etalon tuning

Use this section to discuss "standard" Baader/Coronado/ Lunt SolarView/ Daystar, etc… filters, cameras and scopes. No mods, just questions/ answers and reviews.
Post Reply
User avatar
EricF
Ohhhhhh My!
Ohhhhhh My!
Posts: 198
Joined: Sat Jun 05, 2021 9:51 pm
Location: Vancouver Canada
Has thanked: 1243 times
Been thanked: 270 times
Contact:

etalon tuning

Post by EricF »

More questions. This time about etalons.

For PST and SM-II, internal etalons, we compress the etalon to tune the centre wavelength. And Lunt changes the air pressure (refractive index) to tune CWL of internal etalon. so far, so good?


For single Coronado SM-II or SM-III external etalon these have rich-view (compresses the etalon) and T-max (tilts the etalon), so we could use either method to tune CWL?

However, isn't it best to remove all tilt, and then use the compression tuner?

What's the point of the tilt adjustment? Isn't the compression (or pressure) tuning enough?


User avatar
MalVeauX
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1858
Joined: Tue May 09, 2017 7:58 pm
Location: Florida
Has thanked: 1171 times
Been thanked: 1360 times

Re: etalon tuning

Post by MalVeauX »

Hi,

Tilt is still needed in a multi-etalon system to move the reflection of the 2nd etalon away from the primary disc. Yes, it would be best to have no tilt and simply have excellent perfectly uniform air compression between the etalon plates. But you still have reflections when lining several of these up. If using a single etalon, no tilt is needed if you can tune via pressure.

Very best,


User avatar
Bob Yoesle
Almost There...
Almost There...
Posts: 994
Joined: Thu Mar 08, 2012 7:24 pm
Has thanked: 539 times
Been thanked: 807 times

Re: etalon tuning

Post by Bob Yoesle »

It all comes down to how these methods are implemented.

Assume that the air-spaced etalon in question is uniform in etalon plate spacing and plate flatness, so these issues are as good as can be reasonably attained.

Overall, air pressure tuning is the best possible method of tuning for a single air-spaced etalon. It can remain normal to the optical axis with no tilt, and tuning is accomplished by changing the refractive index of the etalon gap, which is accomplished by changing the air density (via air pressure increase/decrease). This change in refractive index is therefore very uniform for all portions of the etalon. These etalons are made to have a native CWL less than 656.28 nm, and increasing pressure red-shifts the etalon to come on-band. Even if the manufacturing of the etalons results in varying etalon spacer thicknesses between etalons, all that is needed is a little more or a little less pressure to get the particular etalon on-band. As long as the spacers have uniform thickness, contrast uniformity should remain optimal.

Tilting can also be used to tune the etalon, which is made to have a normal CWL just slightly above 656.28 nm. The CWL can be shifted blue-ward to get on band with a minimal degree of tilt, and still have excellent contrast uniformity. However, it seems some etalons have a native CWL that is higher than needed, and too much tilt to get on-band will result in a varying contrast (differing CWL) across the etalon face. As Marty notes, a little tilt to get on-band is ideal for double stacking etalons so as to both get on-band and move the retro-reflections between the double-stacked etalons completely off the primary image.

Mechanical pressure tuning is used to tune etalons by giving a small amount of etalon spacer compression, but appears to be the least ideal method of tuning because any mechanical pressure needs to be applied very equally to all etalon spacers. This is theoretically possible, but difficult to implement. It is also implemented differently in its various embodiments. As with the tilt-tuned etalon, the native CWL is high, and compression decreases the width of the etalon gap and blue-shifts the etalon. In the PST for example, pressure is applied around the peripheral spacers - and there is no central spacer needing compression. If the compression is applied equally, the etalon remains uniform in contrast and FWHM across its diameter. If pressure is applied unequally, the FWHM changes across the etalon, and the contrast will vary as well.

With the RichView etalons, pressure is applied only to the central spacer. Therefore unless the etalon's native CWL is very close to being on-band (just as with tilt-tuned etalons) the pressure needed to come on-band can be disproportionately excessively applied to the center and not the periphery, and the FWHM and contrast will vary from center to edge, and the effective overall FWHM broadens and contrast is reduced. RichView etalons can therefore require tilt to get them closer to being on-band, hopefully without introducing excessive contrast variation ("banding"), and then use hopefully minimal amount of central spacer compression to get on-band without broadening the FWHM too much and decreasing overall contrast performance.

Lastly, heating of solid etalons changes the gap spacing once the etalon reaches a uniform temperature - which can take several minutes or longer. These etalons are made to have a lower CWL at room temperatures, and heat is applied to cause thermal expansion of the etalon gap, shifting the etalon red-ward to come on-band at a temperature significantly above any ambient temperature to be encountered, and therefore remain on-band despite environmental temperature changes.
Last edited by Bob Yoesle on Sun Jul 25, 2021 12:29 pm, edited 2 times in total.


Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

Dark-Sky Defenders
Goldendale Observatory
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: etalon tuning

Post by Merlin66 »

I’d like to see a comparison of the Coronado Rich view tuning and the Lunt pressure tuning.


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
EricF
Ohhhhhh My!
Ohhhhhh My!
Posts: 198
Joined: Sat Jun 05, 2021 9:51 pm
Location: Vancouver Canada
Has thanked: 1243 times
Been thanked: 270 times
Contact:

Re: etalon tuning

Post by EricF »

Thanks guys, just processing all the information.

I've seen some earlier Lunt 60 scopes, which appear to have a internal etalon, with a tilt mechanism. I guess for this we are we are counting on the etalon being very close to 656.28 and not having to tilt very far to tune the etalon ?


User avatar
Bob Yoesle
Almost There...
Almost There...
Posts: 994
Joined: Thu Mar 08, 2012 7:24 pm
Has thanked: 539 times
Been thanked: 807 times

Re: etalon tuning

Post by Bob Yoesle »

Yes, exactly.

Just as with the early original Coronado Tucson etalons, these require higher spacer thickness tolerances and are therefore more difficult (and expensive) to produce.


Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

Dark-Sky Defenders
Goldendale Observatory
Post Reply