Imaging Sirius B has been on my astrophotographical bucket list for some time now. Several years ago I tried with an old Meade SN8, but the scope had some optical issues and although it was a reasonable wide field imaging platform, it just wasn’t up to the task of resolving close binary stars.
My next attempt was with a classic 8 inch SkyWatcher Newtonian. I’m still convinced that this system may be able to resolve Sirius B, but I’ve been stymied by the bright halo around the bright A component. The f/5 scope has a large secondary so diffraction is an issue.
These days I’m using an Esprit 120 APO refractor and I thought I’d give it another try. The scope is a joy to use with a very stable focuser and great optics. February 15, 2018 around 7:00 PM I rolled the scope on a ScopeBuggy out of our garage and set it up to cool in the driveway. It wouldn’t be until around nine before Sirius cleared the trees so I went back in to watch the news and plan the evening’s attempt. I had downloaded several images that all supposedly showed the dwarf, including one from the national RASC site on the Sirius observing challenge that I was going to use as a reference. Around 8:30 I went back outside and polar aligned the scope, pointed it at Sirius and started up the autoguider, not that easy a task at -12 degrees!
Heading back inside I have the advantage of using a VNC connection to my tablet running the scope so I could stay warm for the rest of the evening. Since I had no idea of what exposure would work I tried a variety starting at 30 seconds and reducing in two second increments until I saw something close to Sirius A three frames in a row. I was able to see a faint star with exposures from 10 to 5 seconds so I settled on 8 seconds and shot 64 frames. There was a slight breeze so I used the best 42 and stacked them to produce the image below.
Sirius (full field)
Zooming in on the main target
Now things were getting exciting, in close to Sirius A is a faint blue star, slightly elongated due to the breeze moving the scope in some of the subframes. This was the same star labelled as the dwarf in all of the images I had downloaded. From here is where everything went wrong.
I took a couple of calibration frames where I used a planetarium program to send the mount one arcminute north and took a quick image so I would be able to determine direction in the image. I took another image after sending the scope another arcminute north thinking that the first would run out any backlash in the gearing system and I could use this image to measure scale. Finally I took another image with the north button held down and the shutter open, trusting the mount to move north at the sidereal rate. After working through the math I came up with a position angle of 68 degrees (actual is 72) so I was in the right ballpark. When I checked the separation I got almost 17 arcseconds which was off by quite a bit. This was puzzling so I decided to sleep on it and retake my calibration images on the next clear evening.
The next night I planned to take new calibration frames and image Rigel. Rigel is a binary with close to the same separation as Sirius at the moment, but the stars are closer in brightness so resolving the system is much easier. The idea was to use a sequence of images to home in on the correct exposure for the next attempt at Sirius. It turns out that my initial attempts were done with much too long an exposure, saturating the entire area where the dwarf would be located. After determining that two seconds was correct for Rigel, I was able to image and easily resolve Rigel. I took some proper drift shots by exposing for ten seconds with the mount turned off allowing the star to drift through the frame in order to calculate the correct value for the image scale.
With the new drift calibration the image scale was found to be 0.315 arcseconds per pixel. I checked the separation on Rigel and got an answer of 9.8 arcseconds with the real value being around 9.5. Much better agreement!
Using the correct image scale of 0.315 arcseconds per pixel I went back to the Sirius image to find that the faint star was definitely not the dwarf. The central crops of Sirius and Rigel, shown above, are both to the same scale. The separation of the two stars in the Rigel system is close to the spacing between Sirius A & B so it is obvious from the two images that the actual dwarf is still within the saturated area of the brighter star. It is interesting to note that the star I had mistaken for the dwarf has been labelled as Sirius B in several images online, including one on the RASC Sirius Observing Challenge page so I’m not alone in my error.
Stay tuned as I’m not ready to give up on this one just yet. Since the target is bright, I can keep trying from home whenever it is clear.