150mm/Lunt35/Coronado BF15 mod

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150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 » Fri Apr 25, 2014 6:18 am

Hi all,

A bit of a mixed bag to produce a Ha scope, but here goes......

I've been using a 150mm modified PST for a while now, but have always been frustrated with the 'sweetspot' issue. Sometimes it is worse than others, but it never goes away. Maybe I just have a less well performing etalon.

Encouraged by what some others have done on this forum using Lunt 60s and so on, I thought I'd have a go with a Lunt 35 which is more closely price-matched to the PST, especially the basic 35.

This required me to make a few changes (reversible ones) to my PST stage 2 modified scope, starting with the removal of the PST etalon housing and all the back end stuff. I then introduced a biconcave collimating lens within the OTA from the rear using a custom lens holder which threaded into the drawtube of the focusing unit retained from the doner scope (the doner scope is actually a custom built scope made from scratch fitted with an Istar Optical 150mm/f10 objective). The collimating lens takes the converging f10 light cone and expands it into an almost parallel format ahead of the Lunt 35 mounted at the rear of the assembly. I already had a Coronado BF15 from my modified PST, so I used this in place of the 4mm BF which came with the basic Lunt 35. Actually, I also removed the helical focusing unit from the Lunt BF assembly and added that directly to the rear on the Lunt OTA using a custom adapter and then placed the Coronado BF with the 45 degree mirror into that so that I had some additional focussing ability.

The Lunt is held using tube rings mounted on an extension plate attached to the doner scope focusing unit. The advantage of the focusing unit here is that the position of the collimating lens can be adjusted up and down the OTA to attain optimum position. The Lunt can be moved forwards and backwards, likewise, to follow. The Lunt etalon sits inside a cup designed within the lens holder.

The biggest challenge was obtaining a suitable collimating lens, which I wanted to be greater than 35mm and have a negative focal length of 350mm to give a 35mm collimated light source to feed into the Lunt 35. I managed to obtain something close from the US after a seven week wait. I finally ended up with a 50mm diameter lens at -300mm, which I considered good enough.

Some pictures of the assembly.
DSC_0202.JPG
Lens holder.
DSC_0201.JPG
Optical path.
Lens Configuration.jpg
Lens Configuration.jpg (48.76 KiB) Viewed 3748 times
First light. The conditions were not ideal and there was a lot of cloud around, including high haze, but for my first efforts with the new scope I'm quite pleased. My first impressions were that the sweetspot was no longer an issue, although more thorough testing and comparisons need to be made.
Capture 24_04_2014 14_59_50..jpg
Capture 24_04_2014 14_59_50..jpg (180.35 KiB) Viewed 3748 times
Capture 24_04_2014 12_44_40..jpg
Capture 24_04_2014 12_44_40..jpg (80.61 KiB) Viewed 3748 times
With better conditions I should be able to really test it out.

Thanks for looking.

Stu.
Lunt LS60THa B1200 PTFT
150mm H-alpha Solar telescope with Lunt35 mod
DMK41, Basler acA1920-155
NEQ6 Pro-mount
Fluxgate Magnetometers (1s and 150s Cadence)
More images at http://www.flickr.com/photos/solarcarbon60/

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by jp-brahic » Fri Apr 25, 2014 6:26 am

Hello Stu

Very beautiful modification which seems good to work!!!! :)

thank for sharing

JP
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by marktownley » Fri Apr 25, 2014 6:33 am

That's pretty damned impressive Stu! Sweetspot size, in my experience, is related to the ratio of the size of the objective to the etalon. So you will get better results with a 35mm etalon compared to a 20mm etalon of the PST. I look forward to seeing more results from it.
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Sungazer » Fri Apr 25, 2014 8:16 am

Hello Stu, A very nice conversion, and the Lunt can be used on its own. First class pictures.!! Out of interest, i have converted my 100mm F8 scope to take a PST in the 2" focuser with the lens removed, and a sub apature 60mm ERF down the tube, and have 4 different size front apature stops to vary the focal ratio. Giving, F13, F10, F9, and F8 .to suit the seeing, works very well. John.

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 » Fri Apr 25, 2014 2:15 pm

Thanks guys. I appreciate your comments.

A couple of additions.
Capture 24_04_2014 14_59_50.Colour.jpg
New.jpg
New.jpg (181.02 KiB) Viewed 3716 times
I just need some clear skies to really give it a good go.

Stu.
Lunt LS60THa B1200 PTFT
150mm H-alpha Solar telescope with Lunt35 mod
DMK41, Basler acA1920-155
NEQ6 Pro-mount
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More images at http://www.flickr.com/photos/solarcarbon60/

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by marktownley » Fri Apr 25, 2014 3:32 pm

Good additions!
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Robin » Fri Apr 25, 2014 4:12 pm

Mind blowing images, great result
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by swisswalter » Fri Apr 25, 2014 10:13 pm

Hi Stuart


first of all just congratulations on the good job
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 » Fri Apr 25, 2014 11:12 pm

Mark,
You once again mention the D/d impact on the "sweet spot". I know this was raised also by Valery, but I have problems getting my head around the fact that a collimated beam (Not a telecentric arrangement) should give such a problem.....
Can anyone give me an (optical) answer......??
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 » Sat Apr 26, 2014 5:40 am

Thanks for the kind words everyone.

