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

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

Post by hk160 »

> So they should have a very similar sweet spot?

The effective (refractive) index of Mica is 1.6 vs. 1.0 for the air spaced etalon. Therefore the quark can accept beams more inclined, which is equivalent to a larger sweet spot.

Another way to see it is that a 21mm quark is equivalent to a 1.6x21mm=33.6mm air spaced etalon.


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

Post by MalVeauX »

hk160 wrote: Tue Mar 02, 2021 12:05 am > So they should have a very similar sweet spot?

The effective (refractive) index of Mica is 1.6 vs. 1.0 for the air spaced etalon. Therefore the quark can accept beams more inclined, which is equivalent to a larger sweet spot.

Another way to see it is that a 21mm quark is equivalent to a 1.6x21mm=33.6mm air spaced etalon.


Klaus
Thanks! That perfectly explains what I was remembering. Quite interesting.

So does this mean that a 60mm F15 aperture refractor with a Quark Combo (21mm aperture air, but 33.6mm mica?) mean it has a d/D ratio of 0.56 and therefore can put a full disc in the entire sweet spot? This seems too easy. Probably can't be true.... but I gotta ask!

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

Post by AndiesHandyHandies »

Hi

What does the size of a sweet spot say about the configuration?

I have a Rumak180mm F10, perfect flat field 'orthoscopic', with 1.5A Omega seconds as a Blocker and a Meade PST which looks 0.75A.

Tuning the PST I get a sweet spot about 1/4 diameter of the field and then it goes into a doughnut.

As its only showing proms its well big enough.

Cheers. Andrew.


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

Post by hk160 »

So does this mean that a 60mm F15 aperture refractor with a Quark Combo (21mm aperture air, but 33.6mm mica?) mean it has a d/D ratio of 0.56 and therefore can put a full disc in the entire sweet spot? This seems too easy. Probably can't be true.... but I gotta ask!
Well, in a telecentric design you need f/#=30, so with a 60mm aperture, that is a fl=1800mm, giving a disc size of 17mm so that would fit. There is no sweet spot in that case, just the fwhm goes from 0.7A to 0.8A.

For a collimated design, (which is discussed here), you need to create a pupil image of a certain size and turn the beams parallel. By geometry, if I make the pupil 1/n th the size, the angles get n times larger. So if you take the sun as an example, you start with .5 deg, and you need to go from 60mm aperture to 20mm pupil=20mm etalon aperture --> angle increases to 1.5 deg. Now you can dial in that the cwl is right half way out from the center, so really you need to accommodate 0.75deg.

For the mica etalon, this leads to a CWL shift of 0.2A, occurring in the center and at the edge of the disc. Whether this is acceptable or not depends on how picky one is.


I also want to point (again) to Christian Viladrichs excellent analysis on his page here:

http://www.astrosurf.com/viladrich/astr ... lar/FP.htm

I took the took the cwl shift and FWHM value from there, too lazy to calculate them myself ... ;)


Hope that helps,
Klaus
Last edited by hk160 on Tue Mar 02, 2021 3:09 pm, edited 1 time in total.


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

Post by Bob Yoesle »

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?
As Klaus describes, you can't mix the imprecise rule-of-thumb about objective and etalon diameters used for collimated systems with telecentric systems, which ideally keep the converging cone field angles below the acceptance angle threshold to maintain the specified bandpass. However, in my experience you can also get a Jacquinot spot (in addition to bandpass widening) if the telecentric system isn't optimized to produce a truly telecentric axial cones normal to the etalon.

Etalon locations.jpg
Etalon locations.jpg (114.48 KiB) Viewed 335 times

Again, the Jacquinot spot is a result of what the etalon acceptance angle is (varies based on etalon gap refractive index), and the field angle of the extended object you are observing:

The Jacquinot spot is defined as the field about the optical axis within which the the peak wavelength variation [ Δλ ] with field angle does not exceed √2 of the etalon bandpass. This angular field can be used to perform close to monochromatic imaging.

So you can see that monochromatic imaging can only be accomplished to the degree in which the field angle doesn't exceed the acceptance angle.

