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  #41  
Old 13-02-2020, 03:53 PM
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multiweb (Marc)
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Originally Posted by gary View Post
Each mirror has 36 hexagonal segments and the surface shape of
a segment depends on its distance from the center. It turns out that there
are six different surface shapes.
Oh wow. Is the whole telescope design a Ritchey Chrétien or other? Must be a manufacturing nightmare. How do they independantly test segments that have those "exotic" asymetric shapes I imagine yet focus each area of light to the same point? Do they use some kind of scafold in the factory with the elements in their respective position from the optical axis for figuring and testing?

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So they carry spares which they can swap-out during re-aluminizing.
Each segment is 1.8m across and as you mentioned it is a lot easier to
handle something of that size compared to the logistics of each time they
re-aluminize the 3.8m AAT.
1.8m? Those trolleys didn't look that big in your photo, it was hard to have a sense of scale but they're massive on their own.
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  #42  
Old 13-02-2020, 03:55 PM
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multiweb (Marc)
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The author at the Keck Visitor Center at Waimea.
This is also the headquarters for the Keck and at 800m altitude is a more
comfortable elevation for many of the workers and astronomers to work
rather than being at the summit, where one's judgement can become
impaired. Waimea is the greener transition zone between the dry and
wet side of the island.

The group at the Visitor Center. The place to pick up your Keck T-shirts and caps.
Great shot to see the opening hours.
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  #43  
Old 13-02-2020, 04:29 PM
gary
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Originally Posted by multiweb View Post
Oh wow. Is the whole telescope design a Ritchey Chrétien or other? Must be a manufacturing nightmare. How do they independantly test segments that have those "exotic" asymetric shapes I imagine yet focus each area of light to the same point? Do they use some kind of scafold in the factory with the elements in their respective position from the optical axis for figuring and testing?
Hi Marc,

It is an RC design.

As I understand it, to fabricate a segment, the clever part was to polish
a blank as a sphere whilst applying specific differing forces around the
edge of the glass. Apparently polishing a sphere is relatively easy
(Mark Suching was here a short while ago and I could have asked him).
When they then released the forces the glass which had been under stress
magically popped into the right shape and was then ready for final figuring.

When they built the AAT back in the 70's it was the first large digital
computer controlled telescope. We take computer control of telescopes
for granted now but at the time the AAT was built it was regarded as a risky
decision. However in the case of the AAT the computer is just performing
the task of pointing and tracking.

With the Kecks, not only is the mount itself computer controlled but
the optics are computer controlled. In order to maintain its figure,
sensors can measure multiple positions on each segment with micron
precision and actuators then in realtime change its position or shape
to compensate as the primary shifts its position as a function of gravity.

It's funny thinking of the actual optics of your telescope being
computer controlled but without it the Kecks would not be able to
achieve their incredible optical performance. These are the telescopes
that first detected extrasolar planets.

One other notable characteristic of the Kecks is their ability to image
into the infrared. The optics, instrumentation and siting so high
up in the dry atmosphere were all designed to achieve this. So a lot
of their observations are done at infrared wavelengths which is precisely
what you want for deeply red-shifted targets.
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  #44  
Old 13-02-2020, 06:45 PM
gary
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Originally Posted by big-blue View Post
Thanks Gary. I enjoyed reading your great write-up of a fantastic trip. Brought back memories of my own trip up there last year, albeit at the mass-tourism level. Even so ( for those of us who are not fortunate enough to have your local connections), I highly recommend the Manu Kea astro viewing tour by 4WD bus that starts with sunset viewing at the observatories, then descends to 9000ft for viewing of glossies via C11. An unexpected highlight for me was the spectacular sunset shadow of MK extending many kms over the cloud base below. The Big Island is an amazing place to visit, on many levels, and deserves to be on everyones bucket list.
Thanks Big-blue!

It is an amazing place.

With the road blockage up the mountain by the protesters up until
only 6 weeks ago, not only were all the observatories shut down for a couple
of months but the operators who offered the summit sunset and astro
adventure tours.

In talking to one of our contacts at the Keck, it set back some of those
tour operators enough that they had not resumed operations.

When we were there, the Visitor Information Station at the 9200' point
had been cleared out of all merchandise and the only thing they had
on offer was self-service instant coffee. They had a couple of solar
scopes set up outside and there was an AP mount sitting in the corner
inside.

Hopefully the operators will get up to speed again as they fill a valuable
niche but the spectre of the protest tent city at the road junction still
remains and with it the possibility that the road might get blocked again
if the truce is broken.

My advice to anyone going to the Big Island is to check in advance.

For dedicated enthusiasts one might also try contacting the West
Hawaii Astronomy Club (WHAC) for additional ideas. Some of the
members work at the Keck.
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  #45  
Old 13-02-2020, 07:09 PM
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Where Captain Cook was killed - Kealakekua Bay, Big Island

On the west coast of the Big Island is Kealakekua Bay, which is the
place where Captain Cook was killed by the native Hawaiians on the
14 February 1779. That will be precisely 241 years ago tomorrow.

I wasn't prepared for what a beautiful place it was. The water is an aqua
blue, there is a steep cliff face that dramatically rises above the water and
there is a rocky black lava rock beach landing.

In some ways it is probably no coincidence that this is such a beautiful
place. The cliff face was the chosen burial place of Hawaiian royalty
and the royals lived there.

In the third photo below you might make out a white obelisk on the
opposite shore which marks the spot where Cook died.

