Thoughts on modding my AP130GT with LS80 PT Etalon

[chrishet]

I am fairly new to solar imaging but so far I am having a blast and have really enjoyed my Lunt LS80 scope. Here is a link to some of my works, including an APOD of the transit a few days ago!

Imaging The Cosmos Solar Gallery

I also have an AP130GT that I use for deep sky imaging and I was curious if there are any advantages/disadvantages to adapting my Lunt PT module, plus FT focuser, to the back of my 130?

Roland insists that his oiled spaced triplet objectives can easily handle direct sunlight, i.e. using a Baader Hershel Wedge, but an ERF rejection filter could be used and I would probably go that route. What I like about a modification to the 130 Gran Turismo is that this scope is meant to be disassembled easily, basically it breaks into 3 parts for easy travel. If I could get an adapter to take the PT etalon plus FT focuser that is on the Lunt and replace the focus module on the GT I will have a quick way of turning my 130 from a very nice wide field deep sky imaging scope into a killer 750mm solar scope and back again. If I understand it correctly, the etalon has to be 200mm back from the focus point, that is why I will want to keep the PT module (etalon) and FT module (focuser) together – this doesn’t change the Lunt specs. The key IMO will be making the adapter and understanding the new reflection points.

Thoughts?

Regards,
Chris Hetlage

[Merlin66]

Chris,
Welcome to the forum.
The "majic" -200mm applies ONLY to the PST etalon mod.
We (using the Lunt 60) are working on the possibility of successful Lunt mods...still under investigation. Check out the scope mods area for details.

[swisswalter]

Hi Chris

welcome to that wonderful TOS free site. Have fun and show us more of your wonderful shots

[chrishet]

Chris,
Welcome to the forum.
The "majic" -200mm applies ONLY to the PST etalon mod.
We (using the Lunt 60) are working on the possibility of successful Lunt mods...still under investigation. Check out the scope mods area for details.

Thx..will do.

I would assume then that each etalon has a sweet spot distance...is that determined by the overall focal length of the system? or is it based on the etalon optics? In otherwords, would this distance change when going from a 80mm f7 to a 130mm f6.3? Thx!

[Merlin66]

Chris,
At this stage I think all we can say is that there's a relationship between the donor objective focal ratio, focal length and the "off-set" position for the etalon assembly.
What the final details are/ will be is still to be confirmed.

[chrishet]

Chris,
At this stage I think all we can say is that there's a relationship between the donor objective focal ratio, focal length and the "off-set" position for the etalon assembly.
What the final details are/ will be is still to be confirmed.

Sounds as if you are a working group...I would very much like to follow that thread, can you post a link or title? Thx Merlin!

[Merlin66]

http://solarchat.natca.net/index.php/en ... unt-etalon

The story so far.......

[colinsk]

I have never read that thread all the way through. Are we missing any measurements now?

[Merlin66]

yes,
Viljo, is still "tuning" the system......

[colinsk]

I can dis-assemble mine if needed. Let me know what measurements we need.

[solarchat]

Chris Hetlage is a friend of mine and member of our local astronomy club. Please welcome him to our forum.

He even came out to a CBSAP event a couple weeks ago and examined the imagining setup as we walked through some imaging.

He is apparantly a quick learner...He He... if only we could get him off that sissy nighttime stuff and full time solar..

[chrishet]

Chris Hetlage is a friend of mine and member of our local astronomy club. Please welcome him to our forum.

He even came out to a CBSAP event a couple weeks ago and examined the imagining setup as we walked through some imaging.

He is apparantly a quick learner...He He... if only we could get him off that sissy nighttime stuff and full time solar..

Thx Stephen, the night has always been a friend of mine...its hard to let her go.

[chrishet]

I can dis-assemble mine if needed. Let me know what measurements we need.

Same here....let me know if I can provide any data.

I have no problem making adapters to fit the PT module to the 130 OTA and focuser but I am concerned about the distances and any potential reflections. As soon as we have this info I can get to work on the adapters. Are the distances we are concerned with here 1) from the OTA objective to the PT module or is it 2) from the PT module to the focus point? or both? Thx

[colinsk]

The two numbers I would use for your PT80 are the focal length of the objective and the distance inside of focus to the first lens in the etalon module. Important for figuring out the field of view would be the entrance aperture to the etalon module and the focal length and ratio of the donor scope.

