Rear Mounted Lunt 60 Modular Mod (HA) | Complete

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Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

Hey all,

This topic is centered on the idea of taking an air spaced internal etalon assembly and mounting it on the rear of a telescope to scale resolution higher at the cost of a smaller jacquinot spot, using an air-spaced etalon instead of mica-spaced, non-destructive to the etalon assembly. Think PST mod, but instead, a Lunt mod without having to break the Lunt.

Why Air instead of Mica Spacing?"

Mica-spaced etalons are ideal for rear-mounted options with large apertures and limited FOV where you're not seeing the full disc image. However, they all share a few things that I have issue with (my own personal thing). I don't like the really long elaborate imaging trains to get a telecentric amplifier involved, to get a long focal-ratio (F40+) through a mica-spaced etalon for its optimal performance, get these huge focal ratios, and then use focal reducers and spacers to get the focal-ratio reduced for sampling and inevitably deal with vigngetting, more dust on more surfaces, placement issues, flexure/tilt, etc. And worst of all, at least to me, is the waiting for temperature to tune the etalon and keeping it on-band and the need for electricity. I don't want to wait for my etalon furnace to get to temperature in Florida and I don't want it to overheat and go offband (which is super easy in Florida). And I really just don't want to deal with electricity needs which means eventually it will break and need repair and render the etalon useless. So for these reasons, I put a lot of effort into making an air spaced etalon system work for me to have real time tuning, simple mechanics, no electricity needs, just simple and nothing to immediately fail.

When Lunt announced its modular series, I was excited. I've wanted a Lunt etalon for a long time as they're some of the higher quality etalons currently that come to market for fair prices and have been great with their uniformity and contrast. But more importantly the modular series comes apart without any destruction. The modular series screams "mod me!" with how they come apart without having to break anything. That's a big deal to me at least because I like most things to be reversible and keep it modular. So I placed my order and got a Lunt 60 modular series scope (single stack) with crayford focuser and a 12mm blocking filter after about 3 months of waiting. Once I had it in my hands and took it apart with its modular thumb bolts, the wheels starting turning on how easy this thing should be to mod into a rear mounted etalon setup that can scale to any aperture, but to keep it reasonable for me, I'm going to scale it to 150mm on a refractor as that's my most common solar imaging instrument. It took me a few iterations but got it working on the most simple approach finally after getting a crucial measurement correct.

Why the Lunt 60 Modular (LS60MT):

A quick bit on why the Lunt 60 versus other options from Lunt. I've done a successful PST mod with a very good PST etalon, all my work over the past 2+ years has been with that PST etalon. It's clear aperture is 20m and it's sweet spot was very small, and that got smaller as I increased my scale to my 200mm scope. I have wanted a high quality air-spaced etalon that was larger aperture to get a larger sweet spot without resorting to a mica-spaced etalon because I got tired of dealing with long focal-ratios, sampling with enormous pixels, needing focal reducers, and the complex and long and heavy imaging trains. Plus all the problems that come with temperature regulation and electric handling over time. I cast away all my mica-spaced stuff and went all air-spaced on etalons a while back. So moving forward, I want to stay air-spaced still. Also, I wanted a pressure tuned etalon for its benefits instead of a tilt-tuned one, so only the modern options have this.

There is a fantastic thread for learning more about jacquinot spots here that had influence on this project:
viewtopic.php?f=9&t=30881

From that above thread, I had a few `Ah-ha' moments about figuring what size etalon I needed to target. I knew 20mm was too small, but needed to know more about why, at least, with air-spaced options. The Lunt 50 has a 20-ish-mm clear aperture etalon (the internal one). That's the same size as the PST roughly so that was off the table. The Lunt 60 has a 35mm clear aperture etalon, so this was an option. The Lunt 80 DS II has a 51mm etalon accordint to Brian. The larger Lunt systems do not have larger etalons. Note the Lunt 152mm has a 52mm clear aperture according to Lunt, so its likely the same etalon(s) series and size as what's found in the Lunt 80 and above options. So really, I don't see the point of spending on any internally mounted Lunt etalon option beyond the 80mm series knowing this. Basically after calculating the difference between a 35mm clear aperture etalon's resulting jacquinot spot versus something with a 40mm clear aperture vs 51mm clear aperture for cost and the ability to easily mount them, I settled on the Lunt 60 option for a few reasons despite knowing it will have the smaller jacquinot spot: 1) It can still easily image a full disc when reversed to its stock form; 2) its easier to adapt and mount as a mod without a bunch of custom work; 3) the jacquinot spot is still larger than the FOV with smaller sensors on a long scope, so the extra aperture wouldn't matter on the etalon unless you're using huge sensors which isn't likely with high speed high resolution work. Brian has an excellent thread with examples of the Lunt mod with the internal 80mm Lunt options, so take a look at that if you want to max out this option with custom adapters.


