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What Determines Sweet Spot Size in an H-alpha Scope?

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#1 Gregory Gross

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Posted 01 August 2019 - 12:20 PM

Out of all the various elements in the optical train of an H-alph solar scope, what particular element or combination of elements determines how wide the sweet spot is?

My understanding all this time has been that the size of the etalon determines how wide the sweet spot is. But that thinking is more in the context of internal etalons, specifically comparisons among the 50, 60, and 80mm Lunts. My experience with H-alpha scopes with front-mounted external etalons is limited, I confess.

Part of what compelled me to start this thread is recent experience I've had comparing a 50mm Lunt with 50mm DS module and a 60mm Lunt with the smaller 50mm DS module. I was really impressed with the performance of the latter, making me wonder if the smaller size of the internal etalon in the 50mm scope was responsible for limitations in performance (namely sweet spot) as compared to the 60mm Lunt even when its aperture was reduced by the 50mm DS module. Clearly the reduction in aperture would result in reduced resolution. But in my mind, increased contrast is more of an imperative than increased resolution where H-alpha solar observing is concerned.

In other words, I'm trying to get at the rationale for why one would buy a 60mm scope with 50mm DS module rather than simply going for the cheaper 50mm scope/50mm DS module combo. Clearly, the optical train of the 60mm scope is going to offer some advantage over the 50mm scope no matter what size of a DS module is attached to it. That advantage, in my mind, is larger sweet spot with the 60mm scope. So is it the larger size of the 60mm scope's internal module that is chiefly responsible for that wider sweet spot?

#2 Eddgie

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Posted 01 August 2019 - 01:59 PM

The Lunt 50 is $750, but this is with only the BF600, so the only thing you can do with the Lunt 50 is monoviewing.  The BF600 will not support any kind of binoviewing and limits you to using very small chips for imaging.

 

Next, the 50 uses a helical focuser that is really only good for visual mono-viewing, but again, without the BF1200, that is about all you can do with the 50 (not that you can't image, but most people will want to use larger chips). 

 

And while the DS 50 offered no advantage of over the double stacked 50, if you unstack it, you have a much larger aperture for high magnification of prominence and sun spot activity (not that we have had a lot of that lately.)

 

Now, you could buy a BF1200, but that now puts you within rattlesnake striking distance of a Lunt 60. 

 

If you look at Lunt's pricing, you will see that this is a common theme.  To pimp out a particular model with a big BF, you are within spitting distance of the next larger model.   That is actually a good marketing model because while it makes the decision difficult for us, many will simply spend the extra money and upsize to the next model up.

 

So. mostly it is excellent marketing by Lunt, and in the case of the 50 vs 60, you can see exactly what I mean. If your only intention is to observe visually using a single eyepiece, then you can save a lot of money over the 60/50DS by buying the 50DS.  If though you want to image and/or binoview, most people are going to want the BF1200 and you would have trouble giving away a BF400. 


Edited by Eddgie, 01 August 2019 - 02:02 PM.

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#3 bandazar

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Posted 01 August 2019 - 06:25 PM

I think I had a similar experience as well.  The lunt 60mm appears to be much better than the lunt 50mm.  The lunt 50mm is closer to the pst in performance, than it is to the lunt 60mm from what I've seen.  Not a bad telescope, it was just too close in performance for me to really like it over the pst once you factored in the issues of owning a pressure tuned telescope (having to deal with air loss, trying to screw the pressure tune knob once it was off sometimes took a very long time, until I figured out that the rubber ring was put in the wrong place).



#4 BYoesle

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Posted 01 August 2019 - 06:42 PM

The "sweet spot" (more formally known as the "Jacquinot spot") is determined by the etalon's central wavelength, the etalon FWHM (aka "bandpass') and the refractive index of the etalon gap. See here. It also determines what the "Acceptance Angle" of the etalon will be (0.5 time the Jacquinot spot diameter) - e.g. how far from normal a light ray can deviate before the etalon begins to go "off band." See Etalon Basics.

 

The size of the sweet spot is therefore not determined per se by the size of the etalon, but rather its optical construction and location in the optical system and whether or not the angles passing through it are significantly modified by the collimating or telecentric optics that are employed to mitigate the etalons location within the optical system.

