sweet spot and f/d effect

[DSobserver]

hello

just a quick question :

presuming I modify a lunt 50 f:d 7 presure tuner, how will f/d scope impact the sweet spot?

to be clear, what do you think about using a 127mm mak with f:11 or f/15 ratio vs my current 130 f/7 scope?

I assume that Bandwidth would be positively impacted, but sweet spot???

[marktownley]

Try it, but I don't think it will work anywhere near as well. The lunt module is designed for f7. Your frac will have much higher contrast than the Mak.

Why do you think bandwidth of the etalon will be better in a longer focal length beam? This might be the case with a telecentric etalon but not a collimated etalon.

With a different focal ratio feeding the module than it is designed for, you will likely find the sweetspot will show a shift of both bandwidth and cwl as you move from the centre of the fov.

[pupak]

That combination will not be usable. Lunt etalons work acceptably +- 1 value from the original focal ratio, i.e. F6-F8.

[DSobserver]

And did somebody already tried to remove the collimating lens from the lunt 50 module and using a simple barlow x2 on a F15 scope?

[pupak]

That doesn't work either. You would have to use a large diameter focal reducer in front of the etalon and a negative lens of same optical power behind the etalon.

[Bob Yoesle]

"And did somebody already tried to remove the collimating lens from the lunt 50 module and using a simple barlow x2 on a F15 scope?"

Hmmm - where to begin. Removing the collimator lens from the a pressure tuned etalon module may render it useless as you will no longer be able to pressure tune it from the native CWL which is significantly off-band into the blue side of the H alpha line.

There also appears to be a bit of confusing collimator lens systems with telecentric lens systems. While similar in design, they achieve their results very differently:

Zetner telecentric design.jpg (242.06 KiB) Viewed 3767 times

Peter Zetner


A collimator system (bottom) attempts to render the axial and field rays going through the etalon parallel enough to be within the etalon's acceptance angle, and a refocusing lens re-converges them to form an image. The refocus lens can be of any desired FL (it will determine the EFL) but will not affect the etalon performance. The FL and positioning of the collimator is critical in producing a sufficiently collimated output for the etalon.

A telecentric system (top) again uses a collimator method to achieve afocal collimated ray bundles. However, the refocus lens acts as an amplifier to render the axial and field rays going through the etalon nearly parallel and within the etalon acceptance angle via the f30 + parallel light cones. The FL and positioning of the both the collimator and refocusing lens are critical to its ability to have a telecentric output. See Christian Viladrich's detailed description here.


All H alpha narrow-band filters have an "acceptance angle," which defines how much of an angle can pass through an etalon before the design wavelength (or center wave length - CWL) shifts un-acceptably "off-band." This is naturally one-half the diameter of the Jacquinot Spot diameter, otherwise known as the "sweet spot." 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.

Equation 1: Δλ = √2 x FWHM

The field (tilt) 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 an air-spaced 0.7 Å FWHM etalon. Outside this "sweet spot" radius H alpha detail will begin to blue-shift off-band and fade.

Note that if you have a narrower FWHM (as with double stacking) the Jacuinot spot will be proportionally smaller. For two 0.7 Å etalons in series, the cumulative FWHM is ~ 0.5 Å, and the Jacquinot spot is reduced to about 0.84 degree. The forgoing also reveals why a solid-spacer filter with a greater n (refractive index) will have a larger Jacquinot spot to a comparable FWHM air-spaced etalon.

"I assume that Bandwidth would be positively impacted..." No; an increased focal ratio improvement of bandpass is only applicable to telecentric lens systems, not collimator lens systems.

"With a different focal ratio feeding the module than it is designed for, you will likely find the sweetspot will show a shift of both bandwidth and cwl as you move from the centre of the fov." Actually this is what happens when you use a non-optimized telecentric lens system - both the axial and field rays begin to exceed the etalon acceptance angle and begin to broaden as well.

The sweet spot diameter size for an internally located etalon with a collimator is simply a function of the Objective FL/Colliamtor FL = field angle magnification. The more magnified the field angles, the smaller the sweet spot becomes. The central on-axis ray FWHM will not be affected by the field angle magnification, but the CWL will shift more rapidly the farther off-axis you go. However, by definition the CWL within the sweet spot remains within +/- FWHM x √2.

