Sweet spot, banding and tilted etalons ...where are the limits ?

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Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

Hi,
**************************************************************************************************************************************
To the moderators, I am not quite sure to what section this post belongs to, so feel free to move it in the correct one.
******************************************************************************************************************************************

As you might know, most front side etalons are to be tilted in order to tune them on the Ha line. With some etalons, the tilt is small (a few 0.1°), with others the tilt is "larger" (from 0.6° to more than 1°).
At last Zoom meeting, Bob Y rose the issue of "banding" formation when the tilt of the etalon gets large. Let's have a look at it.

First, let's remember the definition of the sweet spot : this is the area projected on the sky where the center wavelength of the etalon is equal to Ha +/- a given tolerance. Let's take a tolerance of 0.25 A.

If the etalon center wavelength is precisely at Ha at normal incidence, the"sweet spot" is a circle of 1° diameter. This is good news since the solar disk is only 0.5°.
http://astrosurf.com/viladrich/astro/in ... t/SS-2.png

Now, let's assume the etalon is 0.6A from Ha at normal incidence (this was the case of the SM III 60 I tested recently). Then the "sweet spot" is no longer a spot but a "sweet ring". Tilting the etalon on Ha moves part of the ring on the solar disk. In other words, the part of the Sun tuned on Ha is a "band"(so the term "banding")
http://astrosurf.com/viladrich/astro/in ... t/SS-4.png

The bad news is that the width of this band is only 0.33°, which is smaller than solar diameter ....

Let's assume a more extreme situation with an etalon 1.25 A away from Ha (this was the case of the SM III 60 RichView I tested). Then, the width of the "sweet ring" is only 0.22° (smaller than the solar radius). This is not exactly good news ...
http://astrosurf.com/viladrich/astro/in ... t/SS-5.png

Indeed, as soon as the center wavelength of the etalon at normal incidence is farther than 0.3 A from Ha, the width of the "sweet ring" is smaller than the solar diameter.

At this end of the day, it is much better to have etalon very close to Ha at normal incidence. But ... this is more difficult and more expensive to do.
http://astrosurf.com/viladrich/astro/in ... t/SS-6.png

The same holds (with different figures) for narrowband filters (1 to 2 A FWHM Ha, Ca K or H filters).

The whole page is there :
http://astrosurf.com/viladrich/astro/in ... -spot.html

Christian


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Great information Christian, thanks!

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

Thanks Christian! Very well put JAPP.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

justapictureposter wrote: Fri Feb 26, 2021 8:05 pm Very well reported Christian, and this is unfortunately the humpty dumpty story of solar filters!

companies will not throw underperforming glass filters in the trash unless there is a severe defect; companies will attempt salvage of the rejects by allowing an extended acceptable tolerance range.

This is why all of our filters are different, it is impossible to conform to such strict standards as designated by these requirements. IT is much easy to increase the acceptable tolerance.


with such extreme circumstances, a company must produce 9 bad filters so the 10th can be good. The 9 bad filters will be sold to a group of unsuspecting individual's (US) ; when the users realize their filter is not performing as it should; the warranty/return period has expired.

:( Sadface :(
I am afraid you are probably right :-(


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by krakatoa1883 »

Very informative reading, Christian. Btw something like the sweet ring you described can be observed in some pressure-tuned etalon, may be worth of investigation.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Some questions, if they may be entertained:

1) What's the ideal way to get a larger sweet spot on a rear-mounted etalon (or at the very least, a sweet spot large enough for a narrow FOV to not see the non-sweet spot)?

2) Is it possible to get a full disc inside of the sweet spot in Lunt's 60mm modular HA scope's etalon?

3) What would happen if one were to mount the etalon from a pressure tuned Lunt 60mm modular scope on the back of a longer scope in terms of how the sweet spot would change?

4) Is there any way to improve the sweet spot or preserve the sweet spot of a PST etalon?

From what I'm reading, it looks like the best way is essentially to get an etalon that is as close to on-band with the least amount of tilt needed to get there in the first place (which is near impossible for most of us), or basically a perfectly even pressure tuned etalon with the largest aperture (also near impossible to find).

