150mm/Lunt35/Coronado BF15 mod

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150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

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 14585 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 14585 times
Capture 24_04_2014 12_44_40..jpg
Capture 24_04_2014 12_44_40..jpg (80.61 KiB) Viewed 14585 times
With better conditions I should be able to really test it out.

Thanks for looking.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by jp-brahic »

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 »

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 »

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 »

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 14553 times
I just need some clear skies to really give it a good go.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by marktownley »

Good additions!


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Robin »

Mind blowing images, great result


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by swisswalter »

Hi Stuart


first of all just congratulations on the good job


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 »

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 »

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.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Valery »

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.

There is NO free lunch.
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 »

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 »

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 »

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?


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

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 14431 times
Thanks for looking.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by swisswalter »

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 ;)


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by marktownley »

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 »

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.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by sullij1 »

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 »

Thanks, Joe.

Hopefully this weekend will bring good weather for further trials.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by MapleRidge »

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 »

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 »

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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Maple Ridge Observatory
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Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
ASI1600MM, ASI432MM, ASI294MM Pro, ASI174MM, ASI462MM
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

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 14332 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 14332 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.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 »

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...)


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by MapleRidge »

Hi Stu...

Thanks for the updated specs.

Brian


Brian Colville

Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
ASI1600MM, ASI432MM, ASI294MM Pro, ASI174MM, ASI462MM
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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

I managed a few minutes on the new scope before the sun set behind my neighbour's roofline and unfortunately there was not enough time to grab what I would call a good AVI. In my rush after work to set up I had not quite aligned my mount and tracking was aweful. I had to try to keep on target manually which resulted in a really unsteady image. So much so that AviStack 2 was having trouble creating a suitable image to work with.

Anyhow, out of 5 AVIs I managed one which was 'reasonable' and here is the result of further processing using PixInsight. Overall, the new scope looks very promising and things will only improve when I get my act together!

A better quality version is on my flickr site.
20140514.jpg
20140514.jpg (123.96 KiB) Viewed 16257 times
Thanks for looking.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Merlin66 »

Majic!!
I think this set-up has the potential to give you some excellent results....
Well done.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by swisswalter »

Hi Stuart

congratulations, you are on the right track


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

Just a quick update on the mod.

In order to attain focus using the Lunt35, the draw tube from the doner scope, together with its collimating lens, I've found, has to be positioned 30mm rearwards from its intended -300mm from the prime focus position of the donor scope objective. In other words, it is now at -270mm. This no doubt gives the light rays some degree of 'toe-in' as they enter the Lunt objective which can then focus them at about the same point as they would be focussed using the native Lunt alone. Looking by eye with a 25mm fl eyepiece I noticed that the image does, indeed, have a sweet-spot, but this appears large enough to encompass the entire field of view from the DMK41. I suspect that with larger format cameras, then some degradation at the edges of the image will be observed. It does appear to be minor and around the edges of the observed image, and might be addressable.

I'm experimenting with the position of the collimating lens to see if this makes much difference to the size of the sweet-spot, but need to extend backwards the rear position of the BF and camera in order to achieve focus as I get closer to the original -300 position (moving the collimating lens forward in the OTA). I've reached the limit of movement and need a new adapter! I feel a visit to my friendly machinist coming on.

Thoughts on this approach would be welcome.

Cheers

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by marktownley »

Makes perfect sense Stu, the Lunt 35 is designed for a f108 beam, not a perfectly collimated one, so some deviation from the -300mm position is to be expected I think.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by swisswalter »

Hi Stuart

I watch that mod very close. Good luck on it. Maybe I'll try it with a LUNT 100 ;)


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by daslolo »

Stu, the results are impressive but after reading it a few times I don't think I understand it.
So please correct what's wrong:
  1. No modification to the ls35
  2. a holder rail with rings to alleviate the weight of the ls35, connected on the other end to the 150mm/s dovetail
  3. you built a custom adapter to fit the nozzle end of the ls35 to the 2" focuser of the donor 150mm
  4. had a shop, Optosigma, custom build a type of barlow with a specific lens, put that inside the above mentioned 2" adapter
  5. this barlow also holds the D-ERF so the D-ERF is inside the tube of the 150mm