Equation 1: Δλ = √2 x FWHM

The field angle verses wavelength change can be found with formula for the CWL shift:

Equation 2: Δλ = ½ (CWL / n^2) θ^2

We can now solve for θ:

√2 x FWHM = ½ (CWL / n^2) θ^2

θ^2 = √2 x FWHM ÷ ½ (CWL / n^2)

For an air spaced etalon (n = 1.00) with a FWHM of 0.7 Å at the H alpha line (6563 Å), with θ in radians (1 radian = 57.2957795 degrees):

θ^2 = 1.4142 x 0.7 ÷ ½ (6563 / 1.00)

θ^2 = 0.98994 ÷ 3281.5

θ^2 = 0.000301673

θ = √0.000301673

θ = 0.017368736 (radians) x 57.2957795 degrees

θ = 0.9951553 degree

Therefore the Jacquinot spot is ~ 1.0 degree, and the “acceptance angle” (field angle) for this size a spot would be ~ 0.5 degree, as is the frequently cited acceptance value for a 0.7 Å FWHM etalon. Outside this "sweet spot" radius H alpha detail will begin to fade into continuum.

Next, for a double stacked etalon system with a 0.5 Å FWHM, and assuming a DS system follows the same rules, we get the following;

θ^2 = 1.4142 x 0.5 ÷ ½ (6563 / 1.00)

θ^2 = 0.7071 ÷ 3281.5

θ^2 = 0.00021548

θ = √0.00021548

θ = 0.0146792 (radians) x 57.2957795 degrees

θ = 0.841058 degree

Therefore the Jacquinot spot is ~ 0.84 degree, and the “acceptance angle” (field angle) for this size a spot would be ~ 0.42 degree. Christian Viladrich produced a great graph of these relationships for various band-passes and refractive index materials which is quite useful:

Jacquinot spot.jpg
Jacquinot spot.jpg (85.58 KiB) Viewed 335 times


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

Post by MalVeauX »

Thanks guys, the math helps a lot!

:bow :bow

It really helps to emphasize that large etalons that are front mounted are rather important for a double stack with respect to the full disc FOV to ensure that a double stack sweet spot keeps a full disc within the sweet spot FOV.

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

Post by marktownley »

It's finding that winning combination Marty, just takes time!


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

Post by MalVeauX »

marktownley wrote: Tue Mar 02, 2021 4:57 pm It's finding that winning combination Marty, just takes time!
Ain't that the truth.

The problem is there's very little selection and no standard for quality. I'm not willing to drop $6k on a Quantum (or more) and really don't feel like doing $8k~16k on a SolarSpectrum, looking at the rear mica spaced etalons with larger apertures for high res. I realize there's no free lunch, so I'm always looking for what's a good balance between affordability and functionality and being able to be modular/flexible.

I've never had a good full disc HA setup. I tried a few different double stacks, went through SM40, SM60 double stacks, pre and post Meade at this point, and none of them were good. Even my pre-Meade had problems. It's sad that my PST etalon (pre-Meade) is better than all those etalons I had and my past Quark. Sold them all, kept the PST etalon and I still have a Sm60II double stacking etalon laying around just because its near worthless to sell and I can double stack with it.

Based on this thread, I realize I probably should just get a Lunt 80mm for full discs. But, that price tag is not happening for me right now. And I'm not gambling on Solarmax since there's no support and I cannot test them or see them before buying, so the SMIII series 90's, while more affordable, are just not likely going to cut it without cherry picking one somehow. And I don't have high expectations for them.

So, I've been toying with the idea of how to optimize some other affordable options until I can get into a Lunt 80 or something.

So then I wanted to explore, what would it take to explore optimizing a PST 20mm etalon for a full disc and potentially when using it in a large system for high res, to get a bit bigger sweet spot. The PST etalon I have is great, good finesse, great contrast, but a small sweet spot. So I was just seeing what can I do to get a bit more out of this etalon for now. If I could use it with a collimator system to get a full disc with something that has about a 60mm aperture in terms of resolution, that would be great.

Then I started exploring, what if I got a Quark Combo unit to hold me over a while, and just use it with F15 refractor at 60mm. Even if I had to put a 2x powermate between it and the frac, a reducer would bring it back down to F15 and the 25mm blocking filter and a big sensor should be able to full disc. But, I didn't know how to best calculate the sweet spot to know if the disc would be within the sweet spot on such a system. The benefit here would be the ease to double stack a Combo unit and the ease of use on larger apertures with my big scopes and DERF since I already have those working.

The other thing I'm looking at is the Lunt 60mm modular that has a pressure tuned etalon and is operating at F7, being able to set this up in a rear mounted configuration with a larger etalon aperture than the PST and being pressure tuned might be interesting, while also being competent for full discs on its native 60mm scope. So this has me highly interested as a versatile mod. I was just exploring what the sweet spot would be in its 60mm form and when mounted behind a 120mm or 150mm refractor at F7 with a DERF of course. I'm not sure what the etalon clear aperture is on this though.

