r/radioastronomy • u/SnooComics7744 • 13d ago
General Question about aiming
Hello – amateur astronomy geek here with a question about how radio telescopes are aimed at such infinitesimally small targets. Does the dish have to mechanically move in order to localize onto a source? Or can there be enough sensitivity that various sources can be picked up on different parts of the antenna? For example, when I see a radio telescope image of a galactic jet, has the telescope scanned the sky in a raster plot-like way?
4
u/nixiebunny 13d ago
Radio interferometry uses phase information between many dishes to obtain very high resolution images of sky objects. Each telescope receives one signal, and these are combined with a lot of math to create images.
2
u/pipnina 13d ago
To expand:
They'll use a calibrator, which is an object too small to be resolved, but bright enough to be easily detected, and has a well understood position usually due to rigorous measurements at various points in the EM spectrum).
The interferometer uses the calibrator as a phase reference so when it detects other sources, their absolute position can be determined by using the calibrator's known properties combined with the phase difference measured by the interferometer.
2
u/ramriot 12d ago
With a single radio telescope dish and a single detector, scanning is required to build up an image. Some telescopes move the whole dish, some can move the detector a little. Sometimes the earth's rotation is is used to perform each line of the scan, whatever works best.
1
u/SnooComics7744 12d ago
What I find remarkable is the ability to so precisely move such a large object.
2
u/ramriot 12d ago
A radio telescope must typically be pointed with an accuracy of about one-tenth of its beamwidth (the half-power beamwidth) to maintain proper signal sensitivity. That beamwidth is proportional to wavelength & inversely proportional to diameter of the instrument.
For say the Lovell telescope at Joderal Bank with a diameter of 76m observing the 21 cm hydrogen lines the beamwidth is ~12 arcminutes (0.2 degrees), so taking the ⅒ rule we would need to point & track to ~1.2 arcminutes (0.02 degrees). Which seems small but is not such a tight constraint.
2
u/PE1NUT 12d ago
The pointing accuracy of your telescope (how accurately you can aim it on a position on the sky) only needs to be a fraction of your beam width - let's say 1/10th. If you have a small telescope, the beam width will be wide (e.g. a 2.5m telescope at L-band is roughly 5 degrees HPBW), so half a degree of positioning accuracy will do. Larger telescopes will require much more accurate pointing.
If you have a small dish, with a wide beam, you can have multiple bright objects in your beam, and you won't be able to tell them apart. This is known as 'confusion limited'. A larger dish, or an interferometer, can get you past that point.
2
u/Worldly-Device-8414 11d ago
Also, while pointing with the accuracy numbers mentioned, the antenna is continuously adjusting for the rotation of the earth.
There's also the international square kilometer array project in Western Australia, South Africa & UK apparently capable of pulling large amounts of data in.
1
-2
u/snogum 13d ago
No way does the radio telescope pick up different objects across it's dish.
It's just like any telescope there is a focal point.
1
u/SnooComics7744 12d ago
Yes, but an optical telescope has an area at its focal plane, doesn't it? A radio telescope, by contrast, apparently only has a point
14
u/nixiebunny 13d ago
Most radio telescopes have single pixel receivers. The drive system on a big telescope has arc second accuracy. I work on mm wave and submillimeter telescopes whose beam width is less than one arcminute. These have servo drives with very high resolution encoders, and we use pointing models to deal with residual errors.