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I The terrible distortions of the Hubble Telescope

  1. May 28, 2016 #1
    I'm not talking about the well known, miss-grinding of the main mirror which was corrected long ago . This picture illustrates well , my beef with the current images ....

    phot-40c-09-fullres.jpg
    Every image of every star has four streaks of light , this is not so noticeable in pictures of galaxies where the stars are very close to point sources but the streaks are still there.
    These streaks are cause by the four struts holding the secondary mirror.
    There must be many ways to hold the secondary mirror, particularly in zero gravity , without making this mess of the picture , magnetic suspension ....or a network of thousands of supper fine wires/nano tubes like a spoked bicycle wheel.... or fixing the mirror to a transparent sheet covering the whole field of view would only lose a few % of the light .

    Is this the best the tax payer can expect? Incompetence at the design level !!!
     
  2. jcsd
  3. May 28, 2016 #2

    andrewkirk

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    Silly NASA! What a pity they didn't have access to your advice at the time they were planning the mission.
     
  4. May 28, 2016 #3

    Drakkith

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    Certainly. One of my own telescopes has a ten-inch schmidt-corrector made out of glass at the aperture that also holds the secondary mirror. But remember that the HST is 8 feet across. That's some serious weight, which is the reason you don't see large refractors. The glass simply weighs too much. For the HST the glass would have had to have been thick enough to survive launch, so you couldn't have made it very thin just to hold the secondary mirror or you risk it breaking during launch. I believe glass also absorbs UV light, which may or may not have been a reason not to use glass to hold the secondary mirror.

    Wires or mesh have the problem that they aren't nearly as sturdy as a solid spider (the struts + secondary mirror holder), and the last thing you want is for the secondary mirror to move around as the HST twists and turns in space to move between targets. I believe magnetic suspension has a similar problem.

    The fact is that a solid spider is the simplest, most reliable method of holding the secondary mirror in place to within the necessary tolerance.

    Nonsense. This isn't incompetence. There are many issues that the HST engineers had to account for, and this was one of them. Remember that the HST is primarily a scientific instrument. Diffraction spikes have little effect on the quality of the image in terms of scientific data. If one of the spikes is going to be in the way of your primary imaging target, you can put into your instructions to have the telescope rotated. I believe their are ways of digitally removing the spikes after-the-fact as well.

    In addition, diffraction spikes are very much a subjective issue. Some people love them, some don't. I hear there are amateur astronomers who place wires over the front of their refractors just to get diffraction spikes in their images.
     
    Last edited by a moderator: May 30, 2016
  5. May 28, 2016 #4
    You seemed to be under the illusion that great geniuses run these projects, The company employed by NASA couldn't even grind the mirror correctly, or check it properly before launch!...And NASA didn't even check it was OK before launch!!! No one cares, no one sacked...It's only tax payers money.
     
  6. May 28, 2016 #5
    Glass or Quartz( transparent to UV and IR) seem a good choice. Of course the secondary mirror would be added after launch could be thin for use in Zero gravity, no problem surviving launch lay it horizontally, just need a launch vehicle to accommodate the mirror laying flat .
     
  7. May 28, 2016 #6

    Drakkith

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    There were certainly problems with the HST, and not just with the primary mirror. However, I wouldn't say that no one cared. The company that manufactured the primary mirror ended up paying 25 million dollars to the federal government just to avoid a lawsuit. And remember that the HST was launched in 1990 while the primary mirror was finished in 1982. Many of the people involved in the project were no longer around to fire in 1990. Bud Rigby himself, project manager for the mirror, was retired by then.

    The HST was launched pre-assembled. Putting it together in orbit was never an option.
     
  8. May 30, 2016 #7

    russ_watters

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    I have an SCT and a refractor, neither of which have support struts. I sometimes tape wires over the end because I think the diffraction spikes look artistic:

    Horsehead-HaRGB.jpg
     
  9. May 30, 2016 #8

    mfb

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    What exactly makes you think you would have done better than the hundreds of people involved in the Hubble project? Apart from perfect hindsight, of course.
    The mirrors have been tested. Every single component has been tested multiple times. The grinding error escaped all those tests - an unlikely event, but not impossible.

    The engineers and physicists studied all the things you suggest here, in much more detail than you ever will. This includes knowing that the telescope has to be rotated, for example, and needs a very stable position of the secondary mirror, which excludes support structures that are too weak. Also, in-orbit assembly makes everything much more complex. They chose the rigid spider because it was the best option for the science goals of Hubble.
     
  10. May 30, 2016 #9

    Drakkith

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    Here's an easy to read article on the fabrication process, focusing on how and why the mirror was ground incorrectly: http://people.tamu.edu/~v-buenger/658/Hubble.pdf
    I like the title. "Hubble Error: Time, Money and Millionths of an inch"

    The short version is that the fabrication of the mirror took place in an environment where engineers were stressed for time, money was so tight the project kept being threatened with closure, good communication between key players did not take place, and management failed to properly recognize and address issues that cropped up. In such an environment the key problem, that of a faulty null-detector being used, was capable of going uncorrected. There were multiple times in which engineers were essentially staring the problem in the face and a phone call to the designer of the null-corrector would have most likely identified that that there was an issue.

    What's interesting is that these are the nearly the exact same problems that have led to a fair number of disasters, including more than one nuclear meltdown and both space shuttle disasters.
     
  11. May 30, 2016 #10

    mfb

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    All those systems are designed to have a lot of redundancy and cross-checks in normal operation. Many things have to go wrong at the same time to get a really negative outcome. For every such outcome, there are tens to thousands (depending on the way to count) of problems that get caught early on before anything gets damaged.
     
  12. May 30, 2016 #11
    Can't deconvolution deal with those spikes if one really wants them gone?
     
  13. May 30, 2016 #12

    1oldman2

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  14. May 30, 2016 #13

    Drakkith

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    Not sure. The only deconvolution I've done hasn't gotten rid of the spikes.
     
  15. May 30, 2016 #14

    mfb

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    Well, if you know the spike pattern, you can reduce them significantly. Won't be perfect, of course, but you get an image with much less prominent spikes.
     
  16. May 30, 2016 #15

    D H

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    But only at the cost of not seeing what you want to be seeing, which in the case of Hubble (and the Very Large Telescope, and many other prominent telescopes) is to see very far. Those diffraction spikes are a necessary evil. It is not bad design.
     
  17. May 31, 2016 #16

    davenn

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    yup, agreed
    in fact the new Astronomy Tools action set I have just got for Photoshop allows me to add the spikes if I so wish :smile:
     
  18. May 31, 2016 #17
    For some, it is all about the picture visuals.
    The real data work would be about spectroscopy, and red shift, and much else.
    The spider diffraction artifacts would not detract from this.
    For me, maybe like other engineering folk, the quality of the clear lines are evidence of how well the optics are working beyond the normal diffraction limit.
     
  19. May 31, 2016 #18

    Andy Resnick

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    Just to clear up another urban legend, the primary mirror figuring wasn't exactly 'wrong', it was ground exactly as it was for the other KH-11 satellites.
     
  20. May 31, 2016 #19

    Drakkith

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    What do you mean?
     
  21. May 31, 2016 #20

    Chronos

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