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How to Tune a Lunt Pressure-Tuned Solar Telescope

tuning a Lund solar scope

Get the best performance possible from your Lunt single-stacked or double-stacked pressure-tuned solar scope.

So, you have gotten yourself one of those beautiful pressure-tuned solar scopes (like the Lunt LS80T) with all its knobs and you’re wondering…how on earth do I drive it?  Well, this is my way! The first step is to stick it on a mount and point it at the Sun. Most of you should have gotten that far by yourself, but if I didn’t mention it someone might complain. 🙂

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Solar Scope Lunt LS80T
My Lunt LS80Tha

How I do it

Note: This tutorial works for both a single-stacked and double-stacked Lunt telescope!

Step 1: Unscrew the large etalon knobs

You want to back each etalon knob right off until they pop or hiss. Don’t worry if they come right off. They are a bit fiddly to screw back on, but I find winding them anti-clockwise until you find the knob falls into the end of the thread then winding clockwise works the best.

Step 2: Set up the scope in single-stacked mode

Remove the double-stack unit, and don’t forget to replace the spacer between the etalon and the focuser! (Ignore this step for single-stacked telescopes) [Figure 1]

Lund solar scope
Figure 1: Set up the scope in single stacked mode

Step 3: Roughly focus the scope

If it won’t come into focus you can loosen the blocking filter screws and move the filter in and out until focus is achieved. The Sun will most likely look like a featureless disk at this point. [Figure 2]

Lund solar scope
Figure 2: Roughly focus the scope

Step 4: Wind the tuning knob inwards until surface detail appears

If you have a camera attached the histogram will conveniently show a minimum at the best tuning point. FireCapture’s horizontal histogram bar is an excellent tuning aid. [Figure 3]

Lund Solar scope
Figure 3

Step 5: Refit the double-stack unit

Replace the double-stack unit, and remember to remove the spacer. (Skip this step for single-stacked telescopes.)

Step 6: Focus and adjust the double-stack unit

Adjust focus on the double-stack unit until more surface detail appears on the Sun. With a camera in FireCapture, this corresponds to a maximum on the histogram. (Skip this step for single-stacked telescopes.) [Figure 4]

Lunt solar scope
Figure 4

Step 7: Mark your settings

If you put a pen mark on both etalon knobs this will give you a starting point for next time. The tuning won’t be exactly the same, but this gives you a starting point. After a few successful tunings you will get the hang of it and find that the double stack unit doesn’t need to be removed. Simply fine tuning around your initial pen marks will work.

For visual observers, you can stop here!

An additional note for imagers

If you are using a camera, tuning on the histogram for the single-stack etalon will not work if the double stack is installed. I just wind the single stack tuner both ways until features disappear then adjust it to the mid point. Then tune the double stack with the histogram.

If you increase the gain on the camera, you will find a hot spot. It’s something that occurs with a double stacked unit. [Figure 5]

I find fine tuning both etalons slightly until the hot spot is centered will give a better image and be less problematic when doing mosaics. [Figure 6]

Lunt Solar Scope
Figure 5: The hot spot
Lund solar scope
Figure 6: Center the hot spot
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About the author

Paul Stewart

Living partly in his home and the backyard observatory he built himself in Timaru, South Island New Zealand, Paul prefers to image the Sun and other dynamic objects like the ISS, asteroids and comets — not the boring stuff that has been the same for millions of years.

An accomplished and published solar (and even deep-sky) astrophotographer, he was short listed for ROG Astronomy Photographer of the Year in 2015, and published in the Astronomy Photographer of the Year collection book.

Learn more about Paul on his website.

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