Imaging Fomalhauts Debris Disk

[Drakkith]

This is my first attempt at imaging a debris disk around a star. I chose Fomalhaut because, as far as I can tell, there are no other stars visible at this time of year in the northern hemisphere with their disk's as widely separated from them as Fomalhaut.

This task is rather difficult. Fomalhaut is VERY bright. Even with a 1 second exposure of ONLY the near-infrared range from 680-1000 nm, I was already maxing out my pixel values in the center of the airy disc. I stacked 1,000 one second exposures, for a total of about 16.6 minutes imaging time. Between imaging time, downloading time, and whatever else my camera does while taking pictures, it took me an hour just to get that 16.6 minutes of exposure time. I then took 600 one second exposures of Deneb, which was the only star I could find that was near the same spectra classification given my extremely limited resources at the time. (No internet, no decent program, and I only decided to give it a go after I had set up for some imaging and noticed Fomalhaut was up, so I hadn't planned ahead very well lol) I was pressed for time so I didn't get as many exposures of either star as I would have liked.

After stacking the images, I aligned and subtracted Deneb from Fomalhaut to get the image attached to this post. I don't know if I managed to get it or not, but it looks like it's possible. I might have caught a hint of the edge of it. The right side of the star where the red mark is at should be the inner edge of the disk. The angular distance in the image matches with the debris disk, and the orientation seems to match up as well. Well, as best as I can tell given that I don't know which direction is what in my image, only which axis goes in which direction. (The lessons you learn...)

So, it looks to me like it could be the disk, or it could be nothing at all lol. I'm going to try a few more things and get more imaging time in as soon as I can. I don't think 15 minutes is quite enough.

The 2nd image linked, the one from the HST, is from the PDF slideshow here: http://www.ast.cam.ac.uk/ioa/meetings/discs06/proceedings/talks/kalas.pdf

 

[chemisttree]

Any idea how much resolution you need to image the disc? Hubble's mirror is pretty big. Seeing is excellent too! How many arc seconds across is the disc? How many arc seconds of seeing at your location? What is the Dawes limit of your setup?

 

[Drakkith]

Any idea how much resolution you need to image the disc?

I'd guess around 1-2 arc seconds. The disk appears to be about 2 arcseconds thick. Honestly I think you could do it with worse, as the key lies in getting the light from the disk to show up instead of the light from the star. So with worse seeing the image is blurrier, but you have some room to play with.

Hubble's mirror is pretty big.

Yeah, it beats my little 8 inch (203 mm) scope I used for this.

Seeing is excellent too! How many arc seconds across is the disc?

The debris disk is about 2 arcseconds thick, and about 36 arcseconds across at its widest point.

How many arc seconds of seeing at your location? What is the Dawes limit of your setup?

Not sure. The FWHM of a perfect airy disk with my setup at 800 nm is about 6-7 microns, and the pixels on my camera are 7.4 microns across. I don't know what the seeing was like last night. I'm fairly sure I was able to get pretty good seeing since the wavelength was pretty long and the exposures were only 1 second.

 

[Drakkith]

Just to make sure you know what I'm saying here, the attached image has the part of the disk that MIGHT be visible circled in red. I really have no idea if I actually managed to remove the light from the star well enough to see part of it, or if it's just a side effect of the image subtraction and my imagination.

 

[mfb]

You could compare the brightness of that spot with the star itself, and look for some reference value of the ratio (depends on the resolution, of course, but it should be possible to get the order of magnitude). In addition, how does that area look in terms of raw (subtracted) numbers? More like 1.2 - 0.2, or more like 100001-100000?