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

daslolo wrote: Mon Feb 17, 2020 2:34 am Stu, the results are impressive but after reading it a few times I don't think I understand it.
So please correct what's wrong:
  1. No modification to the ls35
  2. a holder rail with rings to alleviate the weight of the ls35, connected on the other end to the 150mm/s dovetail
  3. you built a custom adapter to fit the nozzle end of the ls35 to the 2" focuser of the donor 150mm
  4. had a shop, Optosigma, custom build a type of barlow with a specific lens, put that inside the above mentioned 2" adapter
  5. this barlow also holds the D-ERF so the D-ERF is inside the tube of the 150mm
Hi,

Correct, no modification to the Lunt 35. It’s hung from the back of the refractor on the rail and rings to carry the weight. The light cone from the 150mm f10 is substantially collimated through the Barlow lens and fed to the Lunt35 objective. Laser2000 here in the UK sells suitable lenses (Planoconcave in my latest version). The ‘adapter’ simply holds the Barlow lens at the correct distance behind the 150mm objective to produce a substantially parallel beam. The sub-aperture (90mm) DERF is positioned within the OTA, held in place with a separate fixation ring.

Hope this helps.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by daslolo »

DERF inside the OTA!
How is the heat?
Could you take a photo of the inside of the OTA?

I don't know much about optic so i googled planoconcave. they turn convergent beams into // that's to feed the lunt objective what it's designed for - object at infinity?

How do you determine focal length and position of that planoconcave lense?


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

daslolo wrote: Mon Feb 17, 2020 10:20 pm DERF inside the OTA!
How is the heat?
Could you take a photo of the inside of the OTA?

I don't know much about optic so i googled planoconcave. they turn convergent beams into // that's to feed the lunt objective what it's designed for - object at infinity?

How do you determine focal length and position of that planoconcave lense?
Hi,
The heat management is perfectly fine. The DERF reflects the IR back out through the objective.
The build is shown on my Flickr pages (go to the earliest pics) so you’ll see some of the details there.
Yes, the parallel beam feeds the Lunt objective as if at infinity.
The Planoconcave (PC) lens needs to be positioned within the focal point of the main f10 objective lens at a distance equal to the negative focal length of the PC lens. So if the PC lens has a focal length of-300mm then it needs to be 300mm forward of the point of focus of the main objective lens. This will provide a 30mm diameter light beam to the Lunt. A PC lens with a larger diameter than this can be masked off to 30mm, which is a close enough match to the Lunt35.

Hope this helps.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

Carbon60 wrote: Tue Feb 18, 2020 9:04 pm
daslolo wrote: Mon Feb 17, 2020 10:20 pm DERF inside the OTA!
How is the heat?
Could you take a photo of the inside of the OTA?

I don't know much about optic so i googled planoconcave. they turn convergent beams into // that's to feed the lunt objective what it's designed for - object at infinity?

How do you determine focal length and position of that planoconcave lense?
Hi,
The heat management is perfectly fine. The DERF reflects the IR back out through the objective. Take care, though, as this is now a converging beam of intense light and heat which can be damaging to anything at, or close to the point of focus in front of the telescope.
The build is shown on my Flickr pages (go to the earliest pics) so you’ll see some of the details there.
Yes, the parallel beam feeds the Lunt objective as if at infinity.
The Planoconcave (PC) lens needs to be positioned within the focal point of the main f10 objective lens at a distance equal to the negative focal length of the PC lens. So if the PC lens has a focal length of-300mm then it needs to be 300mm forward of the point of focus of the main objective lens. This will provide a 30mm diameter light beam to the Lunt. A PC lens with a larger diameter than this can be masked off to 30mm, which is a close enough match to the Lunt35.

Hope this helps.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by daslolo »

Carbon60 wrote: Tue Feb 18, 2020 9:04 pm Hi,
The heat management is perfectly fine. The DERF reflects the IR back out through the objective.
The build is shown on my Flickr pages (go to the earliest pics) so you’ll see some of the details there.
Yes, the parallel beam feeds the Lunt objective as if at infinity.
The Planoconcave (PC) lens needs to be positioned within the focal point of the main f10 objective lens at a distance equal to the negative focal length of the PC lens. So if the PC lens has a focal length of-300mm then it needs to be 300mm forward of the point of focus of the main objective lens. This will provide a 30mm diameter light beam to the Lunt. A PC lens with a larger diameter than this can be masked off to 30mm, which is a close enough match to the Lunt35.

