File talk:Comparison optical telescope primary mirrors.svg

Mistake in VLT location
"Very Large Telescope - Mauna Kea, Hawaii". Isn't it in Chile?
 * Fixed! cm&#610;&#671;ee&#9094;&#964;a&#671;&#954; 14:16, 12 July 2014 (UTC)

Sizes of mirrors' central holes
Actually the data is out there. The following should be good enough for illustration purposes. These sizes are of the primary mirror's central hole. This is normally in the shadow of some larger central obscuration, such as a secondary mirror or prime focus instrument, which sets the real limit on light-gathering power. But some telescopes (especially the MMT and LBT) have interchangeable obscurations, while the central hole is at least a permanent feature.

Where both are available, I quote the mechanical hole size, which is usually a cm or so inside the innermost optical surface. This is to be consistent with outer diameter, which also normally quoted as the mechanical size. (E.g. Gemini's light-collecting area is 8000 mm diameter, but it's normally called an "8.1 m" telescope) In the case of BTA-6, I was reduced to measuring a high-resolution picture; given that it was designed pre-Internet, as a cold war prestige project, and in Russian, I don't have a lot of hope of finding design documents.

71.41.210.146 (talk) 07:55, 28 July 2014 (UTC)
 * Kepler: No central opening (prime focus) Ref: http://www.esmats.eu/amspapers/pastpapers/pdfs/2008/koski.pdf http://www.nasa.gov/mission_pages/kepler/multimedia/images/PMA_backside_inspection_1_at_SOC.html
 * Hubble: 0.7 m Ref: p. 8 of http://science.nasa.gov/media/medialibrary/2012/11/19/Matthews-NRO-20Telescope-20Summary20-20Approved.pdf
 * Hooker 100 inch: No central hole. Ref: http://www.mtwilson.edu/vir/100/mirror/
 * 200-inch Hale telescope: central aperture 40 inches. Ref: http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/hale/page3.php
 * BTA-6: 38 cm. (Not used optically; Prime or Naysmith focus in front of primary only.) Ref: http://lzos.ru/en/images/2_08_11.jpg http://lzos.ru/en/index.php?option=com_content&task=view&id=209&Itemid=2
 * Measuring that picture, I get mirror: 1154 px wide, Hole (incl. chamfer): 73 px. With an outer diameter of 6.05m, that's 38.3 cm.
 * BTA-6 has two primary mirrors. The above is the original, flawed mirror that was replaced in 1978, being prepared for regrinding starting in 2008, after which it was put back into service.  The grinding removed 8 mm of material, which may have shrunk the chamfered hole by 16 mm or so
 * Prime focus cage (central obscuration) is 2m diameter. Ref: http://www.sao.ru/Doc-en/Telescopes/bta/descrip.html
 * Large Zenith telescope: No hole (liquid mirror, prime focus camera). Ref: http://www.astro.ubc.ca/lmt/lzt/
 * MMT: 889 mm Ref: p. 4 of http://www.mmto.org/MMTpapers/pdfs/ctm/ctm96-2.pdf
 * Magellan: ≈889 mm. I can't find a number, but it's the same size and curvature (f/1.25) as the MMT telescope, made at the same time in the same facility (Seward Observatory Mirror Lab), the sources I can find describe them almost interchangably, and visually they look the same size, which should be fine for an illustration.  Ref: pp. 4–5 of http://www.mmto.org/MMTpapers/pdfs/tops99.2.pdf
 * Gemini telescope: 1180 mm. Optical stops at 8000 & 1220 mm.  Ref: http://www.gemini.edu/documentation/webdocs/spe/spe-o-g0006.pdf
 * Very large telescope: 1000 mm Ref: https://www.eso.org/sci/facilities/paranal/telescopes/ut/m1unit.html
 * Subaru telescope: Central hole 1.2m diameter. Ref: http://www.science-bbs.com/16-astro/bc334c94951dcf49.htm
 * Central obscuration is larger, 30% of 8.2 m = 2.46 m Ref: http://www.naoj.org/Projects/SCEXAO/01project/01srp/01srp_nomenu.html
 * LBT: 889 mm Ref: http://www.lbto.org/optics.htm
 * LSST: 5066 mm (joint line with M3; 5116 mm edge of optical surface). Ref: p. 3 of http://www.loft.optics.arizona.edu/documents/journal_articles/Michael_Tuell_Optical_testing_of_the_LSST_combined_primary_tertiary_mirror.pdf, p. 2 of http://www.inst.bnl.gov/LSST/PDF/SepF2F_BNL.pdf
 * GMT: 2.5 m Ref: p. 6-55 of http://www.gmto.org/slpdr/GMT_SLPDR_Section_06_Telescope.pdf
 * p. 6-48 of that document says 2.4 m, but that's the optical path. The baffle described on p. 6-55 occupies the extra 10 cm diameter (5 cm thick)
 * Off-axis segments have a 50mm central drain hole. Ref: p. 6-449.
 * 2006 conceptual design report says 1.78 m "but may increase". Ref: p.6-2 of http://gmto.as.utexas.edu/science-conceptu.html  Ref above is December 2013, so I'll go with it.
 * The mirror hasn't been cast yet, so technically they still have time to change their minds.
 * OWL: 35 m Ref: §6.2.2 (pp. 194–195, pp. 10–11 of pdf) of https://www.eso.org/sci/facilities/eelt/owl/Blue_Book/6_Telescope_optics.pdf


