I was once with a group at a small local observatory, trying to take a look at Venus, low over the horizon. As the rotating roof rolled into the correct position, it turned out that a Yagi-style antenna mounted outside sat right in middle of the view. It seemed like something that'd ruin your day (or night), but it turned out that it really made no noticeable difference.
I guess it's just down to the percentage of the aperture that is blocked?
There's an amusing picture where a large owl has blocked a substantial portion of the view:
https://apod.nasa.gov/apod/ap030401.html
Yes. Given enough distance, the obstructing object will start to come into focus, but if it's close, it just darkens the image. After all reflecting telescopes have a secondary mirror (along with its mount) permanently obstructing part of the primary mirror.
This was not a telescope, it was an all sky camera, the caption explicitly states the owl scratched the optics with its claws.
Cracks, scratches, dust, fungus, and haze are all invisible until you shoot a photo with strong light in it. It’s mentioned and shown in the article.
It's like driving at night with a dirty windshield. You don't really notice until you meet an oncoming car with bright headlights. Or similar in the daytime, when you're driving into the sun.
Cool. I first learnt about this property when I took my first proper camera with zoom lens to a zoo and was able to take really nice pictures of a squirrel behind some kind of mesh enclosure. the mesh totally disappeared.
Given that, why are specs of dust on glasses so noticeable?
Your eyes are a "wide angle lens." Scratches on wide angle lenses are also very noticeable
Like with scratches on lenses, those floaters are only visible when your eyes have a small aperture, like when you’re outside in broad daylight
Floaters are a bit different because they are behind the lens. It's more comparable with dust on the sensor in an SLR (or dust in the sensor cavity)
That is also more noticeable with lower aperture but for a different reason. With a low aperture the light comes more from a single direction so the shadow from the dust particles is more defined. The same happens with the floaters. The reason you only see them when looking at the sky is because they're pretty transparent and you need a bright detailless surface to see the low contrast they provide.
But it's very different from scratches on a lens surface because those are in front of the optics.
I'd love to get rid of mine ... and there is a procedure, but I looked up photos of it and ... nope. Think I'll just stick to using dark mode when using my computer instead.
Can you share a link to that procedure? I have floaters bad enough to make me consider drastic steps.
Talking to an optometrist isn't usually that drastic.
All the ones I talk to say there is no way to fix floaters.
Thus a link to the process would be useful - even if searching for reviews of that process would show it does not work.
During my last eye exam my optometrist advised me to call the office in case of a sudden increase in floaters, which might warn of a condition common in the myopic and potentially progressing to retinal detachment, but which can be easily treated during an office visit by a process I understood to involve lasers.
I didn't name the procedure because, not yet having needed to study it, I don't know what it is called. (I don't recall the proper name of the floater-producing, dangerous but ameliorable condition, either.) But I understood its intent to be preventing any risk of progressive loss of vision, rather than removing the floaters directly. Maybe we're talking about two different things, though.
Yes if floaters increase or change agree go to hospital or optometrist immediately.
As for asking for the name it is that which helps to see that we talk about the same thing without that it is impossible to know what you are talking about.
Vitrectomy would be what you are looking for. It’s fairly drastic as you are pulling vitreous humour from the eye in order to get rid of the floaters.
And that laser one is the one that there is an issue with.
In the UK my doctors and NHS notes say there is no evidence that this works.
The removal of the vitreous humour does work but the doctors at Moorfields, the top UK eye hospital, do not recommend it as it has what they say is a high risk of making things worse. They will do it but you have to be really really certain that the floaters are impossible to live with.
The one thing that I believe it is successful for is if people have floaters that are suspended directly in their fovea by adhesions to other parts of the eye. They use the laser to cut the ’ligaments’ so it can float away.
I’m not sure ‘breaking up’ floaters would even be desirable, they are still going to be in there making visual noise.
But they aren't, only dust on the sensor is noticeable, dust anywhere else just softens the image somewhat (unless maybe stopped all the way down with some lenses, but it would be unusual).
Edit heh, I was thinking about "lens glasses". I don't find dust on eyeglasses very noticeable unless light catches it in some particular way.
