# How are radio telescopes pointed?

In the past, I worked as a programmer in a radio astronomy station, and since then I have a question on my mind.

To observe a star with a telescope, we locate its location with our eyes and then we try to point the telescope using our eyes. We analyze light signals to point the telescope.

How do we do it with a radio telescope knowing that our eyes cannot discern the radio waves emitted by a star in the sky ? How can we be sure that the radio telescope is pointed at the right object? With a classical telescope, it is simple, we have our eyes but how can we be sure with a radio telescope?

Well, even most optical telescopes aren't steered by hand and guided by human eyes - aside from a small fraction of objects, most sources are too dim to be seen by the naked eye or even with assistance, and observations require precision far beyond the ability of humans. A hobbyist telescope where less precision is needed and most sources are relatively bright could be adjusted by hand; a telescope being used for research couldn't - or certainly shouldn't.

Radio telescopes use a procedure also utilized by telescopes at other wavelengths: observing scripts and scheduling blocks. These contain information about the observing parameters - such as the telescope configuration, receivers and backends used, and the sources themselves - and commands that tell the telescope to slew to a point in the sky, take data, track the source, etc. Here's an extremely simple demo scheduling block for use on the Green Bank Telescope (see Section 6 of Observing with the Green Bank Telescope for additional details on it):

# load the configurations file
execfile('/mypath/myconfigurations.txt')

Catalog('/mypath/mycatalog.cat')

# configure the GBT
Configure(myconfig)

# slew to the source
Slew('B0329+54')

# balance the IF system
Balance()

# now observe the source for ten minutes
Track('B0329+54',None,600)


This block can use Python commands within it, allowing it to, for instance, loop through a list of sources. Notice that it loads in a source from a catalog (the Catalog() command) and automatically slews to the source (Slew()) without any human intervening; you just submit the block and let it run. The catalog contains all the information needed for the telescope to find a source, including its right ascension and declination.

Pointing and focus errors can creep in and do require additional calibrations, particularly for high-frequency continuum observations. This can be somewhat achieved by applying a pointing model (e.g, White et al. 2021) taking into account the telescope's structure, current positioning, thermal deformations, etc. However, for even better pointing correction, calibration sources should be used, which may be chosen from a specific catalog of pointing sources. Again using the GBT as an example, this can be achieved by running AutoPeakFocus() in a scheduling block to perform pointing and focus scans prior to gathering the actual data, or simpler variants to just do one of the two. This will then automatically update the corresponding corrections. Again, this is significantly more important in certain regimes and certain targets.

It is possible for the telescope operator - the observatory staff member on site in the control room coordinating and aiding observations - to interrupt scripts and make unscheduled changes. This is typically in the event of high winds (the GBT, for example, must be stowed if winds exceed 35-40 mph) or if snow or ice has built up on the dish, in which case observations must be paused for the snow to be dumped. That said, this isn't really done by the operator looking at the telescope (two kilometers away from the control room), but by computers that can calculate and move the telescope to the precise position necessary to keep it safe.

• Some are 'steered' electronically, including the one used to discover pulsars. Feb 3 at 18:35
• This doesn't address how you do the fine adjustment. Presumably there are pointing and tracking errors that are important? Feb 3 at 19:34
• @ProfRob I'm not overly familiar with how things are done in the optical, for instance, but pointing and focus corrections sometimes need to be computed - is that the sort of thing you were asking about? I've made an edit discussing it. Feb 3 at 20:50
• I must say that Green Bank telescope turning is a surreal sight to behold, at least when you experience it for the first time Feb 6 at 1:03

There are several ways to point a radio telescope.

1. Some radio telescopes are mounted with an optical telescope pointed in the same direction. The user moves the optical and radio telescopes together while pointing the optical telescope at the region of interest. Credit: Ukita, N. & Ikenoue, Bungo & Saito, Masao. (2008). Optical Seeing Measurements with an Optical Telescope on a Radio Antenna.

1. Some radio telescopes have automated mechanical Azimuth/Elevation mounts. The user can use a celestial map to identify the region of space and pointing angles for the telescope. Inputting this information into the telescope computer allows the telescope to automatically point to the source. An advantage of this technique is that it can also be programmed to track a source as its apparent position in the night sky moves. For a fixed dish like the former Arecibo observatory, the feed is moved instead. Credit: NRAO.

1. Arrays of radio telescopes are not only steered mechanically, but are also steered mathematically by applying signal delays before the signals are combined. The clever application of signal delays can create constructive interference in the region of interest while creating dampening (destructive) interference for signals outside of the region of interest. Credit: Maxter315 wikipedia:
• Are you assuming that if you have the coordinates of a source you can point at it? That isn't the case with an optical telescope where fine adjustment is required. Feb 3 at 19:36
• @ProfRob I think this is dependent on the telescope and the target. When I was observing on the Green Bank Telescope, we were able to point at a source using coordinates, which was good enough for excellent observations which didn't require high angular resolution. Prior to our observations, we performed RF and pointing calibration to other known radio sources to reduce pointing errors. For arrays of radio telescopes, the "fine adjustment" in some sense is performed in the correlator, from which peak interferometry responses can be translated into triangulations with high angular resolution. Feb 3 at 21:17

If an amateur astronomer can see and identify a specific star in the sky with their naked eye, they can point their telescope at that star. Many telescopes come with smaller spotting scopes attached which have a wider view of the sky. If the target star can be seen and identified through the spotting scope the main telesceope will be pointed at that target star.

But there is one big flaw with spotting a star and aiming your telescope at it.

Actually there are only about 6,000 to 9,000 stars that can be seen with the naked eye from Earth, and "billions and billions", not to be overly precise, of other stars which can be detected in telescopes, to say nothing of other astronomical objects invisible to the naked eye but visible in telescopes.

Stars which have been previously detected by various methods have their coordinates listed in one or more astronomical coordinate systems in one or more catalogs of stars. And there are catalogs of other types of astronomical objects.

And when someone discovers a new object, or something new about a known object, they publish the coordinates of that object so others can observe it.

https://en.wikipedia.org/wiki/Astronomical_coordinate_systems

An amateur or professional astronomer who wants to look at a specific star and can't recognize that specific star when they look at teh sky, can look up the coordinates of the object, and have a computer program calculate where to point their telescope to that star for their location on Earth and the time and date. And then the program will instruct the motors on the telescope to turn to point at the star, and to keep turning to keep that star centered until told to stop.

All professional telescopes and all the fancier amateur telescopes have motors to turn the telescopes which can be programmed where to point them.

And of course the same goes for steerable radio telescopes. Their motors can be instructed to point them at any location in the sky that is to be studied.

• That isn't quite how it works for optical telescopes. Fine adjustment is required. Feb 3 at 19:37
• "fancier amateur telescopes have motors to turn the telescopes which can be programmed where to point them".... after you calibrate it by pointing it at the star(s) it says to in the manual. - There's a 'map' we've been making for hundreds of years... lmgtfy, +1 Feb 4 at 4:03
• "Billions and billions" is, if you want to be more precise, exactly one Sagan. It doesn't have a precise conversion to "normal" numbers (but it is at least four billions). Feb 4 at 12:46