The image (a 2016 photograph) needs a caption. Alternatively I would suggest using this, which shows the observatory as it once appeared when in use. If you use it, you don't need the link in the External links section.
Caption added; I prefer the modern image for the lead. Praemonitus (talk) 16:04, 31 December 2022 (UTC)[reply]
N The section is split into four subsections, but only three are given titles. I would give the first section a title as well (e.g. Background).
I'm not seeing anything in the MOS about requiring a header for this introductory section. I think that just adds clutter. Praemonitus (talk) 18:41, 31 December 2022 (UTC)[reply]
My apologies for not being clearer. You won't a requirement (see MOS:SNO), but not all the text down to the subsection D. O. Mills Expedition relates directly to history of the observatory, which is what readers might expect when they see the title 'History'. The idea of a southern hemisphere observatory appears at When Campbell brought the need for a southern observatory.... Everything before this should be in a different (sub)section, and it would make sense if the text starting from this place was in the D. O. Mills Expedition. Amitchell125 (talk) 12:38, 1 January 2023 (UTC)[reply]
Sorry, but I don't agree with you on this. A paragraph break at that point would be inappropriate. The 'D. O. Expedition' section is about the actual expedition, not the motivation, planning, and preparation. (I ended up changing the section title, which is regrettable because the original was better.) Praemonitus (talk) 19:53, 2 January 2023 (UTC)[reply]
Improve the prose slightly by adding a comma after use in May 1895.
I modified the sentence slightly (see next). Praemonitus (talk) 18:41, 31 December 2022 (UTC)[reply]
it proved highly successful – replace it with what is being referred to. (Wherever it, its or a pronoun occurs, you need to check that readers can easily understand what the subject being referred to is.)
On August 12, 1900, the director of the Lick observatory, James Edward Keeler, suddenly died - ‘The director of the Lick observatory, James Edward Keeler, died on 12 August 1900’?
What followed was a delay of a month – ‘After a month-long delay’ sounds better imo.
That sentence wouldn't make sense. Praemonitus (talk) 19:27, 2 January 2023 (UTC)[reply]
Clearly. To improve the quality of the prose, I meant to amend to something like 'After a month-long delay due to a riotous strike in the port, he gear was unloaded, and then transported...". Amitchell125 (talk) 15:42, 3 January 2023 (UTC)[reply]
Changed, reluctantly. I thought it sounded fine when read out loud and it better emphasized the impact of the riot. Praemonitus (talk) 15:51, 3 January 2023 (UTC)[reply]
June 1, 1904 – ‘1 June 1904’ removes a comma.
I moved the date to the end of the sentence. Praemonitus (talk) 19:27, 2 January 2023 (UTC)[reply]
Twenty two – should be ‘22’, for the sake of consistency. I suggest amending the text to something like ‘The astronomers discovered 22 stars with variable radial velocities’.
Wilson was assisted by math instructor Arthur A. Scott from the Instituto Ingles in Santiago, beginning in 1913 until he resigned June, 1917, then by Charles M. Huffer- seems excessively detailed for this article.
technical and economic problems – some explanation is needed here, as it isn’t clear why the observatory was able to continue working after 1982, having (according to the text) been closed for four decades.
Unfortunately there isn't much additional information available. I changed the wording slightly to avoid saying 'closure'. Praemonitus (talk) 16:27, 31 December 2022 (UTC)[reply]
These observations became degraded over time – is an ambiguous statement (the early records deteriorated? It became more difficult to make good enough measurements?)
What has happened since 1995, apart from it becoming a monument? See https://astro.uc.cl/en/observatorio-foster/ (and possibly elsewhere) for more information that needs to be added.
Thanks for the link. I added a few more details, but other links don't seem to be as reliable. Praemonitus (talk) 16:45, 3 January 2023 (UTC)[reply]
Not GA, but it’s worth noting that when I last counted them, 25 of the sources listed in this section (and three in the section below) were over half a century old. Whilst they seem to be reliable enough, more recent sources might be preferable, if they exist (see WP:OLDSOURCES).
I'll just note that this is primarily a history article that uses the original sources for tactical details. The newer sources mostly cover it at a higher level. Sorry but I can't really address this in a significant way. Praemonitus (talk) 15:37, 31 December 2022 (UTC)[reply]
I'm putting the article on hold for a week until 8 January to allow time for the issues raised to be addressed. Regards, Amitchell125 (talk) 14:00, 31 December 2022 (UTC)[reply]
Thanks for the work you've started. I've begun to cross out issued that are addressed. Amitchell125 (talk) 12:22, 1 January 2023 (UTC)[reply]
Thank you for your detailed review. Praemonitus (talk) 16:58, 3 January 2023 (UTC)[reply]
And thanks for your work on the article. I'll be adding a small cross (N) by any comments not crossed out, and coming back to them later. Amitchell125 (talk) 17:41, 3 January 2023 (UTC)[reply]
I can happily live with the article as it now stands—it's clearly now a GA. Congratulations and thanks. Amitchell125 (talk) 18:35, 3 January 2023 (UTC)[reply]