Double Stack Mod with Lunt DSII Modules

[MapleRidge]

Hi everyone...

I have posted several descriptions of the on-going mods I started about 9 years ago using a Lunt DSII module as they evolved over the years. I started to look at options to get a larger solar scope but no PST to use for a donor etalon. I did have a Lunt DSII internal DS module for my LS80T solar scope, so the first mod was to have an adapter made up to fit it to the rear of my Celestron C102 (f10) OTA with a front mounted Lunt 100mm ERF. The second scope used as a mod was a Celestron CR-150 OTA (f8), using an internal mounted 110mm Baader D-ERF.

Both of these mods worked well, but I was wanting to get a narrower band-pass and was having issues with the Daystar Quantum and Quark reaching the correct temps in the winter (yes as most know I'm out at -10C to -20C on a regular basis and the most annoying issue is the delay in reaching proper operating temperature when trying to image off the center Ha band. The Pressure tuned Lunts are quick an deficient in this regard. So my attempt to get a DS system with the Lunts was a shot in the dark. The commercial line of their scopes offer an easy add on of a second pressure tuned etalon fitted on the rear of the OTA, but the individual DSII units were never intended to be stacked...but I figured it was worth a shot and ordered a second DSII.

Once I received the unit I found the first mod needed was an adapter to allow the two etalons to join together...since the cavity at the focuser end is not deep enough to fit the protruding assembly that fit the internal primary etalon...that was planned for. Os, I had a local machine ship make up a short collar to allow them to fit together. The CR-150 was the first scope to have the mod attached and it worked. I have many pics posted through this past winter and spring taken with the modded setup. Most would leave it alone at this point.

I have made a decision to de-commission one of the 3 observatories in my back yard and have fought with options to consolidate solar and wide-field deep sky into one system that can be easily converted form solar to night use. The CR-150 is an achromat, so not the best choice for deep sky work and it is a heavy OTA that was taxing the mount. So my plan evolved to getting a slightly smaller APO scope to meet the deep sky needs and make it solar friendly. I found a used Orion EON 130ED OTA online from a fellow in Winnipeg Canada and had it in hand about the time the spring rush started at work. So I used the Quantum filter for Ha as it did not need any mods to work, and now that I have a little more of my life back I started have the required adapters made up to fit the mods to the Orion.

Below are a few pics of the scope...one thing to note is that I did not get a pic is the internal 2" Astronomic 12nm Ha filter that I have mounted ahead of the first optical element of the first DSII...I need to make some changes to the filter holder in order to mount it in the new adapter.

So, first is the Orion 130 and Lunt LS80T/LS75HA , showing the ERF cell on the 130 as well as the front mounted Lunt LS75FHa etalon used the DS the 80mm Lunt scope. The original Orion focuser was installed when this was taken and has the Lunt Solar Wedge mounted in the focuser:

Orion130_Lunt80-3.jpg (519.4 KiB) Viewed 1755 times

I replaced the original Orion focuser with a 2.5" Moonlite focuser that I had (not begin used), so I got a flange form Moonlite that threaded into the rear of the OTA and provides a large male flange that mates the focuser to the scope. This will work fine for WL/Cak where a standard 2" nosepiece fits the filter to the scope. Below is the a pic of the motorized focuser, and the WL/CaK filters:

WL_CaK-Use.jpg (618.72 KiB) Viewed 1755 times

The ends of the Moonlite focuser tube in M68 threads which allows multiple options to fit different devices to it. In the pic I have a standard 2" eyepiece holder, but for deep sky imaging I can either attached a camera with a 2" nosepiece or remove the 2" adapter and thread the focal reducer onto the M68 thread. Below is a pic of the focuser, reducer and ASI294MC cooled camera:

Night_Use.jpg (767.92 KiB) Viewed 1755 times

Now for the mod Below is a pic I took this past Sunday on the first light, with the parts labeled:

