Stargazing The terrible distortions of the Hubble Telescope

[oz93666]

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 ...

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 !

 

[andrewkirk]

Silly NASA! What a pity they didn't have access to your advice at the time they were planning the mission.

 

[Drakkith]

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 .

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.

Is this the best the tax payer can expect? Incompetence at the design level !

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.

 

[oz93666]

Silly NASA! What a pity they didn't have access to your advice at the time they were planning the mission.

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.

 

[oz93666]

Certainly. One of my own telescopes has a ten-inch schmidt-corrector mad 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.

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 .

 

[Drakkith]

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.

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.

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 .

The HST was launched pre-assembled. Putting it together in orbit was never an option.

 

[russ_watters]

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:

 

[mfb]

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.

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.

 

[Drakkith]

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.

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.

 

[mfb]

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.

 

[Lord Crc]

Can't deconvolution deal with those spikes if one really wants them gone?

 

[1oldman2]

http://people.tamu.edu/~v-buenger/658/Hubble.pdf

Great link, thanks.

 

[Drakkith]

Can't deconvolution deal with those spikes if one really wants them gone?

Not sure. The only deconvolution I've done hasn't gotten rid of the spikes.

 

[mfb]

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.

 

[D H]

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.

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.

 

[davenn]

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:

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

 

[GTrax]

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.

 

[Andy Resnick]

I'm not talking about the well known, miss-grinding of the main mirror which was corrected long ago .

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.

 

[Drakkith]

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.

What do you mean?

 

[Chronos]

For a recap of the foibles and redemption of the HST, see http://www.montana.edu/jshaw/documents/17b Saving Hubble - Jeff Hecht - OPN 2013.pdf

 

[Rob Stow]

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"[

Many thanks for that link - fascinating reading.

 

[Andy Resnick]

What do you mean?

The error was not the mirror shape- the mirror was perfectly ground. The error came from the null corrector used to set the shape. Perkin Elmer, the relevant contractor, was also the contractor that figured the primary mirror for the KH-11 series of spy satellites, upon which the Hubble design is (was?) based. Analysis (http://ntrs.nasa.gov/search.jsp?R=19910003124) shows that the field lens of the null corrector was set 1.3mm too far from the nominal position due to misalignment of the metering rod. That 1.3mm distance error is sufficient to explain the aberrations.

(from the linked report, bf mine):

"With one end of the metering rod presumably located at the center of curvature of the lower RNC mirror, the field lens was then brought up to the end of rod B, but there was no adjustment left in the screws used for this positioning. More adjustment room was made by inserting spacers between the field lens and the lower mirror mounting plate. The adjustment mechanism was found not to be staked. Staking, i.e., securing the mechanism to prevent inadvertent movement, was a specified procedure. The final location of the field lens was then set with the addition of the spacers. As a result, the field lens was about 1.3 rnm too far from its correct position relative to the lower mirror."

All of the alignment jigs were constructed based on a 200 mile object distance, the Hubble null corrector was kludged to set the object distance to be infinity, but not all the jigs were reconfigured properly.

 

[Drakkith]

The error was not the mirror shape- the mirror was perfectly ground.

I'd call being ground correctly to the wrong shape still "ground wrong".

 

[Andy Resnick]

I'd call being ground correctly to the wrong shape still "ground wrong".

That's fine, but it misidentifies the process error.

 

[Drakkith]

That's fine, but it misidentifies the process error.

I don't agree. If the grinding process was supposed to yield a certain surface, and it did not, then the mirror was ground incorrectly.

 

[Andy Resnick]

I don't agree. If the grinding process was supposed to yield a certain surface, and it did not, then the mirror was ground incorrectly.

But that's just it- the grinding process yielded the correct surface. Do you understand the process by which lenses and mirrors are figured? Surfaces are ground to a *reference* shape. Let's say you are grinding me a mirror, because you (rightly) take pride in being the world's best grinder, and I give you the reference shape. Oops- I gave you the wrong reference. Who is at fault?

Now go a step further: I give the reference shape to a third party who installs the reference in your grinding jig. This time, the reference is good but the installation was flawed. Now a wrinkle- I told the installer to adjust the installation procedure, but did not document (or validate) the modified procedure very well. Now what?