If anyone has experience with the above 3 or similar options, I'm all ears, eager to know your experiences. This thread with sweet spot calcs got my attention since it directly is in line with what questions I've had about some of this equipment and how it could potentially be used, without extreme re-modeling.

Part of it, as you said is getting a good combination, since every single one is unique and requires some tweaking. Few are excellent stock. Few are excellent at all. So I'm trying to learn more about the geometry of this stuff to better select potential options to tweak in the first place.

Very best,


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

Post by MapleRidge »

Hi all...

This is quite the read, and need to go over it a few more times.

I have a question about the sweet spot ration from the d/D ratio. Does the result, say .2, have any units associated with it or is it 0.2 of the actual field of view, or some other relative detail?

Just trying to make sense of it all.

Thanks,
Brian


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

Post by christian viladrich »

MapleRidge wrote: Tue Mar 02, 2021 9:42 pm Hi all...

This is quite the read, and need to go over it a few more times.

I have a question about the sweet spot ration from the d/D ratio. Does the result, say .2, have any units associated with it or is it 0.2 of the actual field of view, or some other relative detail?

Just trying to make sense of it all.

Thanks,
Brian
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = sqrt (delta lambda) x f/F
with theta in degree,
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = sqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha and air-spaced etalon. For Ca K (or other wavelengths) or mica-spaced etalon, we need to use a more general formula..


Hope this helps


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

Post by MalVeauX »

Hrm,

Any help with this?

Mica spaced etalon, 21mm aperture (Quark Combo)
60mm aperture F16.67 telescope (1000mm focal length)

Sweet spot size and general thoughts?

I just rough calced it with n=1.6 and the result was 1.59 degrees approximately? This seems to agree with the graph above?

I'm curious how this would work, without anything, at native F16.67, and how it would work with a 2x powermate followed by a 0.5x reducer and if the disc image would fit within the sweet spot on a larger sensor, such as IMX253 (1.1") or IMX183 (1")?

So the above setup, native F16.67, on IMX183 1" sensor would be 0.75 degree x 0.5 degree
Same above with IMX253 (1.1") sensor would be 0.81 degree x 0.59 degree
Disc size should be between 10mm and 9.09mm
Blocking filter is 25mm
1.1" sensor is 14.2mm x 10.4mm (17.6mm diagonal)

I'm missing something.

Very best,


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

Post by Bob Yoesle »

Hi Marty,

You can not determine much because the Quark does not specify a FWHM - you'd have to measure this for your filter before anything else could be determined. Once you do this, you can use the equation I posted above to determine the size of the Jacquinot spot for this filter - which again is dependent on filter FWHM and gap refractive index (e.g. mica).

With a simple objective-etalon system at f16.7, you'd have significant bandpass widening and poor contrast. Generally, the field angle of the Sun's disc of 0.25 degree is equivalent to about a f108 converging cone, so a rough estimation is f108/f16.7 = 6.46 - or about a 6.5 times increase in field angle for the objective etalon-system, and a significant sweet spot therefore seems inevitable.

I you use the Powermate, you will not have a truly telecentric system (converging light cones normal to the etalon) and compound the bandpass widening with increased field angles. Therefore you may have a "sweet spot" develop as well, although it won't be near as bad because the filter now sees a f33.4 converging cone. This is about a 3 x increase in the 0.25 field angle of the solar disc edge (e.g. ~ 0.75 degree) and depending on the FWHM, this might allow a full disc to have uniform contrast at the widened FWHM of the filter at f33.

For any system, this would be independent of a further downstream focal reducer, which only affects the final image size, not the "sweet spot" originating with the increase ray angles presented to the etalon.

Without understanding the mechanisms of the Jacquinot spot, as well as collimator and telecentric optics, it will be difficult to get ideal performance from a home-brewed DIY solar telescope design. Air spaced etalons generally are better designed for objective to collimator systems with larger etalons and shorter EFLs, and smaller mica etalons work better with telecentric systems and longer EFLs.
Last edited by Bob Yoesle on Wed Mar 03, 2021 3:51 pm, edited 1 time in total.


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

Post by MalVeauX »

Bob Yoesle wrote: Wed Mar 03, 2021 3:43 pm Hi Marty,

You can not determine much because the Quark does not specify a FWHM - you'd have to measure this for your filter before anything else could be determined. Once you do this, you can use the equation I posted above to determine the size of the Jacquinot spot for this filter - which again is dependent on filter FWHM and gap refractive index (e.g. mica).

