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Group 70

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#26 JohnH

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Posted 11 April 2019 - 08:55 PM

I too saw the original group 70 at articles in Sky & telescope from Circa early 90s. By a coincidence I was staying cross the bay during the Loma Prieta earthquake, and I was watching a baseball game when the power went out and then I heard a couple of car alarms go off and then some excited people running by and then I went out to place where I could see across the bay I could see Columns of smoke starting to rise from the burning buildings over there where the epicenter was

#27 Lucullus

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Posted 18 April 2019 - 08:34 AM

I thought this very mirror was passed on from the group 70 team to Mike Clements who uses it as a Dobson now?



#28 GShaffer

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Posted 18 April 2019 - 09:15 AM

I thought this very mirror was passed on from the group 70 team to Mike Clements who uses it as a Dobson now?

 

While its possible there may be some relationship regarding the origins of the two mirrors (dont know), they are indeed not the same mirror.


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#29 Chuck Hards

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Posted 18 April 2019 - 09:45 AM

I thought this very mirror was passed on from the group 70 team to Mike Clements who uses it as a Dobson now?

 

No.  Mike's mirror came from US government surplus.  It is a "lightweight" honeycomb-style mirror (only 900 lbs.) originally made for spaceflight on a surveillance satellite.  The Group 70 mirror is monolithic, poured glass that was made before WW2, if memory serves.


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#30 Oregon-raybender

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Posted 18 April 2019 - 10:50 AM

The understanding was the blank was for a 48 inch Schmidt for the Southern skies. A 72 inch diameter, made at Corning Glass back in the 40's. Fire bricks were used to form a slight curve. Beyond that Kevin may know more.

 

Starry Nightswaytogo.gif


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#31 gnabgib

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Posted 14 May 2019 - 03:17 PM

Sorry I have been absent for awhile.  My mom came down with pneumonia (she's 93) and managed to scare my sister and I really well.  She has pulled thru and is back on her feet!    I thought a little update is in order.  We have completed the welding of the  "mirror box" and yesterday drove to Benicia Ca.  to deliver it to Unico for machining of the side plates that will be  the attachment points for the altitude trunnions.  The "small milling machine" to the left of the truck is the mill they will be using.  I was informed that this building is they're small machine shop!  Nice to know these places still exist in the U.S.A.  Hereare a few picts.  Enjoy!

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Edited by gnabgib, 14 May 2019 - 03:19 PM.

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#32 mcolbert

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Posted 14 May 2019 - 04:29 PM

Chuck,

   I am not sure of the dates but we believe the mirror blank was one of two cast for the 48 inch schmidt camera .  With the successful completion of the schmidt camera the second blank was in storage for several years when Dr.Theodore Dunham Jr. who I believe was with Palomar Observatories acquired it.   It was shipped to Hobart, Tasmania where Dr. Dunham planned to build a large southern observatory. He managed to build a 40 inch telescope but the 70 inch also languished due to lack of funds.  This "blank" was not the edged, ground, and generated blanks we see today.  When we brought it back to the states and un-crated it the impressions of the firebricks used to make the mold were clearly visible.  It was very rough.  If I remember correctly it took over 2 months to edge it round using a diamond cup wheel on a 1 hp motor.  The back was then ground flat using a steel manhole cover (don't ask!) and starting with 40 grit.  The first few strokes over that rough surface were nerve racking.  But we prevailed.  Finally after several months the blank was flipped over and the optical work began.  The curve molded in the blank required removing about 3/4 inch from the edge to achieve the desired radius.  When washing off a 70 inch f/3 mirror it takes a while to "fill it up" with water.    By the way before any work began we first had to build a grinding machine.  This mirror is like a cat.  It seems to have "many lives".  Shipping it back from Tasmania was straight forward except for the "dock strike" that delayed shipping several weeks.  When it got to San Francisco the import paperwork was straightforward except the paperwork got lost and required a bit of back-stepping to complete.  Finally it was released so we drove over to the port of San Frisco with a truck to pick it up.  The trip home took us across the bay bridge to Oakland, Ca.  This was 2 days before the Loma Prieta earthquake took out the bridge.  A charmed life?  This was in 1989.  If anyone has the Sky and Tel issues for 1988 perhaps you could find the original ad offering this blank for sale. 

 

Kevin

and therein my friends lies a sad comment about many projects that could have been in australia.  (not meant as a political comment, more one about what can be lost to any country whether object or expertise).  The upside is that it has found life with Group 70.wink.gif


Edited by mcolbert, 14 May 2019 - 04:30 PM.


#33 tim53

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Posted 14 May 2019 - 07:22 PM

Sorry I have been absent for awhile.  My mom came down with pneumonia (she's 93) and managed to scare my sister and I really well.  She has pulled thru and is back on her feet!    I thought a little update is in order.  We have completed the welding of the  "mirror box" and yesterday drove to Benicia Ca.  to deliver it to Unico for machining of the side plates that will be  the attachment points for the altitude trunnions.  The "small milling machine" to the left of the truck is the mill they will be using.  I was informed that this building is they're small machine shop!  Nice to know these places still exist in the U.S.A.  Hereare a few picts.  Enjoy!

Gadzooks! You could mill a car out of solid stock with that thing!


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#34 JohnH

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Posted 15 May 2019 - 08:12 AM

How is it in San Francisco when the Loma Prieta earthquake happened. I have also recalled seeing this telescope project being mentioned in Sky & telescope from time to time in fact it was just in a bunch of back issues I was reading off a few weeks ago

#35 gnabgib

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Posted 15 May 2019 - 11:02 AM

How is it in San Francisco when the Loma Prieta earthquake happened. I have also recalled seeing this telescope project being mentioned in Sky & telescope from time to time in fact it was just in a bunch of back issues I was reading off a few weeks ago

     The blank was purchased in late 1988 after a quick trip to Hobart, Tasmania.  Shipping took several months due to a dock strike.  It arrived at the port of San Francisco  late September 1989 and it took a few weeks of wrangling with customs to clear.  When the paperwork finally cleared we drove over to the impound warehouse and picked up the crate.  This was a few days before the Loma Prieta quake hit the bay area.  YES that was 29 years ago!!  The shaping, grinding, polishing, and figuring took place over the next several years.  In or around 1997 the mirror was crated and stored .  There has been a 20 year hiatus while the membership took care of careers, families, themselves, etc.  Life can due that to you.  A few years ago (5 maybe) the optician who oversaw the finial figuring of the mirror breathed new life into the project.  I was temporarily working in Fairbanks when I got the call.  We got the band  back together and started to make plans to proceed.  So after some growing up here we are.  Right now the telescope construction is our major push.  Estimating 2 years to construct (volunteer effort)!  Were 6 months into that with the tube assembly estimate for completion in June this year.  The mirror cell is being fabricated at Bulling Metal Works right now and the fork, azimuth base and roller drives are under design review.  We are approaching this as a design/build project.  That way minor construction errors are taken into account as the design proceeds.  

 

Kevin


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#36 Bob4BVM

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Posted 16 May 2019 - 12:21 PM

Wonderful story, so full of adventure, and life !

Thanks again for sharing these updates with the ATM community

All the best to the Group 70 team !

Bob



#37 Stardust Dave

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Posted 16 May 2019 - 08:09 PM

"There has been a 20 year hiatus while the membership took care of careers, families, themselves, etc.  Life can due that to you." 
Indeed it can.  

Been reading about group 70 for what ,about 30 years now. Be great to see it completed and look through it.



#38 Ruffin

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Posted 19 May 2019 - 04:15 PM

Great to see that this project is rolling again! I too had heard about it in the early '90s. 


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