Table of Contents (links to sections):


A. Lunt 60 Modular Telescope Tear Down & Overview
B. Measuring & Placement of the Lunt Etalon Chamber
C. Adapting the Lunt Etalon Chamber for Standard 2" Gear
D. Thermal Handling (lots of options)
E. FireCapture WYSIWYG and some Results


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

Lunt60_to_150mm_mod.jpg
Lunt60_to_150mm_mod.jpg (85.42 KiB) Viewed 742 times

Very best,
Last edited by MalVeauX on Mon Dec 27, 2021 5:05 pm, edited 5 times in total.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

A. Lunt 60 Modular Telescope Tear Down & Overview

The Lunt 60 Modular series (LS60MT) completely comes apart without having to break anything or tear up anything. It's actually seriously modular down to the etalon chamber itself. All other parts just unthread or unbolt and come apart.

The LS60MT's base refractor it's installed in is a 70mm F6 ED doublet. This matters a lot because it means there's something else going on with the internal components and placement of collimator lens and all that because the Lunt LS60MT is a 60mm F7 HA refractor, but the base scope is 70mm F6. While it's easy to assume the internal assembly masks the light cone to 60mm and produces F7, it does, it's important to not just get stuck thinking that's all you need to know for placement of the collimator lens to scale it to larger apertures (I made this mistake on my first iteration!).

The base refractor, etalon chamber & focuser all come apart with simple thumb bolts into the three components. The etalon chamber has a threaded on “nose” with a red glass ERF (non-reflective) on it before the collimator lens before the etalon. The etalon chamber inserts deep into the refractor's light cone (this is important to note, this will need a measurement). Nothing was glued or cemented. It's like Lunt wants us to take it apart and do stuff with it.

The important thing to note here are the measurements of the collimator lens location in the light cone in the stock refractor it comes in to know where it needs to be placed to scale up to larger aperture systems and also that the “nose” that threads onto the front of the etalon chamber assembly has example M50x1 threads (this is the same as the PST actually) which makes it super simple to adapt to anything else because M50x1 is off-the-shelf available for adapting (no custom adapters needed!).

Here's a tear down of the LS60MT and its components:

Lunt60MT_01.jpg
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Lunt60MT_02.jpg
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Lunt60MT_03.jpg
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Lunt60MT_06.jpg
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Lunt60MT_07.jpg
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Lunt60MT_13.jpg
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Very best,
Last edited by MalVeauX on Mon Dec 27, 2021 4:17 pm, edited 2 times in total.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

B. Measuring & Placement of the Lunt Etalon Chamber

Placement of the collimator lens is the crucial thing to know, or the “magic number” for these types of mods because the collimator is specific to placement within the light cone. I made the mistake of thinking that an F7 collimator would need -140mm of back focus for placement and work; that also assumes the collimator lenses are F7, which we do not know that for sure at all, it's assumed (and probably not correct)! I found this to not be true in this case at all because I completely overlooked that the Lunt LS60MT is a 70mm F6 refractor with internal masking and so it wasn't as simple as trying to figure out anything based on the F7 result of the aperture masking. The correct way to measure placement is with the disc image at focus and where the collimator lens sits in the entire light cone relative to the disc image at focus.

So after a few iterations of overlooking that, I went back and approached it correctly (after a good laugh at myself) and measured the stock Lunt 70mm F6 refractor's properties and bypass not knowing at all what the properties of the collimator lenses actually are (we assumed everything!). I took out the etalon housing and focuser and all that so it was just the doublet lens pointed at the sun and placed a black metal surface behind it until I saw the disc image focused. I measured from that point forward and found the focal length to be 420mm and it agreed with the specs of the focal length of the doublet. From this point, I needed to measure where the collimator lens sits in the light cone, from the focused disc image. I checked this two ways. 1) I measured from the disc image at focus to the collimator lens plane itself; and 2) I measured the disc image at focus to the plane of insertion of the etalon assembly, and then measured the etalon assembly at that point to its collimator lens and added the distances and they agreed, so I confirmed where that collimator needs to be.

From the disc image at focus to the plane where the collimator lens resides in the light cone was found to be at -264mm back focus. If the collimator lens is F7, then its aperture is around 37~38mm which is larger (appropriately) than the 35mm etalon after it. So the idea of “F7” being used to determine collimator placement is not the correct way to do this or you will be way out of focus resulting in poor results. The correct placement is whatever it takes to get that collimator to sit as near -264mm as possible. It will tolerate not being exactly at that point, but ideally that's where it needs to be for optimal results.

