Four
apertures.
You will look through at least two of them on a public night, and the warden chooses which two from the seeing, not from a rota. Aperture buys faint; focal ratio buys field; steadiness buys detail — and on most nights in these islands it is steadiness that is short.
610 mm
Ritchey-Chrétien, f/8
Alt-az fork, direct-drive. The one you queue for.
355 mm
Newtonian, f/4.5
On the east pier. Wide, fast, forgiving of a beginner’s eye.
127 mm
Apochromat, f/7
Rolled out to the lawn. Doubles, the moon, and Saturn at 180x.
Ha 90 mm
Solar, double-stacked
Daytime only. Prominences at 0.5 angstrom.
610 mm
The one you queue for, and the reason the programme runs to transit times rather than to the clock. It is a hyperbolic-hyperbolic pair, so the field is flat to the corners and there is no coma to argue with — what it gives up is a long cool-down, which is why the dome comes off at dusk and not at dark. On a 2/5 night it will split a 0.4″ double. On a 4/5 night it will not, and the warden will move you to the 355 rather than let you look at a boiling star.
| Focal length | 4,880 mm |
| Dawes limit | 0.19″ |
| Limiting magnitude | 15.6 visual |
| Mount | Alt-az fork, direct-drive |
| Dome | 5.5 m, slit 1.4 m |
| Commissioned | 2004 |
355 mm
Wide, fast, and forgiving of a beginner’s eye — it is the instrument the Double Cluster wants, because at f/4.5 you get the whole of both in one field and the aperture is not the thing that matters. It is also the one that keeps working when the seeing collapses, since a low-power wide field does not care about a shaking atmosphere the way 210× does.
| Focal length | 1,600 mm |
| Dawes limit | 0.33″ |
| Limiting magnitude | 14.4 visual |
| Mount | German equatorial, belt drive |
| Housing | Roll-off shed, east pier |
| Commissioned | 1998 |
127 mm
Doubles, the moon, and Saturn at 180×. Triplet, so there is no false colour on a first-magnitude star and the rings arrive with an edge on them. It is on the lawn because the lawn is where people are willing to stand and talk, and because a scope you can put your own hands on is worth more to a first visit than one you have to be driven to.
| Focal length | 889 mm |
| Dawes limit | 0.91″ |
| Limiting magnitude | 12.6 visual |
| Mount | Alt-az, manual, on the lawn |
| Housing | Wheeled out, stored in the hut |
| Commissioned | 2011 |
Ha 90 mm
Daytime only, and never left standing. Double-stacked to 0.5 Å, which is narrow enough that the disc goes from blank to granular and the prominences come off the limb as structure rather than as a rim. It is the instrument that does the school visits, because a school visit happens at eleven in the morning.
| Bandpass | 0.5 Å at 656.28 nm |
| Focal length | 600 mm |
| Mount | Alt-az, lawn tripod |
| Use | Daytime only, warden present |
| Housing | Hut safe |
| Commissioned | 2016 |
What goes
in the back.
| Eyepiece | Barrel | On the 610 | True field | When it comes out |
|---|---|---|---|---|
| 31 mm | 2″ | 157× | 0.51° | Sweeping, and the only one used on M31 |
| 17 mm | 1.25″ | 287× | 0.29° | The default on the 610 |
| 9 mm | 1.25″ | 542× | 0.15° | Doubles and planets, 2/5 nights only |
| 6 mm | 1.25″ | 813× | 0.10° | Twice a year, if that |
The 6 mm exists so that it can be shown not to work. Twice a year the air is steady enough to carry 813× on a bright double; the rest of the time putting it in makes the star bigger and worse, which is the single most useful thing a visitor can be shown about magnification.
Maintenance · Thrale Field Astronomical Association
The 610’s primary was last recoated in 2019 and reads 94 % at 550 nm. Collimation is checked on the first clear night of each month against a second-magnitude star at 542× and logged on the sheet; it has needed adjustment four times since the recoat.