I should point out that my lens configuration drawing omits the very important sub-aperture energy rejection filter (90mm diameter).

I hope you get your answer, Ken. My philosophy was to try to deliver to the Lunt 35 as natural a wavefront as possible, as if it were front mounted, having the ability to fine tune the position of the collimating lens using the draw tube.

Cheers

Stu.
Lunt LS60THa B1200 PTFT
150mm H-alpha Solar telescope with Lunt35 mod
DMK41, Basler acA1920-155
NEQ6 Pro-mount
Fluxgate Magnetometers (1s and 150s Cadence)
More images at http://www.flickr.com/photos/solarcarbon60/

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Valery » Sat Apr 26, 2014 5:48 am

Merlin66 wrote:Mark,
You once again mention the D/d impact on the "sweet spot". I know this was raised also by Valery, but I have problems getting my head around the fact that a collimated beam (Not a telecentric arrangement) should give such a problem.....
Can anyone give me an (optical) answer......??
All the PST mods are COLLIMATED BEAM schemes - with collimated lens in front of etalon and re-focusing lens behind the etalon. But sweet spot is here with all telescopes larger than 60mm. At 60mm it is there too, but much less pronounced. The LS35 is 1.75x larger than 20mm etalon in the PST. So, a maximal size of the donor telescope which will be free of a distinct sweet spot is 100mm. And the etalon should be in band with no any tilt for a given conditions (barometric pressure and temperature). Any tilt will immediately cause banding. Also, no tilt should be allowed by the mechanical construction. It must be rock solid.
Ask the author of this mod to take full disk images and you will see.

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Last edited by Valery on Sat Aug 08, 2015 2:12 am, edited 1 time in total.
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 » Sat Apr 26, 2014 6:11 am

Valery,
Once again you reiterate the D/d issue without saying "Why this is so?"
If the collimated beam into the etalon is the same as the external arrangement...then why should there be a difference in performance? What causes the difference? The optical quality? (why?) the optical quality of the collimating lens? (what criteria need to be addressed?)

You have also mentioned a few times now the "success" of your LS50 ARIES system but have provided no images/ drawing or technical information - is this ARIES system a commercial venture? Which companies are involved in the optical design and manufacture? Were can it be seen and evaluated? When will it be available to the market?
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Valery » Sat Apr 26, 2014 11:42 am

Merlin66 wrote:Valery,
Once again you reiterate the D/d issue without saying "Why this is so?"
If the collimated beam into the etalon is the same as the external arrangement...then why should there be a difference in performance? What causes the difference? The optical quality? (why?) the optical quality of the collimating lens? (what criteria need to be addressed?)

You have also mentioned a few times now the "success" of your LS50 ARIES system but have provided no images/ drawing or technical information - is this ARIES system a commercial venture? Which companies are involved in the optical design and manufacture? Were can it be seen and evaluated? When will it be available to the market?

1. Acceptance angle for a front mounted etalon is about 1 degree or two solar disk diameter. In the telescopes with collimated beam we have increased angle of incoming light. The magification is D/d where "D" is the objective diameter and "d" is collimator diameter. So, for a telescope with D/d=2 we have 2x smaller acceptance angle on the sky. About 1/2 degree or one solar diameter. With the D/d ratio increasing the acceptance angle on the sky will proportionally decreased. For wider band pass the acceptance angle is larger, for narrower band pass the acceptance angle is smaller.
For D/d=3 the acceptance angle is somewhat smaller than a solar disk, but because we tolerate some bandpass increasing and shifting, it will work despite that D/d is more than 2. But 3 is the limit and works only if you have etalon 0.7 - 0.8A and which does not require a tilt for a given conditions (barometric pressure and temperature).

2. The ARIES solar kit is my own design and manufacturing as I am the owner of the ARIES. The only part we do not make in house is the coatings. And yes, this is a small series commercial project for those who can't afford large (4" and larger) dedicated solar telescope, but still want to observe the sun through a big refractor and with full disk ability.

Sorry for temporar hijacking of this thread.


Valery.
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 » Sat Apr 26, 2014 10:22 pm

Valery,
Thanks for that added information.
Let me spend some time re-reading....

What's the status on the availability of the ARIES system?
Can this currently be purchased from you, if so, what is the pricing?
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 » Mon Apr 28, 2014 6:55 pm

No problem with the hijacking, Valery. It's all interesting stuff.

Anyhow, I've reprocessed one of the images to reduce the noise and increase the quality.
Capture 24_04_2014 14_16_53_2..jpg
Capture 24_04_2014 14_16_53_2..jpg (156.32 KiB) Viewed 3594 times
Thanks for looking.

