I The light from Mercury, the Sun's corona and gravity

[Wondermine]

The question is:
The General Relativity Ideas were confirmed by the position of Mercury by Eddington. How could they be certain that the light deviation was due to gravity rather than the lens effect the Sun's corona may have had?
What process was used,if any,to remove the effect of the corona on the light from Mercury?
The corona is an high energy particle "fog" which would lens light like a crystal. How to remove this in order to be certain gravity bent the light?

 

[phyzguy]

You're conflating two different tests of General Relativity. What Eddington measured was the deviation of the position of stars due to the deflection of the starlight by the sun's gravity. He measured the positions of these stars during a total solar eclipse and again after the sun had moved away. The precession of the perihelion of Mercury was known since the 1800's to be more than Newton's theory predicted. Eddington was not part of this.

Your question still holds, however. I think the deflection due to the lensing effect of the corona is negligible, but I have no references to back this up. The density of the corona is known, so this deflection could be calculated.

 

[PeterDonis]

This paper by Will reviews the history of measurements of light bending by the Sun:

https://arxiv.org/pdf/1409.7812.pdf

He notes on p. 10 that recent measurements have taken readings in multiple frequency bands, to correct for the effects of the Sun's corona (which is expected to deflect radiation of different frequencies by different amounts, whereas the GR effect is the same for all frequencies).

 

[Herman Trivilino]

How could they be certain that the light deviation was due to gravity rather than the lens effect the Sun's corona may have had?

That 1919 eclipse observation is certainly of historical significance. But it may interest you to know that since that time we have observations that are a lot more definitive.
See, for example, this article by Clifford M. Will: http://link.springer.com/article/10.12942/lrr-2014-4#Sec4. Here is a relevant passage:

[...]the development of radio interferometery, and later of very-long-baseline radio interferometry (VLBI), produced greatly improved determinations of the deflection of light.