Ep. 89: Adaptive Optics

Adaptive Optics on the Palomar Telescope. Image credit: Caltech credit:

Since the dawn of humanity, astronomers have wished to destroy the atmosphere. Oh sure, it's what we breathe and all, but that stupid atmosphere is always getting in the way. Since destroying the atmosphere is out of the question, astronomers have figured out how to work with it. To distort the mirror of the telescope itself though the magic of adaptive optics. Episode 89: Adaptive Optics

Show Notes

Episode 89: Adaptive Optics

• "Straightening Out Bent Starlight" – from the Gemini Telescope's website on Adaptive Optics
• Telescopes on the moon:

1. "Lunar Exploration Science" — from Johnson Space Center's Aerospace Scholars website
2. "Recipe for Giant Lunar Telescopes" — Universe Today article
3. "Pristine View of the Universe…From the Moon" – Universe Today article

• "Introduction to Adaptive Optics and Its History" — pdf. from the National Science Foundation
• "How Does Adaptive Optics Work" from Center From Adaptive Optics
• "What is Adaptive Optics?" from ESO
• Info on Adaptive Optics from Mt. Wilson Observatory
• Info on Betelgeuse
• Laser Beam Adaptive Optics from W.M. Keck Observatory
• Comparative Images from the Keck Observatory with and without Adaptive Optics
• List of papers published based on astronomical observing using Adaptive Optics
• List of papers published from observations from Palomar using Adaptive Optics
• Presentation on the Future of Adaptive Optics by Claire Max, Director, Center for Adaptive Optics

Books

• "Adaptive Optics for Astronomy",  Francois Roddier (ed.), Cambridge University Press, 1999

1. Mike Wood Says:
   May 20th, 2008 at 12:59 am

   Episode 89 seems to be very interesting, but unfortunately I'm currently a bit limited by a computer without sound, so MS 'Windows' won't let me see it. However, I'd like to ask, in the context of 'The Atmosphere', if astronomers in North America are stamping on the beliefs of your aptly named Prof. Caldeira and his proposals to spray the atmosphere with sulfur and other particles to shield us from global warming - or maybe the latest Latin American volcanic eruption will have done it for him? If he and his ilk get their way, we can say goodbye completely to meaningful terrestrial observational astronomy. Next year is The International Year of Astronomy. Will you join me in campaigning for clean skies - and a resurgent effort to reduce light pollution in a year that should be notable for grassroots achievement?

   With best regards.
   M.W.
   [louth-observatory.org — soon!]

2. Mike Wood Says:
   May 20th, 2008 at 1:02 am

   Episode 89 looked very interesting, but unfortunately I'm currently a bit limited by a computer without sound, so MS 'Windows' won't let me watch. However, I'd like to ask, in the context of 'The Atmosphere', if astronomers in North America are stamping on the beliefs of the aptly named Prof. Caldeira and his proposals to spray the atmosphere with sulfur and other particles to shield us from global warming - or maybe the latest Latin American volcanic eruption will have done it for him? If he and his ilk get their way, we can say goodbye completely to meaningful terrestrial observational astronomy. Next year is The International Year of Astronomy. Will you join me in campaigning for clean skies - and a resurgent effort to reduce light pollution in a year that should be notable for grassroots achievement?

   With best regards.
   M.W.
   [louth-observatory.org — soon!]

3. Claude Says:
   June 4th, 2008 at 5:02 pm

   Being a dedicated listener of Astronomy Cast, I am normally extrememly impressed by Pamula’s great ability to accurately communicate complex astronomical concepts in straight forward, non-technical terms. Unfortunately, it seemed that this episode failed to dispel some common misconceptions concerning Adaptive Optics. In particular, I felt the difference between Active Optics and Adaptive Optics wasn't clearly addressed.

   The primary difference between Active and Adaptive Optics is in their response times. Active Optics are slow, responding in something like the second range. They are meant to correct for aberrations induced within telescope due to deformations in the telescope optics. This is becoming increasingly important as telescope mirrors become ever lighter, thinner and "floppier". As the telescope moves around the sky, these thin mirrors tend to deform under their own weight. In Active Optics, actuators are typically mounted directly to the backs of the mirrors to "actively" support them in the correct figure. Telescopes move rather slowly so these actuators can respond equally slowly.

   Adaptive Optics have fast response times closer to the kilohertz range (1/1000 second) to respond to the ever changing deformations in the incoming wavefront due to the effects of our turbulent atmosphere. It is normally not possible to warp the telescope's large primary mirror quickly enough so a smaller deformable mirror (DM) is commonly used. In some cases the secondary mirror in a cassegrain is used with an array of high-speed actuators (often piezoelectric pistons). In other cases, an image of the telescope objective is formed onto a much smaller, lighter, less expensive DM. Keep in mind that what ever is done to the real image of a telescope objective is equivalent to doing that to the objective itself. The actuators on the back of a small DM at the image of the objective have much less mass to push around and can respond many times quicker than actuators on the back of the telescope primary.

   Please don't let these few comments insinuate that I don't have the greatest respect for the show and look foreword to each installment!

   Thanks so much,
   Claude

4. flan Says:
   June 29th, 2008 at 1:09 pm

   It may sound a little stupid, but wouldn't it be better to take the results of your telescope on earth with a pinch of salt instead of assuming that your optics are adapting correctly and relying on the wrong results potentially caused by your optics distorting your image more.

5. Claude Says:
   July 1st, 2008 at 4:27 pm

   Hi Flan,

   There are ways of reliably and confidently determining the wavefront aberration entering the telescope. We are not simply relying on guesswork about what the best image "should" look like.

   A common method of determining how the wavefront (or rays) coming from an astronomical source are distorted is to use a Shack-Hartmann (S-H) lenslet array. A S-H is a 2-D array of little lenses. If you focus the telescope pupil (an image of the telescope objective) onto the S-H, each lens in the array images what that portion of the telescope objective is seeing. The result is a 2-D array of sub-aperture images - looking kind of like a flies-eye arrangement. Each sub-aperture, however, is imaging exactly the same image, just moved about due to atmospheric "seeing". Measuring the offset of the image in each subaperture allows you to map out the contour of the incoming wavefront at each moment in time. The deformable mirror (DM) is then set to have the opposite contour, canceling out the distortion.

   The correction is not, however, perfect as seeing is constantly changing and all DMs have a limited number of actuators. So, you want your "wavefront sensor" to update as quickly as possible (something like a 1000 times/second) and to have a DM with as many actuators as possible. Cost normally turns out to be the limiting factor for both of these parameters as fast cameras & large DMs are both expensive!

   Cheers,
   Claude

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