Hope this helps.

Stu.
I can't find the Lunt assembly.
What is this? It looks like the tilt tuning wheel of an etalon.
https://www.flickr.com/photos/solarcarb ... 135704010/

This one is a crazy cool PST mod, it's more than that, it's a full telescope you built, that doublet is a spare part from another scope?
I'm at a point where I can use my 3D printer well, but it seems that you machine your own metal part! Do you have a CNC at home? How do you bind the metal discs to the tube without welding?
https://www.flickr.com/photos/solarcarb ... 9611603402

If you have a blog somewhere I'd like to learn, this is some useful skill.

Got it on the focal length.

I'm surprised the energy leaves the tube without accumulating. When I trace the beams that reflect they cross in front of the PC lens and then form a cone with lower diameter than the aperture but what about diffuse reflection? how come the inside of the tube doesn't get some of that?


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

daslolo wrote: Tue Feb 18, 2020 11:36 pm
Carbon60 wrote: Tue Feb 18, 2020 9:04 pm Hi,
The heat management is perfectly fine. The DERF reflects the IR back out through the objective.
The build is shown on my Flickr pages (go to the earliest pics) so you’ll see some of the details there.
Yes, the parallel beam feeds the Lunt objective as if at infinity.
The Planoconcave (PC) lens needs to be positioned within the focal point of the main f10 objective lens at a distance equal to the negative focal length of the PC lens. So if the PC lens has a focal length of-300mm then it needs to be 300mm forward of the point of focus of the main objective lens. This will provide a 30mm diameter light beam to the Lunt. A PC lens with a larger diameter than this can be masked off to 30mm, which is a close enough match to the Lunt35.

Hope this helps.

Stu.
I can't find the Lunt assembly.
What is this? It looks like the tilt tuning wheel of an etalon.
https://www.flickr.com/photos/solarcarb ... 135704010/

This is an etalon assembly for my RCT scope. There’s a similar PC lens in the front end of this to make the light cone substantially parallel for the etalon and there’s a refocusing lens behind the etalon to form the image on the sensor.

This one is a crazy cool PST mod, it's more than that, it's a full telescope you built, that doublet is a spare part from another scope?

I bought the objective lens from iStar Optical.

I'm at a point where I can use my 3D printer well, but it seems that you machine your own metal part! Do you have a CNC at home? How do you bind the metal discs to the tube without welding?
https://www.flickr.com/photos/solarcarb ... 9611603402

I used a local engineering workshop to machine the parts to my design. The DERF is clamped within the retaining ring and the ring is adhesively bonded within the OTA.

If you have a blog somewhere I'd like to learn, this is some useful skill.
I also wish I had the skill and machines to do this type of work. :)

Got it on the focal length.

I'm surprised the energy leaves the tube without accumulating. When I trace the beams that reflect they cross in front of the PC lens and then form a cone with lower diameter than the aperture but what about diffuse reflection? how come the inside of the tube doesn't get some of that?

The majority of the heat is reflected out of the front of the scope. I’ve never noticed any issues with heat within it.
Answers above.

Thanks for your interest.

Stu.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by antonello »

As Valery has already written, the system use a scheme with internal collimation. The scheme of Carbon60 is not very different from this,
lunt-tal_100.jpg
lunt-tal_100.jpg (269.82 KiB) Viewed 14395 times
with the only difference that he has kept, as a positive lens, the original achromatic lens of the Lunt 35, and therefore had to look for only the negative lens. Instead, I have searched and found (easily) both simple lenses from the EKSMA catalog (-300mm and + 300 mm).
I did not leave the achromatic objective of the Lunt because I wanted the highest possible quality. Indeed this is the calculation result of my combination.
lunt_35_with_100_1000.jpg
lunt_35_with_100_1000.jpg (305.18 KiB) Viewed 14395 times
The result would be identical if instead of the Tal 100/1000, I used the 150/1500 Istar H-alpha (which I have), with the same simple lenses (-300 mm and + 300mm.