 

For the Coronado and Lunt optical systems that employ collimator lens systems for their internal etalons, what matters is the field angle magnification, and how much it reduces the size of the Jacqunot spot. The field angle magnification is simply the focal length of the objective divided by the focal length of the collimator lens. To keep the field angle magnification larger than the size of the solar disc, the magnification should be no greater than 2 for a 0.7 A FWHM air spaced etalon.

 

Given the geometry of the focal lengths and fully illuminating the etalon, this generally results in needing to have an internal etalon clear aperture being no smaller than 1/2 the objective diameter used with a collimator of no less than half the objective focal length. Using a smaller etalon with higher magnification collimating optics will make the "sweet spot" smaller than the Sun's disc.

 

Therefore smaller field angle magnification means longer focal length collimators and larger internal etalons for good full-disc contrast performance.


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#5 Gregory Gross

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Posted 02 August 2019 - 03:01 PM

Thank you very much, Bob, for your wonderfully detailed and informative post. Because I'm primarily interested in understanding internal etalons, let me focus my attention on the part of your post that discusses them.

I assume two points:
  • Since the purpose of the collimating lens is to make the rays of light from the Sun parallel before they make their way to the etalon, theoretically that collimating lens should have the same focal ratio of the objective lens.
  • The collimating lens has the same aperture as the internal etalon.
I know based on discussions with Lunt that their 60mm solar scope has an objective lens whose focal ratio is f/2. I therefore also know that it has a focal length of 120mm.
 

For the Coronado and Lunt optical systems that employ collimator lens systems for their internal etalons, what matters is the field angle magnification, and how much it reduces the size of the Jacqunot spot. The field angle magnification is simply the focal length of the objective divided by the focal length of the collimator lens. To keep the field angle magnification larger than the size of the solar disc, the magnification should be no greater than 2 for a 0.7 A FWHM air spaced etalon.

In the case of my 60mm Lunt, to keep the field angle magnification larger than the size of the Sun's disk, the collimating lens must have a minimum focal length of 60mm. In other words, 120mm (objective focal length) divided by 60mm (minimum collimating lens focal length) would get me to 2 (maximum field angle magnification).
 

Given the geometry of the focal lengths and fully illuminating the etalon, this generally results in needing to have an internal etalon clear aperture being no smaller than 1/2 the objective diameter used with a collimator of no less than half the objective focal length. Using a smaller etalon with higher magnification collimating optics will make the "sweet spot" smaller than the Sun's disc.

Therefore smaller field angle magnification means longer focal length collimators and larger internal etalons for good full-disc contrast performance.

Again I use my 60mm Lunt as an example: for the "sweet spot" to be the size of the Sun’s disk, the internal etalon needs to have a minimum clear aperture of 30mm, and the collimating lens in front of the internal etalon must have a focal length of 60mm.

I learned via this post in another CN thread that the 60mm Lunt actually has a 35mm internal etalon. Using the formula for calculating field angle magnification, and having faith that the two assumptions I outline above hold true, 120mm (objective focal length) divided by 70mm (collimating lens focal length if it is a 35mm lens figured at f/2) equals approximately 1.7. This actually corresponds nicely with my real-world experience at the eyepiece. I find that the edge of the "sweet spot" in my 60mm Lunt is comfortably but not enormously outside of the full solar disk, and a field angle magnification of 1.7 would seem to quantify this experience well.

#6 dscarpa

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Posted 02 August 2019 - 05:45 PM

 My single tilt etalons Lunt 60's sweet spot is about 80% of the FOV of whatever eyepiece is used. You get a lot bigger sweet spot with a 82* vs 50* eyepiece. My SM 90 III with full size external etalons is the sweet spotless  scope in single etalon or DS mode even with 100* eyepieces. The negative is it's crazy nose heavy when DSs and I can barely reach the inner etalon's tuner looking through the eyepiece. David


Edited by dscarpa, 02 August 2019 - 05:47 PM.


#7 markthais

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Posted 08 August 2019 - 11:13 PM

Bob always does a great job explaining things.

But , I think your missing a few things. 

