<p>Micrometers and <a href="https://en.wikipedia.org/wiki/Blink_comparator" rel="nofollow">blink comparators</a> allowed considerable precision.</p>

<p>Pluto was discovered with the aid of the latter.</p>

CC BY-SA 3.0
2016-04-03T15:38:42.287Z
2016-04-03T15:38:42.287Z
Wayfaring Stranger
<p>Prior to digital imaging then photographic plate negatives were analysed with scanning microdensitometers to produce astrometric catalogues. Many of these catalogues are still in use today, they are valuable sources of early epoch positions that enable proper motion measurements.</p>

<p>For more details you could look at the descriptions of the SuperCosmos project <a href="http://ssa.roe.ac.uk//" rel="noreferrer">http://ssa.roe.ac.uk//</a> (which is based on Schmidt plates) or the UCAC4 catalogue, <a href="http://ad.usno.navy.mil/ucac/readme_u4v5" rel="noreferrer">http://ad.usno.navy.mil/ucac/readme_u4v5</a>, which uses plates to get positions and proper motions for faint stars.</p>

<p>In the good old days, before even these catalogues existed, astronomy groups would have copies of the whole Schmidt and Palomar sky surveys. You would put the relevant plate on a massive, concrete-based, hydraulic X,Y measuring machine, with a binocular microscope. You would measure sets of Standard stars, get a 6-coefficient fit to convert x and y into RA and Dec,then find your objects on the plate, measure x,y, calculate RA and Dec. Then snap a polaroid to use as a finder chart at the telescope.</p>

<p>I was doing this as late as 1995 before the advent of the Digitised Sky Survey. </p>

CC BY-SA 3.0
2016-04-03T19:43:28.427Z
2016-04-03T19:43:28.427Z
ProfRob
<p>Did they just put a ruler on the photographic plate? Quite some precision must have been required to discover Neptune from its influence on Uranus for example.</p>

CC BY-SA 3.0
2016-04-03T17:19:13.610Z
Neptune was predicted by Le Verrier in 1845, astrophotography: not much before 1880.  Observations of perturbations in Uranus orbit were done by eye.
James K
<history><photography><astrometry>
How was precision astrometry done before digital imaging?
LocalFluff
# How was precision astrometry done before digital imaging?

(Asked by: username_0 on 2016-04-03)

Did they just put a ruler on the photographic plate? Quite some precision must have been required to discover Neptune from its influence on Uranus for example.

**Tags:** history, photography, astrometry

> **Comments:**
> **username_1**: Neptune was predicted by Le Verrier in 1845, astrophotography: not much before 1880.  Observations of perturbations in Uranus orbit were done by eye.  

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## 2 answers

### Answer by username_3 on 2016-04-03. Score: 5 | Accepted answer

Prior to digital imaging then photographic plate negatives were analysed with scanning microdensitometers to produce astrometric catalogues. Many of these catalogues are still in use today, they are valuable sources of early epoch positions that enable proper motion measurements.

For more details you could look at the descriptions of the SuperCosmos project <http://ssa.roe.ac.uk//> (which is based on Schmidt plates) or the UCAC4 catalogue, <http://ad.usno.navy.mil/ucac/readme_u4v5>, which uses plates to get positions and proper motions for faint stars.

In the good old days, before even these catalogues existed, astronomy groups would have copies of the whole Schmidt and Palomar sky surveys. You would put the relevant plate on a massive, concrete-based, hydraulic X,Y measuring machine, with a binocular microscope. You would measure sets of Standard stars, get a 6-coefficient fit to convert x and y into RA and Dec,then find your objects on the plate, measure x,y, calculate RA and Dec. Then snap a polaroid to use as a finder chart at the telescope.

I was doing this as late as 1995 before the advent of the Digitised Sky Survey.


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### Answer by username_2 on 2016-04-03. Score: 0

Micrometers and [blink comparators](https://en.wikipedia.org/wiki/Blink_comparator) allowed considerable precision.

Pluto was discovered with the aid of the latter.


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