Etalon Tuning

[AndyWB]

I've just been out at lunch to have a look at the Sun (some big sunspots, and something is coming around the side of the Sun). I noticed, while I was out, that the number of turns it took to tune the etalon on my Lunt 35 was rather more than normal. Based on experience, I'd expect a couple of turns to be enough; it nearly always is. This time it was 4 or 5 before that "things appear" moment. I'm just wondering - what might have caused that change?

I guess that this is why the etalon is designed to be tuned, but what causes the difference? It's quite warm here, today - ambient temperature, maybe?

[Zakalwe]

Atmospheric pressure. The Etalon works by altering the refractive index of the air inside the Etalon chamber. Air's refractive index varies with pressure

[AndyWB]

Hmm. Okay, that explains to me what's meant by a 'pressure tuned' etalon (not what I thought!), but I don't think that's how the Lunt 35 works. It has a small brass wheel that seems to change the angle of the ... something. I'm not sure what to call it. The front of the scope. I think it's actually tilting the etalon, but I'm kinda hazy on this.

[michael.h.f.wilkinson]

Atmospheric pressure. The Etalon works by altering the refractive index of the air inside the Etalon chamber. Air's refractive index varies with pressure

The LS35 is tilt tuned, not pressure tuned. Temperature does have an impact on many etalon designs. Air pressure also influences etalons with an air gap.

[callump]

yes, on non-pressure tuned etalons in the Lunts, tuning is done by a slight tilt to the etalon.

I used to know the physics of this, but that was a long time ago...

You can see a similar effect with deep sky filters (like OIII) - if you hold them to your eye at the eyepiece and tilt it slightly will alter the central passband, so you can see some objects (like 'stellar' PNe) blink.

Callum

[AndyWB]

Hmm. Well, maybe it's temperature then. It just struck me as weird, and I guess there was a thought of 'Oh God, is something loose' (which seems unlikely). Still worked well enough, just needed more turns. I think I'll try again one morning, see if that makes a difference.

Callum - I'll have to give that a try!

[Peter Drew]

Tilting the etalon if an airspaced one increases the length of the air gap compared to the light beam, this alters the refractive index of the air, as does temperature.    

[Zakalwe]

The LS35 is tilt tuned, not pressure tuned. Temperature does have an impact on many etalon designs. Air pressure also influences etalons with an air gap.

Oops. Of course, you are correct Michael. I was thinking of the Ls50.

[Merlin66]

Just a correction...the tilt increases the air gap between the etalon plates and hence the central wavelength. Tilting moves the wavelength from the red towards the blue.

Atmospheric effects are superimposed on this tuning.

Not sure why the amount of tilt would vary...just check the obvious connections of the etalon to the scope.

[michael.h.f.wilkinson]

In theory, there is another effect: Doppler shift due to the rotation of the earh. East of meridian the line is slightly blue shifted, west slightly red. The effect is minute: 0.01 Angstrom at most.

[Pig]

I have tried mine today Andy and the tuner did take a few extra touches to get the best view. However,the old position is still as good as always so I think there may be another factor that is giving a better than normal view

[AndyWB]

Well, I mean to write an update on Friday - on Friday morning, while it was still cooler and the Sun was lower in the sky, I had another gander. The etalon did need much closer to a 'normal' number of turns to tune it. Still a bit more, but not as much. The scope was cooler, coming from my flat rather than having been in a hot car, and it was 0800. 

So, temperature or air pressure, I think.