With a simple objective-etalon system at f16.7, you'd have significant bandpass widening and poor contrast. Generally, the field angle of the Sun's disc of 0.25 degree is equivalent to about a f108 converging cone, so a rough estimation is f108/f16.7 = 6.46 - or about a 6.5 times increase in field angle for the objective etalon-system, and a significant sweet spot therefore seems inevitable.

I you use the Powermate, you will not have a truly telecentric system (converging light cones normal to the etalon) and compound the bandpass widening with increased field angles. Therefore you may have a "sweet spot" develop as well, although it won't be near as bad because the filter now sees a f33.4 converging cone. This is about a 3 x increase in the 0.25 field angle of the solar disc edge (e.g. ~ 0.75 degree) and depending on the FWHM, this might allow a full disc to have uniform contrast at the widened FWHM of the filter at f33.

For any system, this would be independent of a further downstream focal reducer, which only affects the final image size, not the "sweet spot" originating with the increase ray angles presented to the etalon.
Thanks Bob, that puts that to rest, I figured something had to be fundamentally wrong with this, it just seemed too easy of a solution. Will dump the concept all together.

Very best,


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

Post by christian viladrich »

christian viladrich wrote: Wed Mar 03, 2021 9:38 am
MapleRidge wrote: Tue Mar 02, 2021 9:42 pm Hi all...

This is quite the read, and need to go over it a few more times.

I have a question about the sweet spot ration from the d/D ratio. Does the result, say .2, have any units associated with it or is it 0.2 of the actual field of view, or some other relative detail?

Just trying to make sense of it all.

Thanks,
Brian
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = n x sqrt (delta lambda) x f/F
with theta in degree,
n : index of the etalon gap (n=1 for air-spaced etalon, n=1.6 for mica-spaced etalon)
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = n xsqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha. For Ca K (or other wavelengths), we need to use a more general formula.
=> this is valid only in a collimated beam.


Hope this helps


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

Post by christian viladrich »

I updated the formulae for mica-spaced etalon.


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

Post by christian viladrich »

MalVeauX wrote: Wed Mar 03, 2021 2:55 pm Hrm,

Any help with this?

Mica spaced etalon, 21mm aperture (Quark Combo)
60mm aperture F16.67 telescope (1000mm focal length)

Sweet spot size and general thoughts?

I just rough calced it with n=1.6 and the result was 1.59 degrees approximately? This seems to agree with the graph above?

I'm curious how this would work, without anything, at native F16.67, and how it would work with a 2x powermate followed by a 0.5x reducer and if the disc image would fit within the sweet spot on a larger sensor, such as IMX253 (1.1") or IMX183 (1")?

So the above setup, native F16.67, on IMX183 1" sensor would be 0.75 degree x 0.5 degree
Same above with IMX253 (1.1") sensor would be 0.81 degree x 0.59 degree
Disc size should be between 10mm and 9.09mm
Blocking filter is 25mm
1.1" sensor is 14.2mm x 10.4mm (17.6mm diagonal)

I'm missing something.

Very best,
Hello Marty,
I 've updated the formulae for mica-spaced etalons.
Regarding your configuration, I have not understood if you are using a collimated beam or not ? If so, what is the focal length of the collimator. These are the data you need to calculed the radius of the sweet spot. The formulae I gave are valid only for a collimated beam.
BTW, what would you use a 2x Powermate followed by a 0.5x reducer ?


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

Post by MalVeauX »

christian viladrich wrote: Wed Mar 03, 2021 5:10 pm
MalVeauX wrote: Wed Mar 03, 2021 2:55 pm Hrm,

Any help with this?

Mica spaced etalon, 21mm aperture (Quark Combo)
60mm aperture F16.67 telescope (1000mm focal length)

Sweet spot size and general thoughts?

I just rough calced it with n=1.6 and the result was 1.59 degrees approximately? This seems to agree with the graph above?

I'm curious how this would work, without anything, at native F16.67, and how it would work with a 2x powermate followed by a 0.5x reducer and if the disc image would fit within the sweet spot on a larger sensor, such as IMX253 (1.1") or IMX183 (1")?