It seems the legendary gear is already out there, and no new legendary class gear is being made, with respect to etalons?

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

justapictureposter wrote: Sat Feb 27, 2021 1:59 pm
MalVeauX wrote: Sat Feb 27, 2021 1:03 pm Some questions, if they may be entertained:

1) What's the ideal way to get a larger sweet spot on a rear-mounted etalon (or at the very least, a sweet spot large enough for a narrow FOV to not see the non-sweet spot)?
I successfully enlarged the sweet spot on my pst etalon; simply by removing the collimator and placing a much larger one deeper into the optical tube.

On my 127mm x 1200mm telescope i had installed an 80mm x 800mm plano concave; minus 800mm into the ota.

Next i took the original negative planoconcave lens out of the pst etalon, and left the face of the etalon bare.

. After further investigation, I had learned that the system was no longer using the full 127mm aperture; because the parallel beam output from the 80mm lens was 80mm in diameter; and the face of the etalon is only 21mm. The system loses input light by the overlap of the beam around the etalon. See deiagram.

To further on this concept, we may be able to squeeze the 80mm output down using mulitple lens elements.; I.e -800 to 800 , -400 to -400, -200 to 200. This would be a very complex beam expander; operating in reverse; however I do not know about optics to understand if the sweet spot would shrink to original size on the stock PST.

So in theory, yes it is possible to expand the sweet spot by enlarging the circumference of the solar image with a sacrifice of light; and further investigation is needed to determine if advanced optics will allow full objective power to be obtained.

This is the real world result, the sweet spot is no longer visible across the entire full disc;
I would be very interested to try this on either my 80mm F7.5 (masked to 60mm F10, or whatever is needed) or my 120mm F8 (I usually mask to 100mm F10 for this, or whatever is needed). Do you recall where you got the different lenses? Even just getting a little more FOV within the sweetspot would be a huge help.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by p_zetner »

Hi Christian.

I'm assuming banding wouldn't be an issue for etalons that are tuned slightly to the red for normal incidence? Here is a series of interferograms taken in H alpha (discharge tube + 10nm fwhm interference filter) for the Lunt LS50FHa etalon.
Montage_131626-742-821_levels.jpg
Montage_131626-742-821_levels.jpg (61.93 KiB) Viewed 422 times
From left to right: Etalon tilt adjustment wheel is turned completely CCW (left image), adjusted to "maximize" central spot (centre image) and completely CW (right image) with wheel rotation direction as viewed from the etalon back end. I didn't measure the etalon tilt by any independent means although this could be done optically or via a precision tilter like that used in the Cyril Bazin and Serge Koutchmy paper. The central spot in the centre image has a 1.5 degree fwhm which seems a bit high but is very advantageous if true!

It's apparent from the figure that the fringe pattern shifts in the field of view with changing etalon tilt. In the centre image there is a 0.6 degree offset between the centre of the fringe pattern and the centre of the fov. The left hand image gives a 2.5 degree offset! These interferograms were taken with an Edmund Optics 1" interference filter which constricted the field of view but allowed me to roughly measure the centre of the fov by using the "Measure" function in ImageJ to return centre coordinates for a circular selection, as shown in this figure:
131626 fov coords siz.jpg
131626 fov coords siz.jpg (37.09 KiB) Viewed 422 times
I'm guessing the etalon plates are slightly wedge shaped in order to account for this shift?

