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question - pre filters for SHG

Posted: Thu Aug 10, 2017 5:58 pm
by highfnum
page 95 of Ken M Harrison's spectroheliograph book
has a paragraph about using ERF or color pre filters to help in focusing sun image on slit

today I tried a slightly different approach
I placed a polarizing filter in front and as I turned it
I could tell edge of sun would come into focus and then out

and
1) image would get lighter and darker as if there was another polarizing filter in system (there is not)

2) an image that appears in good focus without filter is out of focus when I place polarize filter in front

has anyone tried this before?

I wonder if this will lead to better images because of more precise focus

since slit has a "direction" I wonder if light waves of one direction enter slit in a more orderly manner


on next occasion I will try a more detailed test

Re: question - pre filters for SHG

Posted: Thu Aug 10, 2017 11:43 pm
by Spectral Joe
Two separate things are happening. In case 1), it's important to know that gratings have different efficiencies for parallel or perpendicular polarization, so changing the polarization going in will change the brightness coming out. In the second case, the polarizer has optical thickness, and if it is put into a converging beam (like in front of the slit) it will shift the focus. For glass with an index of around 1.5 the shift will be about 1/3 the thickness of the glass. Both of the things you are seeing are real effects. The polarization relative to the slit will not have an effect.

Re: question - pre filters for SHG

Posted: Fri Aug 11, 2017 12:06 am
by highfnum
thx

so is in case 1 s using P filter useful?

Re: question - pre filters for SHG

Posted: Fri Aug 11, 2017 1:11 am
by Spectral Joe
For adjusting intensity, yes, but only if you have no other way of adjusting exposure.