 I assume two points:

Since the purpose of the collimating lens is to make the rays of light from the Sun parallel before they make their way to the etalon, theoretically that collimating lens should have the same focal ratio of the objective lens.
The collimating lens has the same aperture as the internal etalon.
I know based on discussions with Lunt that their 60mm solar scope has an objective lens whose focal ratio is f/2. I therefore also know that it has a focal length of 120mm.

First, the rays are only parallel in a collimated system when using a point source. The sun is not a point source. It has a field angle, so if you looked at a ray trace, you would find that only the center ray in the bundle is able to be parallel. The rest of the bundle is not. These rays are the same as tilting. So the center wavelength will shift blue from the center of the etalon outward. Then it's back to what Bob said. How much angle can it takes and still look OK. 

 

I would think that the objective  of the 60mm scope would be somewhere around F/6-8 and not F/2. 

If they are using a singlet at F/2 ,there is going to be a lot of spherical aberration.  By using a collimating lens system you usually will end up with a EFL somewhere near the FL of the objective.

I'm not really sure how they get a 1/4 wave optical system with a singlet at F/6-8. Unless the neg lens is helping  correct the objective. I have never heard that the objective is aspherical.

Mark W. 


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#8 Great Attractor

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Posted 09 August 2019 - 12:36 AM

I know based on discussions with Lunt that their 60mm solar scope has an objective lens whose focal ratio is f/2. I therefore also know that it has a focal length of 120mm.
 

No, for Lunt 60 it's 500 mm (f/8.3) (see https://luntsolarsys...pressure-tuned/). For Lunt 50 it's 350 mm (f/7). The collimating lenses' focal lengths are about 1/2 of that, which gives (approximately) 2x field angle magnification (similarly with Coronado PST, 40 mm aperture and 400 mm f.l., collimating lens: 200 mm f.l.).

 

And as for the beam collimation: if the telescope's optical axis is pointing at the center of the Sun, the rays from disk's center hit the objective and the etalon at 0°; and the outermost rays from the limb, which enter the objective at ±0.25° (as the Sun's angular diameter is ~0.5°), are refracted by the collimating lens to an angle ~2x larger (±0.5°), which usually is the maximum etalon acceptance angle to stay on-band. So each of these scopes (assuming correctly lined up optical train components) should be capable of showing the full disk on-band. It may not work that well for tilt-tuned systems, if the local air density requires a significant tilt (the on-band area may be then non-circular). For pressure-tuned etalons it's easier.


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#9 jwestervelt

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Posted 09 August 2019 - 05:12 PM

 My single tilt etalons Lunt 60's sweet spot is about 80% of the FOV of whatever eyepiece is used. You get a lot bigger sweet spot with a 82* vs 50* eyepiece. My SM 90 III with full size external etalons is the sweet spotless  scope in single etalon or DS mode even with 100* eyepieces. The negative is it's crazy nose heavy when DSs and I can barely reach the inner etalon's tuner looking through the eyepiece. David

Not sure how that works out.  The etalon is before the EP, so the EP should be irrelevant.  Maybe I am misunderstanding the way you are wording this...


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#10 dscarpa

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Posted 10 August 2019 - 01:50 PM

  The sweet spot covering 80% of a 82* eyepiece's FOV is a lot bigger than  the sweet spot covering 80% of a 50* eyepiece's FOV. David



#11 BYoesle

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Posted 10 August 2019 - 06:30 PM

The APPARENT size of a sweet spot will vary depending on the field of view of the eyepiece and the magnification (e.g. eyepiece focal length)...

 

The ABSOLUTE size of the sweet spot is fixed by the type of etalon (gap refractive index), its bandpass (FWHM), and the instrument and field angles presented by any of the ancillary optics (collimation or telecentric lens systems if present) to the etalon ahead of the eyepiece. It will therefore be invariant.


Edited by BYoesle, 11 August 2019 - 08:24 AM.

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#12 Gregory Gross

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Posted 11 August 2019 - 12:33 PM

I know based on discussions with Lunt that their 60mm solar scope has an objective lens whose focal ratio is f/2.


I would think that the objective of the 60mm scope would be somewhere around F/6-8 and not F/2.


No, for Lunt 60 it's 500 mm (f/8.3)

I know that Lunt specifies a focal ratio of f/8.9 for their LS60THa solar scope. But I was standing in person at Lunt in Tucson this past spring when I heard their production manager tell me that the focal ratio of this scope's objective lens (different from the effective focal ratio for the scope as a whole) is f/2. I'm 99% sure that the convex refocus lens that sits behind the etalon and that bends the light coming out of the etalon back into a cone shape and is figured at f/8.9.
 

