Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

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Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

I posted this on SGL, but should of course post this here too.

I am thinking of modding my little Lunt LS35THa solar scope. I do not want to take it apart (much) as I would like to be able to use it as-is too. The idea is very simple: Take a 70mm objective (from old bins) and using a (non-achromat) negative lens turn it into a 2x Galilean telescope, with a 35mm exit pupil. If this is put in front of the front mounted etalon of the LS35THa, it turns it into a 70mm F=800mm effective solar scope. This would require a bigger blocking filter for full-disk use (or a 0.5x reducer? (probably not)), and maybe also an energy rejection filter. I have understood that an ERF can also be put halfway down the tube in PST-mods, so maybe the 35mm front ERF of the LS35 is sufficient. Alternatively, I just get a 40mm ERF.

I have worked out that even if I get myself a 12mm blocking filter and a Lunt ERF I get a 70mm solar scope for much less than the standard prices of a 60mm

Any ideas why this should not work (apart from collimation/alignment issues)?


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
peter drew

Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by peter drew »

Hello Michael, welcome to this excellent forum. I can't comment with any authority on your proposed Lunt mod but this is what I would do. I would sell the LS35 and purchase a PST, this would probably make a much better Ha mod as the system is well documented. A larger than 70mm mod could be constructed at a cost still below that of a bespoke 60mm Lunt and the PST can easily be reassembled for grab and go use. ;)


MjrTom

Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by MjrTom »

Hello and welcome to the forum Michael :)

Sorry cant comment on your proposal but I personally own a Lunt LS35THa and just bought a PST for the purpose of modding it.

The bandpass of the Lunt etalon is much narrower but less suitable for modding like the PST.
However saying that there was a thread on here a while back where the Lunt etalon was used in a mod scope.

Mark


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by swisswalter »

Hi Michael

welcome to that wonderful TOS free site. Have sun, have fun.

The cracks are still at work or sleep. You'll get decent answers in a couple of hours, I'm sure.


PS: I'm modding around here and there and with the help of the "big players" here it seems to be easy and for sure it makes fun. I prefere to mod around with a PST.


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by marktownley »

I posted this on SGL, but should of course post this here too.

I am thinking of modding my little Lunt LS35THa solar scope. I do not want to take it apart (much) as I would like to be able to use it as-is too. The idea is very simple: Take a 70mm objective (from old bins) and using a (non-achromat) negative lens turn it into a 2x Galilean telescope, with a 35mm exit pupil. If this is put in front of the front mounted etalon of the LS35THa, it turns it into a 70mm F=800mm effective solar scope. This would require a bigger blocking filter for full-disk use (or a 0.5x reducer? (probably not)), and maybe also an energy rejection filter. I have understood that an ERF can also be put halfway down the tube in PST-mods, so maybe the 35mm front ERF of the LS35 is sufficient. Alternatively, I just get a 40mm ERF.

I have worked out that even if I get myself a 12mm blocking filter and a Lunt ERF I get a 70mm solar scope for much less than the standard prices of a 60mm

Any ideas why this should not work (apart from collimation/alignment issues)?

Hi Michael!

Welcome to the forum! :)

Ok, my initial thoughts; yes, should work but I can see some 'buts'

The negative focal length lens; You would be needing a 40mm diameter lens I figure (stepped down to 35mm), now i've only really seen these go upto -120mm fl (-f3), if your 70mm binocular lens is f3 then all is well, but I suspect not. Say the 70mm bins lens is f5 you will need the negative lens to be operating at -f5 to get a collimated beam into the 35mm etalon. If you know of anywhere that sells a 'range' of decent negative focal length lenses let us know!

Then there are I suspect likely resultant sweetspotting or in the case of this etalon type banding issues. This is a result of the field angles increasing in the system. Sweetspotting increases proportionally as result of the ratio of the objective size to etalon size. Using a focal reducer on the system will I bet money give unusable results - focal reducers also accentuate sweetspotting issues in Ha scope mods like this.

I would always use a Baader ERF over a Lunt one too - the Lunts are much thinner and are not figured to the same accuracy as the Baaders.

There will certainly be challenges with collimation, alignment and mechanics in a project like this but I think these are the easy part compared to the 2 examples above...

Mark :)


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by Bob Yoesle »

You will likely have more success if you use a positive collimator, for the following reason:

The diameter of the collimator is not a critical parameter - as long as it is larger than the etalon, If using a negative lens, the diameter of the negative lens must be larger than the objective’s converging cone where it is located. The field angles will be magnified (increase) exactly based on the ratio of the objective focal length divided by the collimator focal length. Keeping the field angles of the sun’s limb under 1 degree should result in decent performance. This means keeping the focal length of the collimator lens (negative or positive) to no less than 1/4 the th focal length of the objective, and the longer than that - the better. As Mark notes, suitable negative lenses with larger diameters and longer focal lengths will be (more) difficult to find.

