Lunt 35 modification to larger aperture

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Lunt 35 modification to larger aperture

Post by GreatAttractor »

In this thread I'll chronicle the progress of my Lunt LS35 mod(s). As with Omega Optical filters, for now I'm using my Bresser Messier R102 achromat (102/1000 mm) with a front-mounted Baader 90 mm D-ERF. The mod is non-destructive, as the whole Lunt 35 front etalon assembly (+its ERF) simply goes into one of the adapters.

I decided to start with a collimated beam arrangement, reusing my old negative meniscus (D = 44 mm, f = –145 mm), now coupled with a Surplus Sched achromat (D = 43.9, f = 154 mm) for an effective system focal ratio of f/12.22 (eff. f.l. = 1100 mm).

Unlike in the previous OO mod (“get me the adapters, I'll figure out how to mount things inside later” - didn't work too well), this time I thought everything through before ordering. The lenses are held squarely by rings, which themselves are affixed by small headless M3 hex screws, at any point inside their respective adapters:
rings1.jpg
rings1.jpg (149.46 KiB) Viewed 6222 times
The lenses rest on small strips of adhesive velour:
rings2.jpg
rings2.jpg (75.35 KiB) Viewed 6222 times
That's how it looked today for eyepiece testing (for imaging the camera is held by a low-profile T2-C-mount adapter – which replaces the eyepiece holder – directly above the blocking filter); note that T2 helical focuser is missing due to insufficient backfocus at the moment:
all.jpg
The masking tape marks the estimated position of a good beam collimation (the nosepiece is inserted into the focuser to this point). Right now the collimating lens is only more or less correctly placed – I judged it yesterday visually by looking at distant terrestrial targets at night (with just the negative lens inserted) and deciding when I had a sharp picture with the eye fully relaxed to infinity, which should mean I reached a well-focused Galilean configuration.

I'll have to reposition the positive lens to get a bit more backfocus, as today I could not focus with the helical focuser in the train. Nevertheless, I did some quick imaging tests. Note that it was done through an open window, with Sun at 14 degrees elevation, AND through a high cloud – and focused by sliding diagonal's nosepiece inside its holder – but I'm optimistic already :cool:
test1.jpg
test1cn.jpg
test2cn.jpg
test3.jpg
test3.jpg (70.83 KiB) Viewed 6222 times

The highlight in the prominence shot is caused by the front D-ERF and is not an issue – normally I just rotate OTA in the rings by 180 degrees to get rid of it. The vignetting at the bottom is probably due to diagonal slop (it had to be extended quite far, and was held just by a single screw) or the positive lens not entirely squarely mounted. Finally the ring-nonuniformity of illumination can be hopefully alleviated by adjusting the negative lens to optimal position.

Finally I tried to have a visual peek (the Sun was at only about 10 degrees). I had to remove the holder from the diagonal and just hold the eyepiece above the blocking filter. I used a SWAN 15 mm (~73x) and once I steadied myself enough... I was simply blown away. Not just by the resolution increase, which was expected, but the image was so much more contrasty than in the Lunt 35! No more looking for 5-10 s at the disc for your visual cortex to start noticing the details. No, here all the structures were instantly visible. Plus the difference in brightness between the disc and the spicule layer + prominences was much lower compared to LS35 (which means they were also more readily noticeable, especially the big faint one). I can think of 3 reasons for this:

- the etalon behaves better in a collimated beam (even not-too-well, as I had today) than in an almost-collimated direct light from the Sun (this I think least likely)

- the cloud did some filtering and photosphere was less blinding

- the addition of Baader D-ERF (which is only 35 nm wide) helped prevent photosphere light leakage

I didn't have the time to dismount everything and take a look through reassembled Lunt 35 for direct comparison.
Finally, the nonuniformity could be barely seen. I'm already predicting many a happy hour with a binoviewer attached to this contraption :D


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Re: Lunt 35 modification to larger aperture

Post by GreatAttractor »

Correction: the negative lens has h = –142 mm, giving an effective f/12.05 (f.l. 1085 mm).


