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Lunt 50 mod with f/7 and f/11 refractors

Posted: Fri Aug 06, 2021 9:42 pm
by stefanr
I was inspired by this blog post (link) and after reading a lot about Lunt mods here on SolarChat and on other astronomy forums I decided to try it out on my 130mm f/7 refractor.

I have a Lunt Solar Telescope LS50THa with B600 blocking filter so I bought the adapters listed in the link above and screwed a 2" UV/IR cut filter into the nosepiece (this acts as an internal ERF before the etalon). I am planning to buy a full aperture ERF when I know what telescope I am going to use as a Lunt 50 mod scope going forward (just to be safe).

My telescope is a TS-Optics Photoline 130 mm f/7 FPL53 Triplet APO Refractor.
One good thing about this scope is that I can remove a 90mm tube extender to get a total of 213mm working distance.
With the extender removed I had 51mm on the telescope focuser distance and put in the Lunt etalon with UV/IR cut filter and B600 blocking filter into the focuser. A small adjustment on the helical focuser and I got a crisp and clear view of the sun in my TeleVue 25mm eyepiece. I saw a lot of h-alpha detail in the chromosphere and eveything seems to work fine.

Here is a photo of the setup:
hele setuppet.jpg
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And here is a photo of the Lunt 50 etalon with adapters, UV/IR cut filter and B600 blocking filter:
Lunt50 delen.jpg
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I tried to put on my ZWO ASI 174MM camera and as expected I experienced vignetting, but I guess that does not matter if I am imaging small features like prominences and sunspots. And I can always put the Lunt etalon back into the small Lunt 50 telescope if I want to image the full disc.

Actually, I think that using my 130mm triplet is a bit too much for this purpose so I am planning to buy a 102mm f/11 ED telescope to use as solar telescope but I am not sure that an f/11 scope will be usable with this Lunt 50 mod? The telescope am planning to buy has a working distance of 270 mm with the tube extender removed.

How do I calculate the perfect distance needed between the lens of a telescope and the beginning of the Lunt etalon?
Any other suggestions and advices are welcome.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Sat Aug 07, 2021 6:37 am
by marktownley
Nice mod and welcome to the forum!

The Lunt 50 uses a 20mm clear aperture etalon, and the native Lunt scope is f7. So, the front collimating lens in the etalon assembly will need to be 140mm ahead / inside the focal point of the f7 donor scope. Using a scope with a different focal ratio will just affect the size of the sweetspot.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Sat Aug 07, 2021 8:05 am
by GreatAttractor
stefanr wrote: Fri Aug 06, 2021 9:42 pm Actually, I think that using my 130mm triplet is a bit too much for this purpose so I am planning to buy a 102mm f/11 ED telescope to use as solar telescope but I am not sure that an f/11 scope will be usable with this Lunt 50 mod?
It will work (see this reply), though closer to f/7 would give a larger (in terms of angular size) sweet spot.

stefanr wrote: Fri Aug 06, 2021 9:42 pm How do I calculate the perfect distance needed between the lens of a telescope and the beginning of the Lunt etalon?
Any other suggestions and advices are welcome.
You can measure it precisely with a camera; see the first post in this thread.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Sat Aug 07, 2021 3:36 pm
by MalVeauX
Looks good!

Can you share more about the adapters used to make it possible to insert into 2" standard gear?

I'm doing similar with a Lunt 60 modular version that already is made to insert into an OTA. Just trying to work out an adapter for it.

Very best,

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Sat Aug 07, 2021 4:32 pm
by stefanr
MalVeauX wrote: Sat Aug 07, 2021 3:36 pm Looks good!

Can you share more about the adapters used to make it possible to insert into 2" standard gear?

I'm doing similar with a Lunt 60 modular version that already is made to insert into an OTA. Just trying to work out an adapter for it.

Very best,
The adapters used for my Lunt 50 mod are these:
TS-Optics Adapter with M68 male thread and M48x0.75 female thread
TS-Optics M68 System - Conversion Adapter with continuous M68x1 female Thread
Baader M68 extension tube, length 20 mm

And the last part is just a normal 2" nosepiece extender

Here you can see how the parts are put together:
adapter.JPG
adapter.JPG (191.78 KiB) Viewed 692 times

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Sun Aug 08, 2021 4:52 pm
by stefanr
GreatAttractor wrote: Sat Aug 07, 2021 8:05 am
stefanr wrote: Fri Aug 06, 2021 9:42 pm How do I calculate the perfect distance needed between the lens of a telescope and the beginning of the Lunt etalon?
Any other suggestions and advices are welcome.
You can measure it precisely with a camera; see the first post in this thread.
Thank you. Using the method descibed in the post I found that 53mm is the perfect distance on the TS-Optics Photoline 130mm refractor.