Since the purpose of the collimating lens is to make the rays of light from the Sun parallel before they make their way to the etalon....

I probably didn't make a good choice of words when I used the term parallel to describe the light coming out of the collimating lens and going into the etalon. Rereading Ian Morrison's article "H-alpha Solar Telescopes – an in-depth discussion and survey," there's a better way to describe the light path going through the collimating lens, etalon, and refocus lens. Morrison writes, "The light passing through [an internal etalon] must still be in the form of a plane wave so a diverging (concave) lens is placed in front to provide a collimated beam through the etalon with a second (convex) lens following the etalon that is used to bring the sunlight to a focus."

#13 BYoesle

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Posted 11 August 2019 - 04:12 PM

I know that Lunt specifies a focal ratio of f/8.9 for their LS60THa solar scope. But I was standing in person at Lunt in Tucson this past spring when I heard their production manager tell me that the focal ratio of this scope's objective lens (different from the effective focal ratio for the scope as a whole) is f/2. I'm 99% sure that the convex refocus lens that sits behind the etalon and that bends the light coming out of the etalon back into a cone shape and is figured at f/8.9.

 

Not likely. The focal length to the LS60 telescope is 500 mm and thus it is f8.3, and is so listed as such by Lunt. Generally for these internal etalon designs, the refocus lens simply restores the original focal length of the objective, so there is no way the objective is f2 (e.g. FL120 mm). Generally, the etalon is half the diameter of the objective, and the collimator lens/etalon assembly would therefore lie at about half the focal length of the objective away. Therefore the objective would have to be somewhere around f8, and appears to be this form the OTA length, as well as the optical layout diagram published on-line:

 

LS60 original singlet.jpg

 

As can be seen the collimating lens attempts to provide light rays passing through the etalon are rendered normal (perpendicular / collimated) to the etalon  - and therefore within the etalons acceptance angle. A refocusing lens is therefore needed to render this "afocal" collimated bundle of light rays capable of forming an image. 


Edited by BYoesle, 11 August 2019 - 04:21 PM.

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#14 Hello_There

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Posted 02 January 2021 - 10:29 PM

How would the sweet spot compare in the new Lunt 40mm vs. the Lunt 50mm? because if i'm understanding this correctly the 40mm has an external etalon while the 50mm has an internal? How would this effect it?



#15 ch-viladrich

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Posted 04 January 2021 - 03:59 PM

To build on Mark W remark, and to make a long story short, in an optimally designed setup, the angular size of the sweet spot depends on the ratio between the etalon diameter and the telescope lens diameter.

It increases when this ratio increases and is maximized when the etalon has the same diameter as the lens (= front etalon situation).

Optimally designed setup means, as explained by Mark, that the f-radio of the collimator is equal to the f-ratio of the refractor lens. In this condition, the etalon has the same diameter as the collimating lens.



#16 vincentv

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Posted 04 January 2021 - 04:23 PM

With the lunt 60 things are tricky. It is a singlet objective and the collimator/refocuser are not exact opposites as in most designs. The difference is used to cancel out aberrations from the singlet.



#17 BYoesle

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Posted 04 January 2021 - 05:57 PM

 

How would the sweet spot compare in the new Lunt 40mm vs. the Lunt 50mm? because if i'm understanding this correctly the 40mm has an external etalon while the 50mm has an internal? How would this effect it?

Theoretically a front etalon has the largest possible "sweet spot." Not to get hyper technical, but because a "sweet spot" (e.g. Jacquinot spot) is based on the field angles passing through the etalon, there is no field angle magnification with a front etalon. For an internal etalon, this field angle magnification is not determined by the objective and etalon sizes (but due to geometry may be directly related to them), but rather is determined by the collimator lens used with an internal etalon. The field angles passing through the etalon are magnified by the FL of the objective divided by the FL of the collimator lens. Keeping the FL of the collimator (and proportionally similar etalon/collimator lens diameter and a given f ratio) to no less than 1/2 the objective FL, results in the ability to maintain a "sweet spot" as large or slightly larger than the diameter of the Sun's disc.

