A peak as is Sirius B. Again, perfect conditions.

The ratio in practice will be even worse (that is, the brightness at Sirius B will be even higher than quoted above) for 4 reasons: chromatic aberration (refractor, even an apochromatic one); scatter from surface figure errors, which at 5 to 6 rings will be dominated by scatter from primary ripple and which tends to dominate over diffraction as one moves out several rings from the core (the amount varies from scope to scope); scatter from contamination (dust, etc.) which also tends to dominate over diffraction as one moves out several rings; finally, fine scale seeing – the inner bound for turbulence has been estimated at 2 mm, which means that seeing scatter will certainly deflect a small amount of image light out beyond 6 rings. Sirius B will be at least a quarter to a third the brightness of the surrounding illumination, and more likely significantly less than that.

As a comparison, in a 10” aperture, the relative brightness of the rings to the core at 7 arc seconds is about 2 millionths. That is, diffraction in a perfect 10” scope is insignificant (~1/50th) relative to the brightness of Sirius B

Upshot: it is extremely hard for me to believe that Sirius B can be detected in a 102 mm aperture scope.

An additional factor that really nails it down for me is the fact that diffraction rings tend be broken and speckle-like once you look farther than a few rings from the core. With a 4” scope, that would tend to place S-B in a field of varying speckles that are crudely arranged in rings. Which means that the observer must try to pick S-B out of a bunch of bumps and wobbles in the illumination pattern, most of which are brighter than S-B itself. Not much hope if you ask me.


Clear skies.