DS_DSII_Mod.jpg (829.4 KiB) Viewed 1755 times

After disassembly and slight mods to the adapter that interfaces the first DSII to the scope (shortened to provide more in-focus) I took this pic tonight of the pieces laid out in order of assembly. The 'Male/Female' gender refers to the type of flange on the adapter faces:

DS_Ha-Exploded.jpg (598.48 KiB) Viewed 1755 times

The 4x Powermate required a lot more back focus so the 23" extension will not be needed in all cases. I will have try out the various lens assemblies used to increase the focal length to see what give the best image quality for the different seeing conditions I have...anything from prime focus with the ASI290, my 2x or 3x Explore Scientific Foal Extenders, AiryLab 2.7x Telecentric, or the 4x/5x Powermates.

The first images were posted yesterday and I hope to get many more in years ahead.

One other mod I have to pull together is the home built pressure control for one of the DSII units...I have the Lunt remote unit but didn't want to spend the money for a second one...more come on that topic.

One last thought on the mods...I used no math/optical design software/etc to put these evolving mods together. My measurement was to find the focus of the scope on a target like the moon, and focus the DSII on the LS80T scope like it was designed for and had the adapter made to place the DSII focus in the same position. The DSII does not add any significant change in the optical path...on the Lunt scope there is a spacer to remove when the DSII in installed so it made it much easier to mod. Another benefit of the f7 optic in the Orion scope is that it matches the f7 that the LS80T objective has, and as a result has the largest sweet spot' of the 3 mods I've made using these etalons which provides a larger FOV on the chip. I was having to use a smaller than full frame ROI, which I expect will have a big benefit for the large scale features that should appear as we move into the solar maximum.

Oh, one second last though...I did not post any measurements as they will be unique to this setup but if anyone wants measurement I can try to provide them in a PM.

Brian

[Carbon60]

Beautifully engineered mod, Brian. I agree with your pragmatic approach. Unless you know exactly the optics involved (refractive index of the glass, lens shape, lens configuration etc), precise modelling can be quite a challenge. It can be useful to make approximations, of course, to obtain a basic design, but final tweaking will always be necessary.

Looking forward to more images from this instrument

Stu.

[marktownley]

Love the methodology and reasoning here Brian, makes a lot of sense to do this. The pictures are much appreciated to see what is going on. Looking forward to seeing the results!

Mark

[MalVeauX]

Thanks for the update Brian, appreciate it, was curious how you got those Lunt 80's to stack in there!

Very best,

[Rusted]

Thank you, Brian, for sharing your remarkably sophisticated mods in such detail and so well illustrated.

I wondered whether you have tried relative rotation between the twin Lunt etalons?

[MapleRidge]

Thanks everyone for your comments and feedback on my 'Frankenstein'

Really, it has been rewarding to see it work out as well as it has...and the support of everyone here from day one of my mods has been very much appreciated.

Rusted, I will make a more concerted effort to see if there is any need to rotate the etalon relive to each other, the main OTA, and the B1800 but so far I have not seen much difference how I set it up. I'll have to do it when I can get a friend who is interested in astronomy to help...my wife and kids want to cut and run after 2 minutes of help

Brian

[Rusted]

Thanks everyone for your comments and feedback on my 'Frankenstein'

Really, it has been rewarding to see it work out as well as it has...and the support of everyone here from day one of my mods has been very much appreciated.

Rusted, I will make a more concerted effort to see if there is any need to rotate the etalon relive to each other, the main OTA, and the B1800 but so far I have not seen much difference how I set it up. I'll have to do it when I can get a friend who is interested in astronomy to help...my wife and kids want to cut and run after 2 minutes of help

Brian

Thanks Brian.

It doesn't sound as if you have the family properly trained.

My diminutive, septuagenarian wife volunteered to help me move 12 m^3 [~25 tons] of self compacting gravel for the observatory base.
40 yards/meters by shovel and wheelbarrow took us a couple of days. It was blocking the drive!
A local builder made a verbal agreement to move it with a machine. Then he couldn't be bothered with such a small job.

