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"Magic" distance of Lunt LS50PT etalon from focal plane?

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"Magic" distance of Lunt LS50PT etalon from focal plane?

Post by Roel80 » Sat Nov 23, 2019 8:45 am

Hi everyone!

After building several PST mods years ago (with great help from this forum), I recently aquired a used Lunt LS50T B600. It appears to outperfom my old PST, and I'm thinking of making a stage 1 mod using this Lunt. Visual use only BTW.

I have a 75mm front ERF, so a 70mm f=500mm (f/7.14) donor scope should be the way to go. At least not much bigger considering the size of the B600.

The only thing I'm not able to get confirmed is the required distance of the etalon unit from the focal plane. In the PST this was quite critital.

Any thoughts?

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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by marktownley » Sat Nov 23, 2019 11:42 am

Yes, it's 140mm (20mm x 7). Good luck with the mod.
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by Roel80 » Sat Nov 23, 2019 12:02 pm

Ah great! I guess that's the diameter of the etalon x the focal ratio of the objective - makes sense.
That 140mm is measured from the focal plane to which part of the etalon unit?

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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by bart1805 » Sat Nov 23, 2019 9:29 pm

Roel80 wrote:
Sat Nov 23, 2019 12:02 pm
Ah great! I guess that's the diameter of the etalon x the focal ratio of the objective - makes sense.
That 140mm is measured from the focal plane to which part of the etalon unit?
To the back of the unit, so the back lens.

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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by Roel80 » Sun Nov 24, 2019 7:19 am

Thanks!

Still one thing I was wondering: I understand that the little correction lenses bring a f/7 tot parallel, and after the etalon back to f/7 again. What exactly is the effect of increasing the focal ratio on the image? Let's say I would like to use a f/7,5 objectieve instead of an f/7.

If this has no effect on the performance of the etalon, it might be possible to move the etalon unit a little bit further up the tube, before it will vignet the light cone. This can be useful to fit a more decent focuser behind the unit and reach focus. However, if this is uncharted territory, I'll stick to f/7 just to be sure...

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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by marktownley » Sun Nov 24, 2019 9:10 am

bart1805 wrote:
Sat Nov 23, 2019 9:29 pm
Roel80 wrote:
Sat Nov 23, 2019 12:02 pm
Ah great! I guess that's the diameter of the etalon x the focal ratio of the objective - makes sense.
That 140mm is measured from the focal plane to which part of the etalon unit?
To the back of the unit, so the back lens.
I'd say the front lens Bart, but sat here reflecting on it all with my coffee this sunday morning, I think there is a valid discussion for both our answers ;)

The front lens needs to be -140mm ahead of the focal point so it intercepts the converging beam where it is 20mm in diameter, this is because the etalon has free diameter of 20mm, and we want to maximise our etalon surface area usage- this is what costs the $$$. The collimated beam that passes through it is 20mm in diameter, and then the rear, re-converging lens, then turns this back into a converging beam with a focal length of 140mm from the back of this lens. The front collimating lens and rear re-converging lens both have a focal ratio of f7 so any aberrations created are cancelled out.

It will work if you use the back of the unit for the -140mm marker distance, however the diameter of the converging beam passing through the front collimating lens will be greater than 20mm, so the housing of the lens cell will act as a field stop on the etalon, resulting in a smaller exit pupil diameter than you could potentially have using the logic above. Effectively, you're now running at a back focus distance of 140mm + thickness of red etalon housing (maybe closer to 200mm? - don't have my LS50 handy). This might not be an issue as the field you would be losing could be outside the jacquinot (sweet spot)) diameter, and, so, might not be usable field anyway. ???

So, simple answer, the distance is a starting point, work with that initially and tweak with what works best as you become more experienced using the system. If you're running a f7.5 system, the numbers will be slightly different, but go with the above in the first instance and refine as you go along.
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by GreatAttractor » Sun Nov 24, 2019 9:22 am

Roel80 wrote:
Sat Nov 23, 2019 12:02 pm
That 140mm is measured from the focal plane to which part of the etalon unit?
I suggest you measure it precisely yourself. Here's a simple method I used: viewtopic.php?f=9&t=19238
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by marktownley » Sun Nov 24, 2019 9:44 am

GreatAttractor wrote:
Sun Nov 24, 2019 9:22 am
Roel80 wrote:
Sat Nov 23, 2019 12:02 pm
That 140mm is measured from the focal plane to which part of the etalon unit?
I suggest you measure it precisely yourself. Here's a simple method I used: viewtopic.php?f=9&t=19238
Forgot about this thread! Nice contribution to the discussion.
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by bart1805 » Sun Nov 24, 2019 10:53 am

marktownley wrote:
Sun Nov 24, 2019 9:10 am
bart1805 wrote:
Sat Nov 23, 2019 9:29 pm
Roel80 wrote:
Sat Nov 23, 2019 12:02 pm
Ah great! I guess that's the diameter of the etalon x the focal ratio of the objective - makes sense.
That 140mm is measured from the focal plane to which part of the etalon unit?
To the back of the unit, so the back lens.
I'd say the front lens Bart, but sat here reflecting on it all with my coffee this sunday morning, I think there is a valid discussion for both our answers ;)

Interesting Mark! In my opinion you should start with the putting the front lens at minus 140mm. Then there is the etalon, the new focal point is at 140mm behind the back of the etalon. What I mean is that both distances should be 140mm, starting with the front.