However, some people think that I am mad to use just a 2" UV/IR cut filter in front of the Lunt 50 etalon.
The argument is that the full visible spectrum still builds up a lot of heat that might damage the etalon.

The same people suggests that it will be safer to use a 2" Baader 3.5nm H-alpha pass filter as an ERF in front of the etalon.
Before I go out and spend a lot of money on this filter I just want to check if any of you have any comments regarding this?

I am planning to measure the temperature of the light coming out of the etalon next time I get a chance, but I cannot measure the temperature inside the telescope since I do not have a full aperture ERF on it.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Mon Aug 09, 2021 4:03 pm
by MalVeauX
Thanks, appreciate seeing that! Definitely different on that one than the new modular ones.

Here's what I'm trying to sort with the new Lunt 60mm Modular series; attaches with thumbolts. Nothing threaded:
Lunt60MT_06.jpg
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Lunt60MT_04.jpg
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Lunt60MT_01.jpg
Lunt60MT_01.jpg (63.4 KiB) Viewed 647 times
Very best,

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Sun Aug 22, 2021 9:14 pm
by VasilevIvan
Nice decision Stefan! Have a nice solar captures!

MalVeauX would you be so kind to send photo of your telescope with Lunt60Mt?

Best wishes, Ivan.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Mon Aug 23, 2021 11:51 am
by torsinadoc
Marty did you sort an adapter?

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Tue Aug 24, 2021 7:45 am
by AndiesHandyHandies
Hi

People use the 35nm Baader as an internal ERF. Not 3.5nm.

I found a problem with reflections off one and a PST etalon following it. Just use a UV-IR as a ERF now. It reflections are invisible for visual.

A Beloptik KG3 after the Etalon to stop long IR, which the 35nm filter passes.

Cheers. Andrew.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Tue Aug 24, 2021 11:57 am
by Rusted
AndiesHandyHandies wrote: Tue Aug 24, 2021 7:45 am Hi

People use the 35nm Baader as an internal ERF. Not 3.5nm.

I found a problem with reflections off one and a PST etalon following it.
Just use a UV-IR as a ERF now. It reflections are invisible for visual.

A Beloptik KG3 after the Etalon to stop long IR, which the 35nm filter passes.

Cheers. Andrew.
After the etalon? Is there some particular reason for this placing?
I have my Baader 35nm stacked with the KG3 just behind it.
Placed well up the main tube on several 2" extensions.
I also have a full aperture D-ERF for my 150/10.
The idea being to spread the [still hot] light cone across the full filter area.
To avoid localised heating causing internal thermal issues.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Tue Aug 24, 2021 12:50 pm
by AndiesHandyHandies
Hi Rusted,

The PST was after the Baader 35nm.

People complain about the bright reflection off the front of the PST etalon and I assume that was coming back off the Baader 35nm.

For my RuMak180 a 1.25" UV-IR inside the baffle brings it down to about a 6" in terms of heat load. Peter Drew has used a 6" PST Mod 2 without any ERF for years.

Cheers. Andrew.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Wed Aug 25, 2021 6:05 am
by Rusted
Thanks Andrew.

I used a 6" with an internal Baader D-ERF and still cooked my ZWO camera!

Reflections are only really noticeable when pushing the gain hard for prom exposure.
Perhaps the reflections are there all the time but go unnoticed?

Somebody should invent a wedged 2" filter ring [or adapter] with built-in tilt.
It has to maintain the optical axis on the outer screw threads. A simple enough task for a Chinese CNC machinist.

Not the usual tilter plate. Which is total overkill for any support task short of a Pro DSLR.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Wed Aug 25, 2021 8:18 am
by AndiesHandyHandies
Hi Rusted,

I am doing visual only on the RuMak.

Cheers. Andrew.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Sat Aug 28, 2021 4:36 pm
by krakatoa1883
The adapters I described in my blog are not actually necessary if one uses a front D-ERF because in this case the barrel of the red glass (arrowed below) can be removed and the M68 thread on the red block can be directly used even if the focuser has a 2" drawtube only.
Adapter.jpg
Adapter.jpg (65.52 KiB) Viewed 405 times
On most refractors with a M68 thread at the end of the focuser drawtube, this allows to place the collimating lens at the right distance inside the primary focus.