Hubble was a process error, not a grinding error.

 

[mfb]

the grinding process yielded the correct surface.

It might be a matter of semantics, but I don't think so. The "correct surface" is the expected shape for the telescope - the surface that would produce sharp images in space. The grinding process yielded a surface that passed the flawed test, which means the surface had the wrong shape.

It is not the fault of the grinding process - they got it wrong because the test setup was wrong. That doesn't make the resulting shape "right". Being responsible and being correct are different things.

 

[fizzy]

What do you mean?

Wasn't the focal length that got ground, that which would be needed to focus on the Earth's surface rather than at infinity?

 

[fizzy]

Can't deconvolution deal with those spikes if one really wants them gone?

This was my first thought too. However, when dividing by the FT of the perturbing defect, there are zero or near zero values which make the result unstable. This means you need to add some noise to prevent certain point blowing up. That degrades the result.

My guess is that they'd rather keep the finesse of the overall image and put up with the 'artistic' effects caused by the supports.

BTW the early, more distant pictures of the Pluto fly-by were obviously using deconvolution techniques and were producing orange-peel type artefacts on detail of the surface. This what happens if you turn up 'gain' too much. It gives the illusion of being sharper but a lot of it is fictional detail which is just amplified noise. They may have been trying to correct slightly out of focus images.

 

[Drakkith]

Hubble was a process error, not a grinding error.

I don't agree. The grinding process would seem to include the tests to determine the shape of the mirror's surface during grinding and polishing. This was done incorrectly because of an error in assembling the null corrector. Therefor the grinding process itself was not done correctly. If you want to break things down further into grinding vs testing, then that's up to you. I'm still going to say it was ground incorrectly and I don't think I would be incorrect.

Wasn't the focal length that got ground, that which would be needed to focus on the Earth's surface rather than at infinity?

No idea.

 

[Andy Resnick]

Wasn't the focal length that got ground, that which would be needed to focus on the Earth's surface rather than at infinity?

Yes.

 

[fizzy]

The company that ground the Hubble mirror did most of their work for DoD making optics for surveillance satellites. I don't have a reliable reference for this, but my understanding was that the mirror got ground to the usual curvature for those satellites. They would not have wanted to talk about the capabilities or number of such mirrors they make for obvious reasons at the time. A lot more information is available about that now.

In fact I recall reading that DoD have offered to donate one such set of optics, that they no longer need to put into service, for scientific use if someone can muster the funds needed to get it into orbit. Presumably they'd just need to make similar optical correction devices as those developed for Hubble and fit them before launching.

Wow, a free Hubble replacement. Maybe someone should crowd fund paying SpaceX to get it into orbit ;)

 

[fizzy]

Getting back to the deconvolution question. What is ideally required is an image of a single star in otherwise empty section of the sky with the current optics. That probably already exists as verification of the corrective devices fitted to compensate the grinding error. If anyone knows where to find such an image please post.

That can be used to create the deconvolution mask. Since every point of light in a nebula suffers similar distorting 'cross wires' this must degrade the image. It would be interesting to look at but my gut feel is that the processing would introduce more artefacts than the defect it is intended to remove would gain.

It gets very processor intensive and rises exponentially as image size increases, so you'd need a super computer to process a full resolution Hubble image. However, it should be possible to test on a smaller scale of a few megapixels on a PC.

In fact, not just the cross wires but a lot of the flare around each star could potentially be removed

 

[Laurie K]

The 'HST Optical Systems Failure Report' from NASA is interesting reading.

https://www.ssl.berkeley.edu/~mlampton/AllenReportHST.pdf

 

[mfb]

It gets very processor intensive and rises exponentially as image size increases, so you'd need a super computer to process a full resolution Hubble image. However, it should be possible to test on a smaller scale of a few megapixels on a PC.

Most deconvolution techniques don't rise exponentially with the image size, but none of them is perfect.

 

[fizzy]

Most deconvolution techniques don't rise exponentially with the image size, but none of them is perfect.

