Lunt LS50THa modification to 90 mm aperture

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Lunt LS50THa modification to 90 mm aperture

Post by GreatAttractor »

I present my latest (and probably final) 90 mm H-alpha mod. I wanted the sweetspot to comfortably cover my 1/3" sensor (ICX445), as it does with my Lunt 50 and a 2x Barlow in the camera's nosepiece (1.6x effective). Since in the collimated beam arrangement the sweetspot's linear size depends only on the final positive lens f.l. (including any Barlows), I wanted the donor scope to closely match the Lunt's f/7, so that with the Barlow I get a similar sampling rate relative to aperture, which seems to be optimal for 3.75 µm pixels. This translates to ~f/11.2.

Luckily I found Celestron SkyScout Scope 90 (http://www.teleskop-express.de/shop/pro ... Scout.html) – f/7.33, inexpensive, compact, light-weight and well-made. TS would not sell me just the OTA, but the tripod w/head turned out to be pretty good, will be useful.

The mod turned out to be completely non-destructive, the donor has enough back-focus. In order to find the precise position for Lunt's etalon assembly, I first focused Lunt OTA+etalon asm.+camera (without the blocking filter) on a distant terrestrial target, then removed etalon asm.+camera (still connected) and held them carefully behind the donor OTA (also pointed at a distant object). With this combination in one hand, a ruler in another, and looking at the live video from camera, I was able to find the correct distance to less than 2 mm accuracy.

Donor OTA adapter (dimensions in mm):
_LS50_CSS90.png
_LS50_CSS90.png (51.35 KiB) Viewed 6564 times
The extension tube which connects to the adapter (see pictures below) has female M68x1 threads on both sides (2 mm thick walls). In my case, it is exactly 49 mm long. I don't know if Lunt 50's collimating f.l. is consistent, so the tube's length should be probably measured individually in each case (as outlined above).

The 90 mm Baader D-ERF is mounted in a cell machined from polyamide. There's some wiggle room between cell's collar and the donors lens cell, so it can be tilted before securing with screws:
oprawa_m.jpg
oprawa_m.jpg (142.96 KiB) Viewed 6564 times
I got a Coronado BF15 converted to straight-through configuration to supplement my B600 (as B600 is vignetting noticeably on a 1/3" sensor with the longer donor f.l.). I'll be using BF15 for surface shots, as it is a few times darker than the B600, so exposure on fainter prominences would become uncomfortably long. B600 is still good for proms, as some slight vignetting doesn't matter much there.
bfs.JPG
bfs.JPG (151.68 KiB) Viewed 6564 times
In prime focus (f/7.33) the sweet spot is too small, as expected:
swspot1.jpg
swspot1.jpg (51.51 KiB) Viewed 6564 times
With the Lunt OTA (f = 350 mm) it just about fits the whole disc. I'm not going to use the f/7.33, as it is (at 660 mm) close to the Barlowed Lunt (560 mm).
With 1.6x Barlow the sweetspot looks much better (but flats will be required anyway for mosaics):
swspot.jpg
swspot.jpg (97.28 KiB) Viewed 6564 times
The scope seated on a SW HEQ5:
mod.JPG
mod2.jpg
b600_barlow.JPG
bf15_barlow.JPG
Compared to my previous attempt with a thick-walled, long 100 mm f/10 donor and massive LS35 adapters, this setup is much more compact and lighter.

I posted first light pictures and animations in viewtopic.php?f=4&t=19223
Last edited by GreatAttractor on Fri May 12, 2017 6:44 pm, edited 2 times in total.


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 scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt LS50THa modification to 90 mm aperture

Post by marktownley »

Superb! I've been looking forward to reading about this mod after seeing the excellent results you got from it. Very impressive indeed!


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Re: Lunt LS50THa modification to 90 mm aperture

Post by GreatAttractor »

Thank you, Mark.

I forgot to mention that unlike the LS35 mod, here the visual contrast is as good as with the stock Lunt OTA. And no thin halo along the limb. So far I only quickly tried a 9 mm Antares Ortho w/out Barlow (sweetspot covers then most of FOV). I'll be getting a pair of 12.5 mm Plössls for my binoviewer; together with 1.6x GPC they'll give me 53x and 85x, should be just fine.


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 scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt LS50THa modification to 90 mm aperture

Post by Derek Klepp »

A great project thanks for the detailed description


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Re: Lunt LS50THa modification to 90 mm aperture

Post by GreatAttractor »

...but if, like me, one's not willing to shell out for a new ED/apo ;) there's another smaller achromat (80/600 mm): http://www.teleskop-express.de/shop/pro ... ptics.html


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 scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt LS50THa modification to 90 mm aperture

Post by _RA_ »

I'm curious if there's any followup on this...

