Helios 120mm mod to Lunt L60

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Re: Helios 120mm mod to Lunt L60

Post by Valery »


If I get results 3/4s as good as his then I will be delighted.



OK, I looked at them. Thanks.
Will all respect to Devid Cortner's efforts, to be honest, these results are not good enough if compare even with some PST mods. The reason, I believe is that when peoples doing PST mods, they typically choose F/10 or so donor objective and it works well as the replacement of PST's native 40mm F/10 objective.

The David's mod with 120mm F/8.3 works better, than with 152mm F/6. The reason is that not a full aperture of the collimator and refocusing lenses works. Aberrations are lower than with F/6 when larger aperture works.

A proper mod for the Lunt 60mm scope will be:

1. Use the 60mm telescope as is, with it's own objective.
2. A donor scope must be used in junction with collimator lens with D=65-66mm (to eliminate vignetting).

Valery.


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Re: Helios 120mm mod to Lunt L60

Post by Allcart »

You're not wrong Valery, but surely that method would produce a much longer focal length. This has been tried before with folded scope designs, but collimation of the system and alignment of components was harder to maintain and it was much harder to build. Also I have found it impossible to find a suitable 65mm collimator lens. If you know of such a lens, please tell.


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Re: Helios 120mm mod to Lunt L60

Post by Valery »

Also I have found it impossible to find a suitable 65mm collimator lens. If you know of such a lens, please tell.

No manufacturer produce such a collimation lenses. These lenses must be:

1. Specifically designed for a given mod. For every given mod - it's own lens. For example, to keep the system of a reasonable length, 120mm F/5 SW telescope + 65mm collimator lens + 60mm Lunt telescope. Fine! But for a 100mm F/5 SW telescope and 60mm Lunt scope we already need another collimation lens, because if we will use here a lens, designed for 120mm F/5 + 60mm Lunt, then a spherical aberration will pop up. May be not too high, need to be seen in an optical simulation software. May be a lens for 120mm F/5 SW + Lunt 60T-Ha will be a good compromise for all three telescopes.

2. Be of at least two components cemented. But better three components cemented. This need to eliminate SA, coma and astigmatism.

3. Every given series of such a lens need to be corrected for glasses melt data, because if we will not do this, we can introduce SA. And, if a donor objective has, for example, a SA of 1/4 wave (very common) and our lens will introduce it's now 1/4 wave SA and aberrations will be of the same nature both positive or both negative, then the resulting SA of the system will be 1/4 + 1/4 = 1/2 wave SA! Not good. Of course, it can be opposite and then they both annihilate each other. Very nice!
So, this will be try and error, or lottery process.
Better to correct the design for each series of the lenses. But this makes such lenses more expensive.

In order to make such production economically feasible, the lens must be at least an order of 10 - 20 pc.
Where to find such an army of modders :?


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Re: Helios 120mm mod to Lunt L60

Post by Merlin66 »

Valery,
I appreciate the input and comments...
The Lunt 60 mod done last year proved the validity of exhanging the Lunt objective with a suitable larger donor...
IMHO there's no need to go to the complexity of large collimating lenses.
The solar chat links previously posted above show the great results.


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Re: Helios 120mm mod to Lunt L60

Post by Carbon60 »

I suppose that, at the end of the day, if we want 'perfection' then we should be honest with ourselves and go ahead, take the plunge and purchase a large aperture Lunt. The reality is, we are interested in doing these mods because we want to save some cash (large scopes are expensive). These mods may not be 'perfect', but judging by some of the results they can provide very acceptable results; indeed, under the right conditions, some results are outstanding.

For me, I'm happy to accept a little imperfection and go ahead with the mod.

My plan is to make the scope adjustable between the Lunt etalon and the focus point of the donor (152mm f10) utilising the existing focuser on the donor scope(mine is a home made scope and I have sufficient room behind the focusing unit to fit the Lunt 60 - less objective and tube). This will give me some 'wiggle room' to tune it as required. We'll see.

Cheers

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Re: Helios 120mm mod to Lunt L60

Post by Allcart »

I do not see any bottom lines or problems of any sort with this type of mod. It does not have any negative effects on the donor scope or the Lunt 60. I have several refractors which have lost value due to being shortened for pst mods. This does not happen when modding the Lunt 60. Both scopes can be returned to normal use in a matter of minutes and used or sold in their original condition.

All that is required is an erf and a method of holding both scopes in alignment. I chose to build an aluminium cradle based on David Cortners design because it is versatile (can be used with more than one scope) quite cheap, and could be made with the minimum of tools.

If you can find a second hand donor scope, then go for it. You have very little to lose.


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Re: Helios 120mm mod to Lunt L60

Post by swisswalter »

Hi Stephen

good luck on the mod, very good read. I vote for imperfection, fun and modding ;-)


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Re: Helios 120mm mod to Lunt L60

Post by Allcart »



My plan is to make the scope adjustable between the Lunt etalon and the focus point of the donor (152mm f10) utilising the existing focuser on the donor scope(mine is a home made scope and I have sufficient room behind the focusing unit to fit the Lunt 60 - less objective and tube). This will give me some 'wiggle room' to tune it as required. We'll see.

Cheers

Stu.

Not sure I understand your intention here Stu. The focuser on the donor scope plays no part in the mod whatsoever and can be removed to make the assembly lighter. Also with the focuser removed, you have a convenient place for a sub aperture erf.


