Great Equatorial ROG

[markse68]

I was lucky enough to have my first look through the Great Equatorial telescope at the Royal Observatory Greenwich last night- lucky because it’s quite a telescope- the largest refractor in the country at 28” clear aperture (!) and it was a clear night!

The astronomer who was operating it mentioned on my questioning something interesting about the objective and i’ve just found this article about its history:

http://www.royalobservatorygreenwich.org/articles.php?article=1174

 

“The object-glass of 28 inches clear aperture and 27 ft. 10 in. focal length was made by Sir Howard Grubb from discs supplied by Messrs. Chance, of Birmingham. It is of special form, adapted to photography as well as to eye-observation, on the plan proposed by Sir George Stokes (in a letter to me dated 1886 August 16) of reversing the crown lens to correct for the spherical aberration introduced by the further separation of the lenses necessary for photographic correction. Mechanical means are provided for readily effecting this reversal and separation without risk. It is found by trial that the further separation required for photographic correction is about 3.5 inches, and that the focus is thereby shortened by about 23 inches. When the crown lens is in the position for visual observation, the radii of curvature of the surfaces are:-

Crown

1st surface146inchesconvex

2ndsurface134inchesconvex

Flint

3rdsurface138inchesconcave

4thsurface1000inchesconvex

and the edges of the lenses are separated by 0.4 inch, the minimum focal length being for rays about midway between D and E.“

My question to the knowledgeable is why would it have needed this ability to be reconfigured for visual or photographic? Seems they abandoned the photographic soon after commissioning in favour of a dedicated photographic scope anyway though.

Mark

[Cosmic Geoff]

Despite having visited the Greenwich Observatory some years ago, I was unaware that such a large refractor was there, or that it had been at Herstmonceux (which I have also visited).  A most interesting post and links.

[Stu]

Looks amazing @markse68! What did you look at? I guess visually you are limited in where you can look or did they have a ladder available??

I’m sure someone clever will know, but I can’t think why the configuration for photography wouldn’t work with for visual as well.

[markse68]

Hi Stu, it was an amazing opportunity and very exciting but alas a little disappointing visually. We had been promised Jupiter which i was really excited about but it proved to be too low at the early hour we were there and couldn’t be reached. Unfortunately the scope is a bit of a kludge- in the astronomy arms race of the time they decided they needed this huge scope but to save costs they decided to fit it in the mount and dome of the previously installed 12” scope. The dome is too small really and limits southerly views to 30 deg above the horizon. The northern sky is almost completely blocked by the design of the English equatorial mount. I really would have liked to have a go at Dubhe as i’ve failed so far to split it with my 8” dob, but it wasn’t reachable. Suggestions for v difficult doubles in southern higher altitudes appreciated for next time

We did look at M15 but couldn’t resolve stars which surprised me. Apparently the objective hasn’t been cleaned for 10yrs so that might have had something to do with it.

M31 was just a blur of the core with a 50mm ep. The fov of the scope is less than a moon disk so no wide field views- it was designed for double star study after all.

Albireo looked more colourful through the tiny little 6” finder (  ) though it was seen as pinpoint stars at far greater separation than i’ve seen through mine. Chromatic aberration was very much in evidence though.

Uranus was a very convincing blue green disk 

I wish we could have seen Jupiter- it’s getting too low now i think- maybe next year. Mars would be amazing i’m sure next year. The moon must be incredible too.

I wonder if they purposely compensated for the then current eyepiece design limitations so deliberately designed in field curvature for visual and converted to flatter field for photography? 🤔

[Captain Scarlet]

Is that you in the photo Mark? 😄

[Stu]

The seat looks more like a torture chair!

[markse68]

Just now, Captain Magenta said:

Is that you in the photo Mark? 😄

Haha- it was too dark for me to get a photo last night and alas they got rid of the comfy chair- we had to crawl around on hand and knees for M31 😂

[Cosmic Geoff]

Can general visitors to the RGO see the Great Equatorial Telescope? I had a good look online but am none the wiser.

[markse68]

Hi Geoff, I don't know about daytime to just look at the scope/dome but I'd have thought so? They have "Evenings with the stars" for observing but it's a bit pricey especially with no refund if weather doesn't play ball:

https://www.rmg.co.uk/whats-on/royal-observatory/evening-stars

Mark

[markse68]

The need for a different arrangement of the objective lenses for photographic use seems to have been due to the sensitivity of available photographic plates and optimising for them:

https://en.m.wikisource.org/wiki/Page:Encyclopædia_Britannica,_Ninth_Edition,_v._23.djvu/154

Mark