How to measure a 21 cm EM wave if it is Doppler shifted?

[atomiclaser]

TL;DR Summary

21 cm was considered as the first light after big bang. How does people measure this emission ?

we know that all emission from asctrophysical context is doppler shifted. So, how to make sure the doppler shifted 21 cm not contaminated by some other emission?

 

[trurle]

Summary:: 21 cm was considered as the first light after big bang. How does people measure this emission ?

we know that all emission from asctrophysical context is doppler shifted. So, how to make sure the doppler shifted 21 cm not contaminated by some other emission?

The statement is incorrect. The observations of 21cm line is an useful probe for redshifts up to 150, while CMB (which is more correct to call "first light") is redshifted by ~1100 times.
The wikipedia section
https://en.wikipedia.org/wiki/Hydrogen_line#In_cosmology
gives a good overview on effects of redshift on 21cm observation. Contamination of signal is indeed a very serious issue.