Has it held up well? Things to do differently if doing it all over again?
Also a couple of things I'm unclear on (sorry to be obtuse). The Lunt did not have to undergo any alterations, but what about the SkyScout? And was the adapter in the first schematic something custom machined?

How would this perform for strictly visual? Would the sweet spot be too small? Vignetting?

Thanks!


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Re: Lunt LS50THa modification to 90 mm aperture

Post by GreatAttractor »

_RA_ wrote: Mon Mar 15, 2021 9:29 pm Has it held up well? Things to do differently if doing it all over again?
It's held up very well, with one small exception - after 5 years the Borg focuser started falling apart (if retracted too far). Perhaps it could be fixed by doing something to the "locking/holding pin", but anyway I was considering replacing it with a 1.25" mini-Crayford (Feather Touch or the Lunt-dedicated Moonlite) - I likely will this year.
_RA_ wrote: Mon Mar 15, 2021 9:29 pm Also a couple of things I'm unclear on (sorry to be obtuse). The Lunt did not have to undergo any alterations, but what about the SkyScout? And was the adapter in the first schematic something custom machined?
No changes to the SkyScout either. Just unscrewing the original focuser and attaching my adapters - they were indeed machined to order.

And with a 2" replacement focuser, the tube appears to be a good performer in CaK (see this thread).
_RA_ wrote: Mon Mar 15, 2021 9:29 pm How would this perform for strictly visual? Would the sweet spot be too small? Vignetting?
Thanks!
Just fine! The sweet spot covers about 1/4 of the Sun. I usually observe with a 9 mm Antares Ortho, then the whole FOV is on-band. Also tried a binoviewer (Baader Maxbright + 1.6x GPC and 2x15 mm Plössls). No vignetting with any on these.


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 scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt LS50THa modification to 90 mm aperture

Post by Boso36 »

Hi Great Attractor/Solar chat forum members,

I have plans to build a scope along the lines of your "Lunt LS50THa modification to 90 mm aperture". Those video files I saw earlier last year and the images are absolutely incredible, the best I have ever seen !

Please can someone/Great Attractor, explain if my optical train is correct. I am also not sure where the extension or donor adaptor is in all of this.

2) Components of this modified scope are (from front to back):

a) The 90 mm Baader D-ERF
b) the donor OTA body of Celestron SkyScout Scope 90 (http://www.teleskop-express.de/shop/pro ... Scout.html) – f/7.33. Basically the metal tube and the 90mm objective lens
c) Donor OTA adaptor (not sure of where this is, or dimensions)
d) extension tube of 49mm length (M68)
e) Coronado BF15 converted to straight-through configuration - for surface imaging of sun (lunt BF600 for imaging of proms can be used)
f) Barlow 1.6x (to obtain better sweet spot focusing)
f) ? Borg helical focuser
g) Camera - PGR Chameleon 3 mono (ICX445)

Thank you.
Magnus
London


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Re: Lunt LS50THa modification to 90 mm aperture

Post by GreatAttractor »

Hi Magnus,

The component list is correct. Clarifications:

c) see the image in the first post. The adapter is axis-symmetrical (not counting the 3 M4-threaded holes), its right side goes in the OTA in place of the original focuser.

e) I ended up not using it much. Since my setup has guiding capability during recording, even for long time lapses the slight vignette from the B600 stays in one place relative to the solar feature of interest, so an animation doesn't show any annoying flickering (I'm not even using flats - only for mosaics).

f.1) Mine is SW-branded, looks very similar to this model (I don't have the front filter thread). The lens element can be detached and screwed directly into camera's 1.25" nosepiece (for an effective 1.6x in my case).

f.2) looks exactly like this TS model. But even the vanilla helical focuser is OK-ish (even though it's rotating), I had used it in a few sessions.

g) This one: CM3-U3-13S2M-CS. Parameters aren't very impressive compared to newer cameras, but it shows no image artifacts in Hα or CaK, unlike many current CMOS sensors - no Newton rings, fixed-pattern noise, Bayer-mask-like nonuniformity on mono models due to fabrication or firmware issues. Reading about all that made me very wary of upgrading to something bigger and faster.


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 scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt LS50THa modification to 90 mm aperture

Post by Boso36 »

Hi Great Attractor,
Thank you very much for your quick reply and so detailed as well, I appreciate it. I will make detailed notes on all your components and read up on my optics knowledge.
I will now start preparations to make a solar scope along your lines. I may have to use a slight smaller or bigger donor OTA, maybe a 100mm or 80m or 120mm, as I have difficulty finding the Solarscout 90mm scope at the present time.

But the principles and details should be enough. Hopefully, I can get some advice if I run into difficulties. You guys in SolarChat are really helpful and friendly !

Magnus


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Re: Lunt LS50THa modification to 90 mm aperture

Post by _RA_ »

@GreatAttractor, thanks so much for the reply.

