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Maybe I'm wrong, but it feels like a significant event in extra-atmospheric astronomy that happened on June 10 passed almost unnoticed. The matter is that the space flagship of our and German astronomy - the Spectrum-RG telescope - completed the first full scan of the entire sky.
I'll try to fix it. To do this, I will try to explain in a nutshell how different telescopes differ, why this apparatus is important for world science and describe the features of its operation. Moreover, judging by those past publications, sometimes the authors do not understand what the phrase means about its record accuracy, relating it generally to all telescopes of this range.
The fact is that orbital telescopes have long been divided into two types: survey and detailed. The former are designed to search for new objects in the celestial sphere, preferably with a complete map of the sky. The latter are already needed for a detailed study of the revealed new sources with an elucidation of their physical nature and additional characteristics.
For the former, it is permissible to use wide-angle optical systems, for the latter, systems with the smallest possible angle. In addition to the inspection area, this often imposes restrictions on the observation time. The closest analogue is exposure in photography. The wider the angle of the lens, the faster the shutter speed for one shot and vice versa. This will be shown below as an example.
Before starting the second, the first stations should go. This is what will allow the scientists who control the first spacecraft, as one astrophysicist told me when talking about the Spectr-RG: "Stop poking around at random like blind kittens."
This can be best illustrated with real telescopes. For example, the first hard gamma-ray surveillance vehicle was the COS-B, launched in 1975. He had a field of view of the order of 30x30 degrees, that is, to completely cover the celestial sphere, he needed to make 50 observations. Due to the specifics of its orbit, it was expected that it would build a gamma ray map of the sky in 4 years, but in reality, in 6 years of work, only about half of the entire sky. But this was also a very significant result.
For an example of a detailed station, you can take NEAO-2 Einstein, which studied the soft X-ray range in 1978-1981. Its field of view was about 1 degree, the resolution was up to 2 arc seconds, and the sensitivity of the sensors required exposure of about 10 4 seconds (2.7 hours).
If this telescope were required to map the entire sky, it would take about 100 years to do this. During his work, he viewed only 3% of the sky, but from a qualitative point of view, it was a very important 3%. He studied representatives of almost all classes of X-ray sources and even discovered new ones.
And he would not be able to do this if the scientists did not know in advance where to look, thanks to even less detailed maps of the whole sky obtained by survey telescopes.
Since the whole sky survey is a qualitative result, it usually makes no sense to repeat it to other devices with the same result. Unlike detailed stations. The latter is desirable to have as many as possible, albeit with equal resolution. This will allow you to quickly explore open areas.
In survey systems, for each next map, it is desirable to increase the resolution by orders of magnitude, which is not easy. And the problem is not even that you need to digest an order of magnitude more traffic from the device.
From a technical point of view, the survey device must be made with a stabilized rotation so that in one revolution it captures a narrow strip in the sky. And after each turn a new one. It was this scheme that was implemented for Spectra-RG. For the first time for our devices it was brought to the Lagrange point, after which it took a constant orientation to the Sun and began to scan the sky. This is clearly seen in the diagram from the "Bulletin" of the Lavochkin NPO.
The complete period of rotation of the apparatus around its axis is about 4 hours. During these hours, due to the movement of the Earth, the plane of rotation of the apparatus has changed by approximately 0.17 degrees and new areas of the sky have entered the field of view of the telescopes.
It looks simple, but each next card is given with more and more difficulty. It can be seen that the scanning time, data transfer and scanning system parameters must be rigidly synchronized.
But the narrower the field of view, the faster the object passes through it. Say, at an angle of view of 10x10 degrees, an object from the ecliptic plane will be in the field of view for 10 5 seconds (almost a day), and at an angle of 1x1 degree, the maximum possible exposure drops a hundred times to 10 3 seconds (16 minutes). The requirements for receivers have increased 100 times, and the linear resolution is only 10 times. And if we demand to take the next step, that the maximum possible exposure will be reduced to a few minutes. And with such a shutter speed, even in the optical range, there can be problems, not to mention the X-ray.
As a result, if initially there were fairly simple receivers on the survey devices, then on the same Spectra-RG, the most complex oblique incidence telescopes were used, some elements of which can be produced by literally several enterprises in the world. And it is not a fact that when all the scientific discoveries of Spectra-RG will be studied with detailed telescopes, the creation of the next observation station will only run into financial problems, and will not face complex technical and scientific constraints.
Comparison of one section of the sky from different telescopes. ART-P / Garnet (detailed), ART-XC / Spectr-RG (overview), NuSTAR (detailed)
However, this is still far away. The first overview map of the sky in the X-ray range was constructed, over the next few years the station will further refine it, will scan the sky several times. After that, the study of new objects will be delayed for decades, both according to Spectra-RG data and with the help of more detailed stations.