Crude diagram of what I measured:

Disc image at focus to the plane where the etalon chamber fits into the telescope (210mm length from focus to this plane). Then I measured the plane to the collimator lens to be 54mm. That's the -264mm of back focus needed into any light cone.

Lunt_FocusPlane.jpg
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Most scopes will not have -264mm back focus which means to properly place this you would have to shorten the OTA (if using a refractor, which is ideal anyways). So this is the one point where a destructive mod will take place. Just like a PST mod or other mod using collimator lenses with air spaced etalons, you likely will have to shorten the donor OTA. So don't do this with your fancy ED/APO. Do this with an even better for the job achromatic doublet that is between F7 and F8 and you'll be in business without much complication. F8 is super common at nearly any aperture in achromatic doublets, so this is a great way to go, it does not have to be exact, to get good results.

Summary on focal-ratio to target on a donor scope, F7 to F8 as a range.

Ideal common refractor configurations for this (ED doublet or ideally just an achromatic doublet):

80mm F7 to F7.5 to F8
90mm F6.67 to F8
102mm F7
120mm F8
140mm F7
150mm F8

Note, with proper thermal handling (a full aperture D-ERF) you can easily use an SCT design telescope for this since you can move the primary mirror to get it closer to the needed position for the collimator. The C8 Edge with the 0.7x reducer for example makes a 200mm F7 option for this (I have this and a full size Aries DERF to run this too). Note, this mod is too heavy for a newtonian most likely. So this is probably only suitable for refractors (ideally) and some SCTs. I will share results with the C8 Eedge & 0.7x reducer and this mod soon when my weather and seeing allow it.

I used a Celestron CR150HD 150mm F8 achromatic doublet in my example here.

I shortened this Celestron CR150HD (C6R today) 150mm F8 achromatic doublet so that I could get my collimator to -264mm with a 2 inch focuser in place, so I could move it forward and backwards by a few mm to be able to precisely position it. That was a couple of inches removed from the tube (my tube was already shortened previously, so I'm not sure the total I removed from this OTA, but the simple way to get this exact is to point your OTA at the sun with nothing in the imaging train and see where the disc image comes to focus; from that point measure forward into the light cone -264mm and that's where the colliamtor needs to be and adjust for your focuser and other things like the adapters to put the collimator in that position). I used a dremel to cut a guide trough. Then a hacksaw to finish it off. It was not super precise, but it needs to be. I had some tilt in mine, so I drilled my focuser flange to be more precise and got it fairly level that way (I still had some minor tilt). Next time I think I'll go to a machine shop and have them precisely cut a tube. But my ultimate goal is to not have a tube at all. The next iteration of this will be a 3D printed truss system to avoid cutting anything ever again and just be able to print the parts and attach trusses and space it as needed. So much easier for anyone to do likely.

So again, the best way to approach modding (shortening) your donor OTA is to measure the disc image at focus plane and go -264mm into the light cone and that's where the collimator lens needs to be. Adjust for your focuser and adapters and anything that will be involved with placement. So I measured my CR150HD from the disc image at focus and measured back -264mm and marked my tube. I then adusted it for the distance of my focuser and the adapter rings I used (the M50 to M48 rings) so that the final position of my collimator was at -264mm (with a 1~2mm tolerance so I can adjust it with my OTA focuser). So total reduction of the OTA is going to be significant to allow for the focuser and other stuff and still place the collimator at -264mm, so take care measuring and calculating this position!

C6R_backfocus_measure.jpg
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Shorten_C6R_minus50mm.jpg
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TL;DR: The LS60MT collimator lens needs to sit at approximately -264mm back focus and will probably require you to shorten your refractor OTA for this.


Very best,
Last edited by MalVeauX on Mon Dec 27, 2021 5:10 pm, edited 9 times in total.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

C. Adapting the Lunt Etalon Chamber for Standard 2" Gear

Originally I used a 3D printer and had parts printed (thanks Michael!) to custom mount by inserting the entire stock Lunt etalon chamber into the back of a larger refractor. It worked, but it was not ideal I found because it required exact placement and could not be adjusted after the fact. So while this is an option, it's not ideal. It also requires a 3D printer and some custom work to be done. I wanted to avoid this at this stage and find a solution that was easier for others to replicate without needing 3D printed stuff. My goal was how to keep it simple and keep it replicatable. This is partly why I went with the Lunt LS60MT because it has a threaded nose in front of the etalon chamber, which makes it dead simple to adapt to anything without custom work.