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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by marktownley » Sun Nov 24, 2019 11:07 am

I suppose we should define front and back too - my 'front' would be objective side, my back would be etalon side ;)
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by bart1805 » Sun Nov 24, 2019 11:42 am

marktownley wrote:
Sun Nov 24, 2019 11:07 am
I suppose we should define front and back too - my 'front' would be objective side, my back would be etalon side ;)
We agree. (-;

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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by Valery » Sun Nov 24, 2019 3:22 pm

I have heard that etalon diameter in this telescope is 26mm, not 20mm.
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by Roel80 » Sun Nov 24, 2019 3:44 pm

This afternoon I totally took apart my 50mm Lunt. I took several measurement, some of them did not match up the specification mentioned above. Some remarkable findings about possible vignetting also... I will post the results in another topic this evening, while it might be interesting for al who want to mod their LS50T.

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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by Roel80 » Tue Dec 03, 2019 10:52 am

marktownley wrote:
Sat Nov 23, 2019 11:42 am
Yes, it's 140mm (20mm x 7).
I've been thinking about this for a while now, and I'm not quite sure it's that simple. While it seems logical, the original Lunt 50mm is not built that way: the first collimating lens is at 168mm inside focus. While the objective is an f/7.14 it "should" be at -143mm.

I believe we can pretty much assume Lunt designed the first collimating lens to get the beams of the objective parallel through the etalon. That means that even when using a longer focal ratio objective (f/7,5 or f/10), the first lens always has to be at -168mm to get that parallel beam. When you move the position of the etalon box, you might get the most light through the etalon, but you change the angle of the light beams hitting the first lens, resulting in a non-parallel beam.
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20191203_113334_resized.jpg (244.32 KiB) Viewed 119 times
I made a small drawing to explain what I mean. Taking 2 equal diameter objectives with different focal ratio's, the optimal position of the fist collimating lenses (considering illumination) are at different positions: with f/10 farther back than with f/7. But then the incoming angles (especially at the edge of the collimating lens) changes as you move its position, resulting in a non-parallel beam.

I believe the angle of the incoming beam is more crucial to the working of the etalon than optimal illumination. I think sticking to Lunt's -168mm rather than the calculated -150mm position for my f/7,5 mod might be the best choice. Still 18mm difference between outcomes is quite a lot, and I can only cut the tube once...

Any thoughts on this?
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by marktownley » Mon Dec 09, 2019 10:05 pm

Slow on the reply here sorry!

If the objective is f7.14 (and I don't doubt that!) then yes, for the etalon enclosure (20mm opening) to be not operating as a field stop then the distance is -143mm.

If the etalon enclosure is sat at -168mm (25mm further in than -143mm), then it is doing this to act as a field stop. This is to reduce the maximum field of view of the scope. A 50/350 has about a maximum fov of about 4 degrees, this is not necessary with the sun only having an angular diameter of half a degree.

The collimating lens in the etalon assembly turns that converging beam from the objective in a (parallel) collimated beam. The 'strength' of this collimated lens will be matched to a particular focal ratio of objective lens, changing that focal ratio will result in a less than perfect collimated beam - ie you might get a somewhat diverging beam passing through the etalon, or, a very high focal ratio still converging beam. Either of these would result in a smaller sweetspot than with an etalon that operates in a true collimated beam. The larger the sweetspot the larger the usable field of view.

Not sure what focuser you use with your f7.5 scope, but if it has an m68 fitting you can always add extension tubes to it to get the etalon further out, cutting 'short' isn't necessarily a bad thing.
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Re: "Magic" distance of Lunt LS50PT etalon from focal plane?

Post by Roel80 » Tue Dec 10, 2019 2:59 pm

marktownley wrote:
Mon Dec 09, 2019 10:05 pm
The 'strength' of this collimated lens will be matched to a particular focal ratio of objective lens, changing that focal ratio will result in a less than perfect collimated beam - ie you might get a somewhat diverging beam passing through the etalon, or, a very high focal ratio still converging beam. Either of these would result in a smaller sweetspot than with an etalon that operates in a true collimated beam.

Let's assume the configuration of the original Lunt LS50 is ideal given the parts' dimensions. (Which I think is true, considering the nice big sweet spot. I'ts larger than the sun's diameter at f=350mm).
Obviously a parallel beam is more important for optical perfomance than the amount of vignetting.
So I also think the original -168mm for the first collimating lens is to be used in all mods from f/7 and longer. It is the only position where that lens diverges a incoming beam to parallel. To be sure I'm going to make a sketch of what I mean, but for now I believe that for example an f/12 cone can still be collimated to parallel, but only when it's placed at -168mm.

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