As shown previously, the typical air-spaced etalon with a 0.7 Å FWHM has an acceptance angle of 0.5 degree, and therefore a sweet spot diameter of 1 degree. Used with a collimator lens with 1/2 the FL of the objective, the field angle magnification is 2, and the sweet spot is reduced to 1/2 a degree, but the FWHM remains 0.7 Å. Similarly, using the collimator with a longer FL objective will further reduce the diameter of the sweet spot by the reciprocal of the magnification.

Ideally you'd like the f/D geometry of the collimator lens to be equal to the f/D geometry of the objective - and this is generally used in the commercial designs, and is why some believe the ratio of the objective to etalon (or collimator) diameter determines the size of the sweet spot, but this is only reflective of the similar f/D geometry of the lenses: Do/Dc = FLo/FLc.

The f/D ratio of the objective does not affect the sweet spot, instead it affects whether or not the etalon is vignetted or the effective aperture is reduced (f/D of the objective being larger or smaller than the collimator's). This is due to the geometry of the foci of both the objective and the collimator needing to coincide to render the axial rays exactly parallel (collimated).

Internal etalon geometry.png (100.95 KiB) Viewed 3797 times

Note: conceptual to show f/D geometry and does not show the collimated output or refocusing lens. Obviously an equal to or longer f/D objective is preferable to a shorter f/D objective.


The Lunt 50's FL is 350 mm. Assuming the collimator is 1/2 the FL of the objective = 175 mm. As described previously, the sweet spot will be 1/2 a degree - just large enough to fit the full-disc of the Sun.

Given the forgoing collimator FL assumption, for the OP's 130 f/7 (FL 910 mm) and using the LS 50 collimator/etalon, the field angle magnification is 910/175 = 5.2x. Therefore the sweet spot will be 1/5.2 degree = 0.19 degree, or about 1/3 the sun's diameter. Due to the similar f ratios, there will be no vignetting of the etalon or reduction of the aperture.

For the OP's 127 f/11 mak the field angle magnification is 1397/175 = 8.0x. Therefore the sweet spot will be 1/8 degree = 0.125 degree = 1/4 the sun's diameter. This would be the size of the sweet spot regardless of the objective's f ratio. However, due to the longer f ratio of the objective compared to the collimator, the etalon will also be vignetted (not fully illuminated).

Obviously, the 127 mak used at f/15 would have an even smaller sweet spot due to the longer FL, and the etalon will be further vignetted.

Bob

[OlegLviv]

Very good BOB! I bought TS 152/5.9f(900mm) and have Lunt 60 with include 2x barlow inside A collimator system f4 + main Ota Lunt 60 have f8
I bought f5.9 because my Lunt have f4 but not f8.....my previous telescope for my lun60 had f9.5 and had normal result....but f5.9 dont know because in way to me...my friend say me Oleg you will be have big field but same sweet spot like you have with f9.5.

What do you say about my future setup TS 152/5.9f(900mm)?

What do you say about my Lunt 60mm f4.....why lunt did 2x lens and we have f8....?

[Bob Yoesle]

Hello Oleg,

I'm not really sure what you're describing to me - and I'm not sure where the f4 and 2x Barlow is coming from - here's what I see for the LS60 layout, and showing the internal etalon collimator and refocusing lens positions:

Lunt LS60 layout.jpg (298.7 KiB) Viewed 3639 times

The lower drawing appears to be the dimensions used in the LS80.

It sounds like you want to use the LS60 with a larger 152 mm f5.6 objective.

There are two basic ways you could do that, and the first is the traditional removal of the etalon module and placing it at the proper position inside the focus of the larger refractor objective. However, as I described above, the LS60 should have about an f8+ collimator (objective is 500/60 = 8.3), and therefore you'd be effectively reducing the larger telescope's aperture, which partially defeats some of the mods purpose. You could possibly use a 2 inch Barlow lens ahead of the module to extend the 152's focal ratio to above f8.3, and then place the etalon module at the appropriate distance inside the new objective/Barlow focal point. A better solution would be to sell the 152 f5.9 and get a 152 f8 ;-)

The other rarely employed method you might try would be to get a 60 mm + diameter negative element ahead of the complete LS60 telescope in order to collimate the larger refractors output, and feed this collimated light to the complete LS 60 telescope. You could design this so that the LS60 can be easily repositioned in the larger refractors output so that the LS 60 could be used both independently for full-disc viewing and imaging, and repositioned in the larger refractors collimated output for higher resolution viewing and imaging.