Yesterday I ordered a SkyScout90 from eBay listed in "excellent" condition for around US $80. It's scheduled to arrive next week, so I'm holding my breath until then and hoping it checks out.

I'm hoping to order the LS50 soon too, and after I've had time with both scopes separately, I may want to return here with more questions about the hybridization. I'll try not to be tedious, but this would be my first round with ATM, and I understand the seriousness of messing with solar.

The answers to @Boso36's question are also very helpful.

Until then, thanks for the followup, and for your pathfinding.

~RA
Austin TX


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Re: Lunt LS50THa modification to 90 mm aperture

Post by Boso36 »

GreatAttractor wrote: Tue Mar 23, 2021 10:23 pm Hi Magnus,

The component list is correct. Clarifications:

c) see the image in the first post. The adapter is axis-symmetrical (not counting the 3 M4-threaded holes), its right side goes in the OTA in place of the original focuser.
Hi, Great Attractor,


Some more questions now come to light.

1) How did you make the custom adaptor in c) which replaces the SS90 ota's focuser? I looked online and found an online service, and even a basic adaptor cost 271 usd. Can you advise where you can make one precisely to your needs.

2) Is the M68 extension, 49 mm long with female threads also custom made?

3) In regards to safety, how do you centre the sun in your scope, a sol-finder device on the finderscope bracket, or some other means?

Many thanks.
Magnus


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Re: Lunt LS50THa modification to 90 mm aperture

Post by Boso36 »

_RA_ wrote: Wed Mar 24, 2021 7:51 pm @GreatAttractor, thanks so much for the reply.

Yesterday I ordered a SkyScout90 from eBay listed in "excellent" condition for around US $80. It's scheduled to arrive next week, so I'm holding my breath until then and hoping it checks out.

I'm hoping to order the LS50 soon too, and after I've had time with both scopes separately, I may want to return here with more questions about the hybridization. I'll try not to be tedious, but this would be my first round with ATM, and I understand the seriousness of messing with solar.

The answers to @Boso36's question are also very helpful.

Until then, thanks for the followup, and for your pathfinding.

~RA
Austin TX
Hi, RA,

You are very lucky, I searched ebay and Amazon and could not find SS90 ota. Maybe I will need a worldwide search. Can you also clarify your order for LS50. Is that a whole working Lunt solar telescope, LS50, or just parts of it?

Do you know how to get your custom adaptors, maybe extension tubes as well?

Thank you.
Magnus
London


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Re: Lunt LS50THa modification to 90 mm aperture

Post by GreatAttractor »

Boso36 wrote: Thu Mar 25, 2021 5:45 am 1) How did you make the custom adaptor in c) which replaces the SS90 ota's focuser? I looked online and found an online service, and even a basic adaptor cost 271 usd. Can you advise where you can make one precisely to your needs.

2) Is the M68 extension, 49 mm long with female threads also custom made?
Both were made by an "astro-machinist" specializing in such things (I only paid ca. 35€), but no longer active. I imagine many local workshops with a lathe could make it for you (you might need to get it black-anodized separately) - maybe someone on UK-specific fora can point you towards one? If that fails, here's another astro-machinist I know of in Poland: http://serwisastro.pl/ (and some example stuff he makes). I'm quite certain adapters like mine would be cheaper than $270.
Boso36 wrote: Thu Mar 25, 2021 5:45 am 3) In regards to safety, how do you centre the sun in your scope, a sol-finder device on the finderscope bracket, or some other means?
I've been meaning to build a nice "solar searcher", but for now I just use the shadow-minimizing method (i.e., hold something flat behind the telescope, slew until the shadow is as small and round as possible).

Lastly, my D-ERF cell has 88 mm clear aperture (also made to order, by another machinist). There are metal ERF cells on www.teleskop-express.de, but quite expensive. A 90 mm filter is rather small and light, if I couldn't source a machined cell, I'd attempt to build one with some plywood & screws perhaps.


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 scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: Lunt LS50THa modification to 90 mm aperture

Post by _RA_ »

Boso36 wrote: Thu Mar 25, 2021 5:49 am
Hi, RA,

You are very lucky, I searched ebay and Amazon and could not find SS90 ota. Maybe I will need a worldwide search. Can you also clarify your order for LS50. Is that a whole working Lunt solar telescope, LS50, or just parts of it?

Do you know how to get your custom adaptors, maybe extension tubes as well?

Thank you.
Magnus
London
Hi Magnus,

We'll see if I was lucky. The seller doesn't accept returns, so it's a bit of a gamble. I'm hoping it will be here in a few days, and for conditions to allow me to check it out. I'll report back once I have an opinion. The package includes the original tripod, finder scope. and bracket for the Personal Planetarium™, which is not included (thankfully?). I'm unclear if any eyepiece comes with it. All in all, if it checks out, not a bad entry-level scope for the price.

