Helios 120mm mod to Lunt L60

Zakalwe · Post by **Zakalwe** » Thu Oct 10, 2013 10:39 pm

I'm going to have a bash at modding my Lunt L60. I've picked up a Helios 120mm refractor. Its a f8.3 scope, so will match the Lunt's native f-stop.

I'll probably opt for a Baader 110mm D-ERF and front mount it, though it would be cheaper to go for a sub-aperture ERF. I'll investigate this when the OTA arrives. I believe that the OTA is baffled, so there will probably be an internal baffle that I can mate up to a smaller ERF. I'm hoping to end up with something similar to Allcart's modded scope:
http://solarchat.natca.net/index.php/en ... =120#70249

This will be a slow build, so don't expect any finished scope this side of Christmas!

Any comments or ideas?

RobBower · Post by **RobBower** » Fri Oct 11, 2013 1:04 am

Im curious to see how this goes! I was thinking of doing mine into my meade 127mm
Good luck moving forward! :rockon:

Post by **marktownley** » Fri Oct 11, 2013 6:13 am

Will be interesting to see how you get on

Carbon60 · Post by **Carbon60** » Fri Oct 11, 2013 6:30 am

Is your Lunt a PT version and do you know if it makes any difference to the 'magic' number (focus point of the donor scope)if it is pressure tuned or tilt version?

Stu.

Zakalwe · Post by **Zakalwe** » Fri Oct 11, 2013 8:14 am

Is your Lunt a PT version and do you know if it makes any difference to the 'magic' number (focus point of the donor scope)if it is pressure tuned or tilt version?

Stu.

Its a pressure tuned version.

My thinking, and it's probably totally incorrect) is that both donor and new scope are both f8.3. If I measure the point where the donor objective comes to focus and position the new objective to come to focus at the same point then (by my reckoning) the light cone hitting the lens infront of the Etalon should be the same? That seems to be the way that Allcart seems to have done it?

Allcart · Post by **Allcart** » Fri Oct 11, 2013 12:09 pm

Hi Stephen, your assumptions are not quite correct, but you almost have it. Its the etalon front lens that has to be placed at the correct distance inside the light cone of the Helios 120mm. When the Lunt is combined with the donor scope, the focal point is extended further away from the original donor focal point by just over 200mm. So all you have to do is find the correct place to position the etalon inside the light cone. I can't remember what measurement worked for my mod, but we have basicly the same scope, so what works for mine should be ok for yours. I'll measure mine over the weekend and let you know the details.
There are several methods of combining the two scopes, but I chose my method because I can use the aluminium cradle with various scopes just by changing the position of the lunt. It does however add a lot of weight to the system, so a decent mount is essential.

Post by **solarchat** » Fri Oct 11, 2013 12:13 pm

me? Stephen?

Allcart · Post by **Allcart** » Fri Oct 11, 2013 1:31 pm

No, Zakalwe is Stephen also.

Zakalwe · Post by **Zakalwe** » Fri Oct 11, 2013 3:05 pm

me? Stephen?

Hey..there's more than one Stephen in the world!

Hi Stephen, your assumptions are not quite correct, but you almost have it. Its the etalon front lens that has to be placed at the correct distance inside the light cone of the Helios 120mm. When the Lunt is combined with the donor scope, the focal point is extended further away from the original donor focal point by just over 200mm. So all you have to do is find the correct place to position the etalon inside the light cone. I can't remember what measurement worked for my mod, but we have basicly the same scope, so what works for mine should be ok for yours. I'll measure mine over the weekend and let you know the details.
There are several methods of combining the two scopes, but I chose my method because I can use the aluminium cradle with various scopes just by changing the position of the lunt. It does however add a lot of weight to the system, so a decent mount is essential.

Allan,

Thanks very much for the clarification. I intend to plagiarise shamelessly from your design!

Re the mount: I'm using an EQ6, so it should be man enough for the job.

Zakalwe · Post by **Zakalwe** » Fri Oct 11, 2013 3:18 pm

Its the etalon front lens that has to be placed at the correct distance inside the light cone of the Helios 120mm. When the Lunt is combined with the donor scope, the focal point is extended further away from the original donor focal point by just over 200mm. So all you have to do is find the correct place to position the etalon inside the light cone. I can't remember what measurement worked for my mod, but we have basicly the same scope, so what works for mine should be ok for yours. I'll measure mine over the weekend and let you know the details.

What I was thinking of doing is something like this:
Measure the distance from the donor objective to the front lens of the Etalon.
Take the objective off the donor scope
Point it at a light source and measure the diameter of the light cone at the point as measured in point 2 above
Mount the new Helios OTA and find the spot where the light cone is the same size (by moving a piece of translucent paper back and forth.
Mount the Etalon in a collimatible cell at this point.

