Quark and Lunt 40mm etalon FSR and FWHM measurements with a DIY spectroheliograph

[Oak]

Hi All,


I recently completed a 3D-printed DIY SHG described here: viewtopic.php?p=401631#p401631

It’s cloudy this weekend so I set up a (hopefully) collimated beam and used my new SHG to observe the transmission lines from a quark and Lunt 40mm etalon in single and double stack in a few configurations. All without blocking filters to increase transmission. You can see an increase in free spectral range and decrease in FWHM in the quark+Lunt40DS vs the quark alone. Focus likley wan’t perfect and a few different exposures needed to be used. Angstroms/Pixel are based on the theoretical resolution of the system and have not been validated against any sort of standard yet, so take the actual numbers with a grain of salt. Images below.


Basic setup: Collimated light source (ST80+HalogenBulb at focus) → ~12foot gap → BaaderERF160mm → C6 SCT → TZ4(f40) → Quark → Lunt40mmEtalon → SHG (150mm lenses x 2, 2400l/mm grating) → ASI174MM

TZ4(f40) → Quark



TZ4(f40) → Quark → Lunt40mmEtalon



TZ4(f40) → Lunt40mmEtalon



Collimated light source → Lunt40mm Telescope With Etalon (no blocker)



Image-based background subtraction was applied with a 50mm radius rolling ball. These are the resulting profiles (15 pixel high ROI across the image - maybe I should use a smaller ROI in the future).

TZ4(f40) → Quark



TZ4(f40) → Quark → Lunt40mmEtalon



TZ4(f40) → Lunt40mmEtalon



Collimated light source → Lunt40mm Telescope With Etalon (no blocker)



I then used FITYK to fit Lorentzian profiles to the peaks and came to these results:

TZ4(f40) → Quark
FSR: 28.3A
FWHM: 0.91A

TZ4(f40) → Quark → Lunt40mmEtalon
FSR: 141.4A
FWHM: 0.585A

TZ4(f40) → Lunt40mmEtalon
FSR: 12.5A
FWHM: 0.78A

Collimated light source → Lunt40mm Telescope With Etalon (no blocker)
FSR: 12.5A
FWHM: 0.91A
Blue Shift: 1.98A

There is also a blue-shift compared to the collimated beam, although the grating wasn’t locked down during measurement so could be a spurious result.


Since focus is likely not perfect and there was no real calibration done, these are not likely to be exactly accurate numbers, but the relative differences are interesting. Particulalry interesting is that the FWHM of the air-spaced Lunt etalon was smaller in the telecentric beam vs the collimated one. This should be the other way around. I will repeat the measurements using the sun as opposed to a DIY light source and see if the difference remains…

Thanks for reading!


-Derek

[Dennis]

Thanks for measuring this, interesting with the TZ4 and the Lunt40.

BTW do you know the aproximate peak transmission of Quark etalons?

[marktownley]

Nice analysis Derek.

[Oak]

Thanks for measuring this, interesting with the TZ4 and the Lunt40.

BTW do you know the aproximate peak transmission of Quark etalons?

Haven't measured overall transmission. Will attempt when the weather clears. I needed to remove the quarks blocker to get enough signal to measure. Certainly the lunt etalon has a much higher transmission than the quark. (Polarizers and other stuff in the quark that isn't present in the lunt)

[christian viladrich]

Well done Derek !

As you said, the result of the comparison between colimated / telecentric is quite unexpected.