FYI - The final effective focal length will also be magnified by the ratio of the objective focal length divided by the collimator focal length.

Bob B)


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

Thanks all! The idea of a positive collimator is interesting (though it needs to be the same size as the negative, i.e. 35mm or more). Some time ago I did find a site listing several plano-concave lenses (multi-coated) at 2" diameter. I will dig that up again. The focal reducer idea is out, as it needs to be in front of the blocking filter, resulting in too little out-focus. A positive collimator would yield a longer overall length (by twice the focal length of the collimator), but on the positive side, I have a couple of mounted 50mm objectives from a defunct Russian pair of bins lying around, so maybe I could lash something together out of that (I could even turn them into a giant achromatic symmetric EP (remember those?), and use the combo as collimator).

I might proceed as follows:


[ol]
[li]Measure up the focal lengths of the lenses I have lying around, and see if I can lash up a 2x70 Galilean (negative collimator) or regular astronomical (positive collimator) scope. If not, a colleague of mine got a box full of lenses from a garage sale for 5 euro, and there is bound to be something of use there. This costs little or nothing. [/li]
[li]Get an ERF (Baader is best, I am told), as I will need that should I want to go for the PST-mod later anyway. Modest cost, and not wasted[/li]
[li] Align the Galilean scope with the LS35 (and its standard blocking filter) and see what we get. This cost nothing[/li]
[li]Only if I am happy with the result will I look into a new blocking filter. Expensive[/li]
[/ol]

If this fails I will get myself a PST and mod that.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

You will likely have more success if you use a positive collimator, for the following reason:

The diameter of the collimator is not a critical parameter - as long as it is larger than the etalon, If using a negative lens, the diameter of the negative lens must be larger than the objective’s converging cone where it is located. The field angles will be magnified (increase) exactly based on the ratio of the objective focal length divided by the collimator focal length. Keeping the field angles of the sun’s limb under 1 degree should result in decent performance. This means keeping the focal length of the collimator lens (negative or positive) to no less than 1/4 the th focal length of the objective, and the longer than that - the better. As Mark notes, suitable negative lenses with larger diameters and longer focal lengths will be (more) difficult to find.

FYI - The final effective focal length will also be magnified by the ratio of the objective focal length divided by the collimator focal length.

Bob

Cheers, Bob,
At 2x the field angle should be about 1 deg, so the band-pass should not increase too much.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by Bob Yoesle »

Hi Michael,

You should always use the largest etalon possible for internal etalon solar telescopes.

I would use the Lunt 35 and find suitable lenses. Using the PST will result in poorer performance due to the smaller size of the etalon.

The ability to control the field angels is critical to filter performance. The rule of thumb is that the larger the etalon, the smaller the field angle magnification (increase) and the better the off-axis performance (contrast uniformity / sweet-spot suppression.)

The drawing below demonstrates why this occurs - even though it uses positive collimating optics, the same applies to using negative optics.

You can see form the geometry that if you use a smaller etalon, the collimator lens geometry becomes smaller, requiring a smaller focal length collimating lens. Since the field angles are magnified based on the ratio of the objective focal length to the collimator focal length, a smaller etalon results in greater field angles, and the sweet spot will become increasingly an issue.

In the example below, we use a 100 mm f/10 objective. If we are using a 20 mm etalon, the collimator lens will have an effective diameter of 20 mm (but could be larger). It will need to have a similar f/10 FR and hence a FL of 200 mm. The field angles form the sun’s limb (0.25 degree at the objective) will be magnified by 1000 /200 = 5 x. 5 x 0.25 = 1.25 degrees.

Using a larger 50 mm etalon and the same 100/1000 objective, the collimator geometry becomes one of using a lens with a 500 mm FL, and the field angle geometry is much better: 1000/500 = 2 x. 2 x 0.25 = 0.5 degree.




The EFL of the refocusing lens will also be magnified by the same amount: in the examples above appled to the Lunt 35 with a 400 mm FL objective - which becomes the refocusing lens - this EFL would become 2000 mm and 800 mm respectively. So you can see obtaining a target EFL is again dependant on the choice of the collimator and refocusing lens FL's.

This should pretty much cover the main parmeters to properly design a decent internal etalon solar telescope.

Hope this helps.

Bob B)


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

I forgot to divide the field angle by 2 (I used full disk rather than half disk). With a 35mm etalon I can get a 0.5 deg field angle. Good to know. I have measured the focal length of the 70mm lenses, and they come to F/5 as expected (f=350mm). My 50mm lenses are F/4 (200mm) I could use those and effectively reduce the size of the 70mm to 61mm, which is not what I want. I think my parents still have a pair of very old 8x40 bins. I might give them a better pair as a present anyway, and see if I can scavenge parts from these bins.If the lenses are F/4 (160mm) I would get 2.19x magnification and a field angle of 0.54, which is still fine.