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Re: Helios 120mm mod to Lunt L60

Post by Carbon60 »

Hi Allan,

It's just that my scope is set up already as a PST mod. I have a sub aperture ERF (90mm)in place already, part way down the OTA. I was intending to make the switch between the PST mod and the Lunt 60 mod as easy as possible, just by substituting my PST etalon and adapters with another machined adapter and the Lunt 60 minus its objective/tube. This has the advantage (in my view) of giving me some latitude for fine tuning the focus point in relation to the Lunt Etalon. I understand that this is not necessary if all the measurements and the adapter are 'bob on', but.......

Stu.


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Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
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Re: Helios 120mm mod to Lunt L60

Post by Zakalwe »

Valery,

With respect, when theory does not match real-world observations, then it is almost always the theory that needs revising.
I think that you are over-complicating this.

With regards to the David Cortner blog, if you look at this page:
http://www.davidcortner.com/slowblog/20120717s.php
he provides a back to back comparison of the native Lunt and the 90mm Franknscope. The extra focal length gives more scale, but there is also a lot more resolution (look at the detail in the sunspot), which you would expect from having the larger aperture.

Allcart has also reinforced my earlier point....if it doesn't work then it is a small job to return both scopes to stock.
Win-win situation as far as I am concerned.

Finally, there's now been number of people that have completed this mod, and the details can be found in this forum.

Unfortunately, as I said earlier, this won't be a fast mod, but I will update this thread as and hen things happen. :rockon:


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Re: Helios 120mm mod to Lunt L60

Post by Allcart »

Ok Stu, I think I understand the concept now. My only concern would be that the drawtube on your focuser may severely vignette the lightcone entering the lunt etalon. But crack on and see if it works. Having the ability to tweak the etalon position would not be a bad thing. I can just slide my Lunt back and forth on its own dovetail.


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Re: Helios 120mm mod to Lunt L60

Post by Valery »

Valery,
I
IMHO there's no need to go to the complexity of large collimating lenses.
The solar chat links previously posted above show the great results.

OK, if no need then no need.


However for others I need to explain to the end. If the focus point in the system ( Donor + objectiveless Lunt 60) is the same in this case the incoming rays goes through etalon being collimated and etalon works as need.
However at donor's F/8,3 the etalon works only at it's 54% of aperture. What does it mean?
1. Negative result: the angular filed with acceptable field angle is about 54% of possible for such etalon size.
2. Positive result: such etalon stops also stops both collimator and refocusing lenses, decreasing their aberrations.

May be the factor (2) is in charge for good results (I mean image quality). However in the mod with significantly faster donor objective (Meade AR 512) I see worser image quality. If to use an F/5 donor objective the images must degrade even more.
No free lunch here. Native 60mm F/4.5 singlet is too different to larger achromats.

There is the rate of possible mods for Lunt 60 with larger achromats.

1. Full size special collimator which works with Lunt 60 as is with it's native objective. With 120mm donor telescope working field will be about 0,5 solar disk diameter. With 150mm donor telescope working field will be about 0,4 solar disk diameter.

2. Special small slightly positive power lens (for F/5 or F/6 donors) which need to be installed prior to Lunt's 60 collimator. It must make a beam as F/4.5 (to make etalon works at it's full aperture) and correct the aberrations. Such a corrective lens also
can bring the possibility to have the largest possible working field like in case (1).

3. The mod as described in this thread with different donors with different results. Better to have F/8 donor or slower (however with smaller working field).

Sorry for taking so much of your time, guys.


Valery.


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Re: Helios 120mm mod to Lunt L60

Post by swisswalter »

Hi Valery


no sorry at all. Thank you for the explanations


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Re: Helios 120mm mod to Lunt L60

Post by Valery »

So, is there any bottom lines here? Only yesterday I was close to buying a 120mm for this mod only to read Valery's comments which brought proceedings to a halt :whistle:

All depends of what you require or expect. If a smaller working field with good enough quality is OK for you, then 120mm F/8.3 is a way to go (Photonist's mod).
If you want to squeeze out all the possibilities of Lunt 60 - to have maximal working field with best possible quality, then see other possible mods as I have described above.

Photonist's one is surprisingly good for smaller working field and the most economical. I would not advise to use faster and larger donor's.


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Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
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Re: Helios 120mm mod to Lunt L60

Post by Zakalwe »

I received the helios OTA yesterday...the objective looks to be in great condition:





And also a new Chameleon camera (tiny, ain't it compared to the TIS DMK!)



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Re: Helios 120mm mod to Lunt L60

Post by Carbon60 »

Great. Good luck with the mod Stephen.

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
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Re: Helios 120mm mod to Lunt L60

Post by Allcart »

Hi John, all the info i have seen states that the lunt has a 60mm singlet chromatic objective. I don't know if your 100mm singlet will work as a replacement for the 60mm. You would obviously have to find out the focal ratio of the lens and mount it at its optimal position relative to the etalon. If the lens comes in a cell, you will have to find a tube to mount it in, and then devise you method of attaching the lunt as well. It all adds to the expense.

Would it work better than a 100mm achromat??? not sure. I already have a couple of 100mm OTA's hanging about, so if the lens was around F7 or f10, I would probably buy it just to experiment with, but thats just me.

Do you have any info on the lens?


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Re: Helios 120mm mod to Lunt L60

Post by Allcart »

Not too cheap I hope. :ohmy:

I'm not an optics expert, (far from it) but i wonder if you found an airspaced doublet and just used the front lens, would it work.??


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