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.