I know that at the end of the Receivers in Radio Telescopes, after the final Amplifier the analog signal will be given to the ADC. Now this ADC converts the analog signal into a stream of binary bits. I want to how do we convert this stream of binary bits into a FITS image of the area of the sky that I'm observing. I'm asking this with respect to a single dish observation and not with interferometers.
$\begingroup$
$\endgroup$
5
-
2$\begingroup$ To make an image with a single dish, you must scan. Details of image construction depend on how you do the scanning. $\endgroup$– John DotyCommented Jul 8 at 13:22
-
$\begingroup$ Can you explain in detail please. Also I'm planning to observe a particular location in the sky for 6 hours continuously by locking onto the location initially and moving the antenna with the rotational speed of the Earth using stepper motors. I hope this detail gives you an idea about my observational method. $\endgroup$– G.B. RaghavkrishnaCommented Jul 9 at 4:08
-
$\begingroup$ So, you're not scanning. You can't make an image that way. $\endgroup$– John DotyCommented Jul 9 at 9:41
-
$\begingroup$ @JohnDoty I don't understand what you mean by scanning. Please explain in more detail. $\endgroup$– G.B. RaghavkrishnaCommented Jul 10 at 4:09
-
$\begingroup$ Well, you point the antenna someplace. Measure the flux. Then point the antenna at a different place. Measure the flux. Repeat until you've covered the area you wish to image. $\endgroup$– John DotyCommented Jul 10 at 11:16
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
5
it is my first time to answer a question on this website!
There are two main ways to observe an astronomical object using radio telescopes. One is scanning and the other is just tracking a point.
If you want to get an image, you need to scan a region (e.g. a rectangle area) centered on your target and keep the samplings to satisfy the Nyquist Sampling Law (ensure the sampling density both spatially and time). Then you could record the sampling points into grids and retrieve an image made up of pixels. Before imaging, some necessary calibration steps also need to be considered. But fortunately, professional radio telescopes will have very complex data reduction software to finish these jobs.
If you are planning to observe a particular location for 6 hours continuously, that means the telescope is tracking a single point in the sky. The only information you will get is a spectrum inside the beam over the sky field. The benefit of this observation mode is to increase the signal-to-noise ratio with a 6-hour integration. Then you also have to do the calibration procedures to convert the binary bits into physical units.
Hope my answer will help you :D
-
$\begingroup$ My question is, how this gridding is done ? In telescopes using CCD each pixel will be the output of a photodiode and their analog signal can be digitized and arranged in to form of an array of pixels and that forms the FITS image. But in radio telescopes, all the radiation coming from the field of view of the dish will be compressed and will be sent along the same coaxial cable and how can we make the output of ADC into an image in this case ? $\endgroup$ Commented Jul 10 at 7:17
-
$\begingroup$ Also how are you saying we will get a spectrum ? Because I'm looking at the same location to get high SNR as well as high sensitivity, so I will make an image right ? How will it become a spectrum ? $\endgroup$ Commented Jul 10 at 7:19
-
$\begingroup$ Yeah, as your first comment, radiation inside the beam has been compressed and we cannot distinguish them. That is why the resolution of the single-dish radio telescope = FWHM of the beam. If you want to make an image, the dish should sample on different coordinates and record the pointings (RA, Dec). Every pointing is a spectrum. Then the gridding process is to put nearby spectra into a grid, using some weighting average methods. You can get many pixels with different positions or coordinates, which is different from optical imaging, it photos once and records all the pixels. $\endgroup$– astroR2Commented Jul 10 at 9:48
-
$\begingroup$ Can you tell me how to do the gridding and to attach the coordinates to each pixel ? $\endgroup$ Commented Jul 12 at 4:17
-
$\begingroup$ Complex, it is... We usually generate a grid using
astropy.wcs, and then spread the spectra into the pixel based on a gaussian weight. It is difficult to explain here, maybe this python code will help you . $\endgroup$– astroR2Commented Jul 14 at 7:45