My intention is to buy a new LS50, basic package but with the BF600 and zoom eyepiece. My plan is to take my time with both scopes getting well acquainted before I attempt any customization. I expect to go slow, learn all I can, and gather the necessary parts piecemeal.

You and I seem to be on parallel trajectories. I love that we've got people leading the way, and others like us who can share our process of catching up. Your questions about the adapter are also on my mind. Between custom machining and the front-mounted D-ERF, the costs could start to add up.

Regarding the D-ERF, I find myself wondering if a smaller aperture [i.e. cheaper] internal mount would be feasible. If it were placed between the objective and etalon assembly, it would only need to be the diameter of the light cone at a given placement, plus tilt. Just a thought, and I'd definitely want guidance from those with expertise.

In any case, I'll be very excited to compare notes. Thanks for connecting!

~ RA


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Re: Lunt LS50THa modification to 90 mm aperture

Post by Boso36 »

Hi RA,
Its a pleasure to "talk" to others on SolarChatForum. I will think about the D-ERF as well. I don't know much about D-ERF, but a front attached ERD may give better images and easier to get the focus and clarity. Putting the D-ERF further back in the optics could be difficult, maybe you can ask the experts on SCF !

I am really considering this project as the images by Great Attractor, are truly extraordinary. I did notice he had setup his scope on grassland, so this must be to optimise the ""seeing"". Anyway, I am trying to find out how the Lunt LS50 can be taken apart, and the etalon could be re-assembled back into the original Lunt configuration, i.e. it is reversible. I hope its easy to dismantle the Lunt. I am asking Lunt for advice on this.

By the way, it is slightly easier for me to optimise my Quark setup. I am planning to go high resolution with a 120 or 100 or 90 mm achromat, setup the Quark, use a motorised focuser, and find the right camera to avoid or reduce Newton rings. I want to image and follow the magnetic field lines, but not on too high res scale, because you lose perspective, like looking at a part of a leaf in a forest, you lose what you are seeing. So the image scale that Great Attractor got on his LS50THa to 90mm modification looked really good.

This hobby is quite expensive, so I don't want to fail and get rubbishy images !!

Good luck!

Magnus
London


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Re: Lunt LS50THa modification to 90 mm aperture

Post by _RA_ »

Finally, an update, as promised.

The SkyScout 90 came a couple of weeks ago, exuberantly well-packaged, but showing some signs of minor neglect. I've cleaned the dust off as best as I can, and conditions have allowed for some decent views. I performed some simple star tests at modest magnification (66x) and things look good there. I was also able to split some doubles on a night of unusually good seeing for here. Overall, very pleased, given what I paid for it, and how I intend to use it.

What's more, is I just ordered the LS50 today, and hope to take receipt later this weekend. Very stoked!


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Re: Lunt LS50THa modification to 90 mm aperture

Post by Boso36 »

Hi _RA_,
DId you mean the Lunt LS50THa, single stack. Which blocking filter, or did you just order the 50mm Lunt filter. I find all the terminology quite confusing sometimes.

Many thanks.
Magnus


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Re: Lunt LS50THa modification to 90 mm aperture

Post by _RA_ »

I ordered the starter kit with OTA (single stack), which includes case, zoom eyepiece, and the BF600. The package is supposed to include the Celestron CG-4 tripod/mount, but they are back-ordered through October, so the good people at Lunt were happy to just remove that as a line item and deduct the cost. (Luckily the SkyScout came with a sufficiently adequate AZ that will tide me over.) The package has arrived, and unboxing looks good, but it's been gray here, so no first light yet.

OB this topic, printed materials included with order state specifically that disassembly of the scope will void the warranty, but it's not clear what constitutes disassembly. I certainly would not try to get inside the etalon, but surely just unscrewing the module from the tube should be allowed?

In any case, I am interested in knowing more about the anatomy of the scope, and build concepts in general. My understanding is that the ERF is part of the PT etalon module, and is light-forward of the collimating lens. Is this correct, and does anyone know the specs on the Lunt internal ERF for this scope?

Also, despite the amount of reading I've been doing, I'm still unclear on the anatomy of the blocking filter/diagonal assembly. I think the blue glass filter in the nose is the actual blocking filter, the job of which is to trim the harmonics off the Ha wave. But what is the red filter on the ocular side... is that a secondary ERF, or a UV/IR cut filter, or what?

Still learning, and any comments are welcome, especially from those who have successfully performed mods. I'll have more questions as we go along, and will do my best to keep everything pertinent to this thread.


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Re: Lunt LS50THa modification to 90 mm aperture

Post by _RA_ »

NM on the blocking filter/diagonal assembly, I found the info I needed. Knew I'd seen it somewhere before, but there's a lot to absorb here, and I sometimes need a refresher.


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