 * Thank you very much for your excellent work, user at 71.41.210.146. I've incorporated your feedback as on the right. Please let me know if you have further thoughts, e.g. if I missed out any important optical telescopes. Have a nice day, cm&#610;&#671;ee&#9094;&#964;a&#671;&#954; 12:57, 28 July 2014 (UTC)


 * No problem; it was fun, and you did all the hard work. Most were easy, although MMT and Hubble were a PITA to track down.  There are always more telescopes (Spitzer, Sloan Digital Sky Survey, Pan-STARRS, solar telescopes, notable historical ones like Herschel's 40-foot telescope and the Leviathan of Parsonstown), but you have a good assortment and I don't want to clutter the diagram more.  The focus seems to be showing the upcoming generation of extremely large telescopes in comparison to current telescopes (8m monolithic, 10m segmented).
 * The one esthetic comment I'd make is that the LZT looks pale, like it's a planned telescope in China. The TMT, on the other hand, could maybe be made a smidge more pale so it's more distinguishable from existing U.S. telescopes.
 * Oh! The text "lens at same scale" is longer than necessary.  Just "(refractor)" or "(lens)" will do.  Likewise for the sports courts and human; the picture is obviously about size comparisons and there is an explicit scale.
 * On the left-hand side, I might move BTA-6 above MMT, and Yerkes above the Paris Exhiition so they're in chronological order.
 * On a positive note, I really like the dotted lines showing interforometric-mode area!
 * 71.41.210.146 (talk) 09:52, 29 July 2014 (UTC)

What does the colour-coding mean?
What does the colour-coding mean? I can't seem to find that info... Fig (talk) 23:28, 17 January 2015 (UTC)


 * It's the country the telescope is in. Blue = USA (mostly Hawaii), Green = Chile, Orange = South Africa, Yellow = Spain (Canary Islands), Red = China, Pink = Canada, Dark blue = USSR, Black = Space.  Lighter shades mean "not built yet". 71.41.210.146 (talk) 01:21, 18 January 2015 (UTC)

SDSS telescope (moved to separate section)
I should declare a conflict of interest before I make this comment - I'm the Director of EPO for Sloan Digital Sky Surveys. But having said that, I think I can objectively say that the SDSS 2.5m telescope is a telescope which has done a lot of astronomy - for example made images of 1/3rd of the sky openly available, and measured redshifts for millions of objects. I also think it's impressive how small the telescope actually is. Details here: http://adsabs.harvard.edu/abs/2006AJ....131.2332G it's a 2.5m primary, with a central hole which I if I understand Table 1 in that reference right has a 72cm diameter hole. Would be awesome if you'd add it to your graphic. KarenLMasters (talk) 15:48, 18 September 2014 (UTC)
 * Thanks for your interest in the graphic, Karen. The criteria I used to include a telescope were


 * Its size (for its time)
 * Its novelty (e.g. liquid/segmented mirror)
 * Its location (e.g. space)


 * Based on the above, while I can appreciate the contribution SDSS has done for science, I could not personally find a strong enough reason for its inclusion, especially since US telescopes already dominate the selection. I know that it's not fair to compare the size of a survey telescope with the 8+ m monsters but 2.5 m is really just a bit larger than the 100" Hooker from 1917!


 * However, I'll leave the issue open. If several other editors show strong support for it, I'll put it in.


 * Cheers, cm&#610;&#671;ee&#9094;&#964;a&#671;&#954; 00:22, 10 May 2015 (UTC)


 * FWIW, I agree with Karen that it's an extremely significant telescope (third-party confirmation), and the fact that it's the same size as the Hooker is the entire point; the current illustration leaves the incorrect impression that only 8m+ telescopes are significant these days. (Another significant smaller telescope is Pan-STARRS.  I'd list BICEP2/Keck array, but it's not optical.)  My problem is that the diagram (whose overall size is set by OWL) is already full and something else has to be deleted.  LZT is the least significant astronomically, but it's interesting and as you say reduces U.S. bias.