Similarly, you normally don't see any imperfections/dust specks on your cornea - but you will if you look into a very narrow (0.1-0.2 mm) beam of light (e.g., when using a telescope pushed to a very high magnification = very small exit pupil [1]).
[1] https://en.wikipedia.org/wiki/Exit_pupil
Working with a few US, it took me a while to appreciate "eye-glasses", as a thing (noun).
TIL "lens glasses", is also a thing.
I was rather hoping context would imply what the subject matter is.
Also distinguished from "opera glasses" or "field glasses."
It is in the Fourier domain or just non sharp where the dust is.
Never notice them, only big stains. Maybe it depends on the strength and type?
they also have a blog post about a big house fly that somehow got inside the lens but did not affect image quality
if only there was a lens that could ignore dust this way...
How did the mirror not just shatter though?
The mirror (third largest in the world at the time) is composed of fused silica. Apparently manufactured in segments, then joined. The cooling process itself took two years. That's described in one of the linked YouTube videos in this thread: <https://news.ycombinator.com/item?id=41459171>
The result is a glass that's remarkably immune to shattering. Possibly similar to a Prince Rupert's Drop, though that's my own speculation.
<https://en.wikipedia.org/wiki/Fused_quartz>
"Possibly" is doing some heavy lifting in my comment.
The similarity is in the end result, not, obviously, the fabrication process.
The strength of the Prince Rupert's Drop comes from the differential tensions and stresses in the head-end of the drop. The tail, of course, is remarkably fragile and will lead to the explosive failure of the entire drop. The head can withstand hammer and even bullet impacts, as Destin Sandlin has demonstrated multiple times on his "Smarter Every Day" YouTube channel. I'd suspect a similar tensile arrangement in fused silica, though of course I may be wrong on that.
Isn’t the original cooling process you’re describing “annealing,” by which glasses are cooled slowly so as to allow internal tensions to equalize? A task requiring exponentially more time as the size of the slab of glass increases?
I seem to remember glassmakers using a device called a polariscope to actually see those internal stresses [0]. Which makes me wonder if there are optical considerations as well as physical ones—although I guess if the whole thing is getting silvered, probably not.
My understanding, which could be flawed, was that perfectly annealed glass resists cracking because there isn’t a clear fault line to cleave across, something akin to the way hot water in a perfectly smooth glass has trouble boiling if deprived of a nucleation site. While a Rupert’s drop achieves its stability by something more akin to the trope of a “Mexican standoff,” [1], where countervailing tensions are so densely frozen in place that a little extra tension from the outside makes little difference (but when one crack forms, all bets are off and the tensions resolve explosively).
I’m also reminded of one of my very favorite moments of video: the silvering pass during the manufacture of the Rubin Observatory’s 23.5-ton, 8.4-meter-diameter mirror; which took from 2008 until 2019 to manufacture:
https://m.youtube.com/watch?v=ftKsP3do4nM&t=38
[0] https://www.public.asu.edu/~aomdw/GSI/Glass_Strain.html
[1] https://www.backstage.com/magazine/article/how-to-do-a-mexic...
I'm well out of my depth, so, at this point, I don't know.
According to TFA, they're craters, not proper holes. The force of the bullet wasn't enough to puncture the glass, just puverize the points of impact
Part of this that hasn't been mentioned is that the mirror is thicker than you might expect. The observatory's website says 12.5 inches (though it will vary somewhat across the curved surface).
New York Times:
https://www.nytimes.com/1970/02/07/archives/texas-man-fires-...
> An employe of the Mc Donald Observatory, described as drunk by the sheriff and as mentally depressed by his superiors
> Sheriff W. B. Medley said that the shots “completely de stroyed the telescope — it was ruined.” University of Texas astronomers in Austin said the damage was minor.
> Sheriff: Drunk suspect destroyed telescope
> People who know better: Depressed employee did some minor damage
Modern LEO techniques seem to have history.
The article itself says:
"The damage is limited to small craters about 3 to 5cm in radius,
which reduce the light collecting efficiency by about 1 percent"
This is no news by any metric, but I found curious that:
1. It only reduced the telescope efficiency by 1%
2. The original report from 1970 is still up!
This is the video that made me aware of the incident: https://www.youtube.com/watch?v=59cw_bDbEqA
To be fair, it's Texas. Even the state motto is 1% riddled with bullet holes.
