Evaluating the PST etalon with a neon bulb

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Evaluating the PST etalon with a neon bulb

Post by thesmiths » Mon Dec 28, 2020 3:24 am

I stumbled upon this effect while working on something other than astronomy (Zeeman effect, in case you ask). I took the PST hydrogen alpha etalon I had around from a mod a few years ago and put if in front of one of those small neon bulbs. If you position the etalon with the camera side facing the bulb and look at the side that normally faces the telescope objective, the optics forms a nice collimated image of the bulb that your eye can focus on. What you also see are the lovely interference fringes typical of a Fabry-Perot etalon. Here is a photo I took with a Sony mirrorless camera with the bulb placed just outside the field of view (near the top left):
neon spectrum with PST etalon
neon spectrum with PST etalon
273.jpg (5.52 MiB) Viewed 472 times
The neon spectrum is dominated by a bright yellow line at 5853 angstroms and then a whole series of red lines (which combined gives the familiar neon colour). So what we see in this photo are the different yellow and red spectral lines split and then they repeat concentrically. I believe the red spot in the centre has some significance for the hydrogen alpha wavelength (perhaps there are better experts out there who know more).

I took this image and put it into the program RSpec (used for looking at astronomical spectral lines) to evaluate the finesse of the etalon. The finesse is related to the ability to distinguish between closely spaced spectral lines and therefore also related to what we commonly call the "bandwidth" of the etalon. A higher finesse implies the ability to distinguish more closely spaced spectral lines.
neon spectrum pixel map
neon spectrum pixel map
273-pixels.jpg (150.95 KiB) Viewed 472 times
The finesse can be defined as the distance between repeating spectral lines (the free spectral range) divided by the FWHM (full width half max) of the spectral line. My guess is that finesse of this PST etalon is around 7, which would correspond approximately to a bandwidth of 1 angstrom. I could not find much literature on PST finesse measurements, only the paper by Cyril Bazin and Serge Koutchmy, "Photometric properties of new solar Ha commercial Fabry-Perot etalons". Here they measured the front mounted Coronado etalons to have finesse of 8.7 for 40mm, 13.3 for 60mm and 13.9 for 90mm. In their measurements, the larger aperture etalons had higher finesse so it is not surprising that the 20mm aperture PST etalon has a lower finesse.

I think this technique could be useful for rapidly evaluating PST etalons. As I understand it, there is a wide variation in PST etalon quality. The neon diffraction patterns give a good indication if the etalon is aligned well (the rings are perfectly concentric if alignment is good). The sharpness (finesse) of the lines is an indication of the etalon bandwidth, which depends on the flatness and the reflectivity of the internal etalon surfaces. I believe the central colour is an indication of how well tuned the etalon is to hydrogen alpha (presumably close tuning requires less tilting).

If someone wants to try this, I used miniature neon bulbs (ordered from eBay) and powered it with 110V AC through a 0.5W 10k resister in series (this is a lower resister than typically recommended but it gives a brighter light, presumably with a shorter lifespan -- but the bulbs are very cheap).



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Re: Evaluating the PST etalon with a neon bulb

Post by Merlin66 » Mon Dec 28, 2020 5:08 am

A very interesting post.
I think the wide spread of the neon spectrum is not ideal for this sort of evaluation. (See attached)
neon2.gif
neon2.gif (20.15 KiB) Viewed 465 times
Possibly a laser pointer with a defined emission wavelength would be better.
I use Bass Project (https://groups.io/g/BassSpectro) for all my spectral processing. It would allow you to determine the FWHM, the lne spacing and approx finesse.
If the bandwidth of the PST etalon is around 0.7A (which seems to be a good figure for a good PST) and the measured F-P comb line spacing is 10A, then the finesse is close to 7.


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Re: Evaluating the PST etalon with a neon bulb

Post by DeepSolar64 » Mon Dec 28, 2020 1:54 pm

Interesting 🤔


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Re: Evaluating the PST etalon with a neon bulb

Post by christian viladrich » Mon Dec 28, 2020 6:51 pm

It is better to use a Ha lamp.
It is relatively easy to derive the FSR from the measurement. FWHM estimation is more difficult.
Have a look here for this tests and others:
V. I. SKOMOROVSKY. Commercial Narrowband Fabry-Perot Solar Filters, Methods and Instruments for their Examination. Solnechno-zemnaya fizika. 2015. Vol. 1. Iss. 3. P. 72–90.