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Re: Lunt 35 modification to larger aperture

Post by swisswalter »

Hi Filip

what a great mod. Congratulations. The big prom is just fantastic. Now I see where my missing allan wrench is ;)


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Re: Lunt 35 modification to larger aperture

Post by Derek Klepp »

These are great results.


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Re: Lunt 35 modification to larger aperture

Post by GreatAttractor »

Seeing today was terrible, so I didn't event try to capture.

I got rid of the slop and vignetting, but it turned out the sweetspot is not going away. So I'll pop in longer lenses (–308/350 mm), which will enlarge the sweetspot over 2x so it hopefully covers most of the sensor. If the need arises, I can even attach the whole Lunt OTA (giving me a 400 mm final lens) to the rear thread of the etalon adapter (like Stu did in http://solarchat.natca.net/viewtopic.php?f=9&t=11271).

Raytracing showed that Chameleon's outer FOV will suffer some 8% vignetting (by diameter) on the etalon with a –308 mm collimator (as I cannot place the etalon close enough to it without shortening the donor OTA).

Some more photos:
etalon.jpg
The machinist did a nice job of blackening the interior of the nosepiece (a nice rough texture); not as good as velour, but not much worse:
int1.jpg
int1.jpg (147.23 KiB) Viewed 6173 times
int2.jpg
int2.jpg (134.98 KiB) Viewed 6173 times
Today I also came up with another way of determining the collimating lens position (too bad it was after Sun set and I dismantled everything). I'll try it next time:

- Point the following subset at the Sun: LS35 + positive lens + B600 diagonal + camera.
- Set the focus as precisely as possible (even with a short ~150 mm lens, Sun image on Chameleon's sensor will be a few hundred pixels across, so it should be no problem). The subset is now focused for a case where the positive lens gets collimated light on input (as the Sun is at infinity). Lock everything and don't move these elements again.
- Attach the subset to the rest, i.e. to the donor scope (objective lens) + negative lens.
- Set the focus again (by observing image from the camera) using the donor's focuser (or by sliding the whole attachment in drawtube). The focus will be reached if and only if the positive lens once again gets collimated light on input, i.e. when the negative lens's focus overlaps with the objective's.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
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SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt 35 modification to larger aperture

Post by swisswalter »

Hi Filip

very good news


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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Re: Lunt 35 modification to larger aperture

Post by marktownley »

GreatAttractor wrote:In this thread I'll chronicle the progress of my Lunt LS35 mod(s). As with Omega Optical filters, for now I'm using my Bresser Messier R102 achromat (102/1000 mm) with a front-mounted Baader 90 mm D-ERF. The mod is non-destructive, as the whole Lunt 35 front etalon assembly (+its ERF) simply goes into one of the adapters.

I decided to start with a collimated beam arrangement, reusing my old negative meniscus (D = 44 mm, f = –145 mm), now coupled with a Surplus Sched achromat (D = 43.9, f = 154 mm) for an effective system focal ratio of f/12.22 (eff. f.l. = 1100 mm).
Interesting mod and you're quite clearly getting some good results, but unless you're running a beam diameter of about 13mm into that -145mm lens then you aren't getting a collimated output beam into the etalon...


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Re: Lunt 35 modification to larger aperture

Post by GreatAttractor »

Of course, Mark, the diameter is just that (I always do a manual raytrace first) and the OTA has enough backfocus (plus my nosepiece is long enough) that I'm able to place the collimator sufficiently deep. Currently (I think) the lenses are just too short and beam angles are magnified too much (which wasn't bad for prominences with an overexposed disk). I should be able to try a -308/350 and -308/400 combination over the weekend.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
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SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt 35 modification to larger aperture

Post by marktownley »

Good stuff! I thought you must be to be getting the results you are. Keep us updated :)


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Re: Lunt 35 modification to larger aperture

Post by GreatAttractor »

I had a nice sunny day and terrible seeing (it's rarely that bad, when even 90 mm aperture is unusable). At least I verified that the "sweet spot + sweet ring" is gone with a -308 mm collimator, but vignetting due to the neccessary collimator-etalon distance was significant, as predicted. So I'll go ahead and shorten the OTA. Additionally, even with -308/400 lenses (f/14.4) it's too bright (shutter needs to be set dangerously close to Chameleon's horizontal stripes limit), so I'll have to fit an ND filter somewhere.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt 35 modification to larger aperture

Post by GreatAttractor »

The OTA is now shortened (and the drawtube). I've inserted a 25%-transmission ND filter between camera and diagonal (thankfully not causing any reflections, Newton rings or sharpness degradation), which turned out to transmit some 60%-70% in Hα, but it's dark enough.