If one wants to achieve the same result while maintaining the achromatic objective Lunt, must make a negative custom lens which however costs much more than two simple lenses like the ones I put in my system.
As I have written on other occasions, the negative lens of a collimation system is preferable to be Plano-concave. By putting a Biconcave lens the result worsens slightly. My calculation with Oslo for this specific case confirms this result.
biconcave _lens.jpg
biconcave _lens.jpg (725.19 KiB) Viewed 14395 times
plano_concave _lens.jpg
plano_concave _lens.jpg (723.46 KiB) Viewed 14395 times

The photographic results of Carbon60 are good, but can be improved if instead of a biconcave lens it puts a flat-concave lens, with the side flat towards the lens.

In the Eksma Catalog it is disponible a plano-concave lens, diameter 38 mm and focal -300 mm for 28 euro... By


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Rusted »

Very impressive results Stu.

I wonder if separating binocular objectives would provide suitable plano concave lenses at much lower cost?
Inexpensive, Porro Binoculars are readily available secondhand. Though I have no idea as to the likely, negative, focal length.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

Thanks for your responses. Antonello, please note I did replace the biconcave with a Plano concave for the reasons you’ve given.

The images in this thread have since been significantly surpassed with practice and improved processing. Indeed this kit has produced prize winning images (Insight Astro comp).

Looking forward to doing more with it when the Sun and conditions allow ;)

Stu
Last edited by Carbon60 on Wed Feb 26, 2020 6:28 am, edited 1 time in total.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by antonello »

Sorry Carbon60,
You're right, I hadn't noticed the latest messages and I limited myself to the scheme of your drawings.
In any case, my message was the pretext to let know that simple, concave and convex lenses, even with long focal lengths, and at a good price, can be found by searching the internet well.
In my research I also found lenses from -500 mm and + 500 mm from 60-70 mm in diameter, but then I preferred to have them built custom. Unfortunately I didn't memorize the URLs :-(
The purpose of my message was also to inform that those who want to make a modification of the lunt 35 can find in the EKSMA catalog the two lenses of -300 and +300 mm and have an etalon of 35 mm, instead of only 20 mm.
Unfortunately I don't have a PST and I have never compared the two modifications, however, theoretically, I expect the Lunt 35 modification to be much more comfortable than the Modification with the PST, so who has the lunt 35 and wants to use it for a project similar to yours now it can make it (with less than $ 100) and it doesn't have to buy a PST.
Anyway, congratulations for your work and for the enthusiasm with which you carried it forward and transmitted it in this discussion on Solarchat


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by Carbon60 »

No problem :)

Good to know suitable lenses are easily available and it’s interesting to see your ray tracing results. I might try experimenting with the Lunt objective based on your comments, just out of curiosity.

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by antonello »

Rusted wrote: Tue Feb 25, 2020 6:45 pm I wonder if separating binocular objectives would provide suitable plano concave lenses at much lower cost?
No, Rusted,
this is unfortunately not possible. The simplest solution is to buy the two simple lenses, flat concave and flat convex. The other solution is to design and build custom lenses with calculated curves, much more expensive, but which does not include flat sides, with a theoretical advantage in terms of contrast. To put two lenses at random and only a waste of time :-(


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by antonello »

Carbon60,
What you have done is correct. The best positioning of your plano-concave lens is what you can get experimentally after correctly putting the Lunt 35 (only complete Lunt 35) in focus at infinity. In this way it is certain that all the rays that come out of the plano-concave lens and enter the etalon (obviously the rays parallel to the optical axis, those in the center of the field) are all parallel and enter in the etalon in the best situation.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by daslolo »

do the planoconcave lens need to face the flat part to the objective?
and how do i recognize the flat part? the curvature of the PC lens I see is so soft that I cannot tell the flat from the concave by looking at it - perhaps through it.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by antonello »

Yes. In a plane-concave lens of -300 mm focal length, the concave face is easily visible.
In any case, see the face for reflection. The plane face shows the reflection of the same dimension as reality. In the concave face all is much smaller.


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Re: 150mm/Lunt35/Coronado BF15 mod

Post by daslolo »

Oh I see thanks for the tip.


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