Cheers.
Peter


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

Interesting discussion, i've put in the mods section, not because was in the wrong place, but will get lost very quickly on the main board many posts down.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Any thoughts on the Lunt 60mm modular pressure tuned etalon they have available now? It's on an F7 setup. Thumb screws hold it on the back of the OTA. Should be fairly easy to configure to mount on the back of any OTA. I'd like to see if this is possible to mount on the back of big refractors (with proper DERF of course). But I'm curious, how will the sweet spot be on such an etalon? Is this worth exploring? Better than rear mounted PST etalon? Same? The main goal is getting a bigger sweet spot.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

MalVeauX wrote: Sun Feb 28, 2021 3:53 pm Any thoughts on the Lunt 60mm modular pressure tuned etalon they have available now? It's on an F7 setup. Thumb screws hold it on the back of the OTA. Should be fairly easy to configure to mount on the back of any OTA. I'd like to see if this is possible to mount on the back of big refractors (with proper DERF of course). But I'm curious, how will the sweet spot be on such an etalon? Is this worth exploring? Better than rear mounted PST etalon? Same? The main goal is getting a bigger sweet spot.

Very best,
I bet the free aperture of the etalon is less than you would think. Hopefully somebody will measure one.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

marktownley wrote: Sun Feb 28, 2021 6:46 pm
MalVeauX wrote: Sun Feb 28, 2021 3:53 pm Any thoughts on the Lunt 60mm modular pressure tuned etalon they have available now? It's on an F7 setup. Thumb screws hold it on the back of the OTA. Should be fairly easy to configure to mount on the back of any OTA. I'd like to see if this is possible to mount on the back of big refractors (with proper DERF of course). But I'm curious, how will the sweet spot be on such an etalon? Is this worth exploring? Better than rear mounted PST etalon? Same? The main goal is getting a bigger sweet spot.

Very best,
I bet the free aperture of the etalon is less than you would think. Hopefully somebody will measure one.
Agreed, I'm sure its not 60mm. It's internal. So maybe 18 to 26mm? But my question is, what sweet spot could be figured from this if setup on an F7 instrument such as up to a 150mm F7?

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

MalVeauX wrote: Sun Feb 28, 2021 7:36 pm
Agreed, I'm sure its not 60mm. It's internal. So maybe 18 to 26mm? But my question is, what sweet spot could be figured from this if setup on an F7 instrument such as up to a 150mm F7?

Very best,
Yes, it's all just scaling


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

marktownley wrote: Sun Feb 28, 2021 7:42 pm
MalVeauX wrote: Sun Feb 28, 2021 7:36 pm
Agreed, I'm sure its not 60mm. It's internal. So maybe 18 to 26mm? But my question is, what sweet spot could be figured from this if setup on an F7 instrument such as up to a 150mm F7?

Very best,
Yes, it's all just scaling
I'm trying to see if the sweet spot would be larger potentially than that from the PST etalon in a similar configuration?

I currently use the PST etalon but I lose a ton of FOV due to sweet spot, on all configurations. I'd like to expand the sweet spot somehow (if possible). Or just go for a different rear mounted option with a bigger sweet spot (that is NOT a powered option that needs F30+ like Daystar/SolarSpec/SolarScope. So just exploring internal Lunt pressure tuned etalon options.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

Hello Marty,
There is a simple rule for internal etalon in collimated beam :
- in an "optimal" system, the sweet spot diameter is proportional to the ratio d/D, where d is aperture of the etalon (and collimating lens) and D the aperture of the refractor.
Accordingly, the maximum diameter is acheived when d= D, i.e. when you have the etalon in front position.
Hope this helps


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

christian viladrich wrote: Sun Feb 28, 2021 8:58 pm Hello Marty,
There is a simple rule for internal etalon in collimated beam :
- in an "optimal" system, the sweet spot diameter is proportional to the ratio d/D, where d is aperture of the etalon (and collimating lens) and D the aperture of the refractor.
Accordingly, the maximum diameter is acheived when d= D, i.e. when you have the etalon in front position.
Hope this helps
Yikes...

So, a PST etalon at aprox. 20mm clear aperture, without any refractor or anything (just the opening into the etalon and the rear lens) would produce an equal d/D ratio of 1 for sweet spot size relative to the etalon. And a refractor objecitve of 40mm means the PST was at a d/D of 0.5. And so putting it into a 120mm F10 configuration, its only 0.16? And on my C8 Edge, only 0.1? That's why the sweet spot is so tiny on my C8 configuration with PST etalon then!