Seeing was average to poor (only rare glimpses of sharpness), but good for tests. I used Lunt's rings to shoot a full disc using just the positive lens (350 mm planoconvex):
standalone.jpg
And proceeded to check the whole setup (now -308 mm collimator, 350 mm positive lens, giving f/12.6 and 1136 mm f.l.):
full.jpg
Sweet spot found with the shorter lenses is gone, unfortunately there is still vignetting, which I'm trying to diagnose (according to raytrace, every beam reaching the sensor should fit in the lenses/adapters):
vign.jpg
vign.jpg (38.5 KiB) Viewed 6024 times
Resolution is fine though (stacks are not as good as they could be, due to seeing):
monster3.jpg
Rest of the images in the main forum section thread.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt 35 modification to larger aperture

Post by swisswalter »

Hi Filip

that looks good


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Re: Lunt 35 modification to larger aperture

Post by michael.h.f.wilkinson »

That is awesome. I would be very interested in the precise specs of the adapters and tubes you had machined.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt 35 modification to larger aperture

Post by GreatAttractor »

Summary of the latest findings:

1) Image is back to good (I'm almost sure; the seeing during my last attempts was terrible though, so... not 100%), as in the first post, if the etalon is in a slightly converging beam (I don't know the precise F-number; next season I'll find the optimal negative lens's position and calculate from it).

2) The vignetting is due to the blocking filter. Laser collimator shows that the focuser drawtube doesn't point precisely at the center of objective lens, and in this 'frac the focuser mounting makes it v. difficult to correct. Raytracing shows that even a small focuser slop will cause vignetting. As noted before, there is no vignetting when working with just the Lunt 35 OTA or my shorter OTA + planoconvex singlet. Or when I replace the B600 diagonal with a regular 1,25" diagonal (of course I didn't point it at the Sun, but at a distant lamp at night).

Solution: either to get a B1200, OR it seems that the thick compound filter (ca. 20 mm clear aperture) from the Omega Optical pair works as good as B600 (and vignetting is gone). Again, I'm not sure about the blocking effectiveness due to poor seeing at the time I tried this. There were a few slightly darker bands across the FOV, possibly because the OO filter was somewhat tilted. I'll try to correct this next season.

Michael, here are the diagrams (dimensions in millimeters, all metric threads):
element_A.jpg
element_A.jpg (39.28 KiB) Viewed 5817 times
element_B.jpg
element_B.jpg (32.69 KiB) Viewed 5817 times
element_C.jpg
element_C.jpg (46.4 KiB) Viewed 5817 times
element_D.png
element_D.png (51.65 KiB) Viewed 5817 times
element_E.jpg
element_E.jpg (37.21 KiB) Viewed 5817 times
element_F.jpg
element_F.jpg (36.22 KiB) Viewed 5817 times
Elements A, B, C, D, E, F connect with each other in this order (A = back, F = front = 2" snout). A+B can be replaced by the Lunt 35 OTA, which screws directly into element C.

Lens holders:
element_H.png
element_H.png (18.21 KiB) Viewed 5817 times
element_J.png
element_J.png (20.06 KiB) Viewed 5817 times
As shown in the photos, I attach additional adapters (T2 length extender(s), T2 helical focuser) to element A.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt 35 modification to larger aperture

Post by michael.h.f.wilkinson »

Great, many thanks! I already have a B1200, so that bit is sorted. I will check out different machine shops to see whath this would cost me


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Lunt 35 modification to larger aperture

Post by swisswalter »

Hi Filip

thank you very much for sharing that information. That is solarchat style


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
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