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Re: Evaluating the PST etalon with a neon bulb

Post by thesmiths » Tue Dec 29, 2020 1:54 am

I attach an image of the neon spectra which shows the six visually bright spectral lines near the strong yellow line at 5853 angstroms. These are the lines that form the repeating concentric rings of the Fabry-Perot interference pattern in the image at the top.
3823.jpg
3823.jpg (78.18 KiB) Viewed 415 times
The spectrum was taken on the same neon bulb with a home built 1800 line/mm diffraction grating spectrometer using a Canon 100D camera.

The reason I thought this experiment was interesting and potentially useful was a comment I read once that was along the lines of "10% of PST etalons are excellent, 80% are good and 10% should never have left the factory". I think it has also been remarked that the results from a PST mod depend very much on the quality of the etalon you start off with. My suggestion with the neon spectra is that it could be a quick and cheap way to evaluate an etalon's quality. Presumably the normal way involves actually putting the etalon into a large aperture telescope.

If anyone has a large collection of PST etalon's that have been characterised (I think there are a few of you out there), perhaps you could take a look at the neon bulb spectrum visually and see if this is a possible method to do fast quality control.

I would imagine that the main determinants of image quality will be symmetry of the rings (implying parallel alignment of the etalon mirrors) and the extent of the central red spot. I suspect finesse is actually not that much of an issue; the finesse seems to kept intentionally low in order to boost the light throughput. Finesse will increase with higher reflectivity but then light throughput will go down, so small internal etalons are probably deliberately made with lower finesse than the large external ones.



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Re: Evaluating the PST etalon with a neon bulb

Post by Bob Yoesle » Thu Dec 31, 2020 12:16 am

As Christian notes, a better methodology would be to use a Hydrogen lamp with a diffuser. See:

viewtopic.php?f=10&t=15443&start=50#p174440

As seen below, a well performing air-spaced etalon will have a small central ring surrounded by additional tightly focused interference rings (e.g. secondary etalon peaks). This small size of the central ring indicates the etalon CWL is just slightly above the H alpha emission (to allow tilt tuning), and the narrowness of the succeeding rings indicates a relatively good finesse:
SM90 Internal.jpg
SM90 Internal.jpg (111.73 KiB) Viewed 370 times
Air spaced etalon CWL fringes 2.jpg
Air spaced etalon CWL fringes 2.jpg (59.34 KiB) Viewed 370 times
Finesse 3.jpg
Finesse 3.jpg (51.05 KiB) Viewed 370 times
The higher (further off-band) the CWL of the etalon, the larger the inner ring becomes:
SM90ii CWL uncompressed SM.JPG
SM90ii CWL uncompressed SM.JPG (71.47 KiB) Viewed 370 times
This particular etalon's CWL is too far off-band to be brought on-band by tilting alone.

Adding mechanical compression via a center pin, the inner ring can become a tightly compressed "spot," indicating the etalon is now exactly on-band:
SM90ii compressed center SM.JPG
SM90ii compressed center SM.JPG (71.86 KiB) Viewed 370 times
However, if pressure is applied unequally to the etalon periphery, while the center of the etalon is on-band, the periphery may not be, as can be seen when moving off-axis and the spot expands to a larger ring:
SM90ii compressed off axis.JPG
SM90ii compressed off axis.JPG (197.64 KiB) Viewed 370 times
This represents an unequal spacing of the etalon plates; thus the uniformity of the band pass is unequal, and is the sum of all areas of the etalon - part on-band and part off-band - results visually in an image that is produced by a much wider bandpass etalon as seen of the left, versus the uniformly spaced etalon gap tilt-tuned on-band etalon on the right:
Closu compare SM 2.jpg
Closu compare SM 2.jpg (136.72 KiB) Viewed 370 times
My interpretation, which is subject to correction ;-) is that the inner etalon fringe ring will have the appearance(s) below depending of the CWL of that portion of the etalon being viewed:
Etalon fringes and CWL sm.jpg
Etalon fringes and CWL sm.jpg (183.26 KiB) Viewed 370 times
I also conclude that the size of the inner ring represents how far on or off-band the CWL of the etalon is tuned, and the uniformity of the size of this ring/spot across the etalon represents the uniformity of the etalon gap. A perfect etalon will have essentially no change as one views the fringes off-axis. Variation in fringe diameter across the etalon indicates etalon gap variations, which if large enough can degrade the etalon FWHM performance. This variation can represent a spherical deformation, wedge, or other non-uniformity.

For a mica spaced etalon, only the inner ring should be visible (the outer off-band fringe rings - the secondary etalon peaks - will be blocked by the blocking filter). When on-band the ring will be the on-band "spot," not a ring, and the off-axis spot should remain a spot, indicating good etalon thickness uniformity.


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Re: Evaluating the PST etalon with a neon bulb

Post by Merlin66 » Thu Dec 31, 2020 12:35 am

Bob,
Interesting.
As the CWL will vary with tilt (or Rich view tuning, or pressure tuning) and we do want occasionally to record the solar features off-band, what's the relationship of the data you collected to the FWHM performance?
There's a review of commercial etalons:
https://www.researchgate.net/publicatio ... XAMINATION
(I disagree with their Fig 3. The etalons don't have to be off band to each other to gain the reduced FWHM)


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Re: Evaluating the PST etalon with a neon bulb

Post by Bob Yoesle » Thu Dec 31, 2020 3:30 am

Hi Ken,

That's a very good question. These procedures are more qualitative than quantitative. Right now I don't have the equipment that can derive the FWHM. It appears one would need a system such as the one linked below in order to determine the FWHM:

https://www.holmarc.com/fabry_perot_int ... er_ccd.php

I've shared my results with Christian Viladrich, as well as the above link, and he's at work as I write on making some more experiments how to derive both the FSR and FWHM from the etalon fringe assessments. Ultimately I believe a high resolution spectrometer is likely required, something not so easily achievable by the DIY community - but worth trying. Christian and I have also explored examination of etalons for their uniformity and presence of striations. I used a simple pair of magnifying glasses for my on-the-fly collimator set up.
Collimator Striations sm.jpg
Collimator Striations sm.jpg (251.7 KiB) Viewed 356 times
I hope Christian will post some of the findings he's made with his use of better collimator lenses.

Ultimately I hope it will become easier for consumers to be able to objectively judge the quality of the commercial etalons that are being produced, and perhaps reinforce the importance of better quality control with the OEMs.

And agreed, Figure 3 in the cited work is incorrect.


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Re: Evaluating the PST etalon with a neon bulb

Post by Merlin66 » Thu Dec 31, 2020 4:41 am

Bob,
OK.
I have a few (!!!) spectrographs which are capable of resolving down to 0.26A but for this experiment it may not be necessary...

If a reference wavelength (Ha) is used and we image the F- P rings we could determine both the spacing and the FWHM by accurately recording the spacing and FWHM in pixels (Astroart, and I suppose other software can take a profile across the image to allow these measures)
The spacing of the rings is dependent on wavelength. Delta Lambda = [( Lambda 1)^2]/2d where Delta Lambda is the wavelength difference between the fringes, Lambda 1 is the reference lamp wavelength and d the spacing of the etalon gap.
Measuring the fringe spacing we can scale the FWHM measurement into a resolution in Angstrom.

All this is technically interesting....but not sure at the moment as to how the results could be used by the "average Joe" to give him/her guidance in determining whether on not to buy a $10,000 air spaced etalon......


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Re: Evaluating the PST etalon with a neon bulb

Post by christian viladrich » Thu Dec 31, 2020 12:18 pm

Well ... this is a work in a progress. François Rouvière (co-writer of Solar Astro) has worked out the math to derive the FSR and FWHM from the fringe pattern. As a reminder, the fringe pattern seen when the etalon is used as an interformeter, is not the same as when the etalon is used as a filter.

On the basis of our current experiments, it is relatively straigthforward to get a good estimation of the etalon gap and FSR.

FHWM estimation is much more dependant on the conditions of the measurement. More work is need there ...


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Re: Evaluating the PST etalon with a neon bulb

Post by marktownley » Thu Dec 31, 2020 12:30 pm

Interesting thread thanks guys!


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Re: Evaluating the PST etalon with a neon bulb

Post by Bob Yoesle » Thu Dec 31, 2020 4:41 pm

Hi Ken,

"but not sure at the moment as to how the results could be used by the "average Joe" to give him/her guidance in determining whether on not to buy a $10,000 air spaced etalon......"

My thoughts are that right now the typical FWHM specification generically applied to etalons for solar filter purchase are meaningless. The OEM's do not typically provide any sort of actual specifications for their individual filters (DayStar Quantum PE's excepted). So one is left essentially trusting the reputation of the OEM as to whether or not they will supply a filter performing the way it should.

However, if one purchases a filter with a warranty, after the purchase the interference rings can reveal basically if the filter has a CWL close to being on band, and whether or not that CWL is maintained sufficiently across the etalon to produce a good performance. Depending on how the filter is tuned, one can also assess if the filter is capable of coming uniformly on-band.