Telescope collimator.jpg (42.62 KiB) Viewed 3639 times

Bob

[OlegLviv]

Bob thank you for answer!
So the collimator in the previous Lunt 60 LS60THA vs the latest modular Lunt 60 LS60MT are different.
The problem with the LS60 is that the collimation lens also works as a Barlow 1.89 x and the objective is F4.2. It is difficult to find a larger diameter with a similar parameters and, above all, the same aberration, so that the etalon can be used without changing lenses.
The single-lens objective is compensated by a collimation lens and even if it is used, for example, by the OTA 80/F4, the image quality is not good.
The only way is to remove the collimation and output lens of the etalon and replace them with lenses that do not compensate for single-lens objective aberration.
If you want know more about my Lun60 model and the same model Pupak please write this posts second and third pages!
viewtopic.php?t=34331&start=25

[Bob Yoesle]

Hello Oleg,

How do you know the the objective's focal length of 256.6 mm shown above? If that is for the modular LS60 telescope, it is clearly erroneous, as for nighttime use (i.e. with the H alpha etalon module removed) the LS60 modular is listed as having a 420 mm focal length, or 420/60 = f7.

A collimator lens is NOT a Barlow lens by definition. A collimator produces a collimated (afocal) output, and a Barlow produces a focused output. A positive lens could be used after the objectives focal plane as a collimator as well, and similarly you would not refer to it as a "focal reducer." So please don't confuse the general lens shape with the lens function. They are not the same. And in this case, the alleged 2x "Barlow" and the alleged "f4.2" objective would result in a f8.4 converging light cone through the etalon (not the parallel lines shown) - which is also clearly erroneous and not a collimated optical system, as would have been designed by Lunt!

Despite the old LS60 and the new LS60 using perhaps different collimators, they still work the same. The objective and the collimator foci must be coincident to have a collimated output to the etalon. This is determined without knowing the collimator FL by observing where the objective focus is without the etalon module, and seeing how far in towards the objective the module is placed from this focus. This distance will have to be the same for any other objective used with the module. The objective's focus position will also give you the objective's f/D ratio, and will thereby indicate the minimum focal ratio of any other objective you need to use to avoid restricting the objective's effective aperture as I've shown in the above post.

Therefore I would state you do not need to remove either the collimator or refocus lenses. Finding appropriate replacements - and sealing them properly - can be problematic in my first-hand experience ;-) And if there is an issue between the original LS60 using a singlet and the newer LS60 modular using a doublet, you'd be hard-pressed to figure out what will correct any higher order aberrations without knowing the lens prescriptions and being adept with lens and optical design software.

My assumption given the apparently erroneous diagram(s), is that the etalon modules were at some point incorrectly located to give acceptable performance, and this is not due to any significant differences in objective types.

Bob

[pupak]

I had the same Lunt60 and the focal length of the lens is indeed 255mm.
This is not difficult to measure. The collimation and correction lenses are set to compensate for lens aberration and the resulting focal length is 420mm. It's not a Barlow, but just different focal ratios of the collimation and correction lens. The name Barlow is misleading. This etalon is very problematic to implement on other lenses.
Oleg is very capable of doing this. I failed because the results were not satisfactory for me.

[Rusted]

Where the objective focal ratio is too slow [longer focus] then inserting a standard focal reducer in front of the etalon is impossible.

A 2x focal reducer has a focal length of ~10cm or 4". Inserting such a powerful lens in front of the etalon makes it impossible to focus the system. It will focus well short of the blocking filter.

So where do we find a suitably weak, positive lens to place in front of the etalon to alter the objective f/r from f/10 to f/7? A telescope or binocular objective? Approximately what focal length should it have in this case?

[pupak]

The main problem with this Lunt60 is lens aberration compensation. It's hard to do anything with her. Of course, it is ideal to choose AR150/750, which is F5 and there is a chance that the etalon can handle it. Another option is to remove the collimation and correction lens. A few people have already done this and it works on the F30.

[OlegLviv]

My friend say me Oleg you need buy f4 telescope but such telescopes are not for sale. Ok found TS152/F5.9 and waiting arrive this telescope to me.