Stu.
Lunt LS60THa B1200 PTFT
150mm H-alpha Solar telescope with Lunt35 mod
DMK41, Basler acA1920-155
NEQ6 Pro-mount
Fluxgate Magnetometers (1s and 150s Cadence)
More images at http://www.flickr.com/photos/solarcarbon60/

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by swisswalter » Mon Apr 28, 2014 7:53 pm

Hi Stuart

that is an excellent result, congratulations. The first one was already very good. I would go right in the middle of the two. What else would you have expected as feedback ;)
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: 150mm/Lunt35/Coronado BF15 mod

Post by marktownley » Mon Apr 28, 2014 8:14 pm

Looks good to me Stu! Only thing I would do differently is sharpen at a much smaller radius...
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 » Tue Apr 29, 2014 11:27 am

Thanks Walter/Mark.

Good hint, Mark. I'll look into doing that. I guess there is an option to do that in PixInsight's 'Unsharpening' tool settings.

Cheers

Stu.
Lunt LS60THa B1200 PTFT
150mm H-alpha Solar telescope with Lunt35 mod
DMK41, Basler acA1920-155
NEQ6 Pro-mount
Fluxgate Magnetometers (1s and 150s Cadence)
More images at http://www.flickr.com/photos/solarcarbon60/

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by sullij1 » Wed Apr 30, 2014 1:55 am

Great mod Stu I will be looking at the coming images with interest.
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 » Wed Apr 30, 2014 5:51 am

Thanks, Joe.

Hopefully this weekend will bring good weather for further trials.

Stu.
Lunt LS60THa B1200 PTFT
150mm H-alpha Solar telescope with Lunt35 mod
DMK41, Basler acA1920-155
NEQ6 Pro-mount
Fluxgate Magnetometers (1s and 150s Cadence)
More images at http://www.flickr.com/photos/solarcarbon60/

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by MapleRidge » Wed Apr 30, 2014 1:00 pm

Hi Stu...

great setup and images that you are producing. Great mod!

One questions, and maybe I missed it in your notes, but is the LS35 scope "intact"...fitted at the focuser behind your collimating lens? I see that you swapped the Lunt BF out for the BF15. The optical diagram just shows the etalon, but it looks like you have the entire scope still assembled in the pic.

Looking forward to seeing this evolve and what you do for refinements if they are needed.

Thanks,
Brian
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 » Wed Apr 30, 2014 2:35 pm

Brian,
It looks like Stu used the complete Lunt35 scope...
The front etalon is "nested" into the red painted section of the collimating lens adaptor....
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by MapleRidge » Wed Apr 30, 2014 7:16 pm

Hi Ken...

It looked that way to me too, but I wasn't sure if the objective had been divorced from the etalon. I wasn't sure if the collimating lens was compatible with the objective.

Will be following the progress on the mod as Stu posts images.

Brian
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 » Wed Apr 30, 2014 8:52 pm

Hi Ken/Brian,

Yes, this uses the full basic Lunt 35. The lens, D-ERF, Lunt and Coronado BF diagonal configuration is as shown here:
Lens Configuration2.jpg
Lens Configuration2.jpg (52.97 KiB) Viewed 3495 times
Of course, had I used the deluxe Lunt 35 with 1200 blocking filter, then the Coronado BF15 would not have been necessary.

Ken, you are right, the front of the Lunt sits within the red painted section of the lens holder, really just to prevent any stray light getting into the Lunt. Not totally necessary. There is sufficient clearance to allow the Etalon to be tilted.

The lens holder was machined from aluminium as shown here:
Lens Holder.jpg
Lens Holder.jpg (45.21 KiB) Viewed 3495 times
The collimating lens was not specifically matched to either the large objective, or the Lunt objective. I took it on blind faith that it would work. This was actually the most difficult element of the project. I must have contacted almost every lens manufacturer around the world to try to obtain something close to what I needed before finding OptoSigma in the US via their UK distributor, Laser 2000. Most suppliers show lenses on their website, but don't hold stock and require multiple unit minimum order quantities.

The lens was made to order, but reasonably priced at £90.

Thanks for your interest.

Stu.
Lunt LS60THa B1200 PTFT
150mm H-alpha Solar telescope with Lunt35 mod
DMK41, Basler acA1920-155
NEQ6 Pro-mount
Fluxgate Magnetometers (1s and 150s Cadence)
More images at http://www.flickr.com/photos/solarcarbon60/

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 » Wed Apr 30, 2014 9:27 pm

It reminds me of the early PST trial I did - EC90 "El Cheapo PST mod"
This was a folded semi-compact arrangement with no ERF.

I used a 90mm 800mm fl. achromat from Surplus Shed, a couple of 50/50 beamsplitters (to reduce the energy by 1/4) a TV 2" Big Barlow as a collimator straight into the original PST - no additional ERF in the system.

It did work....but the results were much poorer than anticipated...could have been due to any of the many factors involved. This design was not pursued.
(I thought I'd uploaded some images of the EC90, but can't find them with "search" - if there is any interest I can find them and upload...)
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
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"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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