OK, I was speaking quickly . DFT is O(n²), which is similar to exp, it's actually super-exponential ; FFT is O(nlogn)

It still blows up on you quickly if you want to work with Hubble sized images

when experimenting I usually crop down to 512px by 512px to save waiting all day for each test .

 

[fizzy]

It might be a matter of semantics, but I don't think so. .

It certainly is nothing but semantics. It was ground perfectly to the wrong parameters. We are all agreed on that so lets' not spend the rest of the day arguing about it.

The NASA report says it was ground flatter than it should have been. I don't think that is compatible with the idea that it was ground to focus Earth as the usual series mirrors would have been. It should be noted that the report says this explanation is speculation since they never were able to locate the relevant documents.

The mirror was ground in 1982 and the report was done in 1990. I do not expect that DoD would want much discussion of the construction of their spy satellites getting into such a report. Whatever happened, certain details may have been changed to protect the 'innocent.' ;)

 

[mfb]

You can edit your posts if you want to add something.

Exponential would be cⁿ, O(n²) and O(n log n) are polynomial.
Hubble's wide field camera has two sensors with 8 megapixel, some smartphone cameras have more than that, and NASA can certainly afford using 32 to 1000 times the computing power necessary for 512x512 pixel images.

 

[Hornbein]

OK, I was speaking quickly . DFT is O(n²), which is similar to exp, it's actually super-exponential ;

O(n²) is about as polynomial as you can get.

 

[fizzy]

Thanks guys, I do know that n2 is a polynomial.

I thought Hubble images were bigger than 8Mpx, that may be doable on a decent PC if you are not in a hurry for the results.

Clearly it's doddle for anyone with large computing resources. I've already pointed out that NASA were obviously doing deconv processing on the first approach images they got back from Pluto fly-by.

It might be quite fun to try this but it would require a single star image for the mask and unmodified source image in a non-lossy format. Pointless if it's supplied as jpeg.

 

[D H]

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.

There is an urban myth here, but it is that the Hubble mirror was ground exactly as it was for other KH-11 satellites (i.e., to focus on something 200 miles away as opposed to focusing at infinity). You yourself provided a link that shows that this is not the case.

Analysis (http://ntrs.nasa.gov/search.jsp?R=19910003124) shows that the field lens of the null corrector was set 1.3mm too far from the nominal position due to misalignment of the metering rod. That 1.3mm distance error is sufficient to explain the aberrations.

The key technical cause of that 1.3 mm error was an erroneously cut piece of black electrical tape that was used in lieu of an antireflective coating. This caused the technicians to measure a spot 1.3 mm closer than the intended location. A key image from that report portrays what went wrong.

The root causes of the problem were much deeper than that botched tape job and botched measurement. Perkin-Elmer had underbid the contract. This led to rushed operations (e.g., using electrical tape instead of paint), and ignoring independent measurements that showed the mirror was being ground incorrectly. NASA had created a very hostile relationship with all of the companies working on the Hubble, including PE. Reporting problems to NASA was highly problematic. NASA made it very clear at the time that they did not want to hear about any problems, so they weren't told about them.

That Hubble's primary mirror was ground incorrectly ultimately was a failure of management and of culture. Fortunately, the end result in this case was an incorrectly ground mirror that was eminently correctible.

 

[Andy Resnick]

There is an urban myth here, but it is that the Hubble mirror was ground exactly as it was for other KH-11 satellites (i.e., to focus on something 200 miles away as opposed to focusing at infinity). You yourself provided a link that shows that this is not the case.

I admit that my evidence is circumstantial, but it's a lot of circumstantial evidence. The dates when the mirrors were ground, the companies (and the technical teams) involved and the optical designs. are all the same. Finally, note that even though the Hubble mirror was orbiting Earth and not accessible for metrology, optical designers were able to construct a corrector (COSTAR) that perfectly compensated for the aberrations. Given that the date of the report was late 1990 and COSTAR was launched in 1993, it is unlikely that the designers started after the report was issued, implying that the designers already knew (or suspected) the underlying cause and had begun working on a corrector prior to issuance of the report.