[Bob Yoesle]

I stand corrected. I spoke to Brian at Lunt and despite the cutaway diagram above he stated the LS60 singlet was indeed f4, which required a unique corrected collimator lens in order to correct the singlet's aberrations. He stated that it would be very difficult to use it with any other objective.

Therefore it makes the ideal use of that version of the LS60 pressure-tuned etalon to indeed have the collimator and refocusing lenses replaced.

[OlegLviv]

Friends tell me about Lunt 50 etalon and Lunt 60mm diameter?
Lunt 50 have 20 or 25mm?
Lunt 60 have 30 or 35 mm?
Thank you!

[marktownley]

Lunt 50 have 20 or 25mm?

20mm

[RodAstro]

Hi
Just to add a little to this thread that may help others.
My f15 Zeiss 6" has to long a focal length to work with my Quark as my seeing is not very capable of supporting f60 or 9,450mm focal length.
So I had a look at focal reducers and as Rusted says you loose a lot of back focus but I had in my box of bits a reducer for a Sony NEX camera.
Because this reducer has to keep the image from the lens in the same place, the chip, it has a set of lenses that does this so you loose no back focus.
As with other reducers you can change its position in the light path, further in to get more reduction or out to get less, I can just get mine inside the focus just enough to get the Zeiss working at about f30 with the Quark and that suits my seeing.
The other nice thing is it is all air spaced so no problems with heat, well not from my 6" aperture anyway, I have used it for over two years and some days it sits tracking the sun for several hours.

I bought mine on ebay but they are quite common on the net and range in price from £70-140 mine is the cheapest.
The lens unit does unscrew from the camera mount and fits inside a 2" fitting nicely with a little ingenuity.
I hope this is useful.
Rod

[Rusted]

Thanks Rod.

Not easy to find one in Europe it seems.

Do you have any idea of the focal length of this "speed booster?"

I have a lot of binoculars which could become donors for a suitable objective.
If only for experimentation. Objectives are not designed for converging light.
That doesn't stop some of them from achieving near focusing.

[RodAstro]

Hi Rusted

I'm not sure how I would measure the focal length as it has a strange lens design for cameras to keep the original lens at the same focus for infinity.
For a camera it is rated at 0.72 reduction but in a telescope it all depends on where you put it in the light path.
All I can say is with my Zeiss Coude setup there is no real way of shortening the tube without spoiling the scope so this is a must for my seeing.
I have compared it with a Baader TZ2 and Quark combo and no real difference I can see.
What I have found is if the rear lens of the reducer is very close to the Quarks front filter I can rack the focuser almost completely in and obtain around the 0.5 reduction.
The telecentric beam then helps because you have much more room for focusing after the Quark.

I'm not sure how much reduction you could obtain with a air spaced etalon system, probably just the 0.72. I would think the best place for a large Lunt etalon would be just before the camera after the collimated beam.

Cheers Rod

[Rusted]

Hi Rusted

<snip>

I'm not sure how much reduction you could obtain with a air spaced etalon system, probably just the 0.72. I would think the best place for a large Lunt etalon would be just before the camera after the collimated beam.

Cheers Rod

Thank you Rod.

Your last sentence really confused me.

Are you suggesting a "naked" etalon bereft of collimation lenses?
Or are you talking about double stacking?

Normally, the etalon has to be followed by the blocking filter. Which has a physical length.
So the etalon can't be placed closer to the camera than the [following] blocking filter will allow.

To talk of adjusting the focal ratio, to better suit a Lunt etalon, is also confusing.
If the etalon demands f/7 then focal ratio adjustment from [say] f10 [surely] has to be done before the etalon?
Doing so after the etalon will have no useful effect except forcing a change in image scale. Won't it?

Or am I even more confused than I think I am?

[RodAstro]

Hi Rusted

No you are not confused Just my explanation, or lack of.
The front lens of the reducer is only 30mm and the rear lens is only 25mm so using a large etalon may be vignetted by the reducer if it is in forward position before the collimated beam.
It will fit in the rear position no problem as it needs no extra back focus to work there unlike normal reducers but as you correctly say, this will only give you a wider field to the camera and have no effect on the telescopes focal ratio so not helpful at getting F7.
Depending on your etalon size it may be worth a try though.

Cheers Rod

[Rusted]

Thank you for the further explanation Rod.