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Sharpest Images With "Lucky" Telescope
igny writes "Astronomers from the University of Cambridge and Caltech have developed a new camera that gives much more detailed pictures of stars and nebulae than even the Hubble Space Telescope, and does it from the ground. A new technique called 'Lucky imaging' has been used to diminish atmospheric noise in the visible range, creating the most detailed pictures of the sky in history."
One of the main limitations to ground-based optical telescopes (and one of the reasons that Hubble gets such amazing images) is that the atmosphere generates considerable distortion. Random fluctuations in the atmosphere cause images to be blurry (and cause stars to twinkle, of course). The technique they present appears to be taking images at very high-speed. They developed an algorithm that looks through the images, and identifies the ones that happen to not-blurry (hence "lucky"). By combining all the least blurry images (taken when the atmosphere just happened to be not introducing distortion), they can obtain clear images using ground-based telescopes (which are bigger than Hubble, obviously). I imagine the algorithm they've implemented tries to use sub-sections of images that are clear, to get as much data as possible.
Overall, a fairly clever technique. I wonder how this compares to adaptive optics, which is another solution to this problem. In adaptive optics, a guide laser beam is used to illuminate the atmosphere above the telescope. The measured distortion of the laser beam is used to distort the imaging mirror in the telescope (usually the mirror is segmented into a bunch of small independent sub-mirrors). The end result is that adaptive optics can essentially counter-act the atmospheric distortion, delivering crisp images from ground telescopes.
I would guess that adaptive optics produces better images (partly because it "keeps" all incident light, by refocusing it properly, rather than letting a large percentage of image acquisitions be "blurry" and eventually thrown away), but adaptive optics are no doubt expensive. The technique presented in TFA seems simple enough that it would be added to just about any telescope, increasing image quality at a sacrifice in acquisition time.
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Add up 1000 of those frames, and you have a 50 second exposure.
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As the previous poster noted, there isn't any atmosphere and thus the technique isn't useful for HST.
Additionally, while they don't mention details in the article, I presume they have a specially designed camera. This is an old technique, but it's generally limited to very bright objects due to something called readout noise. Basically all CCD's produce an additional signal due to the process of reading out the data. This limits the effectiveness of repeated short observations to sources which are much brigher than this noise, since the noise also grows linearly with the number of images taken.
To image distant galaxies you typically have to take exposures of one to several hours, and thus this technique isn't useful.
Doug
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THere's several pieces of software which do som parts of this - Registax is what I use, but amateurs usually only have enough aperture to make this work for bright objects like planets. You can take a good quality webcam (the top of the line Phillips webcams are the best bang for yout buck), record some video of a planet through a telescope and then pick out the least distorted images before adding them together to create the final image. Now, the trick is getting the best measurement of which images are undistorted, and getting enough light in each frame while keeping the esposure time short enough to beat the atmosphere.
Look at the planetary images here for my attempts at this technique.
TFA mentions that they can achieve images better than Hubble. The sample image they show, of the Cat's Eye Nebula, isn't as sharp as the Hubble image of the same object.
Probably they can push their technique harder than this initial image suggests (it was mainly comparing the "lucky" image with a conventional, blurry, ground-based image)... But I just thought it would be good to show Hubble's pictures alongside.
Additionally, while they don't mention details in the article, I presume they have a specially designed camera.
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They are using a new kind of CCD that somehow lowers the noise floor. Details are at:
http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/L
In fact this site (same basic place) is much more informative than the press release and answers a lot of questions:
http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site/i
Hubble: Cat's Eye Nebula
using 'Blue Peter' technology
Blue Peter is a BBC childrens show. Blue Peter Technology is effectively something so simple a child could do it.
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Actually, near infrared is not blocked by water vapor, in fact water vapor is extremely transparent to near infrared light even moreso than visible light. That's why satellites can use infrared to see through clouds, and also why adaptive optics work so well in the near infrared range.
Far infrared is a different story, and you're absolutely correct there.
Random and weird software I've written.