[MalVeauX]

Hi Brian,

Are there any collimation lenses involved in front of your etalons?

Very best,

[MapleRidge]

Marty...there will be collimating lenses of some sort in the etalon body as designed to work with the LS80, but I have not seen much info on any specs and in my case I am using the 'off the shelf' version with no mods to the etalon assembly. Well beyond my level of rudimentary expertise.

Rusted,I seldom have any interest form my wife or kids to look through the scope let alone help configure anything. It's just my expensive eccentric hobby I do get some help, don't get me wrong, but the interest wanes pretty quick. Glad to hear your wife was willing to help...I expect the blocked drive helped inspire her!

Brian

[MalVeauX]

Ahh ok, so whatever collimating lenses are in it already are stock? I wasn't aware that the DS module had a collimating lens in it at all actually. Interesting. I thought it was a bare open etalon that stacked in with the other that had collimating lenses.

Amazing how hard it is to find info about these designs and optical trains that have been out for many years!

Very best,

[MapleRidge]

Marty...

I truly don't know what is in the unit for optics...I was only presuming some elements would be in place to work with the light cone that comes through the primary internal etalon. The unit is installed 'out of the box' as shipped by Lunt.

Brian

[marktownley]

Ahh ok, so whatever collimating lenses are in it already are stock? I wasn't aware that the DS module had a collimating lens in it at all actually. Interesting. I thought it was a bare open etalon that stacked in with the other that had collimating lenses.

Amazing how hard it is to find info about these designs and optical trains that have been out for many years!

Very best,

Hi Marty.

Yes, these unit have a collimating lens and also a refocusing lens as does the PST. The lenses are sealed with a gasket to keep the pressure in the housing.

Mark

[MalVeauX]

Ahh ok, so whatever collimating lenses are in it already are stock? I wasn't aware that the DS module had a collimating lens in it at all actually. Interesting. I thought it was a bare open etalon that stacked in with the other that had collimating lenses.

Amazing how hard it is to find info about these designs and optical trains that have been out for many years!

Very best,

Hi Marty.

Yes, these unit have a collimating lens and also a refocusing lens as does the PST. The lenses are sealed with a gasket to keep the pressure in the housing.

Mark

Excellent thanks; was curious where they were since that dictates what can be done to it (such as replacement, addition, extension, etc). I assume they are F7~F8?

Very best,

[marktownley]

Yeah, Lunt seems to have settled on f7 for its scopes.

[MapleRidge]

Marty...

I suspect that it would be possible to change the optics in the DSII in order to match a scope that was f8 or f10 for example, but there didn't seem to be a lot of info on where to even start so I left it alone even on th eF8/f10 donor scopes. The sweet spot did improve as I moved from f10 to the current f7 OTA, still not a full disk on band as I showed in the first.

I am only planning to to image on mid-small scale areas so it isn't affecting me. I suppose the performance could be improved if the optics in the DSII were tweaked, but since it was designed for an 80mm aperture I would think there comes a limit to how well you can 'force' it when working behind objectives that are much larger.

Brian

[christian viladrich]

Excellent Brian !
Thanks for sharing.
Christian

[MapleRidge]

Thanks for your comments on the mod Christian

Brian

[MapleRidge]

Hi all...

I have re-mounted the internally mounted Astronomik 2" Ha filter (12nm) so that it is placed ahead of the first Lunt DSII module.

I have mounted a 2" filter holder in the Moonlite adapter flange that mates the DSII module to the flange on the rear of the Orion scope. In the pic below, the male side of the adapter is on the right and the mounted filter is on the left side of the pic.

Internal.jpg (415.28 KiB) Viewed 1514 times

The nice part of this is the filter is easily accessible if I want to use it for deep sky imaging, or offer opportunity to mount other filters should they be needs for CaK work or other needs.