The front nose of the etalon chamber of the LS60MT is a M50x1 thread. This is the same as the PST actually, which is how I confirmed it. Knowing this, I went online and looked for simple M50x1 threaded rings to step up and down things as my goal is to get to M48x0.75 which is standard 2” gear so that 2” extensions will thread on and so I can mount standard 2” filters. This makes it super friendly for having thermal handling covered on smaller instruments (filter wise) and the 2” extensions means you can insert this into a 2” focuser and have fine adjustment of placement of the collimator for perfect results and still have a 2nd focuser for fine focus of the disc image.

RAFCamera has this stuff off the shelf. This guy has all the size and thread options out there for this stuff. So I found a M50x1 Female to M65x1 Male threaded ring adapter. And then I used a M65x1 Female to M48x0.75 Female threaded ring adapter. These adapters are still very short and occupy a little space, so I took this into account for the collimator lens placement. These two off the shelf adapters arrived in a week from RAFCamera and fit perfectly. This allows me to use any standard 2” gear now. And I used a M48x0.75 x 2 inch extension ring with threads for filters as the new nose. Now it can insert into any 2” focuser and I can thread on any 2” filter to the nose (this is where you can place a 2” DERF now or as a 2nd DERF in a larger system). You can introduce tilt here if needed with a shim in your filter cell.

RAFCamera_M50x1_to_M48x075_2inch_Adapted.jpg
RAFCamera_M50x1_to_M48x075_2inch_Adapted.jpg (62.55 KiB) Viewed 743 times

So in this setup, with a shortened OTA and the focuser placed back on, I have a 2” focuser to allow precise placement of the collimator lens at -264mm give or take a little. And I have the LS60MT focuser after the collimators to focus the image, so the whole system has a lot of room for precision in case something was a little off. It just inserts into a 2” focuser and you're set. The blocking filter and focuser do not change or influence how the stock LS60MT focuses with cameras and its the same when scaled up, so no extra work needs to be done after this stage in this respect for reaching focus.

Lunt60_install_minus50mm_C6R.jpg
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Lunt_60_2inchMod_RAFCam.jpg
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Very best,
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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

D. Thermal Handling (lots of options)

So how about energy rejection and thermal handling? There are lots of options. Lunt used a non-reflective red glass filter on the nose of the LS60MT. Also, Lunt doesn't use much in their larger systems but they all employ a sub-aperture ERF like this and none of their modular series bigger apertures have any kind of front mounted ERF or coatings for it there. Some of their largest scopes have some IR coatings in a few places, but these mid-range modular series ones do not have anything other than an internal sub-aperture option.

From small apertures up to 120mm max aperture on a refractor, a sub-aperture 50mm option will work. So on the 2” nose, you can install a 2” dielectrically coated filter there for primary energy rejection on smaller apertures. Good examples are the Baader Red CCD-IR Block 2” filter, Baader HA 35mm 2” filter, any Chinese (ZWO, etc) dielectrically coated Red 2” imaging filter, etc. Basically just any dielectrically coated filter that limits UV, IR and visible spectrum B & G and only passes Red wavelengths to include 656nm. Note, most of these red imaging filters pass about 100nm of red wavelengths and is not much different from Baader's HA 35nm option as its not that narrow. So depending on availability, any of these will work fine up to 120mm aperture on your donor refractor's aperture.

Baader-Red-CCDIRBlock_DERF_subaperture.jpg
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For apertures beyond 120mm, I suggest a full aperture D-ERF instead. And you can then use a sub-aperture DERF in this same location on the nose of the assembly described above as the 2nd D-ERF to completely eliminate any heat from the system. So if you target 130mm, 140mm, 150mm or larger (it scales to whatever you want), then I would use full aperture front mounted D-ERF for this. Note, if you put a 2nd D-ERF inline with this, you may need some tilt if you have reflections (this is easy to accomplish with a shim in your filter holding cell).

For my 150mm mod, I'm using a full aperture Aries D-ERF which I down stepped to fit my refractor. My Aries D-ERF is 214mm (8 inch) full aperture. I had a 3D cell designed and printed to hold this and mask it to 150mm and securely fit to my 150mm refractor (thanks Michael!). You can of course use any D-ERF at this point as long as its high quality glass and reflects unwanted UV & IR but remember that visible spectrum carries a lot of energy too and of all these options UV carries the least here, so the most heat rejection will come from visible spectrum and IR!

C6R_214mmDERF_Cell_Assembly.jpg
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C6R_ERF_Cell_Comparison.jpg
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C6R_214mmDERF_150mmMasked.jpg
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So to summarize you can do this with a smaller 80mm to 120mm refractor and manage the energy rejection inexpensively with a 2” sub-aperture D-ERF filter as described above on the 2” nose that inserts deep into the light cone on this etalon chamber assembly. Over 120mm, and I recommend a full aperture D-ERF completely after a few years of testing both methods.

