150mm/Lunt35/Coronado BF15 mod

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Re: 150mm/Lunt35/Coronado BF15 mod

Post by MapleRidge »

Hi Stu...

Thanks for the updated specs.

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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 16256 times
Thanks for looking.

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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 14394 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 14394 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 14394 times
plano_concave _lens.jpg
plano_concave _lens.jpg (723.46 KiB) Viewed 14394 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.

Stu.


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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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