Allcart · Post by **Allcart** » Fri Oct 11, 2013 6:53 pm

I'm not sure I understand your methodology Stephen. When you say "donor scope" do you mean the helios or the lunt?
What you need to do is remove the focuser from the helios and point it at the sun. Then using a piece of card or something held at the rear of the scope, find the focal point. Now measure the distance between the focal point and the end of the helios tube and write it down for reference.
With the objective lens removed from the lunt 60, you have to find the optimum position for the etalon inside the light cone of the helios. I based my initial trials on the info found here, http://solarchat.natca.net/index.php/en ... t=60#29491 (info at bottom of the page)and from David Cortner's blog. I then tweaked it to suit.
If I can, I'll try and clarify my measurements over the weekend.

And feel free to plagiarise my design all you want. I will consider it a compliment. After all, I plagiarised everyone elses.

Zakalwe · Post by **Zakalwe** » Fri Oct 11, 2013 8:28 pm

I'm not sure I understand your methodology Stephen. When you say "donor scope" do you mean the helios or the lunt?
What you need to do is remove the focuser from the helios and point it at the sun. Then using a piece of card or something held at the rear of the scope, find the focal point. Now measure the distance between the focal point and the end of the helios tube and write it down for reference.
With the objective lens removed from the lunt 60, you have to find the optimum position for the etalon inside the light cone of the helios.

Allan,
thats exactly what I meant

And feel free to plagiarise my design all you want. I will consider it a compliment. After all, I plagiarised everyone elses.
:rockon:

Allcart · Post by **Allcart** » Fri Oct 11, 2013 9:06 pm

Ok, thats good Stephen. If you need any additional info or pics just ask. Advice is, as always, free of charge and willingly shared.
Keep us posted on your progress.

Zakalwe · Post by **Zakalwe** » Fri Oct 11, 2013 10:47 pm

I want to do this with my Lunt 60 PT so watching with interest. I dare sat this will work ok with an F8 6" achro.

Allan (Allcart) has it working with a Celestron C6.

I was tempted to go with a longer focal length scope (f10) to get a bit more "reach", but I found the Helios first. It has the same f-stop as the original Lunt scope, so that might make it a bit easier.

The trouble with using a larger objective is that it becomes more sensitive to the seeing and atmospheric turbulence. Plus its a LOT bigger.

vikeljay · Post by **vikeljay** » Fri Oct 11, 2013 11:01 pm

What advantages do you gain with this setup?

JohnM · Post by **JohnM** » Fri Oct 11, 2013 11:17 pm

I got a good price on a 4"
Should be better under usual seeing than my old 6".
Plus considerably less for a full sized ERF.

Allcart · Post by **Allcart** » Fri Oct 11, 2013 11:37 pm

Yes the C6-R is a lot bigger and heavier and more awkward to mount , but it is manageable and somewhat satisfying when you use it. It does seem to suffer more with bad seeing, and if I'm honest, I use the 120mm more. The modding was just as easy with either scope, and its actually simpler than pst modding.
I found that the guidesope rings on the lunt and a laser are essential for collimating the system.

Post by **marktownley** » Sat Oct 12, 2013 7:21 am

What advantages do you gain with this setup?

Aperture and hence resolution...

Valery · Post by **Valery** » Sat Oct 12, 2013 9:59 am

I'm going to have a bash at modding my Lunt L60. I've picked up a Helios 120mm refractor. Its a f8.3 scope, so will match the Lunt's native f-stop.

I'll probably opt for a Baader 110mm D-ERF and front mount it, though it would be cheaper to go for a sub-aperture ERF. I'll investigate this when the OTA arrives. I believe that the OTA is baffled, so there will probably be an internal baffle that I can mate up to a smaller ERF. I'm hoping to end up with something similar to Allcart's modded scope:
http://solarchat.natca.net/index.php/en ... =120#70249

This will be a slow build, so don't expect any finished scope this side of Christmas!

Any comments or ideas?

Let me add two cents.

1. F/stop in the Lunt telescope is not necessary the same F/stop for the objective (singlet). The F/stop you did mean is the F/stop of the equivalent system.

2. The collimator and refocusing lenses in the Lunt 60 are designed for the singlet objective. And this is straight - through designed system. All aberrations of the singlet objective are nullified by collimator and refocused lens.

3. An objective in a Helios 120mm F/8.3 refractor is an achromat. It has absolutely another aberrations than a singlet 60mm lens in the Lunt 60mm scope.

If your system will work, it will likely work poorly. In any case at much lower level than 120mm properly designed telescope can do.

Not recommended.

Zakalwe · Post by **Zakalwe** » Sat Oct 12, 2013 10:50 am

Thanks for the comments Valery.

Your comments seem to fly in the face of others' experience. Other people have successfully done this mod and have posted the results.
http://solarchat.solarastronomy.org/ind ... -completed

Why do you say that it will work poorly, if at all? Have you practical experience of the proposed modification?

If the mod doesn't work (and I am not convinced that it won't) then all I have to do is screw the original objective back onto the Lunt.

