Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

How to keep older borosilicate professional telescopes competitive?

  • Please log in to reply
1 reply to this topic

#1 Lucullus

Lucullus

    Viking 1

  • -----
  • topic starter
  • Posts: 609
  • Joined: 14 Feb 2012

Posted 08 August 2019 - 04:34 PM

The "Big Eye", the Palomar 5m Hale Telescope, was built with a Pyrex mirror and it was probably the glas with the best thermal expansion coefficient behaviour at that time. Several materials with negligible coefficients were invented since then, e.g. Zerodur in the late 60ies. The MMT also had borosilicate mirrors until 1998. Admittedly, the Hale and MMT have/had honeycomb mirrors to decrease thermal acclimatisation times. The William Herschel 4.2m, the Blanco 4m, the Anglo-Australian Telescope 3.9m, and CFHT all have mirrors made from Cer-Vit in the late 60ies - according to the "Optics" Chapter of the William Herschel Telescope (version at the time of this writing) - similar to ultra low expansion materials and, thus, much better concerning thermal expansion properties. The 4m Mayall telescope, first light 1973, also has a mirror of the advantageous material fused quartz. The ESO 3.6m Telescope in La Silla started it's life also with a fused silica mirror in 1976. So it seems that the professional large telescopes onwards from the 60ies didn't use borosilicate-like materials anymore. Apart from the Hale, the Shane Telescope 3m mirror also has Pyrex - actually it was a testing blank for the Hale. Maybe it's probably the last huge (in amateur perspective!) telescope being commissioned with such a material in 1959. E.g. the Otto Struve 2.1m telescope (1939-...) also has a Pyrex mirror.

I could not find out what material the 3m NASA IRTF mirror is made of, but it's probably of a similar material. To see what other mirrors of telescopes built in the 60ies were made of I tried to find out about the UH88 and 2.3m Bok telescope but could not find out what they are made of. But the 2.7m Harlan J. Smith telescope is made from fused silica. In general, there are not so many 2-3m class telescopes to read about in Wiki, which was my main information source and from where I went on to get more information.

When looking for information what the 100-inch Hooker Telescope is made of I only found in several websites that it is glass. Despite it being 2.5m in diameter it is now the largest telescope solely dedicated to public observing.

 

Anyway, I am sure there are still many professionaly involved active telescopes in the 1-3m class with borosilicate-like glass and without the advantage of honeycomb structure. Nevertheless, I guess they have incorporated ways to mitigate the thermal expansion disadvantages in order to still be scientifically productive besides student training purposes and be competitive against more modern ultra-low expansion mirror telescopes of similar size. I am wondering what these improvements are; water cooling tubes aroung the mirror or vans blowing air across/at the backside of the mirror to make it reach environmental temperatures faster and increase observing time?



#2 EJN

EJN

    Skylab

  • *****
  • Posts: 4400
  • Joined: 01 Nov 2005
  • Loc: Between what is and what's not there

Posted 09 August 2019 - 04:46 PM

When looking for information what the 100-inch Hooker Telescope is made of I only found in several websites that it is glass.


Plate glass. Poured in 3 layers. With lots of bubbles in it at the layer interfaces.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics