Viewing the PST etalon sweet spot

[thesmiths]

I'm not sure if this has been shown on Solarchat (at least recently). I put the etalon from a Coronado PST in front of a hydrogen spectral tube. A piece of Rubylith red plastic filter was used to filter out the blue hydrogen beta line. A diffuser from a 35mm slide copier was used to provide a uniform light source. The size of the uniform and filtered window is approximately 24mm x 36mm, so matches the size of the etalon fairly well.

Both collimating lenses of the etalon were in their normal positions but the etalon was viewed from what would normally be the telescope side. This allowed the camera to come to focus as it was effectively seeing the light source near to infinity. This photo shows the tuning ring positioned close to its "sweet spot".

PST etalon tuned to sweet spot -- looks a bit like the HAL 9000 computer

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Turning the tuning ring on a PST etalon shows how the interference pattern changes. This video shows the tuning of the etalon from one extreme to the other. Somewhere in between, when there is a single spot in the middle, is, I believe, the optimum "on-band" tuning.



Sorry for the image jumping around a bit (the camera was on a tripod but the etalon was handheld). For the video, the camera used autofocus and autoexposure so the fringes are not a sharp as in the still image (for which I could use manual exposure and manual focus).

I had disassembled the etalon (following the video which Mark did some years ago) in order to tighten things up and recentre the various spacers. By using the H-alpha light source, I was able to position the tuning ring to put the sweet spot near the centre of the tuning range. Previously, it was way off to one side and the tuning felt a bit soft and wobbly. Now it feels much firmer and doesn't jump around.

I also used some very nice grease called Nyogel 795A to make the tuning smoother (it is used in electronic volume controls). Having taken the etalon apart and seeing how it is constructed, I also think it is better to store the etalon turned all the way to the position where it puts the least compression on the spacers. That way, they won't get squashed over time and will be able to put sufficient pressure on the etalon to reach the sweet spot. I suspect if stored in a completely compressed state, the etalon would have to be readjusted more frequently.

[thesmiths]

If you are able to take some good photos and with appropriate software, you can calculate the "finesse" of the etalon and thereby how good the "bandwidth" is.

[christian viladrich]

Hello,
The use of the term "sweet spot" here can be a little bit misleading. By definition, the "sweet spot" is the part of the field which is "on band", meaning close to Ha. It depends on the optical setup, the size of the etalon, and the distance allowed from Ha.
Some information is provided here :
http://astrosurf.com/viladrich/astro/in ... -spot.html

What we see with the interference pattern has nothing to do with the sweet spot. Still, you can calculate the FWHM and the FSR of the filter from the interference pattern.
Examples are provided here :
http://astrosurf.com/viladrich/astro/in ... lters.html

Detailled procedure is presented here :
http://astrosurf.com/viladrich/astro/in ... rview.html

[Bob Yoesle]

Hi Doug,

That's an excellent video!

I also think it is better to store the etalon turned all the way to the position where it puts the least compression on the spacers. That way, they won't get squashed over time and will be able to put sufficient pressure on the etalon to reach the sweet spot. I suspect if stored in a completely compressed state, the etalon would have to be readjusted more frequently.

I agree, relieving the mechanical pressure placed on the etalon would be ideal to keep the resiliency of the compressive material from degrading over time.

As Christian notes the "sweet spot" refers to what is technically known as the Jacquinot spot, which is the area considered to be able to perform monochromatic viewing and imaging - defined to be the region over which the change in wavelength does not exceed √2 times the etalon bandpass - and outside of which the etalon will fall off-band.

In solar telescopes which use such collimation optics, the field angles through the etalon are magnified by the ratio of the objective FL divided by the collimator FL, and the Jacuinot spot is decreased from what the "naked" etalon would be due to this field angle magnification. So if the etalon has a native Jacquinot diameter of 1 degree (the typical air-spaced 0.7 FWHM etalon), and the FL of the collimator is 1/2 the that of the objective, the Jacquinot spot would therefore shrink to 1/2 a degree; and a collimator with 1/4 the FL of the objective has a Jacquinot spot of only 1/4 a degree - or one-half the Sun's diameter, etc.

To carry out Christian's detailed etalon FSR and FWHM analysis, you probably would have to remove both the collimator lens and refocusing lens to get the most accurate measurement parameters.

[marktownley]

An excellent thread! Thanks guys!