Misalignment of the spacing rod was *a* critical cause of the failure, not the only cause- another cause was the use of spacers, which had not originally been present.

But I agree, the root causes of the failure were not simply technical assembly errors- project reporting and management were the root causes.

 

[fizzy]

Sure PE made a whole series of those mirrors and the spec. was very similar. That is a very good reason to give them the contract and why they may have been able to put in a competitive bid: there was virtually no development needed.

The paint flake story I find a bit flakey and it is stated to be only speculation but the report says the mirror was too flat. That seems to be going in the wrong direction to be shorter focal length grind.

Just what is the 'circumstantial' evidence that the mirror was ground at the same radius as a spy satellite?

"Given that the date of the report was late 1990 and COSTAR was launched in 1993, it is unlikely that the designers started after the report was issued,"

So your whole argument is "unlikely" without stating why this makes it unlikely. There is a principal in science of falsifiability. If you do not make a specific claim or statement, your point is not falsifiable and thus unverifiable.

Once the investigation had determined the nature of the error, I should think they would be able to make the corrective optics quicker than a bureaucratic committee can write a report and get it security vetted to make sure they did not reveal any details they should not about PE's other mirror grinding work.

I don't see any circumstantial evidence that he mirror was short focusing rather than long focusing as the report claims. What am I missing? It's quite a tempring story but I don't see any evidence to support it.

 

[Drakkith]

I admit that my evidence is circumstantial, but it's a lot of circumstantial evidence. The dates when the mirrors were ground, the companies (and the technical teams) involved and the optical designs. are all the same. Finally, note that even though the Hubble mirror was orbiting Earth and not accessible for metrology, optical designers were able to construct a corrector (COSTAR) that perfectly compensated for the aberrations. Given that the date of the report was late 1990 and COSTAR was launched in 1993, it is unlikely that the designers started after the report was issued, implying that the designers already knew (or suspected) the underlying cause and had begun working on a corrector prior to issuance of the report.

From the website of the company that designed COSTAR:

Perhaps the most famous of our Hubble instrument is the Corrective Optics Space Telescope Axial Replacement (COSTAR) that helped correct Hubble’s hazy vision. We designed and produced the complex COSTAR optics in only 28 months instead of the typical 48. When COSTAR and JPL’s WFPC-2 camera corrected Hubble’s vision after the first servicing mission in 1993, Ed Weiler, NASA’s chief Hubble astronomer, said the telescope was "fixed beyond our wildest expectations."

http://www.ball.com/aerospace/programs/hubble-space-telescope

 

[fizzy]

"fixed beyond our wildest expectations."

Yep, they really pulled a minor technical miracle recovering a situation like that.

 

[Drakkith]

The paint flake story I find a bit flakey and it is stated to be only speculation but the report says the mirror was too flat.

Well, you can see a picture of the missing portion of the anti-reflective coating on the null corrector on page 50 of the PDF here: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910003124.pdf
That doesn't prove that this was the primary cause of the error, but I'd say it's the most likely explanation.

 

[sophiecentaur]

"fixed beyond our wildest expectations."

Yep, they really pulled a minor technical miracle recovering a situation like that.

That 'repair' sequence was some of the best space TV I have ever seen.

 

[Chronos]

As I understand the NASA technical reports, the null corrector was incorrectly figured. The HST was matched to the null corrector, so it ended up misfigured.

 

[Lord Crc]

Getting back to the deconvolution question. What is ideally required is an image of a single star in otherwise empty section of the sky with the current optics.

I stumbled over a BBC Horizon episode from 1991 about the issue with Hubble's mirror, and the specifically mentioned deconvolution there. The explanation they gave for why it was not suitable was that to get the whole image clear rather than just one star, they had to split the image in small regions and optimize for each one separately. And back in 1991, that simply took too much CPU time.

The episode also highlighted a point I had not considered, namely that the different instruments on board suffered differently from the mirror flaw. At east from what I could gather, deconvolution would have been useless for some of the other instruments.

On a completely unrelated note, it's amazing how nice it is to watch older tv documentary shows from before the ADHD producers took over...

 

[1oldman2]

How could anyone not love this.