Lunt LS80THA solar scope.

The Lunt LS80THA on top of the C11 EdgeHD.
Figure 1: The Lunt LS80THA on top of the C11 EdgeHD.
Piggy-back on the Celestron C11 XLT EdgeHD rides a Lunt LS80THA pressure tuned 80mm f/7 single stack etalon solar telescope. The Lunt is used both for visual observations using the standard Lunt zoom-eyepiece and for solar imaging (for examples see the Sun page of this website).
In basic principle the etalon consists of two parallel glass plates with a small air gap in between. By changing this air gap one can influence which wavelengths are being cancelled out or magnified. For this there are two methods available: tilting and pressure tuning. When tilting the gap changes width in direction the light passes through it. Using a pressure tuner, like on my Lunt LS80THA, the gap is changed by increasing the pressure in the gap between the glass plates. As a result the gap widens, affecting other frequencies.
Over time this pressure leaks away, but by turning the pressuriser until it detaches the pressure can be reset and the etalon pressurised again.

The Lunt LS80THA ready for imaging with a 4x barlow and monochrome camera.
Figure 2: The Lunt LS80THA ready for imaging with a 4x barlow and monochrome camera.
At the observer end of the telescope a star diagonal contains a B1200 blocking filter. When imaging I use a TeleVue 2″ 4x PowerMate Barlow to image details of the sun. To be able to focus, the Barlow must come after the blocking filter. Lunt supplies straight-through housing blocking filters that allow to have the Barlow and camera in line with the telescope, but it is expected that most users will use a diagonal blocking filter just like I have. Adding a Barlow and camera to the diagonal looks a bit funny, but works just fine.
In order to connect a Barlow to the diagonal, Lunt supplies a special adapter ring that can be threaded onto the blocking filter, a much more affordable solution than a second straight-through blocking filter. The adapter is attached by unscrewing the 1.25″ eyepiece holder is first, after which the 2" Barlow adapter ring can be mounted. Once it is in place, the Barlow is inserted and, using a 2″ to 1.25″ adapter, the camera (I use a ZWO ASI174MM for the purpose) is inserted into it.

The 7" monitor in use with the Lunt LS80THA.
Figure 3: The 7" monitor in use with the Lunt LS80THA.
As the Lunt is regularly used for visual observing I did not (yet) add a motor focuser to it. That, however, in combination with its high position above the floor of about 2 metres, made it extremely difficult to properly focus the scope for imaging. From a befriended amateur-astronomer I learned that the NUC has an USB-C port that can be used as a second display port. He lent me an 11" monitor so that I could see if having a second monitor close to the Lunt would help focusing it, which indeed appeared to be the case. I then acquired a 7" full-HD Field-monitor, which was fitted piggy-back on the Esprit 150ED and now allows me to operate the focuser while seeing the result directly next to it.

Imaging set-up

The QHYCCD QHY163 imaging-train of the Lunt LS80THA.
Figure 4: The QHYCCD QHY163 imaging-train of the Lunt LS80THA.
Although most of the images up to 2022 have been taken with a ZWO ASI174MM (with or without a TeleVue 4x PowerMate), the acquisition of a QHYCCD QHY163 mono camera changed the set-up entirely in May 2022.
As the Lunt is used for visual observation as well, it was acquired with a B1200 blocking diagonal. The imaging train comes after this diagonal. The T2 to 1.25" eyepiece-holder is swapped for a T2 to 2" adapter in which a TeleVue 2x PowerMate is inserted. Into that goes a 2" to 1.25" adapter that holds a ZWO ADC (see below why). Using a T2 to 1.25" nose-piece the QHYCCD QHY163 mono is inserted into the ADC.
The ADC has a twofold function: it removes the Newton-rings that otherwise would be visible (the levers should be at at least 90 degrees) and it allows to move the sweet-spot to the centre of of the camera (for this the set-up should allow to rotate the ADC as a whole, the levers may need to be moved to the maximum setting of 180 degrees as in this example). For more info see this post on Stargazers Lounge.

If you have any questions and/or remarks please let me know.

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