Brian

[Bob Yoesle]

Hi Birian. Very nice work, especially the interconnecting bits!

there will be collimating lenses of some sort in the etalon body as designed to work with the LS80, but I have not seen much info on any specs and in my case I am using the 'off the shelf' version with no mods to the etalon assembly. Well beyond my level of rudimentary expertise.

I suspect that it would be possible to change the optics in the DSII in order to match a scope that was f8 or f10 for example, but there didn't seem to be a lot of info on where to even start so I left it alone even on th eF8/f10 donor scopes. The sweet spot did improve as I moved from f10 to the current f7 OTA, still not a full disk on band as I showed in the first.

I am only planning to to image on mid-small scale areas so it isn't affecting me. I suppose the performance could be improved if the optics in the DSII were tweaked, but since it was designed for an 80mm aperture I would think there comes a limit to how well you can 'force' it when working behind objectives that are much larger.

Your system probably could be improved by using a single collimator lens to provide optimally collimated rays through both etalons. Your optimum system would match both the effective aperture of the collimator lens to the diameter of the etalon. The focal ratio of the objective and effective focal ratio of the collimator should be as similar as possible.

Just thinking out loud about some ideas I've had about a similar system of DS PT etalons which would be applicable to your system: You could remove the existing collimation/refocusing lenses and then replace and seal the first etalon's PT module collimator with a negative COTS lens with a known FL for precise coincident focus placement to render an ideally collimated ray set to the first and second etalons. You'd replace the first PT etalon refocusing lens with a sealed optical window, followed by a high-quality circular polarizing filter mounted in the etalon module connector for reflection suppression. The second etalon could be sealed similarly with an optical window and positive refocusing lens to achieve a specific EFL. Ideally the internal windows would be of the appropriate AR coating and slightly tilted when sealed.

[MalVeauX]

Brian,

Do you have any polarized filters between these Lunt DS modules? I see you put a HA filter on the nose of the first one, to complete the blocking after your DERF I assume?

Your custom adapters to join the two etalons with spacers; did you fabricate or have them done? Somewhere anyone else could order or just a local place?

Anyone know some basic measurements of that Lunt DS module? Like it's overall diameter of the housing, flange diameter on each side, etc?

Thanks!

Very best,

[MapleRidge]

Bob, thanks for your feedback on the system

I am not much of an optics design person, so designing/testing is not my forte If it was possible to add a corrective type of lens assembly ahead of the DSII module it would certainly be worth a try. I'm not prepared to do any open etalon surgery on the DSII's though.

If it were possible to improve the size of the sweet-spot to the point where an on-band full disk image was possible it might inspire me to look more at modifying the lenses, bu will need significant help to design/implement it.

Brian

[MapleRidge]

Hi Marty...

Like you mentioned, I use the 2" Ha filter to trim off the extra spectrum that the Daystar ERF transmits. The 6" version of the mod had a 110mm Baader D-ERF mounted internally, but no easy way to make it easy to install/remove between Ha work and other solar or deep sky sessions where it isn't needed. At some point I'd like to add a tri-band style ERF for all solar work.

I do not have a polarizer between the two DSII units but certainly could be an improvement for the longer exposures for proms...a lot of stray light. I do have the Lunt filter that was designed to go between the DSII and the primary internal etalon on the LS80T that was to address some of these issues. I think it is still mounted in the LS80T...I should remove it and see if it will fit the mod.

I had a local machine shop make all the adapters I use...took int he parts and let him build them but no measurements or diagrams. Precise Parts has the ability to make the female fitting that the DSII fits into, but limited to making parts they have pre-existing dimensions for. I could make some measurements but they will hardly be accurate enough to take in to have parts made.

I have the mounting plate that I used on the the original mod to mate the DSII to my old C102...not being used and if you can make use of the part I can send it to you. It is machined to accept the DSII, you will need to have an adapter made up mate it to the donor OTA.

Drop me a PM if interested.

Take care,
Brian

[MalVeauX]

Thanks Brian,

All good; was just curious if there was measurements. Hard to find those things!

Very best,