Valery · Post by **Valery** » Sat Oct 12, 2013 2:24 pm

Thanks for the comments Valery.

Your comments seem to fly in the face of others' experience. Other people have successfully done this mod and have posted the results.
http://solarchat.solarastronomy.org/ind ... -completed

Why do you say that it will work poorly, if at all? Have you practical experience of the proposed modification?

If the mod doesn't work (and I am not convinced that it won't) then all I have to do is screw the original objective back onto the Lunt.

Hmm... It works. Didn't read this before.

However I don't know how this is possible that these parts - a 120mm F/8,3 achromat and collimator - refocused system lens in the Lunt 60 are so well compatible. Simply unbelievable!

Really, no question about achromat objective, but who know (except the designer) the data of the singlet lens objective in the Lunt 60 scope?
A singlet objective should have really huge aspherics on the front surface - to eliminate a spherical aberration and coma in the image. A two lens achromat does not have SA and coma. So, the achromat being as a donor (instead of singlet) will suffer from SA and coma because the collimator - refocus system has them as a negative to SA and coma in the singlet.

The only guess why does it work is that the objective in the singlet is not a singlet.

PS. Biggest surprise for me ever (in optics).

Zakalwe · Post by **Zakalwe** » Sat Oct 12, 2013 3:07 pm

Hi Valery,

It'd be worth looking at David Cortner's blog too. He has successfully used a 90mm F10.1 achro in place of the Lunt single objective:
http://www.davidcortner.com/slowblog/20120717s.php

and also a 152mm f5.9 achro
http://www.davidcortner.com/slowblog/20120817s.php
http://www.davidcortner.com/slowblog/20120914s.php
I believe that the Kuming 152mm OTA is the same OTA that Lunt use in their 152mm solarscope.

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

Every day is a learning day!

Allcart · Post by **Allcart** » Sat Oct 12, 2013 4:52 pm

Hi Valery, I agree that it is surprising how well the large aperture scopes work, but you must remember that this mod is for imaging purposes rather than visual observing. When we image at this scale with ccd cams we are cutting out a lot of the abberations and coma and only seeing the best part of the image. When used for visul observing the Lunt 60 will give a better view.
The Lunt 60 objective is basicly a 60mm f4.5 singlet, but when combined with the collimating lenses of the etalon, it becomes an f8.3 system.
I plan to use this same modification on a faster f5 scope in the future to see if it works better than the slower f8.

Post by **marktownley** » Sat Oct 12, 2013 5:36 pm

I look forward to seeing all the results

Zakalwe · Post by **Zakalwe** » Sat Oct 12, 2013 7:08 pm

The Lunt 60 objective is basicly a 60mm f4.5 singlet, but when combined with the collimating lenses of the etalon, it becomes an f8.3 system.
I plan to use this same modification on a faster f5 scope in the future to see if it works better than the slower f8.

Thats interesting. Something like a Startravel 120 might be a good fit as they are f5. They are also as cheap as chips and can be picked up second hand for not a lot of money at all.

Valery · Post by **Valery** » Sat Oct 12, 2013 7:55 pm

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.

Allcart · Post by **Allcart** » Sat Oct 12, 2013 8:18 pm

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.

Valery · Post by **Valery** » Sun Oct 13, 2013 2:54 am

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

Post by **Merlin66** » Sun Oct 13, 2013 4:11 am

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.

Carbon60 · Post by **Carbon60** » Sun Oct 13, 2013 8:03 am

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

Stu.

Allcart · Post by **Allcart** » Sun Oct 13, 2013 8:58 am

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.

swisswalter · Post by **swisswalter** » Sun Oct 13, 2013 9:03 am

Hi Stephen

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

Allcart · Post by **Allcart** » Sun Oct 13, 2013 9:04 am

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.

Carbon60 · Post by **Carbon60** » Sun Oct 13, 2013 10:17 am

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.

Zakalwe · Post by **Zakalwe** » Sun Oct 13, 2013 10:36 am

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:

Allcart · Post by **Allcart** » Sun Oct 13, 2013 11:08 am

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.

Valery · Post by **Valery** » Sun Oct 13, 2013 1:33 pm

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.

swisswalter · Post by **swisswalter** » Sun Oct 13, 2013 1:37 pm

Hi Valery

no sorry at all. Thank you for the explanations

Valery · Post by **Valery** » Sun Oct 13, 2013 1:43 pm

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.

Zakalwe · Post by **Zakalwe** » Tue Oct 15, 2013 12:01 pm

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!)

Carbon60 · Post by **Carbon60** » Tue Oct 15, 2013 5:48 pm

Great. Good luck with the mod Stephen.

Stu.

Allcart · Post by **Allcart** » Sun Oct 27, 2013 8:42 am

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?

Allcart · Post by **Allcart** » Sun Oct 27, 2013 10:21 pm

Not too cheap I hope.

hmy:

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.??