First, electromagnetic waves span a spectrum. It is a chart-like-scale humans (scientists) devised. All electromagnetic waves travel through free space at 300,000 km/sec.
So the waves have wavelength and frequency (so many cycles of propagating waves in a second).
Frequency = 300,000,000 (m/s) / wavelength (m) or
Wavelength = 300,000,000 (m/s) / frequency (cycles/s).
Now a new name was adapted for frequency called 'Hertz' (1 cycle/s = 1 Hz).
If you look into any introductory book of higher Physics, the 'EM' spectrum is given in the appendix.
At long wavelengths, Radio waves (lengths of several kilometers to 0.3m or 30 cm) can be detected ('received' is the word) by Radio telescopes using a particular repertoire of techniques & technologies. Microwaves (0.3m to 0.0005m or 0.5mm), still radio waves, but with a little bit different techniques can be also received. All our satellite communications systems employ it. Since our atmosphere heavily absorbs them, one needs to put up Microwave antennas above in the satellites.
Then comes the Far Infra Red (FIR) region with wavelengths down to ten millionths (a millionth of a meter is called 1 micron, 1 |u) there are various techniques again. About a century & quarter ago JC Bose did pioneering work for which he was accorded the honour 'now' of co-inventor of Radio. But that work needs to be taken up afresh. Then comes the middle IR. Same is the case here. In near IR (NIR) till 0.7 micron, vigorous work is in progress. This band is mainly for 'Remotes' for electronic equipment. But in all IR (FIR, MIR, NIR) space applications are limited except taking photos with those filters because atmospheric molecules interact freely producing IR and absorbing, as basically IR is 'Heat waves'. One must go above the atmosphere for good work to be accomplished. The Optical 'Fibre' operates at 0.8 |u and is the mainstay in optical communications.
Then comes the work horse - 0.7 to 0.375 micron band in which we see the colours (VIBGYOR; Violet end near 0.375 micron & Red end at 0.7 micron). It is called the Visible Region. Telescope is meant for this region of spectrum. Eye is the main receptor/sensor for this part in electromagnetic waves. Almost all our knowledge of space is due to this window in the 'EM' spectrum and is called Optical window. If you are looking at a star or galaxy (either with aids like a 'Telescope' or without) or seeing its picture in all fantastic colours, you are indulging in Space (Astronomy) studies. 99% knowledge, work, analysis, Red-shift theories are all in Optical spectrum, a mere sliver of the spectrum with almost an 'Octave' bandwidth (it means the ratio of the highest to the lowest wavelengths is 2 : 1).
Beyond 0.375 micron (as I told you a micron is a millionth of a meter) we have Ultra Violet portion again divided into NUV, MUV & FUV merging with soft X rays of Nanometer (a billionth of a meter). Some UV cameras get sent in all missions in the Space probes and such programmes.
X rays : A telescope 'Chandra' is orbiting the Earth now to take pictures at wavelengths (my guess) of a fraction of a nanometer.
After that comes the 'Gamma rays' portion the sensors are complicated and expensive. These high energy and still higher energy radiations, being high energy radiations readily interact with the stuff of our atmosphere and the energy gets transferred into the excitation of those molecules thus decimating the 'information' content. So, one must go much above the atmosphere to build a telescope. Here (in this part of spectrum Electrons don't have separate existence as they combine with Positrons to produce Gamma ray quanta).
Beyond these lie the zone of extremely high energy particles expressed in Energy units (we need to switch from the nomenclature of billionths to billionth billionths).
The whole spectrum (to my reckoning, in wavelengths spans the known region of 10^6 meters to 10^-12meter; a range with a ratio of end to end of 10^18 that we call 18 decades or that is about 60 octaves) and every region or octave has a different technique and equipment. You can't have the same antenna to receive TV signals, Citizen band, satellite communications and data receiving, isn't it? My effort is to outline here that all this constitutes the domain of 'electromagnetic waves'.
The idea of a telescope (whether Optical, IR, Visible, Radio or UV) is to point it. I don't want to venture into it even because it is a different kettle of fish altogether.
I haven't got into the preliminary details even.