 

See post number 4 above, and here and here for more information.


Edited by BYoesle, 04 January 2021 - 06:56 PM.

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#18 Gregory Gross

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Posted 04 January 2021 - 09:06 PM

In post #5 above, I wrote that the Lunt advised me that their 60mm dedicated solar scope has an objective lens whose focal ratio is f/2. This caused some back and forth in this discussion thread. I never followed up on this until now.

 

On a bright, sunny afternoon this past July, I took the objective lens off of my Lunt and did a few measurements. I don't have an optical bench, but my garage workbench filled my need for this experiment.

 

Anyway, I positioned the objective lens so that its front was pointed out my open garage door. I also put a white sheet of paper taped to a box behind it. I moved the box back and forth until I was able to achieve a focused picture of the neighborhood off in the distance on that sheet of paper.

 

60mm Lunt objective lens demo 1

 

I did so under the assumption that doing so would enable me to determine the approximate focal length and thus the focal ratio of the objective.

 

I found that the distance between the objective lens, whose diameter as it sits in its lens cell is actually 64mm, and my sheet of paper which was acting as a projection screen was about 11 inches or approximately 279mm.

 

60mm Lunt objective lens demo 2

 

279 divided by 64 equals approximately 4.36.

 

So based on all of this, it seems that the objective lens of the 60mm Lunt (the older dedicated version with the singlet objective) is f/4.36.

 

Further, as far as I understand, the purpose of the collimating lens in front of an internal etalon is to receive the cone of light from the objective lens and to make those light rays parallel again before they enter the internal etalon. Would it be correct to assume that the collimating lens has the same focal ratio as the objective lens it is paired with?

 

Bob, you wrote:

 

Keeping the FL of the collimator (and proportionally similar etalon/collimator lens diameter and a given f ratio) to no less than 1/2 the objective FL, results in the ability to maintain a "sweet spot" as large or slightly larger than the diameter of the Sun's disc.

If my conclusions stand up to reason, what we are missing here is the aperture of the collimating lens. That is, if we know that the focal ratio of the objective on the dedicated 60mm Lunt is f/4.36, and if we can assume the collimating lens has an equal focal ratio as its paired objective lens, one could determine the focal length of the collimating lens by multiplying its aperture (diameter) by 4.36.

 

To look at these numbers a different way, if the aperture (diameter) of the collimating lens was greater than or equal to 32mm, and if that 32mm or greater lens had a focal ratio of f/4.36 and thus had a focal length of at least 139.5 (half the focal length of the objective lens), then the "sweet spot" size would be as large or slightly larger than the Sun's disk. Increasing the diameter of the collimating lens while maintaining its focal ratio would cause this "sweet spot" to increase in size (a very desirable thing).

 

I would love to know what the value of this missing spec is. (Actually, having hard specs for the focal length and focal ratio of the collimating lens would be better.) It would enable me to determine a hard number that would indicate the "sweet spot" -- the Jacquinot spot -- of my 60mm Lunt solar scope.


Edited by Gregory Gross, 04 January 2021 - 09:07 PM.

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#19 briansalomon1

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Posted 04 January 2021 - 09:49 PM

Not likely. The focal length to the LS60 telescope is 500 mm and thus it is f8.3, and is so listed as such by Lunt. Generally for these internal etalon designs, the refocus lens simply restores the original focal length of the objective, so there is no way the objective is f2 (e.g. FL120 mm). Generally, the etalon is half the diameter of the objective, and the collimator lens/etalon assembly would therefore lie at about half the focal length of the objective away. Therefore the objective would have to be somewhere around f8, and appears to be this form the OTA length, as well as the optical layout diagram published on-line:

 

attachicon.gifLS60 original singlet.jpg

 

As can be seen the collimating lens attempts to provide light rays passing through the etalon are rendered normal (perpendicular / collimated) to the etalon  - and therefore within the etalons acceptance angle. A refocusing lens is therefore needed to render this "afocal" collimated bundle of light rays capable of forming an image. 

Interesting. Actually a four element arrangement for internal systems like this.