Thanks again for the input.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by Bob Yoesle »

Since you're only using 35 mm of the aperture of your 50 mm lens, the FR is 200/35 = 5.7 which is pretty close. In fact it would probably be difficult to find a better match in FL's. You won't be reducing the effective aperture from a resolution point of view, but you might have a little vegnetting (light loss) - which may be unnoticable. Of course with your objective the ideal FL of the collimator would be ~ 175 mm.

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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

At F5.7 you are clipping the outer part of the F/5 light cone of the 70mm. 350mm / 5.7 = 61mm (you are clipping in an aperture plane, not an image plane, so no vignetting but loss of light over the entire FOV). A 160mm gives me a 32mm diameter illuminated part of the etalon. I will scout around for a better option (+6 or -6 diopter lens, does not need to be an achromat?)


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by Bob Yoesle »

I'm not an expert in optics, but my understanding is that in the collimator’s position it would act as a field stop, not an aperture stop. Therefore it won't affect the effective aperture as far as resolution is concerned, which to me is the main reason for doing such a design. My limited understanding is that such a field stop acts to "vignette" the image - either reducing the field size or reducing the image brightness, or both.

If the collimator reduces the field size, using your figures it would be 61/70 = 87%. Therefore, if for example you have a 2 degree maximum field of view, it would be decreased to 1.7 degrees. Still acceptable for full-disk viewing...

Perhaps someone with more knowledge of optics than I could address this.

Bob B)


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by marktownley »

Michael,

I'm going to be perfectly honest here, and I hope you don't take offence, if you want a scope that is going to compete in performance with a commercially available scope then I do believe you are on a road to nowhere with what you are proposing. If you are just wanting a tinkering around and a bit of experimental fun then sure go for it, but, long term you will find that this produces a scope that varies in both bandpass and also centre wavelength across the field of view.

Just my 2c...

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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by swisswalter »

Hi Ollopa


welcome to that wonderful TOS free site. Have sun, have fun and mod ;)


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

Michael,

I'm going to be perfectly honest here, and I hope you don't take offence, if you want a scope that is going to compete in performance with a commercially available scope then I do believe you are on a road to nowhere with what you are proposing. If you are just wanting a tinkering around and a bit of experimental fun then sure go for it, but, long term you will find that this produces a scope that varies in both bandpass and also centre wavelength across the field of view.

Just my 2c...

Mark

Cheers Mark. Thanks for the input.

I intend to experiment a bit cheaply, and just see where this leads. If it leads nowhere, I have had some fun with lenses lying around (not gathering dust, they are stored carefully), and I will convert the 70mm objectives into giant finder scopes. I might then either get a PST for a mod (though that has a smaller etalon, so field angles increase more rapidly), or save up for a bigger solar scope. In the mean time, the proposed mod means I do not disassemble the LS35THa.


I studied optics when I did my MSc (Astronomy), so I have a fair understanding of optics. I did some design work (mainly thermal control) on a F-P etalon-based stellar seismometer, so am aware of the physics of Fabry-Perot interferometers. In theory, what I am proposing is the same optical train used in any scope with a centre-mounted etalon. Whether it works in practice is a very different matter.

Thanks again for the input

Michael


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

Although I agree with Mark, (unless you have some spiritual connection with the redfire,)

The ls35 has some fun attributes which can be quite frankely, exploited.

1. disconnect the etalon from the objective and you have a 62mm thread to work with.

2. The entire telescope ota is not dissimilar to a 2" to 1.25" converter systerm.

3. The lens is f10, and the system already contains a hydrogen alpha doped objective.


With this in mind, and some imagination.

Reverse the lenses of an everyday televue 3x powermate. (for f30 @ f10)

Place this Pfrankenmate'r in the ls35 1.25" adapter.
using a 1.25" to t adapter, convert the powermate to retrofit your 100mm donor focuser.

I dont want to build the scope for ya, so its just a hint from an old wise man.

starting with just a focuser- Progress foward by building backwards..


Thanks. I have been considering taking the LS35 apart, but was a bit worried I would damage things irreparably. If, as you say, this can be done easily, I could give it a shot. I have been considering using my two identical 50mm objectives, mounting them on either side of the etalon as collimator/imaging lens pair, and placing this assembly in or near the focuser of a donor scope. The thing is, I do not have a donor scope (yet).

A TV powermate has a fairly small diameter, which means I will not be able to use the full aperture of the etalon (leading to larger field angles) A 2" Barlow could be considered.

Cheers

Michael


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

I'm not an expert in optics, but my understanding is that in the collimator’s position it would act as a field stop, not an aperture stop. Therefore it won't affect the effective aperture as far as resolution is concerned, which to me is the main reason for doing such a design. My limited understanding is that such a field stop acts to "vignette" the image - either reducing the field size or reducing the image brightness, or both.

