Focal reducer to match Lunt 60MT etalon to f/10 optics.

[Rusted]

The Lunt 60MT expects to see an f/7 light cone entering the etalon's optics.
So those of us with f/10 optics must apply a focal reducer to match the Lunt's needs.

The Lunt 60MT etalon's nosepiece has a coarser thread than the standard 2" filter size.
So the T-S focal reducer will not screw directly into it. Because the T-S reducer has a standard 2" filter thread.

I can easily drop the reducer into my modified 35mm long x 2" extension. Which sits on the etalon's nosepiece.
Which brings the reducer to the minimum distance from the Lunt's optics.
The reducer is only pushed away by the nosepiece by the length of the reducer's own thread. About 5mm.

The problem is that having done this I can't find a focus with the etalon attached.
I have the scope tracking the sun as I add and remove extensions and rack the main focuser in and out.
With the reducer in place there is no sharp, bright sun focused anywhere behind the etalon.
Without the reducer there is a sharp, solar image projected onto a card.

Does the reducer need to be much closer to the etalon's front correction lens?
I can only manage about 30mm minimum spacing.
Does Marty's 264mm inside focus still apply with the reducer in place?

[Rusted]

I have now removed the reducer element from its housing and inserted it into the front of the 60MT etalon module.
It needed a narrow O-ring to maintain its position [without aperture reduction] once the nosepiece was screwed on. Still no focused image.
Removed the main focuser and held the etalon up to the open main tube. Still no focused image behind the etalon.
Refitted the glass for easy handling and the focus is very short behind the reducer lens. Of course it is. It's working in a converging beam.
The reducer has a focal length of about 10cm or 4". Which means that the converging light cone barely enters the etalon before it focuses.

[pupak]

The LS60 collimating lens has a focal length of approximately 260mm, so for a given aperture it is necessary to find a suitable position in relation to the focal point. It will be roughly 260-270 mm from the focal point. No further correction is needed. It is ideal when the optimal distance is determined by experiment. I use the LS80 etalon at 150/F6 and 150/F8 and neither "calculated" position was good. I had to find her by trial and error.
By inserting a reducer in front of the etalon, the total focal point changes. You would have to put a negative lens of the same value behind a etalon to focus.

[Rusted]

The LS60 collimating lens has a focal length of approximately 260mm, so for a given aperture it is necessary to find a suitable position in relation to the focal point. It will be roughly 260-270 mm from the focal point. No further correction is needed. It is ideal when the optimal distance is determined by experiment. I use the LS80 etalon at 150/F6 and 150/F8 and neither "calculated" position was good. I had to find her by trial and error.

As simple as that? I just move the 60MT etalon in and out until it provides the best tuning?

Thanks.

[pupak]

I haven't found a better method yet.

[marktownley]

I just move the 60MT etalon in and out until it provides the best tuning?

Definitely not the way to do it.

It needs to be at fixed point and the camera focused relative to that rearward of the 60MT.

[Rusted]

Mark,

It doesn't seem to be possible, I spent hours today playing with the focal reducer.
It is too powerful to allow the f/10 converging beam to adopt an f/7 beam.
I was even able to insert the bare doublet into the housing of the Lunt 60MT etalon.
It couldn't get any closer than that. Still no focused solar disk external to the MT etalon housing.

Chris

[marktownley]

Mark,

It doesn't seem to be possible, I spent hours today playing with the focal reducer.
It is too powerful to allow the f/10 converging beam to adopt an f/7 beam.
I was even able to insert the bare doublet into the housing of the Lunt 60MT etalon.
It couldn't get any closer than that. Still no focused solar disk external to the MT etalon housing.

Chris

The module (with reducer) will need to be a good 4" inward of where it's position was previously.

[Rusted]

The module (with reducer) will need to be a good 4" inward of where it's position was previously.

Now that makes sense. I was keeping the etalon module at the previous distance from focus without the focal reducer.

Thanks. Something new to try if I see any sunshine. The forecast is not encouraging.

I wish I could remember which way the reducer doublet was original oriented in its housing.
I dropped it out of its housing into a Kleenex for safety and cleanliness and completely forgot to check.

[pupak]

The reducer alone won't solve anything. If it is placed further, the field will be reduced and the etalon will not be used in the entire diameter. It is enough to experimentally find the optimal position of the standard and no reducer is needed. The position of the etalon is then fixed.

[Rusted]

The diameter of the converging light cone can easily be measured where the focal reducer will be positioned.
After inserting the glass element into the front of the etalon module I used a rubber O-ring to secure against any looseness.
This ensured I did not reduce the clear aperture of the focal reducer.
I have been moving the MT60 in and out since it arrived but had imagined this was the wrong solution.
I will continue to try both options {with and without the FR] until I obtain the best results.
Thank you both for your feedback.