So to compare, the Lunt 60mm internal pressure tuned etalon (I don't know the size, let's say its somwhere between 18~26mm?) operating around F7 with a clear aperture collimating lens that is the same size or slightly larger to accept the light cone to it, means that a little 60mm aperture would be from 0.3 to 0.43 d/D sweet spot size ratio (smaller than the original PST?). And if placing it into a larger instrument, say 120mm, it would be anywhere from 0.15 to 0.22 basically? So potentially smaller, to potentially a little larger than the PST etalon in the same configuration?

Does that sound about right?

If so, then the idea is to get the biggest clear aperture etalon with biggest collimating lens possible for this, to ensure the largest possible d/D ratio, 100% limited by the aperture of the etalon.

I need to figure how to calc how that d/D ratio sweet spot then can be related to focal length and disc image size to see if I can see the optimal configuration to generate a full disc image within the sweet spot size of the etalon's configuration with the OTA?

Thanks!

++++++++++++++++
++++++++++++++++

Illustrating real world:

100mm F10 refractor (aperture 100mm); focal length 1,000mm; disc image size 9.09 to 10mm in size
PST etalon with F10 collimating lenses (aperture 20mm)
10mm blocking filter
d/D ratio is 0.2
sensor is 1.1" IMX253

Looking at the preview here, it looks like the sweet spot is around 1/5th the size of the solar disc in this case. But I'm not sure how to translate this to disc image size in mm or how to relate it so I can reverse calc what is needed to achieve a full disc in the sweet spot of a 20mm to 26mm etalon aperture?

FC_Preview_100mm_PSTetalon_IMX253.jpg
FC_Preview_100mm_PSTetalon_IMX253.jpg (100.61 KiB) Viewed 361 times

And here:

60mm F10 refractor (aperture 60mm); focal length 600mm; disc image 5.45mm to 6mm in size
PST etalon with F10 collimating lenses (aperture 20mm) trailing a SM60mm II series DS etalon (front mount)
10mm blocking filter
d/D is 0.33
sensor is 1.1" IMX253

Here, the sweet spot seems to be around 2/3rds of the solar disc, you can see it obviously falls off, it wasn't exactly center, likely due to the minor tilt or bandpass of the SM60II DS etalon on front, but was just to see.

PST_600mm_Test.jpg
PST_600mm_Test.jpg (72.98 KiB) Viewed 357 times
Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

MalVeauX wrote: Sun Feb 28, 2021 9:12 pm
Does that sound about right?
Yup!


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

MalVeauX wrote: Sun Feb 28, 2021 9:12 pm Does that sound about right?
Yes indeed !

As an example, you might remember Antonello Satta's refractor (not sure of his pseudo here). He made a 150 mm f/10 refractor with an internal Lunt 60 mm etalon. It was a kind of truss tube OTA.
This is also the principle at work in the 140/90 mm famous Ha refrator of Bob Y, with a 90 mm internal etalon.
For a given aperture, the larger the internal etalon the larger the sweet spot is.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

BTW, there is also a possible combinaison with a double stack Lunt 80 mm internal etalon. I don't remember what is the actual diameter of this etalon.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

christian viladrich wrote: Mon Mar 01, 2021 8:52 am
MalVeauX wrote: Sun Feb 28, 2021 9:12 pm Does that sound about right?
Yes indeed !

As an example, you might remember Antonello Satta's refractor (not sure of his pseudo here). He made a 150 mm f/10 refractor with an internal Lunt 60 mm etalon. It was a kind of truss tube OTA.
This is also the principle at work in the 140/90 mm famous Ha refrator of Bob Y, with a 90 mm internal etalon.
For a given aperture, the larger the internal etalon the larger the sweet spot is.
Interesting thanks; so realistically the 35mm, 40mm, 50mm, 60mm front mounted etalons are going to be larger etalons and if they were mounted collimated rear-mounted they would provide a much larger sweet spot then since they are all going to be larger than the internal designed etalons.

