Internal vs. external Etalon placement question.

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Internal vs. external Etalon placement question.

Post by RKBerta »

Looking to upgrade my Coronado 60mm Etalon mounted on my TV 85 APO refractor using both BF15 and BF30 blocking filters. I see that some have an internal Etalon and others have it externally on the end of the scope like my existing setup. I would want to get a double stacked setup in either 100mm or 152mm size aiming towards the Lunt although not fully decided yet.

Is there any difference/advantage of having the Etalons mounted internally or externally?

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Re: Internal vs. external Etalon placement question.

Post by marktownley »

You could get a pair or externally stacked Lunt or Solarscope 100mm etalons, but these are five figures cost wise, regardless of currency. At 152mm you are only talking the Lunt. Internal etalons will only do close ups, a pair of externals with a large enough chip will do full disks. These scopes, either, are only really suitable up to about f20/25 imaging wise. If you want closer get a rear mounted etalon. I'd keep that 60mm also if you can.

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Re: Internal vs. external Etalon placement question.

Post by Bob Yoesle »

Ditto to what Mark has said ;-)

Here's a little more detail as to why:

All etalons are sensitive to the angle of light passing through them, and theoretically perform best with only parallel light passing at a 90 degree angle ("normal") to the reflective elements. The Sun, being an extended object instead of a point, will always have associated "field angles" of a minimum of 0.25 degrees from center to edge. As the field angle becomes increased passing through an etalon, that portion of the image falls "off-band" and eventually you do not see any chromopshere detail, only continuum.

Because a front mounted etalon only "sees" this native minimum 0.25 field angle change from the Sun, it will generally perform best for full disc views - all the angles will still be well within acceptable levels to maintain good contrast uniformity over a wide (1+ degree) field of view. Internal and rear mounted etalons require additional optics to attempt to render the light normal to the etalon, but can never fully restore the field angles to their native ones, and/or can add instrument angles to the mix. These magnified field angles and additional instrument angles can contribute to decreased contrast uniformity and "sweet spots" where noticeable contrast changes occur across the disc, and band-pass widening. Contrast issues can become even more noticeable with a narrower band-pass, especially with double stacking imaging.

However, making large etalons for full aperture use becomes increasingly difficult with increased size, and they become quite expensive. Smaller etalons can usually be made that have better overall optical "finesse." Additionally, one generally does not need great aperture or resolution for full disc viewing or imaging, and the advantages of large front mounted etalons becomes almost moot for medium to hi-res close-up viewing and imaging.

Therefore, for full disc viewing and imaging, front mounted etalons of up to 100 mm are ideal. Above this aperture, etalon prices become "astronomical," and internal or rear mounted etalons are favored for their better performance and decreased cost for close-up hi-res viewing and imaging.

For full disc viewing and imaging, a front-front or front-internal double stack is preferred. For close-up viewing and imaging, a large aperture scope using internal and/or rear double stacking is most practical. Depending on your local seeing 100 mm double stack might be ideal over a 152. So either a Lunt or Solar Scope 100 mm front double stack might be best here (the Lunt filters will be more temperature stable due to the "root 3" spacer system). The Lunt or Solar Scope 100 mm front with the Lunt 100 internal etalon telescope would be the next best choice in this aperture. For a little less aperture, the new Coronado SM 90 III front double stacked scopes look good. The remaining choice for 100 mm double stack would be the internal double stack.

At 150 mm of aperture, you're basically limited to the Lunt 152 internal double stack, unless you decide to go the DIY route, which can become quite involved by itself just for single stacking, let alone double stacking ;-) None of these DIY choices will perform as well as the Lunt OEM. A 150 mm or greater aperture telescope with a DayStar or Solar Spectrum rear filter is a good option, but double stacking is more complicated. To get near double stacked performance from a single rear filter you need a filter with a band-pass of < 0.3 angstroms, and these too are quite expensive and tricky to set up. You will need an effective focal ratio of f45+, and this may be quite impractical without additional focal reduction systems.

So you could keep the 60 and double stack a second 60 for full disc viewing and imaging, and for hi-res get the 100 or 152 double stack. The 152 DS can always be stopped down when seeing conditions don't allow for full aperture use.

For more about etalons, see ... on-basics/

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