If the collimator reduces the field size, using your figures it would be 61/70 = 87%. Therefore, if for example you have a 2 degree maximum field of view, it would be decreased to 1.7 degrees. Still acceptable for full-disk viewing...

Perhaps someone with more knowledge of optics than I could address this.

Bob B)

At the collimator, the rays from a single point in the image plane become parallel (it is what a collimator does). This means, the collimator is in an aperture plane, as all image information is carried in the angle at which the light rays traverse the plane, not in their position. In an image plane, the information is carried by the position where the rays pass the plane, not in the angle.


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by marktownley »

I have been considering using my two identical 50mm objectives, mounting them on either side of the etalon as collimator/imaging lens pair, and placing this assembly in or near the focuser of a donor scope. The thing is, I do not have a donor scope (yet).

Don't forget these binocular objectives will need to have the same f-ratio as the donor scope to get a properly collimated beam...

The binocular objectives would also need to sit at their focal length beyond the focal point of the donor ota. Myself and a few others have tried this, the resultant frankensope gets very long very quickly...


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

[quote]I have been considering using my two identical 50mm objectives, mounting them on either side of the etalon as collimator/imaging lens pair, and placing this assembly in or near the focuser of a donor scope. The thing is, I do not have a donor scope (yet).

Don't forget these binocular objectives will need to have the same f-ratio as the donor scope to get a properly collimated beam...

The binocular objectives would also need to sit at their focal length beyond the focal point of the donor ota. Myself and a few others have tried this, the resultant frankensope gets very long very quickly...

The lenses I have have a 200mm focal length, which combined with 35mm etalon diameter would be F/5.71. An F/5.5 objective would lose a bit of light, whereas an F/6 would be OK (at the expense of a slightly larger field angle).

The point about the length is very true. This is why I would prefer using a tele-negative solution, like a barlow.Barlow lenses usually are too strong, dioptrics-wise, yielding large field angles quickly. As others have posted, getting large, high-aperture negative lenses is tricky.

Considering the above situation, the need for a comparatively fast system given the positive lenses I have reduces the length of the scope. If the donor scope has a focal length of about 500, the overall length would be about 1.0 m (say 80cm plus focuser + dew shield). Not terribly compact, but not totally unwieldy.

Food for thought indeed.

Thanks!


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by marktownley »

[quote][quote]I have been considering using my two identical 50mm objectives, mounting them on either side of the etalon as collimator/imaging lens pair, and placing this assembly in or near the focuser of a donor scope. The thing is, I do not have a donor scope (yet).

Don't forget these binocular objectives will need to have the same f-ratio as the donor scope to get a properly collimated beam...

The binocular objectives would also need to sit at their focal length beyond the focal point of the donor ota. Myself and a few others have tried this, the resultant frankensope gets very long very quickly...

The lenses I have have a 200mm focal length, which combined with 35mm etalon diameter would be F/5.71. An F/5.5 objective would lose a bit of light, whereas an F/6 would be OK (at the expense of a slightly larger field angle).

The point about the length is very true. This is why I would prefer using a tele-negative solution, like a barlow.Barlow lenses usually are too strong, dioptrics-wise, yielding large field angles quickly. As others have posted, getting large, high-aperture negative lenses is tricky.

Considering the above situation, the need for a comparatively fast system given the positive lenses I have reduces the length of the scope. If the donor scope has a focal length of about 500, the overall length would be about 1.0 m (say 80cm plus focuser + dew shield). Not terribly compact, but not totally unwieldy.

Food for thought indeed.

Thanks!

Hi Michael,

I do think it is important to match the focal ratios, when I engineered a system that was operating at f9.7 compared to the f10 of the collimating lenses I found the contrast lacking quite noticeably compared when I stopped it down so it was operating at f10 throughout.

Have you worked out a way to be able to tilt the etalon on the LS35 to tune it yet keep the rest of the optical train axially collimated?

Mark


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by michael.h.f.wilkinson »

I will certainly add some stops to ensure stray light caused by mismatching focal ratios. This means stopping down the 50mm objectives to the 35mm of the etalon. If I get a donor scope with f=500mm I will need to stop it down to 88mm in this case to get them matching.

The tilting mechanism is giving me some headaches. Luckily, I do have a colleague with a very good workshop including a lathe (the same with the box of lenses). He is also into astronomy so we may make this a joint project.

I am still firmly in the "considering options" phase.

Thanks for all the input!


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Re: Lunt LS35 mod? Is this a crazy idea? Is it crazy enough to work?

Post by daslolo »

It's really cool to see that this stub of an idea was actualized by Stu in 2014 and became one of the most impressive setup on here!


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