You know the news is from 1970 because back then there was a mental institution for people like him. And it’s not like we have fewer issues now.
Between genealogy and caring for a MI spouse, I learned it went down like this.
Late 18th to early 19th c. people with compassion and insight opened sanitariums to provide care for folks with brain disorders. Much of the care was truly revolutionary and helpful. Some of it was other things. The public was impressed and inspired.
The founders died off and institutions continued on without their vision. The public lost interest; funds waned but need did not.
By the 1980s institutions were no longer safe or caring. The public had moved on. Few in power knew or cared.
Politicians saw funding streams they could grab w/o risking reelection and forced our MI population into homelessness.
We're still there. My 5 state institutions are dangerous hellholes. Homeless are everywhere here.
source: w/o long-term inpatient care, wife left the family to become homeless
I completely agree that mental institutions are important and need more funding/attention. However, your comment implies that all homeless people have mental problems that need treatment. Many are just very poor and most of their problems (mental or otherwise) are caused/exacerbated by homelessness itself. I consider myself sane but if I didn't have a permanent shelter and had to face the streets every night I'd go crazy too
It implies nothing of the sort. What it states, without making implications, is that the very mentally ill who would otherwise be in institutions, are instead homeless. Which is the case.
This sentence:
>We're still there. My 5 state institutions are dangerous hellholes. Homeless are everywhere here.
Implies that "Homeless are everywhere here" wouldn't be the case if the institutions were good. I disagree. I believe that if the institutions were good there would still be homeless people, unless somebody declares to be living on the streets to be a mental condition that warrants forced institutionalization.
A->B =/= B->A.
OPs view is that lack of MI (A) results in homelessness (B). You're taking that further and making the argument that the existence of homelessness (B) implies lack of good institutions (A) - an argument OP didn't make.
A mental institution in Texas in 1970 was likely quite undifferentiated from a prison, and one that you could be sent to without having been convicted of anything. Just needed a state-employed doctor to declare you insane.
Yeah that stood out. There's basically zero mental health resources now after Reagan gutted it. Now the shooter would himself be shot or thrown in jail only to come out worse. Repeat.
The state motto of Texas is "Friendship".
(this is true)
If it was Frıendshıp before the Alamo I'd say that works out about right.
...whereas that of New Hampshire is 'Live free or die'
Incidentally, NH licence plates are stamped out by prison inmates.
Now THAT is cruel and unusual punishment, right there!
Edit: For those not well versed in NH plates, the state motto is embossed on each and every number plate. (This may be the case for every US state, for all I know)
On the subject of bullet holes in 1% of the thing, NH also has way laxer gun laws than Texas. (Or at least had laxer laws 5 years ago, I suppose Texas probably has loosened gun laws in recent years)
No permit for carrying, no duty to inform, no "no guns" signs for buildings that carry any legal weight beyond trespass. NH allows guns in bars, Texas does not. NH you can still protest with guns, which is rare in most states after the 1960s era civil rights protests with guns. Texas nominally prohibits carrying guns for 5 yrs after a violent offense, NH does not. Etc etc.
Yeah, Texas has been all hat no cattle for a long time re: guns. Still looser restrictions than California though.
If you want real ‘wild west’ living, the closest you’ll come is Nevada (except for Clark County), Wyoming (except for Jackson), and Alaska (except for Anchorage). A few other places too.
> This may be the case for every US state, for all I know
Nope. What the jurisdictions choose to write on plates varies, often for a fee you can have something different, either of your choice (within limits) or from some limited selection.
Famously DC has plates quipping about the "Taxation Without Representation" which was notionally the reason the United States wanted independence. The District of Columbia of course does pay federal taxes but does not receive proper democratic representation in exchange, exactly the situation the colonists complained of and with exactly the same retort offered in response†.
[This is a very small hypocrisy compared to say declaring that "All men shall be free" and continuing to literally enslave some of them for example]
† The Congress insists, just like the Westminster Parliament, that these tax payers are represented, but virtually, with the entire institution actually somehow representing their interests. If this strikes you as poppycock for Westminster, it should feel no different closer to home.