I'm curious though about how they deal with some of the "features" you get to see with this technique. It's *very* easy to stack a few hundred images, run Registax's sharpening filter and get some interesting pictures of stuff that doesn't really exist. I'm not sure I really trust the fine detail in my photos- unless I see it in another taken a few hours later it may well not be real.
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See here, for example, for more information.
There are wavelength ranges in the NIR where the atmosphere is indeed transparent (J,H and K bands, for example); but the atmosphere is opaque at most NIR wavelengths (and, even at those IR wavelengths where the atmosphere is transparent, the transmittance is lower than at visible or radio wavelengths). See here for more info.
Even if this technique can eventually produce better pictures at lower cost it is still limited to wavelengths that can penetrate the atmosphere. Some of the most exciting recent discoveries are in infrared (Spitzer) and X-ray (Chandra). The next big telescipe (James Webb Space Telescope) is also for infrared.
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To sum up, the problem is readout noise. The faster you read out the CCD, the more noise you get. When you image a faint object the readout noise exceeds the signal level. The reason amature astronomers can use this technique anyway is because they are imaging bright objects (like planets), so the signal is easily discernable from the readout noise.
Now there is a new type of CCD with a built in digital signal multiplier that precedes the readout step in each individual pixel. You can simply select an appropriate multiplier that gives pixel values that fall nicely in the middle of the register width and when you read out the value, any noise can simply be subtracted away because you know that it will be much less than the signal value you are looking at.
- The Earth rotates in 24h
- Hubble orbits in 90 min
so Hubble cannot peer "hours at a time", but ground telescopes can. Hubble can actually produce millon-second-long exposures. That's 400 orbits, but stacking 21 minute exposures.
So it looks like each frame is shifted as a whole rather than each individual pixel. Which makes sense from the description of the process, since the theory is that the images you're picking in the Lucky Imaging technique are high-quality images with a random offset due to the atmosphere.
I think your suspicions are probably correct.
Lucky Imaging relies on the fact that every so often, a really high-quality image makes it through the atmosphere almost unperturbed (based on the Kolmogorov model of turbulence). While I don't know whether the same model can be applied to cosmic gas clouds, there may be another model that could accurately model the phase distortions those clouds impress upon a wavefront.
To achieve this one must take many very short-exposure (compared to the time-scale of atmospheric turbulence, or gas-cloud turbulence in the case we're considering) images of the source. However, distant (or dim) objects often require reasonably long exposure times in order to collect a large enough amount of light to be able to see the image. The problem with this technique may simply be that the exposure time necessary for the Lucky Image algorithm to work is too short to actually collect enough light to create a good image in the first place.
I emailed the principle researcher on this project, asking him what was novel about his approach, since amateurs have been "stacking" images for years. Below is his response: From: Craig Mackay [mailto:cdm@ast.cam.ac.uk] Sent: Tuesday, September 04, 2007 5:20 AM Subject: Re: What's new with Lucky? Dear Tom Thank you for your message. What is new about this (and gets rather lost with the media coverage) is being able to use lucky imaging on a much larger telescope. With a 2.5 meter telescope we are able to use typically 10% of the images. With a five meter telescope and four times the area we would be able to use only 0.01% of the images, a completely useless fraction! For the first time however we have managed to do it by using an adaptive optic system in front of our lucky imaging camera. That is what is new and that is what has made all the difference. The AO system gets rid of the larger scale low order turbulent distortions leaving lucky imaging to work on the higher frequency ones which it does rather well. Hence the new image quality which is twice as good in terms of resolution as Hubble, something that has never been achieved before either from space or from ground. If you look on the lucky website you will find a lot of information about amateur lucky imaging for which I have a very high regard. Best wishes Craig Mackay.
The amateur stacking program Registax seems to be more sophisticated than this. It allows multiple alignment stars or points and shifts the corresponding subregions of the image as needed. Otherwise, the method for selecting the images is very similar.
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http://www.astronomie.be/registax/html/multi_oper
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