So the above setup, native F16.67, on IMX183 1" sensor would be 0.75 degree x 0.5 degree
Same above with IMX253 (1.1") sensor would be 0.81 degree x 0.59 degree
Disc size should be between 10mm and 9.09mm
Blocking filter is 25mm
1.1" sensor is 14.2mm x 10.4mm (17.6mm diagonal)

I'm missing something.

Very best,
Hello Marty,
I 've updated the formulae for mica-spaced etalons.
Regarding your configuration, I have not understood if you are using a collimated beam or not ? If so, what is the focal length of the collimator. These are the data you need to calculed the radius of the sweet spot. The formulae I gave are valid only for a collimated beam.
BTW, what would you use a 2x Powermate followed by a 0.5x reducer ?
Hello,

I was curious about both. Daystar claims you can use their combo version on F15 refractors, and then of course Mak/SCT. So I was curious why they would claim you could just use the Combo on a F15 refractor in general and what it would produce, knowing full well that it's not likely going to be excellent contrast due to the way the mica-spaced etalon works. However, I was still curious since it wasn't being explicitly stated to use a telecentric with them.

So I was curious how the following configuration would function and the result and the sweet spot size and disc size:

60mm F16.67 refractor with Quark Combo and IMX253 sensor (1.1", 3.45um)

60mm F16.67 refractor with 2x powermate then Quark Combo then 0.5x focal reducer then IMX253 sensor

For a larger aperture, no expectation of full disc, the Quark Combo with a 4x telecentric approximately would be used, as one would expect and reduced on the camera side for sampling.

But I was more interested in how this would function and sweet spot size would work with the above small aperture long focal-ratio refractor approach, but only with the Combo version (not the one with the built in telecentric 4.2x).

Very best,


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

Post by christian viladrich »

Hello Marty,
Ok, I see. Then the calculation is completely different since there is no telecentric or collimated beam falling onto the etalon.

In the first case, you just have a f/16.67 beam. It means you will have both:
- an enlargement of the FWHM (compared to its nominal value) because of the f/16 beam,
- a drift of the CWL away from the optical axis.
The formulae are there :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm
Most probably, it is not very good ...

For case #2, and if the Powermate 2x is telecentric, there is no sweet spot. Still, there is a broadening of the FWHM because of the f-ratio :
http://astrosurf.com/viladrich/astro/in ... FWHM-N.JPG


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

Post by MalVeauX »

christian viladrich wrote: Wed Mar 03, 2021 7:01 pm Hello Marty,
Ok, I see. Then the calculation is completely different since there is no telecentric or collimated beam falling onto the etalon.

In the first case, you just have a f/16.67 beam. It means you will have both:
- an enlargement of the FWHM (compared to its nominal value) because of the f/16 beam,
- a drift of the CWL away from the optical axis.
The formulae are there :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm
Most probably, it is not very good ...

For case #2, and if the Powermate 2x is telecentric, there is no sweet spot. Still, there is a broadening of the FWHM because of the f-ratio :
http://astrosurf.com/viladrich/astro/in ... FWHM-N.JPG
Thanks Christian,

So it will be a bad way to go, no problem, was just curious how it would all work for my own curiosity. For a full disc, it's just way better to get a simple larger etalon smaller focal length non-mica setup, like a Lunt.

Very best,


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

Post by MapleRidge »

Hi Christian....

Just looping back to the math and the additional formulas and terms you sent along...
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = n x sqrt (delta lambda) x f/F
with theta in degree,
n : index of the etalon gap (n=1 for air-spaced etalon, n=1.6 for mica-spaced etalon)
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = n xsqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha. For Ca K (or other wavelengths), we need to use a more general formula.
=> this is valid only in a collimated beam.


Hope this helps
Straight forward, OK :lol:
But walk me through the calculations for my setup if you would please:

The scope is a 150mm, f8 achromat using a Lunt pressure tuned etalon at the tail of the tube. The clear diameter of the etalon is 35.5mm, but I do not have any info on the collimation lenses used in it.

Do you have enough data to fill int he formula?

Please let me know if there are any additional measurement required that I have not provided.

Thanks for your assistance,
Brian
Last edited by MapleRidge on Sat Mar 06, 2021 10:23 pm, edited 1 time in total.


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Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C9.25 Edge + Stellarvue 110, f7 or C14
Deep Sky Work - Canon 60D (Ha mod), SBIG STF8300/FW8, ONAG
Planetary Work - SBIG CFW10, ASI290MM/ASI462+ADC

8' Diameter Dome
MI250 Mount carrying:
Celestron CR-150 150mm, f8 OTA Modular Setup
Ha configuration: 2xDSII/B1800Ha/Daystar ERF as modded Ha Scope
WL & CaK configuration: Lunt WL Wedge or Modded B1800CaK Wedge
Lunt LS80PT/LS75FHa/B1200Ha
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

MapleRidge wrote: Sat Mar 06, 2021 1:19 pm Hi Christian....