A km away is the town of Captain Cook on Highway 11.
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  #46  
Old 13-02-2020, 08:00 PM
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As I understand it, to fabricate a segment, the clever part was to polish
a blank as a sphere whilst applying specific differing forces around the
edge of the glass. Apparently polishing a sphere is relatively easy
Ha ok. Sounds like what Celestron was doing to mass produce their corrector plates. They had a few bases machined with an inverse profile that had the corrector shape then vacuum the glass on top of it, polish it flat then when released the flat machined face would take the right shape.

https://patents.google.com/patent/US3889431A/en
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  #47  
Old 13-02-2020, 08:38 PM
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Very beautiful place. The Japanese telescope on Mauna Kea ,I remember reading , is also a system relying on computerised actuator control to keeps its mirror within minute tolerances and is also an RC although a single 8m mirror I think. I understood this used an actuator system developed after much research in Japan. I think I have seen an online video of the development Are the systems similar?
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  #48  
Old 13-02-2020, 08:52 PM
Sunfish (Ray)
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As indicated in this link
https://subarutelescope.org/Topics/2...27a/index.html
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  #49  
Old 13-02-2020, 10:58 PM
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Hi Gary & All,

Great reports -- much enjoyed. Sounds like a fantastic time was had by all.

Best,

L.
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  #50  
Old 14-02-2020, 01:02 AM
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astroron (Ron)
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Fantastic report Gary,with images to match.
Makes me want to go back again.
Very jealous of your observing with a 22" at the 9000' mark
I used the 11" at what is now the visitors center 30 years ago.
Helped the night guide out so he let me use the scope for a while
after all the visitors had left.
Definitely an observing trip of a life time for you
Thanks for sharing.
Cheers

Last edited by astroron; 15-02-2020 at 12:05 AM.
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  #51  
Old 14-02-2020, 06:24 PM
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Thanks for sharing Gary stunning images btw .



My recent suggested holiday thoughts ,Active volcanoe for a look

or a cruise arnt very popular any more .


How long does it take to fly there and hows accomadation price wise ?
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  #52  
Old 14-02-2020, 09:35 PM
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Thanks for sharing Gary stunning images btw .

My recent suggested holiday thoughts ,Active volcanoe for a look

or a cruise arnt very popular any more .


How long does it take to fly there and hows accomadation price wise ?
Thanks Graham,

In 2018 Kilauea spectacularly erupted on the Big Island and you didn't
need to be on a boat to see hot lava, though where it flows into the ocean
has been a popular vantage point.

Friends who were observing at Mauna Kea at the time could see the red
glow at night 48km away.

One of our Keck contacts provided jaw dropping accounts of lava eruptions
seen up close.

Alas, Kilauea is dormant at the moment, which is probably okay by the
locals, but a friend was pointing out to me the size of the caldera before
2018 compared to post 2018. Unbelievable how much bigger that hole is!

It is 9.5 hours to fly from Sydney to Honolulu and another 30 minutes
from there to Kona on the Big Island.

Flights between the islands are relatively cheap and when specials
are on for Sydney to Hawaii, can be incredibly cheap as well.

The Australian/US Dollar exchange rate doesn't help Australian visitors at
the moment and most things on the Big Island and more expensive than
on the continental US. For example, fuel prices are Sydney prices.

Food is relatively expensive so in a restaurant you find yourself spending
about USD50 (AUD77) per head rather than the AUD50 you might expect
to pay in Sydney.

However, accommodation on the Big Island is comparatively less expensive
than that on Oahu.

An entire modern three bedroom AirBNB rental house can be had
for USD250 per night.

By comparison, a budget hotel in Waikiki in Oahu will cost that per night.

Last edited by gary; 14-02-2020 at 11:17 PM.
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  #53  
Old 15-02-2020, 02:21 PM
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Great write-up & pictures Gary - thanks for posting.

Also brought back some memories for me as I visited the Big Island & Mauna Kea back in early 2017. It's truly like nowhere else on earth!
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  #54  
Old 15-02-2020, 05:10 PM
gary
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Very beautiful place. The Japanese telescope on Mauna Kea ,I remember reading , is also a system relying on computerised actuator control to keeps its mirror within minute tolerances and is also an RC although a single 8m mirror I think. I understood this used an actuator system developed after much research in Japan. I think I have seen an online video of the development Are the systems similar?
Hi Ray,

Thanks for your various comments throughout the thread which is
appreciated.

The remarkable thing about the Subaru primary is that it is monolithic with
an 8.2m diameter.

The Subaru solves a similar problem in that the optics themselves are computer controlled.

All the next generation large telescopes are opting for the segmented
approach (e.g. GMT, TMT, ELT)

In any case digital computers being crucial to the optics ability
to accurately focus light is here to stay.

Last edited by gary; 16-02-2020 at 07:45 AM.
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  #55  
Old 16-02-2020, 08:50 AM
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Wow!!! I am speechless! Totally without speech!
Thanks for such a detailed report Gary
Amazing opportunity..
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  #56  
Old 16-02-2020, 09:07 AM
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Whata trip, what an adventure, thanks for sharing it with us Gary
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  #57  
Old 16-02-2020, 05:39 PM
Sunfish (Ray)
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Thank you Gary. Excelled your yourself with this report. I suppose the Japanese sensors and actuators on their Subaru mirror on Hawaii was novel 15years ago . A mirror made of individual segments does now seem the choice now rather than a single piece used there. It is interesting that both scopes are RC configurations and used for variety of types of observation. Advances in computer technology perhaps drive these changes.

Last edited by Sunfish; 16-02-2020 at 05:41 PM. Reason: Spelling
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