Thoughts on modding my AP130GT with LS80 PT Etalon

[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......