Just looping back to the math and the additional formulas and terms you sent along...
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = n x sqrt (delta lambda) x f/F
with theta in degree,
n : index of the etalon gap (n=1 for air-spaced etalon, n=1.6 for mica-spaced etalon)
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = n xsqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha. For Ca K (or other wavelengths), we need to use a more general formula.
=> this is valid only in a collimated beam.


Hope this helps
Straight forward, OK :lol:
But walk me through the calculations for my setup if you would please:

The scope is a 150mm, f8 achromat using a Lunt pressure treated etalon at the tail of the tube. The clear diameter of the etalon is 35.5mm, but I do not have any info on the collimation lenses used in it.

Do you have enough data to fill int he formula?

Please let me know if there are any additional measurement required that I have not provided.

Thanks for your assistance,
Brian
I am super interested in this Brian, especially how you mounted the Lunt pressure etalon back there and how the collimating lenses play into this (since there's F8.3 and F7 versions of the pressure tuned 60mm, if that's what you used). I'm not sure what collimating lenses are there, if any. Would love to learn more about this setup.

Very best,


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

Post by christian viladrich »

Hello Brian and Marty,
We need an estimation of the focal length of the collimator ...
Is the 35.5 mm aperture the aperture of the Lunt 60 mm pressure tuned etalon ? If so, I understand there are two different versions of this filter: one for f-8.3 refractors and the other for f-7 refractors ?
Is there any way to measure the focal length of the collimator ? The front lens is a divergent lens, the second a convergent lens. Is it possible to detach one of these lens from the body of the filter ?


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

Post by MapleRidge »

Hi Christian...

This is internal DSII etalon for the LS80T solar scope. I am only aware of the scope being made available as f7, and no options for different DSII units to match the OTA.

The system is undoubtedly able to be disassembled, but not comfortable doing so myself. The addition of the DSII unit to the single stack solar scope does not change the focus point much if at all. There is a spacer to use between the scope and focuser in single stack mode but this is removed and replaced by the DSII etalon which takes up the same space. Not sure if this speaks to the collimator lens net effect?

Brian


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Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
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Planetary Work - SBIG CFW10, ASI290MM/ASI462+ADC

8' Diameter Dome
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Ha configuration: 2xDSII/B1800Ha/Daystar ERF as modded Ha Scope
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Lunt LS80PT/LS75FHa/B1200Ha
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MapleRidge »

The clear aperture on the objective side is approximately 35mm. The calipers slipped when I was taking this pic and was trying not to touch the glass aggravating the measurement. If a better measurement is needed I can try this again...won't be -20C tomorrow :lol:

Or, am I measuring the wrong dimension? Is the thickness required?

Brian


Brian Colville

Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C9.25 Edge + Stellarvue 110, f7 or C14
Deep Sky Work - Canon 60D (Ha mod), SBIG STF8300/FW8, ONAG
Planetary Work - SBIG CFW10, ASI290MM/ASI462+ADC

8' Diameter Dome
MI250 Mount carrying:
Celestron CR-150 150mm, f8 OTA Modular Setup
Ha configuration: 2xDSII/B1800Ha/Daystar ERF as modded Ha Scope
WL & CaK configuration: Lunt WL Wedge or Modded B1800CaK Wedge
Lunt LS80PT/LS75FHa/B1200Ha
Daystar Quantum 0.45A SE, Quark Combo Chromosphere,
ASI1600MM, ASI174MM, ASI290MM, PGR Grasshopper Express
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

So, we have :
- a refractor with focal length F = 150 x 8 = 1200 mm
- for the collimator, given that the free aperture is 35.5 mm, let's assume a full aperture equal to 40 mm. Given the f-7 ratio for the collimator, its focal is f = 7 x 40 = 280 mm

Now, if we consider an acceptable limit of 0.25 A shift of the CWL at the edge of the sweet spot, then the radius of the sweet spot is given by :
theta = f/F x sqrt (0.25) = 0.12°, or 0.24° in diameter. This is equivalent to the solar radius.

Hope this helps.


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Co-author of "Astronomie Solaire"
http://www.astronomiesolaire.com/
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