Re: Lunt 50 mod with f/7 and f/11 refractors

Posted: Mon Oct 11, 2021 3:26 pm
by MalVeauX
torsinadoc wrote: Mon Aug 23, 2021 11:51 am Marty did you sort an adapter?
VasilevIvan wrote: Sun Aug 22, 2021 9:14 pm MalVeauX would you be so kind to send photo of your telescope with Lunt60Mt?
Here's what I ended up doing. 3D printed rear cell that replaces the focuser flange on the C6R. It has a lip with thumb bolts so that the Lunt module just inserts and the thumb bolts secure it, just like on the stock scope just scaled up. A basic 2" filter can be used as a sub aperture DERF and botch-threads to the module without any modfication. No modification to the Lunt module at all. Just the one 3D printed adapter and that was it.

C6R_Baffle_Removal.jpg
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Lunt_60_Adapter_01.jpg
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Lunt_60_Adapter_02.jpg
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Lunt_60_Adapter_05.jpg
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Lunt_60_C6R_Adapter_01.jpg
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Lunt_60_Baader_50m_DERF.jpg
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C6R_Lunt60_Mod_FirstLight_Test_01.jpg
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C6R_Lunt60_Mod_FirstLight_Test_ERF_03.jpg
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I'm still tweaking things, as I'm sure it's not optimally placed at all, but its not vignetting at this time. Things I'm doing wrong: it's operating at F10 in this configuration with F7 collimators (reduces my sweet spot); it's not at exactly -140mm back focus, but it's near that area. It came to focus and is on band roughly, but the contrast is low. I see HA features but its probably near the 0.8A range and low contrast in this poor configuration. I will tighten things up with more testing and perhaps with a different donor scope eventually. I'm currently awaiting a new 3D printed cell for my front DERF so I can use it at the full 6 inch aperture and will test further, at F8, which is much better match with the collimator.


Here it is at least working, just not optimal yet:
C6R_Lunt60Mod_FC_FlatField_UniformityTest.jpg
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120mmF10_HA_Lunt60Mod_IMX290_FullField_01.jpg
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120mmF10_HA_Lunt60Mod_IMX290_LImbTest_02.jpg
120mmF10_HA_Lunt60Mod_IMX290_LImbTest_02.jpg (93.56 KiB) Viewed 154 times

+++++++++++++++++++++

I have it too far into the light cone. So that probably is why I'm not getting the results I want, at least, so far.

This OTA (C6R) was shortened by the previous owner for Bino use. So its already short. I took off the focuser flange. And here is a crude measurement using projection of the sun onto the black cap (see the sun in focus, clouds, etc there) showing the plane of focus and that I should put the Lunt collimator lens -140mm from that point, 7 inches towards the scope. That's at about 17 inch mark on my tape measure I'm holding there. So my current setup, the Lunt module is way too far into the light cone, I need to pull it back. But my scope is short already so I need to extend things. This means I'll probably put the focuser back on to get that distance back and explore a 2" insertion mod so that my Lunt can be at the correct -140mm and on a rigid focuser rather than extensions.

20210817_192123.jpg
20210817_192123.jpg (3.93 MiB) Viewed 154 times

So, back to the drawing board!


I would MUCH rather get this sorted to use standard focusers with 2" and use a 2" insert to make this work, instead of the plate adapter, to ensure everything is orthogonal to the light cone. And to keep it ultra simple. The Lunt 50 is better suited to this. The Lunt 60 will require some custom stuff since nothing is threaded, its all insert & thumb bolt on modular series. So will need more work to get it modded like this. That said, I'd suggest the Lunt 50 for the simplicity as shown in this thread above with ready off the shelf adapter rings and 2" gear insertion. Dead simple! The Lunt 60 for me was about getting a full disc scope and having the little 70mm F6 ED frac it comes with as a base for other things too and the crayford; can't stand helical.

Even better than these would be a Lunt 80mm DS II unit, with adapters to mount. Costs more, but its a big etalon for a bigger sweet spot on bigger aperture longer scope options. Requires custom work of course.

++++++++++++++++++++++++++++

Edit:

Noting the above, the etalon is totally in the wrong location in the optical pathway (mine that is). It's way too forward. I'm pulling it back. Oversight on my part because it was "working" and so I left it and it clouded up and I never got back to it.

I also confirmed that the threading on the Lunt 60 modular is M50x1 as my PST gold tube threads directly to it nicely. So I've ordered some M50x1 adapters to convert to M48x0.75 so I can go 2" extension and still thread a sub-aperture DERF on if wanted and will update when I get the parts and have it positioned correctly.

Very best,