The 10th planet, 2003UB313 (Xena) has been imaged by the Hubble Space Telescope. Previous ground-based measurements of Xena had indicated that it was about 30% larger than Pluto, but Hubble's measurements indicate that it is less than 1% larger than Pluto, at a diameter of 1490 miles plus or minus 60 miles, compared to Pluto's diameter of 1422 miles:
Only a handful of images were required to determine Xena's diameter. Located 10 billion miles from Earth with a diameter a little more than half the width of the United States, the object is 1.5 pixels across in Hubble's view. That's enough to make a precise size measurement.Now, I have a problem with this measurement. What is a half a pixel? It can only mean that sometimes Xena shows up as a single pixel in Hubble's instruments, while at other times Xena shows up as being two pixels wide. There is simply no way to get that plus/minus 60 miles (4%) error size when your imaging instrument has a 50% to 100% difference (1 or 2 pixels). In order to get that 4% error margin, Xena's image would have to be at least 50 pixels wide according to the Nyquist sampling criterion. The data simply cannot support the conclusion that Xena is only slightly larger than Pluto.
Technorati Tags: Space, Hubble, Xena, 2003UB313
4 comments:
Perhaps you should actually check some sources before jumping to conclusions. Here is the URL for the paper: http://www.gps.caltech.edu/~mbrown/papers/ps/xsize.pdf
Also, here is Mike Brown's 2003 UB313 page:
http://www.gps.caltech.edu/~mbrown/planetlila/index.html
In short, it looks like they snapped multiple pictures, and then used some sophisticated algorithms to tease the size from multiple low resolutions pics, much like they did for Quaoar.
First of all, it doesn't matter how sophisticated their algorithms were. Using CCDs - such as are available on the Hubble - is a quantized sampling process, and in digital (quantized) signal processing the Nyquist criterion is king. In order to conclusively state a +/- 4% error margin, the image must be at least 50 pixels wide, and no amount of handwaving will get rid of that Nyquist criterion. If that "one and a half pixels" works out to a diameter of 1490 miles, then the best they can do is state that the diameter is between 1000 and 2000 miles.
Secondly, they had to make assumptions about the albedo of Xena, and they figured that its albedo was in the 50-80% range of Pluto or Triton, but when they use that assumption and run through their calculations, they end up with a result of 86 +/- 7% albedo. The low end of this result and the high end of the assumption just barely agree, implying that their initial assumption is incorrect, thus throwing all of their results into question. In addition "On 2003 UB313 even highly reflective methane frosts will irreversibly darken due to long-term photolysis; some uncommon process which continues to maintain a high albedo on 2003 UB313 is required." Basically, they had to invoke magic in order to get an albedo as high as they calculated for Xena.
The data presented simply does not justify the conclusion. I expect that the referees of the Astrophysical Journal will reject this paper.
I may not be an expert on image processing, but at least I know it. Did you even bother to look at the papers? Did you look up how they measured the size of Quaoar?
Lookee Here:
http://www.journals.uchicago.edu/AJ/journal/
issues/v127n4/202444/brief/202444.abstract.html
Quaoar: 43.4 AU, the size they determined: 1260 ± 190 km.
This an accepted paper, as you'll see from the abstract. Using your argument, they would need images at least 14 pixels across. And yet, with an object apparently twice the diameter, and 2.3 times furthur away, only around 1.5 pixels can be gotten.
The fact is, using multiple images to get better resolution is common in astronomy. I suggest you entertain the possibilty that these experenced astronomers might know more about the subject of measuring object size than you do.
Processing of an image captured by a digital device is done by digital signal processing. The Nyquist criterion is the absolute law of DSP. Ignoring Nyquist is tantamount to ignoring all of DSP.
I suggest you entertain the possibilty that these experenced astronomers might know more about the subject of measuring object size than you do.
No amount of appeal to authority makes the Nyquist criterion go away.
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