Very best,
Last edited by MalVeauX on Mon Dec 27, 2021 4:03 pm, edited 2 times in total.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

E. FireCapture WYSIWYG and some Results

With the mod complete we need to see if it achieves focus and if there's any unwanted problems like reflections or excessive tilt and of course to evaluate if our placement and configuration is optimal enough with a good quality etalon to get the results we want. It all hinges on getting a good etalon in the first place with high uniformity, high finesse and the result being good contrast on-band HA. Your biggest issues will be tilt and followed by that if you use several energy rejection filters you may find some reflections (so use tilt to handle that). Before I even moved forward with this mod, I tested the Lunt etalon I received to make sure I could get a uniform disc with high contrast, otherwise, I would have sent it back. I was lucky in getting a good sample on the first go, but this is also why I went with Lunt as the odds are simply higher lately to get a good etalon from Lunt than other options when doing the etalon gamble.

Focus was easy to achieve. The camera achieved focus the same place that it did on the stock LS60MT and when scaled up to my 150mm F8 refractor that I used for this mod; thanks to the collimator lenses. The etalon is luminous and transmission was still high. I'm including a screen shot to show the FOV, uniformity, contrast and transmission via histogram to show some of its properties. I found I was filling my histogram to nearly 90% using an IMX290 sensor with 0 gain and 2.57ms exposure times, resulting in the full 170 FPS of the sensor's potential, that leaves me a lot of room for exposure values. So this is plenty for high resolution imaging. This also means a future double stack is an option but will likely result in some gain use and longer exposure time, but the primary transmission has a big impact on this since the 2nd etalon will grind transmission at least another 45% from whatever it initially is when presented. Just something to consider, it's important to have a luminous etalon if you want to attempt to go that route. This is a single stack at the moment.

FirstLight_Lunt60_minu260mm_03.jpg
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In my first example, tilt is first problem I noticed, I have some sag from the incredible weight of the etalon chamber and imaging train, which is not a problem for my focuser, but it revealed that my OTA being shortened may not have been precisely level enough. Something to keep in mind! Regardless, baring that problem, I noted the sweet spot filled the FOV even on the 150mm aperture with the IMX290 sensor which is a huge improvement over the tiny sweet spot of the PST mod I used previously. So in that way, a big success. From this point I worked on solving any tilt issues and reflections to optimize the results further.

And finally, some examples from this mod using the 150mm F8 achromatic doublet refractor (a Celestron CR150HD or C6R) shortened and using the Lunt LS60MT etalon chamber, a full aperture Aries D-ERF and an internal sub-aperture Baader Red CCD IR block filter D-ERF and an ASI290MM (IMX290) camera.


Image

Image

Image

Image



Here's the LS60MT on a full disc, single stack, on its own with an IMX253 sensor since this mod is 100% modular and reversible, so I can do a full disc in its stock scope, or insert it into my 150mm for high res work.


Image



My next goal is to double stack this, but I'm taking it slow to find the most optimal way to do it without resorting to a mica-spaced system.

Very best,


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by pedro »

Very interesting Marty. I also tried something similar but the results were not great (APM152 F/8 - LUNT60 etalon PT)


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by Bob Yoesle »

Very nice write-up Marty - and some excellent pics and images!


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by torsinadoc »

Excellent write up!


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by pedro »

Hi Marty

Back in 2015 I made a special adapter to mount my LUNT60 TS on a APM 152 F/8. It did not reach focus because I did not cut the tube. Please see the images below. Do you think it will work?. It is not the Modular LUNT60. I also have a Baader 160mm ERF

The adapter will be screwed directly to the tube. I will not use the focuser you see in the first image. How do I calculate the correct distance?

Thanks
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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

pedro wrote: Tue Dec 28, 2021 11:19 pm Hi Marty

Back in 2015 I made a special adapter to mount my LUNT60 TS on a APM 152 F/8. It did not reach focus because I did not cut the tube. Please see the images below. Do you think it will work?. It is not the Modular LUNT60. I also have a Baader 160mm ERF

The adapter will be screwed directly to the tube. I will not use the focuser you see in the first image. How do I calculate the correct distance?

Thanks
Hi Pedro,

I think it will work fine but will require an OTA tube that is shortened. The key is to put the collimator in front of that etalon chamber in the correct position in the light cone for it to achieve proper focus. Don't use the focal-ratio of anything to determine placement, this is deceptive because Lunt internally masks the light cone in all their larger scopes and their internal etalon options, all of them. Instead, use the stock OTA that your Lunt etalon is inserted in and measure the stock OTA's disc image at focus to where the collimator sits in the focal length and subtract that. So the scope pointed at the sun without any filters, and put something that can take some heat (like a metal plate) behind it at a position where it reaches focus and you see a sharp disc image (bright of course). Noting this position, measure from that plane to where the collimator lens in front of your etalon chamber would be, precisely, into the light cone. That back focus distance is what is important and what you must maintain to scale it to a larger system. It will likely be deep into the light cone on another OTA that is larger and not meant for this, which means it has to be shortened for that to work. This is of course why most people do not do this sort of mod on their nicer telescopes.

