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What makes a H-alpha filter work?

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#1 Al Canarelli

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Posted 20 May 2008 - 11:34 AM

I owned a Coronado 60mm SolarMax about 5 years ago and I recently preordered a 60mm Lunt, so I should know the answer to this question. Unfortunately, I don't really. I mean if someone asked me this very question, I may be able to fudge up a passable answer, but I have only a shallow understanding and I'm sure my lack of a complete understanding will show through in my response. Anyone what to take a try at this?

#2 darkstar528

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Posted 20 May 2008 - 11:50 AM

Maybe this will help: H-alpha schtuff

Or Wikipedia explanation

If the person is technically challenged as I am, I would just say light is emitted in wavelengths, the filter blocks all but the h-alpha, how, don't know, don't care...If they are techies by nature, I ask, "why are you asking me, you're the techie!" LOLOLOL

#3 Fish

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Posted 20 May 2008 - 02:11 PM

Al,

Here's a (brief but not all-encompassing) idea of what's happening inside the tube. Note that Coronado and Lunt scope are somewhat different in their design but the general idea is the same. This will focus (!?) on the Solarmax which is what you stated you used.

First, the light passes through an Energy Rejection Filter (ERF) that rejects most of the infrared (IR) and ultraviolet (UV) light while passing most everything else including the band that includes Ha. This is primarily to remove unwanted energy, especially heat, from the rest of the system.

Next, the light passes through the etalon. This is the heart of the system and is an interference filter. Two partially reflected surfaces, spaced a small but precise distance apart, act to pass certain wavelengths very sharply and reject others. Typical bandwidths of this peak are 0.7A FWHM - Full Width at Half Maximum - where 1 A is 10 E(-10) meters. Etalons by design are also "comb" filters as they have multiple peaks. In this case, one of the peaks is around the Ha line and the others are above and below this one.

After the etalon comes a converging lens assembly, basically a refractor objective. Coronado uses a doublet but other designs use only single elements as chromatic abberation is not an issue with an Ha bandwidth so narrow as the one of interest. This lens begins to converge the beam so it may be observed with an eyepiece or camera.

Finally, the light passes through a blocking filter, whose purpose is to filter out the unwanted peaks that the etalon passed in addition to the Ha peak of interest.

The output of the blocking filter heads to your eyepiece, then your eye, then your brain that sends a "WOW - That's cool!" command to your vocal chords. Ah, technology!

As I said earlier, this is an overview. Lunt Solar Systems has similar parts but in other configurations and even Coronado in its PST used a different setup. However, the general idea is the same.

So, now you know the basics. Welcome to the solar community!

Regards,

Marc

#4 ragebot

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Posted 20 May 2008 - 03:41 PM

Seems like I have read something about destructive interference from the first on the first reflective surface and constructive interference from the second reflective surface.

Part of the problem with trying to explain HA filters is you need a good understanding of light and optics; which I certainly dont have.

#5 Fish

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Posted 20 May 2008 - 06:44 PM

OK, let's go down one more level . . .

An etalon is actually very simple in basic design but has tolerances that are extremely critcal for desired operation. It's not the complexity that drives up the price: it's the required precision of the optical surfaces and exacting control of the distance between them.

As I stated before, the etalon is an interference filter. The basic - and by far not the only - equation for its performance is given by:

n * 'wavelength' = 2 * 'spacing' * cos(incident angle)

What this means is that every wavelength that is a multiple (n) of twice the distance between the optical surfaces will constructively interfere - add - and be passed through the etalon with minimal attenuation.

There's the constructive interference you mentioned.

Now, this distance is corrected by the cosine function of the equation if is not directly on-axis. For light directly striking the etalon (angle = 0), the cosine (and correction) is equal to 1. Note that all non-zero angular corrections result in a cosine < 1; this translates to a shorter wavelength coming to constructive interference and a consequent "blue-shifting" of the passband. This can be used to our advantage if we wish to view a phenomena that is racing toward us; the doppler shift of that motion can be accommodated by purposely introducing a non-zero incident angle.

That's what the Coronado "T-Max" does. It tilts the entire filter just enough to add a blue-shifting component.

Does that help?

Marc

#6 David Knisely

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Posted 20 May 2008 - 06:53 PM

Seems like I have read something about destructive interference from the first on the first reflective surface and constructive interference from the second reflective surface.

Part of the problem with trying to explain HA filters is you need a good understanding of light and optics; which I certainly dont have.


Well, not quite. Both plates in a Fabry-Perot etalon are partially transmitting. What happens is that some of the light gets all the way through the first plate and goes on to the second plate surface where it reflects and heads back towards the first plate. It then interacts with light that has just come through the first plate, resulting in interference effects. Light then heads back to the second plate and manages to get through that plate and on to the eyepiece. The wavelength that does manage to get through all of the filter depends on the plate separation and tilt. Clear skies to you.

#7 colinsk

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Posted 20 May 2008 - 10:46 PM

A more technical description of an etalon can be found here:

http://en.wikipedia....iki/Fabry-Perot






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