I'm curious now what the internal clear aperture is of the 60mm pressure tuned, and really interested then in how large the 80mm internal double stack by Lunt is. If it's even bigger or not.

I'm very interested to learn more about the 150mm truss frac with an internal 60mm Lunt etalon.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

justapictureposter wrote: Mon Mar 01, 2021 2:08 pm ;i made the LS50C modkits, with the intention of doubling the size of the etalon for least cost. The closer you get to the limb it goes off band because the solar image size on the etalon requires an -800mm collimator or greater. SO i then developed the -800mm collimator.

The deeper you get into the tube, you require a larger diameter collimator. an 80mm collimator requires an 80mm diameter etalon or else you vignette the aperture by light not entering the etalon.

I advise against wasting your money on a mod for a full disk, they are only good for high resolution high magnfication.

Too many sacrifices to make it viable over what lunt already sells as a complete optimized telescope, intended for full disk out of the box.

Get the lunt 40mm.
Interesting thanks!

If we put a full disc image aside, and just consider it for a rear mounted application for moderate to high resolution use on larger apertures and longer focal lengths, getting into 0.5" to 0.3" per pixel scales basically, I'm curious how the sweet spot size would be with these options (larger since the etalon is larger) and how simple it would be to mount and collimate it. The 40mm and 50mm etalons are pretty affordable for this. My PST etalon works already, I'm just looking to increase the sweet spot size, which may simply call for a new etalon to do it.

It would be great if a modular etalon like the new Lunt 60mm modular one could work, but its a smaller etalon so it probably will not be a bigger sweet spot, at least, not significantly so (I don't know its clear aperture).

With regards to the full disc, mainly I would like to be able to use an etalon that is modular like this with a small short scope for full disc within the sweet spot (like a 400mm to 600mm focal length) and then plug it in behind a bigger scope for high res with small FOV and a smaller sweet spot, but hopefully a bigger sweet spot than the PST provides, so I need a bigger etalon to do that.

But a 40mm~50mm aperture etalon would greatly increase my sweet spot. Even if I had to get a separate one, I wouldn't mind.

Anything to avoid getting an electric powered F30+ Daystar or similar option again; I'd like to avoid this. I sold my Quark after my PST etalon was working as my PST etalon is better than my Quark in all respects except the sweet spot size.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

Hi Marty,

My mindset went this way a few years ago, yes, use the larger etalons to increase the sweet spot, the problem you will have is finding commercially available collimating lenses, hence JAPP producing his own 80/-800 lens.

I went the route of using my good Lunt50mm etalon on a 60mm/f6 frac with a Quark behind - the Quark see's an f7.2 beam before the 4.3x takes the beam through the >f30, I then use the 0.7x baader telecompressor which fits it onto the camera chip. This has given me the most even disk. This one from sunday, open in a new window for full size. As is from camera, just ImPPG.
Ha-FD-DS50.jpg
Ha-FD-DS50.jpg (2.38 MiB) Viewed 308 times
Naturally, which ever way you go relies on getting good etalons in the first place, be they external. internal or rear mounted.

Mark


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

marktownley wrote: Mon Mar 01, 2021 4:06 pm Hi Marty,

My mindset went this way a few years ago, yes, use the larger etalons to increase the sweet spot, the problem you will have is finding commercially available collimating lenses, hence JAPP producing his own 80/-800 lens.

I went the route of using my good Lunt50mm etalon on a 60mm/f6 frac with a Quark behind - the Quark see's an f7.2 beam before the 4.3x takes the beam through the >f30, I then use the 0.7x baader telecompressor which fits it onto the camera chip. This has given me the most even disk. This one from sunday, open in a new window for full size. As is from camera, just ImPPG.

Naturally, which ever way you go relies on getting good etalons in the first place, be they external. internal or rear mounted.