I found the correct position this way on my Lunt 60 in its stock 70mm F6 (420mm focal length) scope:

1) I took out the etalon and focuser, so it was just the bare OTA lens and pointed it at the sun.

2) I used a black metal plate and a tape measure; I held the plate behind the lens pointed at the sun until I found where focus is, a sharp disc image was produced on the plate. From this plane, I measured forward into the system to the point where the collimator lens would be sitting if it were installed. That back focus distance is the critical measurement for placement into a larger system. I found mine was at -264mm back focus, from the disc image at focus, to where the collimator would sit.

3) I measured my donor OTA to scale this to, by also pointing it at the sun and then using the same plate to allow the disc image to be focused and measure from that point and simply use -264mm back focus that I measured from the stock OTA and marked that location. I then measured the length of the focuser and adapter plates and added that distance to my measurement and used that to determine where the cut on the donor OTA needed to be, so that after the focuser was re-installed onto the shortened tube, it would still allow the collimator to be placed at -264mm of back focus with a little room for precision via the focuser itself and with the adapter plates in place I used to convert the M50x1 thread nose to M48x0.75 for 2" gear insertion into the 2" focuser. Of course you can skip any bit of this that is not pertaining to your setup, such as a focuser, or converting threads, etc. But do make sure you account for any adapters or coupling methods you're using that influences position of the collimator.

4) I cut my OTA based on those measurements, drilled new holes for the focuser flange and installed the focuser again. The Lunt module just inserts into the focuser and the collimator was sitting at -264mm backfocus. It achieved focus and as you can see has high contrast and comes on band. Placement is key.

I placed it at a few points, at -140mm, -160mm, -205mm, etc, and while I was able to get focus at a few points, the performance was bad, low contrast. It wasn't until I measured the placement in the stock OTA and replicated that same backfocus I measured into the donor OTA that it was high contrast and looked great. So again placement is key for the collimator.

Since your system will not use a focuser for placement, you simply make sure the donor OTA and any coupling adapters you're using at final position has the colliamtor sitting at its appropriate pre-measured back focus location (pre-measure from the stock OTA, then repcliate that back focus on your donor OTA).

Just measure the stock OTA position of the collimator from the disc image at focus (without the collimator or any filters of course) and that's your back focus needed for the collimator position. How it gets there can be done any way you want. But the collimator lens must be at that precise back-focus location to get good results.

Very best,


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by pedro »

Thanks Marty

I will give it a go. I will have to measure everything and find the correct back distance


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

pedro wrote: Wed Dec 29, 2021 12:43 am Thanks Marty

I will give it a go. I will have to measure everything and find the correct back distance
Measuring is pretty simple.

The hardest part is a level cut on an OTA tube to avoid tilt! :o

Very best,


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by Rusted »

Thank you Marty for your very thorough handling of the subject and excellent supporting photographs. :bow

The next question is whether our 6" f/10s can be made to work with the Lunt mod.
I may have missed this while quickly scanning your text before posting the following:

Cutting a larger tube is as easy as tightly wrapping a roll of paper beside the intended saw line.
Wrap the roll of paper on the tube you want to keep. The paper will protect the finish on the OTA.
Tape the roll to be sure it remains in place after using friction to get the roll nicely tight.
Get the roll edge overlaps, of each layer, to match and the roll must be at right angles to the tube.

I have used wallpaper and lining paper to wrap tubes. The more layers the firmer the edge to guide the saw.
Pencil or mark the line, as a reference, before starting to saw with a 12" hand hacksaw.
Then if the roll slips you don't need to go back to measuring to start all over. Just tighten the roll to the line again and tape.

This paper roll system works on any size of tube. I have used it on 18" PVC tube for a 16" Newt.
I have finished the cut edge of many tubes [up to 18" Ø] using sandpaper glued or pinned to a sheet of ply:
Laid flat on the cut end end of the tube and moved about for a perfectly smooth edge.
Inverting the tube to have the cut edge face down, will avoid dust issues inside the tube.
Last edited by Rusted on Wed Dec 29, 2021 2:10 pm, edited 1 time in total.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

Rusted wrote: Wed Dec 29, 2021 7:33 am Thank you Marty for your very thorough handling of the subject and excellent supporting photographs. :bow

The next question is whether our 6" f/10s can be made to work with the Lunt mod.
I may have missed this while quickly scanning your text before posting the following:

Cutting a larger tube is as easy as tightly wrapping a roll of paper beside the intended saw line.
Wrap the roll on the paper of the tube you want to keep. The paper will protect the finish on the OTA.
Tape the roll to be sure it remains in place after using friction to get the roll nicely tight.
Get the roll edge overlaps, of each layer, to match and the roll must be at right angles to the tube.