The inside distance will tell us the focal ratio of the collimation lens in the front of the etalon.

The distance from the etalon to focus will not change so the back half of the scope can still be used. You may want a larger blocking filter at some point if it limits the field of view.

[chrishet]

The two numbers I would use for your PT80 are the focal length of the objective and the distance inside of focus to the first lens in the etalon module. Important for figuring out the field of view would be the entrance aperture to the etalon module and the focal length and ratio of the donor scope.

The inside distance will tell us the focal ratio of the collimation lens in the front of the etalon.

The distance from the etalon to focus will not change so the back half of the scope can still be used. You may want a larger blocking filter at some point if it limits the field of view.

If the focus distance doesn't change from behind the etalon that does make it somewhat simpler. I have the LS80THa/PT/B1800/FT which includes a nice FeatherTouch focuser as well as the larger BF1800 blocking filter. My hope was that I could simply have one adapter and swap out the PT assembly along with the focuser all as one piece (see photo below) and it looks like that may be possible.

Focal Length of the LS80 = 560mm
Focal Ratio of the LS80 = f7
Focal Length of the AP 130GT = 820mm
Focal Ratio of the AP130 = f6.3
Distance from LS80 Objective (back) to front PT assembly = approx 90mm
Distance from AP 130GT Objective (back) to front of PT assembly = ?

Are there any concerns about internal reflections in such a system? Does the tilt of the etalon need to be adjusted to take into consideration of the new focal length?

Thx for all the comments...this is a fascinating discussion, I just wish I knew more optical calculations...



-Chris

[colinsk]

So, some simple math for you. If you have an donor solar telescope objective of a focal length of "x", and you need to collimate it then you need a lens system such that x+ (f/l of the collimating lens) = the separation.

So if we are using a 10" objective with a 80" focal length and we want to pass it through a 3" etalon, we need to collimate the beam to a 3" diameter. That means that the ratio of the focal lengths of the two lenses must be the same as the beam change. 10/3 is our ratio. So since the f/l of the objective is 80" the collimating lens needs to be 80*(10/3) = 266 2/3 inches.

Since we now need 800"+266 2/3"= separation distance our telescope has become really long. So, what we do is use a negative lens to shorten the total length.

Just because the final focal ratio Lunt publishes is "y" only means that the lens on the exit of the etalon there is a lens reforming the beam at that focal length.

The problem with a negative lens as that it needs to be a larger diameter in order to admit the full light cone.

You will need to cut your tube by the focal length of the negative lens at the front of the etalon assembly. Since the lens will only let in the light cone that it was designed for there will be vignetting on the full field view.

Also, Andy tries to use a singlet for the objective and correct the aberrations with the collimation lens. At least that is what they did on my LS60.

Your next step is to remove the objective and focus it on a piece of paper and measure the focal length as precisely as you can as it might not be what Lunt specifies as the final focal length for the whole system. Then you need to measure the focal length of the first lens in the etalon. This measurement will take a whole thread of it's own as it takes some effort.

[colinsk]

I should note that the PST used a f/10 objective and a f/10 reforming lens so it made some simplifications that we can not count on in all telescopes till we measure. It also was a 400mm fl with a -200mm fl collimation lens and a reforming lens that kept the beam at f/10. It was very simple. It did not have to be that simple. We got lucky on that one. It should have had a 20mm etalon and a larger than 20 mm collimation lens to eliminate vignetting but Ken tells us that there is some vignetting in the system.

(I am assuming a point source here and not a .5 deg extended source like the sun for simplicity.)

[chrishet]

Thx Colinsk...Your understanding of these systems is most impressive. I certainly need to learn more and at some point I hope to fully understand all that you have said, lol! I'm going to run this intel by some optician friends I have here in ATL to get their thoughts as well. I hope we can make this work without too much effort. My goals are to have a fairly simple solution and be able to use the 130 as a multi-purpose instrument...hopefully we can get it there, if not I still love my 80!