Mark
Thanks Mark,

Interesting setup. A lunt 50 on a 60mm frac with a Quark. I'm surprised that produced a full disc. 1548 effective focal length before the blocking filter, disc size is pretty big, it passed the 12mm blocking filter of the Quark and then was reduced by the 0.7x reducer? I did full discs with Quarks a bit in the past, was able to do it with a 400mm focal length base scope, and a reducer spaced away from the sensor, but it definitely didn't look like that which you posted, it was tiny and had bad aberrations from the reducer and was really low res. I'd like to avoid the Quark if I can though at this point.

I'm curious what the internal etalon size is of the 50mm and 60mm etalons (internals; pressure tuned ones).

Ultimately I would be fine with the resolution and disc size produced from a typical 60mm solar scope like a Lunt 60. I'm just seeing if it's possible to use that etalon rear mounted on another scope and if it would have a bigger sweet spot than the PST etalon, on the larger aperture scopes I use since the PST etalon's sweet spot is so small. So if it were 26~30+mm in size that would be great, but it likely is not, so I'm curious just how much bigger, if it would be worth it. The idea being to use it on its 60mm scope and do a full disc with that, stock, and remove it and put it on the back of a bigger scope for high res.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by Bob Yoesle »

Thanks Christian and Peter!
At this end of the day, it is much better to have etalon very close to Ha at normal incidence. But ... this is more difficult and more expensive to do.
You've confirmed what I have observed - namely that tilt-tuned etalon performance suffers if their CWL is too high and requires excessive tilt. And the production realities of keeping the spacer thickness within a very narrow range of tolerance to keep CWL close to the H alpha emission is more difficult to attain and therefore expensive. This explains the general better performance of pre-Meade Coronado etalons, and their and Lunt's higher relative cost. Air pressure tuned etalons are more immune to spacer thickness induced variations in CWL due to the uniform air density/refractive index tuning. Unfortunately these do not exist for front etalon filters.
If so, then the idea is to get the biggest clear aperture etalon with biggest collimating lens possible for this, to ensure the largest possible d/D ratio, 100% limited by the aperture of the etalon.
I just want to note that the objective & etalon diameters are proportional for the sweet spot size due to geometry, but may be imprecise. The actual sweet spot optical size is defined via field angle magnification, which is solely defined via the focal lengths, and the acceptance angle of the etalon.

The field angle at the Sun's limb is ~ 0.25 degree. This is what a front mounted etalon sees, and why it will generally offer the largest "sweet spot" of on-band performance (given the previous constraints noted for CWL).

For an internal etalon, if one uses an objective with a 1000 mm FL, and the collimator lens (such as the PSTs) has a FL of 200 mm, the field angle magnification will be 1000 / 200 = 5, and the limb of the Sun will have a field angle of 5 x 0.25 = 1.25 degrees. If a 0.7 angstrom air-spaced etalon has an acceptance angle of 0.5 degree to be on band, we can see that the extent of on band performance will be smaller - like looking through the narrow tube. A narrower bandpass etalon will have a smaller acceptance angle, and the sweet spot will therefore be even smaller.

If we use a collimator lens with a longer focal length (which also requires a proportionally larger etalon to prevent vignetting) we get better performance: Using a collimator with a 500 mm FL results in a magnification of 1000 / 500 = 2, and a field angle of the Sun's limb is now 2 x 0.25 = 0.5 degree, and most if not all of the disk will be within the sweet spot of the etalons acceptance angle.

The issue of lack of large negative COTS lenses can be overcome by using positive collimation lenses, but obviously the physical size of the OTA will become longer.

Bob


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Well, dang, now I'm confusing myself.

My PST etalon is a 20mm clear aperture.
My old Quark etalon is 21mm clear aperture.

So they should have a very similar sweet spot?

I seem to recall the Quark having a larger sweet spot on the same aperture scope setup. I may be imagining this. Or is it due to the telecentric taking care of the off-axis rays?

Otherwise, it has me thinking about all of this with respect to a 60mm F15 refractor with a Quark Combo (21mm clear aperture, 25mm blocking filter), allowing a full disc to be imaged with a larger sensor. The d/D is 0.35. Despite me not wanting to have a Daystar again, this at least is something to think about on my end, unless I'm missing something (which is likely!).

Very best,


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