I have used wallpaper and lining paper to wrap tubes. The more layers the firmer the edge to guide the saw.
Pencil or mark the line, as a reference, before starting to saw with a 12" hand hacksaw.
Then if the roll slips you don't need to go back to measuring to start all over. Just tighten the roll to the line again and tape.

This paper roll system works on any size of tube. I have used it on 18" PVC tube for a 16" Newt.
I have finished the cut edge of many tubes [up to 18" Ø] using sandpaper glued or pinned to a sheet of ply:
Laid flat on the cut end end of the tube and moved about for a perfectly smooth edge.
Inverting the tube to have the cut edge face down, will avoid dust issues inside the tube.
Heya,

It will work on virtually any focal-ratio setup, but the performance will vary of course. Faster focal-ratio will simply be masked internally to smaller light cone or smaller effective aperture but still work. Longer focal-ratio will work fine, just at the cost of light and a forced finer image scale though it could be reduced after of course if that were the case. I'm sure there's some issues depending on the angles of the light rays entering the collimator, but it seems fairly tolerant of a large range as long as the collimator is in the correct back focus position.

Good tips on the wrapping; I sure wish I had more tricks up my sleeve for that! I had never sawed a tube before and man it was hard with a hack saw. I think next time I would go to a shop and ask someone to cut it with a table saw or something.

Ultimately I want to move away from the tube in general and instead 3D print parts to build a truss refractor that can be replicated by anyone (even just have parts printed and mailed to you, there are services) to avoid needing to cut tubes and to easily get any placement for any mod or function for a solar refractor. Just supply the trusses (maybe right angle aluminum strips). After having did a complete lens holding cell with my 214mm D-ERF and securing it to the refractor above, I'm confident that a simple holding cell for the refractor lenses can be done too and then just go ahead and print the rest of the plates and baffles while I'm at it and a back plate for the focuser that can be moved in any position and just use right angle aluminum strips and be good to go. 3D printed big truss refractor that can be internally accessed, no tube currents, and modular placement of anything in the entire imaging train. This is my next goal, along with double stacking this thing.

Very best,


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by Rusted »

Thanks Marty. :bow

The paper wrapped tube is protected by the roll of paper provided it is made thick enough.
Once the hacksaw blade penetrates the tube progress becomes very much quicker.
It just requires a little patience at the start of the cut. New, fine tooth blade helps.

A truss tube opens up other issues: Like dust build up. Or dewing of the optics.
That said, it provides direct access to a blower or hair drier.

Potential flexibility of an under-dimensioned truss compared to a simple, metal tube.
I would prefer a series of parallel supporting tubes and baffles to allow free adjustability.
A truss is rather limited to the spacing with which its baffles are designed.

Air is not directly heated by passing a beam through it. Though it must avoid hitting blackened metal.
It is possible that convection currents or turbulence occurs within a closed tube.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

Rusted wrote: Wed Dec 29, 2021 2:29 pm A truss tube opens up other issues: Like dust build up. Or dewing of the optics.
That said, it provides direct access to a blower or hair drier.

Potential flexibility of an under-dimensioned truss compared to a simple, metal tube.
I would prefer a series of parallel supporting tubes and baffles to allow free adjustability.
A truss is rather limited to the spacing with which its baffles are designed.

Air is not directly heated by passing a beam through it. Though it must avoid hitting blackened metal.
It is possible that convection currents or turbulence occurs within a closed tube.
I'm good with those issues; my optics and filters and all surfaces dew over every morning no matter what, in 99% humidity Florida, so I have to use a hair dryer to warm everything the moment I go to use them even in my observatory, the moment I open the observatory and the cooler surfaces touch that 99% humidity, they dew over. A truss speeds up this process for me, so I can get to the inside glass to warm it. So for me, this is optimal. A closed tube is significantly more difficult for me to warm the inside part of the lens to get rid of the immediate dew.

The truss system will have a bunch of support plates and baffles.