[colinsk]

I made a simple error. Woops.

So the ratios of 10/3 would be 80 inches to 24 inches. I knew the scope seemed to be too long...

But the idea is the same. Collimation requires two lenses and the sum of their focal lengths.

Changing a collimated beam requires that the ratio of the focal lengths equals the ratio of beam diameters.

Normally I am changing a 3mm laser beam to a 10" diameter beam...

[chrishet]

so are you saying that this modification would require a mod to the existing collimating objective?

[colinsk]

You are replacing an objective of 80mm in dia with one of 130mm. If the 80mm objective is a doublet then you likely can just measure the f/l of the collimation lens and place it the sum of the new objective f/l and the collimation lens f/l apart and get a good result. You will likely lose field of view for much better resolution. You will lose FOV in two ways. One is the likely vignetting of the objective. The other is that you will have larger angles in the etalon and the bandpass will be shifted on the edges of the field.

[chrishet]

To address the adapter placement would something like this rig help?

[chrishet]

at this stage I could at least validate whether there is any reason to move forward....

[Merlin66]

Chris,
Yes, a set up like that would work...consider the option that the etalon module may end up inside the tube of the donor scope!

[chrishet]

Chris,
Yes, a set up like that would work...consider the option that the etalon module may end up inside the tube of the donor scope!

I am keeping my fingers crossed that does not happen...

[Merlin66]

Ignoring all the optical theory for a moment.....
If the original Lunt internal etalon worked OK in the original Lunt scope....
Then for at least initial trials it's just a matter of replacing the original OTA with a suitable (larger!) donor OTA....
The key issues, based on all the prior PST mods:
1. Find the position of a "known" datum point on the etalon module - a fixed shoulder etc. which can subsequently be used to positively position the etalon relative to the donor..
2. Measure the original OTA and determine the focal length and focal ratio.
3. From the "known" datum determine its position relative to the prime focal plane.

With this data you can determine the effective focal ratio and the position of the entrance optics of the etalon assembly in the beam. If the distances are the same (ie "known" datum to prime focus) and the f ratio is similar or greater...then you can position the etalon assembly in the new donor OTA and should get similar results.
Note, the new donor OTA will need an additional ERF up front, and the final position of the focus, relative to the "known" datum will be unchanged.
We have provisional data/ dimensions for a 60mm Lunt set-up. If you can get similar info for a 80mm Lunt then this adds to our knowledge database.
Ken

[swisswalter]

Note, the new donor OTA will need an additional ERF up front, and the final position of the focus, relative to the "known" datum will be unchanged.
We have provisional data/ dimensions for a 60mm Lunt set-up. If you can get similar info for a 80mm Lunt then this adds to our knowledge database.
Ken


Hi Ken

so that should work with a LUNT 100 too. Are the numbers of any interest?

Why do we need an additional ERF? Both the 60 and 100 have an ERF just on the entrace to the etalon

[Merlin66]

Walter,
I'm assuming any of the "internal" Lunt etalons can be adapted to fit a large donor OTA.
I don't have any Lunt equipment for comparison.
If there's a tilted sub-diameter ERF on the etalon assembly this then may still provide sufficient protection. The distance from the rear of the objective to the ERF needs to be assessed. The 99.9% of the incoming energy will be reflected back towards the rear of the object. (See Brian's examples of sub-diameter ERF's)I would normally recommend, as a minimum, that the ERF be >50% the size of the objective.
What is the size of the various ERF's in the Lunt 80 / 100 etc?????

[Merlin66]

Viljo on the Lunt 60 mod is also finding that the final effective focal length is not matching expectations. It seems to be x1.8 longer than the "theory" would give. This is seen from the final size of the solar image. The inference is that the front "barlow" element is infact increasing the EFL....which IMHO it should not be doing.
The more members we can get doing similar mods will allow us to quickly determine the real issues in the assembly.
The story continues......

[Merlin66]

I don't honestly know...even if there was such a lens it would have the same effect in the original Lunt OTA as it would in the Donor OTA...that doesn't seem to be the case. It's almost 1:1 in the original, and 1.8:1 with the donor......