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.


MoonLite focus motor

The original Lunt LS80THA focuser in parts and the aluminium piece that will become a motor bracket.
Figure 5: The original Lunt LS80THA focuser in parts and the aluminium piece that will become a motor bracket.
In August 2022 I decided it had been long enough focusing the Lunt by hand while standing next to it. Reason for that is that the PC-monitor is situated about 80 centimetres above the observatory floor, while the Lunt at is highest pint reaches well over 2 metres. Trying to watch the monitor while turning the focus-knob became a kind of circus act. Initially I tried to solve this using a second 8" monitor that I fastened piggy-back to the SkyWatcher Esprit 150ED (see figure 3), but since that place is taken by an Esprit 80ED, I had to hold it in my hand while focusing.
When I built the GTT60, I bought a MoonLite focuser with a simple non-stepper motor. As the depth of field of that scope is several centimetres, there was no need for this motor on it. So I decided it could do without and wanted it to sit on the Lunt focuser.
The only issue was: how to mount it? The Lunt has no mounting holes to hold this type of motor on its standard Crayford focuser. Perhaps the only solution would have been a Starizona Feathertouch motor, but that would make it impossible to use the focuser manually during visual work. The MoonLite focuser has the advantage that it can be easily disengaged, without the need to remove the motor, and that it is mounted to the direct-drive knob so that the 1:10 fine adjustment knob remains available for manual focusing.
So I had to make some bracket of my own and mount that to the focuser housing. First thing was to disassemble the focuser to see what options I would have.
A few measurements learned that a 6 millimetres thick strip of aluminium would suffice to get the focus motor at the right elevation next to the focuser-axis.


The MoonLite motor mounted to the Lunt LS80THA focuser.
Figure 6: The MoonLite motor mounted to the Lunt LS80THA focuser.
The focuser housing had just enough space at the rear to attach a bracket to. The only downside was that it required me to drill three holes in the focuser and tap M3 thread in it. As the focus tube has a wide collar at the end, the bracket needed to have an opening sufficiently large to accommodate this collar as otherwise it would reduce the focus-range by the thickness of the aluminium bracket of 6 millimetres.
In order to get the bracket to the right dimensions and to drill the holes in exactly the right location I had to disassemble and reassemble the whole focuser some six times, so by the end of that job I was well trained in that.
Another advantage of creating this bracket was that, while the focuser was on my bench in parts anyway, I could adjust the grip of the 1:10 drive. The planetary gear of these Lunt focusers does not grip the central axis firm enough to hold the weight of the TeleVue 2x PowerMate, ZWO ADC and QHYCCD QHY163 mono camera.


The Lunt LS80THA with MoonLite focus-motor in situ.
Figure 7: The Lunt LS80THA with MoonLite focus-motor in situ.
The construction of this planetary gear is similar to those used on my Esprits, with the difference that the one used on the Lunt does not require special tools to adjust (see the SkyWatcher Esprit 150ED page). Instead it has a central nut that first needs to be loosened slightly to break the threadlocker sealant, after which it can be fastened until enough friction is created (care should be taken though not to over-tighten it as that may cause damage to the central axis).
Re-attaching the focuser to the Lunt was a breeze and the first tests have shown that focusing is much easier now, I can now remain seated and watch the large observatory-monitor up close.


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


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