This relatively simple interference ring test of the etalon can therefore form the basis of explaining the etalon's real world performance, and may provide a reason for a less than good performance and possible justification for return/replacement based on an objective and repeatable assessment. If enough consumers avail themselves of this, it might lead to better QC and fewer issues with etalon performance - which in the long run will benefit both OEMs and the consumers of these expensive devices.

The OEM's might also consider supplying better specifications for the filters they make, or segregate etalons based on their optical characteristics, with the less expensive etalons warranted to meet less demanding specifications, and the higher-end filters meeting more strict tolerances. When you as an everyday consumer buy a car, your purchase can be based not only on the historical reputation of the automaker, you can chose different levels of trim and refinement. You also generally are given specifications for fuel efficiency, acceleration, turning radius, etc., and if you get a "lemon" which doesn't measure up, you have the basis for a recompense. There is no reason the same principles should not apply to these optical filters intended for purchase by average Joe consumers like you and me...

BTW - Do you have any links to the hi-res spectrographs you have made or otherwise acquired?


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Re: Evaluating the PST etalon with a neon bulb

Post by Merlin66 » Thu Dec 31, 2020 5:14 pm

Bob,
I designed and manufactured the original Spectra-L200 Littrow spectrograph. This is capable of achieving up to R=10000.
I also have a couple of Classic spectrographs, an "MG80" spectrograph made from readily available commercial components and various spectroheliographs.
Unfortunately the only current commercial spectrographs for the amateur are supplied by Baader (DADOS), Shelyak (ALPY, LISA, LHiresIII) and the Starlight spectrograph.
Many amateurs are turning to the 3D printed LowSpec instrument by Paul Gerlach.
There are some details on my website as well as the forum.
http://www.astronomicalspectroscopy.com/
https://groups.io/g/astronomicalspectroscopy

There's an extensive thread running on SGL discussing the LowSpec, it's manufacture and testing.
https://stargazerslounge.com/topic/3433 ... nt-3732525


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Re: Evaluating the PST etalon with a neon bulb

Post by Merlin66 » Thu Dec 31, 2020 10:07 pm

Building a high resolution spectrograph is not difficult, or necessarily expensive, but it does require some attention to detail and careful workmanship.
An adjustable slit, a couple of binocular objectives from Surplus Shed and a reflective diffraction grating from Optometrics are the main components.
The SimSpec V4 spreadsheet will show the possible performance based on your components.
SimSpec V4_3.zip
SimSpec design spreadsheet and notes
(422.31 KiB) Downloaded 7 times
(I do still have some of the Surplus Shed adjustable slit assemblies, if you need one let me know.)


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Re: Evaluating the PST etalon with a neon bulb

Post by Merlin66 » Fri Jan 01, 2021 5:00 am

Bob,
Looked at your images....
I think to collect and analyse useful data a more robust and repeatable set-up is needed.
Too much variation at the moment . Measuring field angle??
The attached shows the ring spacings and the FWHM of the rings.
(The second image was not good enough to properly analyse - too much background.)
SM90 etalon_yoesle.png
SM90 etalon_yoesle.png (42.85 KiB) Viewed 291 times
A few years ago I tested a SM90 etalon which was performing poorly.
A quick image with the spectroscope clearly showed the etalon was de-contacted...returned to Meade, six months later came back...guess what it was still unusable! I gave them the evidence and they accepted it back once more...almost six month before it was finally returned, I believe OK.
Two minutes with the spectroscope was sufficient. The CWL and FWHM etc could quickly and easily be measured. (I didn't do any detailed field angle v's CWL measurements)
Top LHS the solar spectrum around Ha (the dark absorption feature near centre).
The associated profile shows the width of the Ha line.....
The top RHS is the SM90. You can see the transmission "bands" are not sharp.....
The bottom shows my SM60. Here you can clearly see the transmission bands and sharp gaps between them. Where the "peak" drops down shows the etalon is "on-band" sitting in the Ha line.
SM90_SM60_test_071114.JPG
SM90_SM60_test_071114.JPG (209.38 KiB) Viewed 291 times


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Re: Evaluating the PST etalon with a neon bulb

Post by Bob Yoesle » Fri Jan 01, 2021 7:02 pm

Hi Ken,

Thanks for all the links and additional information, and the very nice analysis of the etalons!

I would indeed be interested in obtaining a suitable slit mechanism, PM me when you have the time.

Happy New Year :-)


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