I want to go this way because its going to be easier I think and accessible for solar refractor mods like this, to avoid having to permanently cut OTAs especially for people with premium scopes. You can just buy the objectives and print the rest (except the truss material of course, will use a common metal from a hardware store, like right angle auminum). Parallel series plates and right angle trusses will be rigid. Then just collimate with a rear plate that holds the focuser. Internal solar filters like sub aperture DERF will be easier to place, and for collimator systems like this Lunt 60 mod or even just a basic PST mod, it makes getting the collimator to its ideal placement possible and isn't permanent since you can adjust where the focuser plate is on the truss and just move it in increments as needed. No cutting. Nothing permanent. And if you botch it you can re-print the plate or get new truss strips. Way better and cheaper than dealing with an OTA tube (especially for premium stuff you don't want to cut).

Very best,


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by thesmiths »

pedro wrote: Wed Dec 29, 2021 12:43 am Thanks Marty, I will give it a go. I will have to measure everything and find the correct back distance
Hi Pedro, I once had a small refractor custom made by Markus at APM (he had a few LOMO lenses left and made a custom made OTA for me). The tube had an 80mm-long piece at the back that could unscrew to increase the back focus (it had threads on both sides). If I were you, I would contact him and have him alter your tube to create a removable piece, once you figure out the proper distances.

I have attempted to cut a few tubes (using masking tape, etc) and it is not so easy to do a nice job. Maybe it's ok to attempt on a SkyWatcher, but on an APM ... I think not. One reason is the APM wall thickness is much greater than on the SkyWatcher type of tubes. Second, the APM has a lovely enamel coating that will surely chip badly. The SkyWatcher type of tube is typically just painted.
Last edited by thesmiths on Wed Dec 29, 2021 7:19 pm, edited 1 time in total.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by Ivan »

Great article and mod, Marty! Your description of how the mod is built will be a great help to all of us. You wrote that in the future you might do a double stack. Will you be using another etalon from Lunt60?

Ivan


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by marktownley »

An impressive article Marty, thanks for sharing!

What is the diameter of the refocusing lens at the back of the modular etalon assembly?


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

Ivan wrote: Wed Dec 29, 2021 5:45 pm Great article and mod, Marty! Your description of how the mod is built will be a great help to all of us. You wrote that in the future you might do a double stack. Will you be using another etalon from Lunt60?

Ivan
I'm not sure yet; either a Lunt 60, possibly a Lunt 80 internal module, possibly a Lunt 50 even. I need to see what would be spaced and integrate best. A mica spaced option would work easiest, but I want to avoid mica spaced and any need for telecentric, reducers, electric power, etc. The issue is when you add another etalon and change the FWHM, you chance the tolerances that result in the sweet spot size, so it shrinks. So to keep a sweet spot large enough to still fill the FOV of a smaller sensor, it likely needs to be a bigger etalon if possible (which may mean the Lunt 50 is not an option with its small 20mm clear aperture).

I'm going to test with a PST etalon if I can and see the real world result of the jacquinot spot with the two etalons and the resulting FWHM and change on the jacquinot spot angular size.

Very best,
Last edited by MalVeauX on Thu Dec 30, 2021 3:01 pm, edited 1 time in total.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MalVeauX »

marktownley wrote: Wed Dec 29, 2021 8:43 pm An impressive article Marty, thanks for sharing!

What is the diameter of the refocusing lens at the back of the modular etalon assembly?
The two collimator lenses are ~38mm (maybe 40mm splitting hairs?) and the etalon has a clear aperture of 35mm (confirmed by Lunt via email).

264/38=6.947 (practically F7); so from this aspect, the idea of "F7" does work out to figure the back focus distance. But only by measuring the aperture of the collimator of course. To avoid making any mistake though, since I did actually the first time, I measured it's location in the light cone to get its focal length so that it didn't matter if I knew F7 or the diameter of the collimator, without any assumptions.

Conversely, the Lunt 152mm has a 52mm clear aperture etalon in it and it's an F6 final focal-ratio. Instead of trying to figure placement from measuring the colliamtor diameter and focal length from the ratio, it would be more precise to simply measure where it sits in back focus as that's measuring its focal length which doesn't change (ratios change of course).

Very best,


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by Highbury Mark »

Great report of a fascinating project.


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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by MapleRidge »

Hi Marty...

I've been slow to reply, but your mod using the Lunt etalon looks great and performs equally well!

Excellent work :bow

One question...with the pressure tuned assembly mounted in the focuser, how well does it handle tuning (as in stress on the focuser tube with the twisting actions to make tuning adjustments)? I expect in warmer climes the twisting motion shouldn't be too stressful, but I'd be afraid of damaging the focuser in the cold.

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Re: Rear Mounted Lunt 60 Modular Mod (HA) | Complete

Post by DanielD »

Marty,

This is incredibly detailed and well thought out. Most of it going over my head. In Layman's terms the collimator is mounted in front of the etalon chamber and then the OTA is shortened to be able to reach focus?

Dan


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