Measurement of the FWHM of two PST etalons

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christian viladrich
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Measurement of the FWHM of two PST etalons

Post by christian viladrich »

Hi,
I recently measured two PST etalons using two different methods : Ha lamp and Sol'Ex spectro.
To make a long story short, if we take the average value over the whole surface of the etalon :
- one etalon has a FWHM of 0.6A and a FSR of 9.3 A,
- the other has a FWHM of 1.0 A and a FSR of 6.9 A.
Both measurements are for a collimated beam.

To my surprise, it appears that these two etalons are tuned, not by tilting, but by mechanical compression (just like the SM III Rich View). I don't know whether this is a general rule for PST etalons.

It would be interesting to share here some pictures of the dismounted etalons to check what is the internal mechanical arrangement. I can't dismounted the two etalons I tested, there are not mine ...

Detailed results are given here :
http://astrosurf.com/viladrich/astro/in ... PST-1.html
http://astrosurf.com/viladrich/astro/in ... PST-2.html

Clear skies !


Christian Viladrich
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Re: Measurement of the FWHM of two PST etalons

Post by christian viladrich »

With Sol'Ex spectro, it is possible to visualized the impact of the f-ratio on the effective FWHM.

The first series of images is taken at f/115 (i.e. nearly collimated beam).
- The first row is the solar spectrum without the PST.
- Second row is the solar spectrum with the PST tuned on Ha. We can see the different fringes of transmission of the etalon. The Ha line is the darkest one near center.
- Third row is the same but with the PST tuned a bit on the red wing of Ha.
The FWHM measured over a very small surface of the etalon is 0.34 A.
http://astrosurf.com/viladrich/astro/in ... -tuned.jpg

Now, with a telecentric at f/27.5 we can see that the Ha line (center) is "embeded" in a larger brighter line. If fact, because of the f/27.5 ratio, the FWHM is larger, capturing photospheric light :
http://astrosurf.com/viladrich/astro/in ... ringes.jpg

With a f/35 telecentric, the effective FWHM because smaller : 0.76 A, but is still much larger that at f/115 :
http://astrosurf.com/viladrich/astro/in ... ringes.jpg

This explains why "air-spaced" etalon are to be used in a collimated beam instead of a telecentric beam (unless a high f-ratio is used).


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http://planetary-astronomy.com/
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Re: Measurement of the FWHM of two PST etalons

Post by MalVeauX »

Excellent Christian!

I can confirm my pre-Meade and post-Meade etalon housing are pressure based, not tilt. They are suspended between foam and threading the top of the housing down applies pressure to the plates inside. It's very NON-specific, no markings, and it's held orthogonal by pressure alone in the cell, so if the pads are off, the whole thing can tilt a little inside there sadly (probably why so many are poor).

Very best,


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Re: Measurement of the FWHM of two PST etalons

Post by Merlin66 »

viewtopic.php?f=9&t=8048&start=50
This was confirmed by Lunt.
The tuning of the PST etalon is by mechanical pressure, similar (but different) to the RichView tuning.
See page 3 of the above discussion.


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Re: Measurement of the FWHM of two PST etalons

Post by Merlin66 »

Christian,
The “double” lines seen with the telecentric don’t look right.
Could there be an optical effect?


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Re: Measurement of the FWHM of two PST etalons

Post by christian viladrich »

Hello Ken,
Thanks for pointing out this old post !

This is the beauty of the Ha lamp test that we can know for sure, and without any mechanical measurement, that the PST etalon (at least the ones I tested) are tuned by mechanical compression. Indeed, tilting an etalon makes no change in the fringe pattern in the Ha lamp test. The one way to have a change is by changing the thickness of the gap.

Regarding the "double line" effect, could you be more specific ? Could you put some arrows on the spectrum to identify them ?


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Re: Measurement of the FWHM of two PST etalons

Post by thesmiths »

Having played around a lot recently with home-made etalons (see for example viewtopic.php?f=8&t=30713, I know how sensitive they are to any mechanical stress. The one shown in the first image in the link is a fairly stiff structure (made with Thorlabs mounts and ADM and Williams Optics parts) but the slightest touch with a finger on the mounts will move the etalon by multiple fringes. Pressing down on the two ends of the red Williams Optics dovetail bar will induce a curvature in the dovetail bar that, with only moderate pressure, will move the interference pattern by maybe one fringe.

So I guess I am not surprised the PST etalon could work by actual compression of the etalon itself. After all, etalons are even used to detect gravitational waves (the stress-strain deformation of space much smaller than the diameter of the atomic nucleus).

By the way, etalons are typically slightly wedge shaped, but this is to prevent reflections from the exterior surfaces from getting confused with reflections from the interior surfaces.


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Re: Measurement of the FWHM of two PST etalons

Post by christian viladrich »

Hello Thesmiths,
Some discussion here on the amount of pressure to be applied on the PST etalon :
viewtopic.php?f=9&t=32638&p=300816#p300816


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Re: Measurement of the FWHM of two PST etalons

Post by Merlin66 »

Christian,
I've sent you and email.


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Re: Measurement of the FWHM of two PST etalons

Post by thesmiths »

christian viladrich wrote: Sun Jul 18, 2021 1:18 pm To make a long story short, if we take the average value over the whole surface of the etalon :
- one etalon has a FWHM of 0.6A and a FSR of 9.3 A,
- the other has a FWHM of 1.0 A and a FSR of 6.9 A.
Both measurements are for a collimated beam.
Christian, so the finesse of the two etalons are quite dramatically different: 9.29/0.61 = 15.2 vs 6.94/0.98 = 7.1
That is really a significant difference in the quality of the two etalons. 15 is rather good; 7 is rather so-so. You can see visually that the one you call etalon 2 has a quite nice finesse. I think this likely has to do with the flatness of the reflective surfaces (since the reflectivity can be quite well controlled). In general, the flatness (or figure) finesse is M/2, where the flatness over a certain area is defined as lambda/M. So, for example, assuming the finesse you measure is dominated by flatness finesse, the finesse of 15 means the flatness is lambda/30 at 656nm, quite a good figure.

It's also worth noting that the fact that the FSR is so different means the etalon spacing is not a very well controlled parameter. Assuming FSR = (wavelength)^2 / 2d (where d is the spacing), the two etalons have air spacings of 0.23mm and 0.31mm.


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Re: Measurement of the FWHM of two PST etalons

Post by christian viladrich »

It may be that the difference of FSR is the result of the difference of generation.
Is there any way to tell the manufacturing date of the etalon from its serial number ?


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Editor of "Solar Astronomy"
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