#20 BYoesle

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Posted 05 January 2021 - 09:19 AM

Further, as far as I understand, the purpose of the collimating lens in front of an internal etalon is to receive the cone of light from the objective lens and to make those light rays parallel again before they enter the internal etalon. Would it be correct to assume that the collimating lens has the same focal ratio as the objective lens it is paired with?  ...Increasing the diameter of the collimating lens while maintaining its focal ratio would cause this "sweet spot" to increase in size (a very desirable thing).

Hi Greg,

 

Only the axial rays can be made parallel and normal (perpendicular) to the etalon by a collimator lens. The off-axis rays (field angles) can not be made parallel and normal to the etalon face. Therefore if the field angle rays exceed the etalon acceptance angle, they will begin to fall off-band.

 

Etalons and field angles.jpg

 

Increasing the diameter of the collimator lens will not affect the Jacquinot spot (the adjacent etalon diameter is the limiting diameter), but increasing the focal length will decrease the field angle magnification and increase the size of the Jacquinot spot. The relative diameters and focal ratios are related to the geometry of the optical system to avoid vignetting, but the actual field angle magnification that defines the Jacquinot spot is established only by the focal lengths of the objective and collimator.

 

In this respect field angle magnification is identical to calculating the "everyday" magnification. The normal single stacked air-spaced etalon Jacquinot spot is 1 degree (e.g. the off-axis field angle is 0.5 degree). As soon as the field angle magnification is 2 x, you've reduced the Jacquinot spot to 1/2 a degree (field angle is 0.25 degree), and only the disc of the Sun itself lies therein. Any collimator focal length shorter than 1/2 the objective's FL will proportionally reduce the Jacquinot spot to less than the Sun's diameter. If the collimator FL is 1/4 the objective FL, then the field angles are magnified x 4, and only half the Sun's diameter (0.25 degree) will lie within the Jacquinot spot.

 

Again this has nothing to do with the lens diameters or focal ratios, other than the optical geometry makes them very similar in real life.


Edited by BYoesle, 05 January 2021 - 10:17 AM.

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#21 ch-viladrich

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Posted 05 January 2021 - 12:29 PM

The radius of the sweet spot is equal to

theta = :n  * sqrt (delta lambda) * f /F

with

theta = radius in degree,

n = index of the etalon cavity (n=1 for air)

delta lambda = max wavelength shift allowed at the edge of the sweet spot, in Angstrom

f = focal length of the collimator

F =focal length of the telescope (same unit as f)

 

In an optimal system, f/d = F/D,

with

d = aperture of the collimator

D = aperture of the teelscope

 

The formula gving the radius of the sweet then becomes :

theta = n  * sqrt (delta lambda) * d/D


Edited by ch-viladrich, 05 January 2021 - 12:29 PM.

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#22 rigel123

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Posted 05 January 2021 - 01:49 PM

You math guys amaze me, I'm just happy when I can set up my scope and see details on the sun!  wink.gif   Call me Oblivious as to how anything actually works!


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#23 Gregory Gross

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Posted 05 January 2021 - 02:56 PM

Thank you everyone for the added information! I love seeing this level of depth in discussions.

 

My earlier hand wringing about the diameter of the collimating lens was part of my speculation about how one might determine its focal length. In other words, I was wondering if one could infer the focal ratio of the collimating lens based on the focal ratio of its paired objective lens. If so, one might be able to derive its focal length by multiplying its diameter by its focal ratio.



#24 briansalomon1

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Posted 05 January 2021 - 04:38 PM

I believe you can measure the focal length of a dedicated solar scope using the same method used to measure it on a night time scope. Remove the blocking filter and while the scope is trained on the Sun, use a plain white ceramic plate to find where the image of the Sun is in focus. It might help to remove the glaze on the plate with #220 grit sandpaper. Of course then measure from the focused image to the end of the focuser.

 

I've never disassembled a blocking filter so I'm not dead certain there is no lens in there that changes the effective focal length.



#25 Gregory Gross

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Posted 05 January 2021 - 04:53 PM

I appreciate the suggestion, but my concern with this method of measuring focal length is that the refocusing lens that sits after the etalon may have its own focal length, one that is different than the objective or collimating lens. I understand that the effective focal length of the dedicated 60mm Lunt is 500mm. But the OTA is nowhere near half a meter long. I believe its effective focal length is 500mm because the refocusing lens makes it so.




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