It seems to me that it makes a great deal of sense for the seat of the federal government to be located in a federal district independent of any state’s control. It also would not make sense for that federal district to be represented as a state — that would end up being a circular dependency, since the federal government is created by the states, and it doesn’t make sense for the federal district to participate in creating and sustaining itself.
Those who choose to live within the federal district have a privilege others in the United States do not have: direct physical interaction with and influence over the individuals composing the federal government. It makes sense to me that the privilege is balanced with a lack of representation in the Senate and House. Note that they do have representation in the Electoral College.
It also makes sense to me to retrocede the majority of the current federal district back to the state of Maryland.
7 10 cm diameter circles have 1% of the area of a 2.7m diameter circle.
Watched this last night and had a chuckle at how his initial attempt to damage it failed - hammer flew right off the surface. Also how seemingly nonchalant they were about the event.
> When the sheriff arrived, the employee was arrested and later committed to a mental health institution. In the report following the incident, the sheriff, clearly being unfamiliar with telescope designs, stated that the mirror had indeed been destroyed as it had a big, circular hole, right in the middle!
"no sir, that bigger hole was there before the incident... yes sir, it is supposed to be there"
As a Cub Scout we went on a tour of the observatory and I think we were able to see the pictured view, but I may just be remembering the story and imagining the image.
They definitely told us the story and all the kids were like “coool!”
Last year I built a Hadley telescope, which is a 3d printed structure with some inexpensive optics from China.
https://www.printables.com/model/224383-astronomical-telesco...
One of the things that surprised me, being new to telescopes (and certainly never having built one) was that it was deemed better to leave dust on the optics than to potentially damage them by trying to clean them. The dust just doesn't affect image quality.
I had no ide athat would scale up to gunshots however! Very impressive.
Is the right to shoot at things (in civil life) really that essential in this day and age? When was the last time you required a gun to ensure your safety? Or even to use it as a bottle opener?
The McDonald Observatory is open to the public (a consequence, as I understand, of it being on the highest-elevation public highway (6,814 ft, 2,077m) in Texas, and hence having right of public access), with both guided and self-guided tours of several of the telescopes / domes, including the 2.7m Smith scope. The damage is visible from the public gallery as I recall.
The scope is primarily used for spectroscopic studies, though it can also be used for direct visual imaging, which might account for why it's even less susceptible to mirror damage than might otherwise be the case.
The region is quite rural, with the nearest settlements being Fort Davis to the southeast and Fort Stockton to the northeast. Among other local attractions are Alpine ("AL-peen"), home to a federal courthouse, and Marfa, a remarkably liberal artist colony.
Remarkably beautiful following a freezing frost in winter time, as well.
Besides the report on mirror damage this telegram also appears to contain observations of "periodic comet Curmumov-Gerasimenko", which had been discovered a year earlier, and happens to be the one ESA's Rosetta mission visited in 2016.
I say this having read the mailing list and specifically the part that mentions the mirror is made of fused silica:
What the hell is that mirror made of?
Is this kind of behaviour a well-documented/practical property of silica? I would assume that it'd shatter after any amount of shots and a hammer blow.
Fused silica is very pure quartz.
Wouldn't that be quite brittle, though?
I wish there was more information about why the guy shot the telescope. But so far I find nothing.
Once ages ago I heard a story like this, itself not young at time of telling, from a professor of physics. That story might have been set in Texas and did involve a handgun being fired into a telescope mirror which was not meaningfully damaged by the experience, but had it as a revolver wielded by a revivalist-type preacher who smuggled the weapon in during a tour to redress what he saw as a violation of God's domain.
It's been ages since I was in touch with the fellow who told the story, so I can't immediately confirm whether these were the same events. But the similarities are striking.
You could try contacting the university to see who worked there at the time. Dr. Smith, mentioned in the article, died in 1991. His obituary is here: https://quasar.as.utexas.edu/BillInfo/HJSmithObits.html
Alternatively, you could try contacting the sheriff's office in Austin to see if their records include a mention of the incident, but that information is probably protected.
you can always tell a reflector scope person because they'll just say "all it did was reduce contrast"
When all you've got is a hammer, but then remember that you're carrying a 9mm...
> McDonald Observatory, University of Texas
And all right in the world.
Proven to be vandalproof!
