What instrument is used to find the temperature of the Sun?

There is core temperature and surface temperature. Neither is estimated by "an instrument" but by calculations based on what we know about energy, gravity, gas, etc...

Here are a couple of overviews of core temp and surface temp:

Finding Solar Temperature (http://www.newton.dep.anl.gov/askasci/ast99/ast99123.htm)
Core Temperature of Sun (http://www.newton.dep.anl.gov/askasci/ast99/ast99562.htm)