 * One possibility is to reformat the text labels of BTA-6 and LZT and place them side by side. Another is to reduce the precision of the location.  Which mountain in Hawaii or Chile is not very important.  I could put the scale bars side-by-side on the top edge of the basketball court and put the human inside the E-ELT central hole.  Nobody would blame you for abbreviating LAMOST (or renaming it "Guo Shoujing Telescope").


 * But that's only if you want to revisit it; the illustration is fine as is. 71.41.210.146 (talk) 04:55, 21 June 2015 (UTC)

Five hundred meter Aperture Spherical Telescope
Need to include China's Five hundred meter Aperture Spherical Telescope, which outclasses Arecibo, the largest telescope in the image. Badon (talk) 08:00, 8 October 2016 (UTC)


 * RATAN-600 is also notable. Badon (talk) 08:02, 8 October 2016 (UTC)


 * Those are radio telescopes. This is, as the title says, a comparison of optical telescopes. 23.83.37.241 (talk) 00:14, 21 April 2018 (UTC)

MMT Observatory Labelling
The MMT Observatory has not gone by the name of Multiple Mirror Telescope since 1998, since it no longer has multiple mirrors. In fact, MMT is no longer an acronym for "Multiple Mirror Telescope" at all, since the name change (MMT just *is* the name). I think the label should be updated to reflect the new name. DenebVegaAltair (talk) 23:43, 30 August 2020 (UTC)

Mirrors for Keck and GTC went missing
For the Keck telescopes and the GTC the figure does not show the actual segmented mirrors anymore. This was included in an early version but these mirrors appear to have been lost at some point. For the Keck, the circle showing the effective aperture of both telescopes is still visible (not really applicable since the Kecks were never really used in conjunction) is still visible but that's all. I guess this requires fixing... And btw: the description of the figure states that the four UTs of the VLT are not used as a single 16m telescope. This statement is outdated. Since November 2018, the new high-resolution spectrograph ESPRESSO located at the incoherently-combined Coude focus can be operated in 4UT-mode, using the light of all four 8.2m telescopes :-) --2001:16B8:118E:E200:232C:8D80:D9DE:CC65 (talk) 00:12, 20 December 2020 (UTC)

Herschel
The selection of telescopes is good. However, what might be missing is the Herschel Space Observatory. Although a mid- to far-IR telescope, the mirror is not too different from a mirror used for optical telescopes and it is with a diameter of 3.5m indeed the largest space telescope flown so far (as of 2020). This might make it significant enough to be shown alongside HST and JWST. --2001:16B8:118E:E200:232C:8D80:D9DE:CC65 (talk) 00:20, 20 December 2020 (UTC)

Keck is missing
The mirrors for the Keck telescopes are missing from the labelled version of the diagram, leaving only the dotted outline. They do appear in the unlabeled version of the image. Further, is no label for the OLT in any of the images, it's just floating there in the background. — Preceding unsigned comment added by 108.168.93.53 (talk) 13:55, 27 January 2021 (UTC)
 * For some reason, PNG previews have lots of stuff missing. For starters, Gran Telescopio Canarias doesn't show at all. If you click the main pic for the proper .svg file, it shows correctly. --2001:14BA:3180:5098:0:0:0:1 (talk) 14:21, 14 February 2021 (UTC)

Summary Description
Many years ago I recall encountering this illustration, and at that time the accompanying text description provided a legend for the colours used, and an explanation of the three largest (grey) curves, two of which only show as sections of arc (as I recall he full grey curve is for the Overwhelmingly Large Telescope, 100m, and the other two may have been Arecibo and FAST, but I have forgotten now. Anyway, various telescope articles link to this image, and it would be of great benefit if the "Summary: Description", which is visible from all those articles, could be edited to include this information again. Thanks 2001:56A:F0E9:9B00:8F8:B5A4:CF7D:E84F (talk) 05:44, 1 February 2021 (UTC)JustSomeWikiReader

SOFIA
Plenty of telescopes have been launched into space, but only one has ever been mounted on a jumbo jet. How about including it? 89.168.67.61 (talk) 00:14, 21 June 2021 (UTC)

Imaging Atmospheric Cherenkov Telescope
A very visual comparison of light reflectors among themselves and with some others. Probably makes sense to add more large reflectors used to observe flash of Cherenkov radiation in the Earth's atmosphere from gamma ray: CTA 8 of 23 m and 40 of 12 m, H.E.S.S. one 28 m and 4 of 12 m, MAGIC 2 of 17 m, VERITAS 4 of 12 m. This will pay attention to the use of large reflectors in order to differ from direct optical observations. --Voproshatel (talk) 09:23, 30 July 2021 (UTC)