Lunt modified 150 mm f/5 achromat

Inspired by a discussion on Solarchat! forum and not fully satisfied of the results I got up to now from my Quark chromosphere, I decided to use of the PT module of my Lunt 50THa scope for making my own solar Frankenscope.

The receiving scope is my 150 mm f/5 achromat equipped with a sturdy Tecnosky V-Power focuser that allows for 190mm backfocus, quite a lot for most purposes. I calculated that transplanting the Lunt into scope required about 150 mm of focus travel so it seemed feasible without modifying the OTA, that is by simply inserting the Lunt module in the 2″ eyepiece holder.

The PT red module of the Lunt can be easily separated from the white tube. These pieces are joined through a M68 standard thread, male on the module, female on the tube. I purchased from TS the following adapters:

– a Baader M68 extension tube 20mm long
– a M68 female/M48 male ring adapter

The M68 extension is necessary because the black tube protuding from the red module (on the left in the below image) is larger than 2 inches by few mm and doesn’t directly insert into the eyepiece holder. However for larger focusers (e.g. 2.5″) the extension can be omitted and less backfocus is therefore required provided the focuser has standard M68 connection.

For inserting the Lunt module in the eyepiece holder of the OTA I made use of a M48 (2 inch diameter) extension usually sold as the 28 mm finetuning ring for Baader Hyperion eyepieces. All pieces were assembled this way:


Now one has only to put the Lunt module into the focuser


In the modified scope the image of the Sun should focus at the same distance from the collimating lens as in the original scope, taking as a point of reference, for example, the rear end of the red module. This requires lot of backfocus and it also means that the focuser of the receiving scope can’t be used and focusing shall be done only at the very end of the system. As the original focuser of the 50THa is almost unusable it is necessary to install a replacement, in my case a FT Crayford focuser.

In front of the 150 mm achromat objective I placed a 104 mm free aperture Baader D-ERF. I can’t tell if it is really necessary to protect the PT components, however it at least prevents internal tube turbulence.

The Lunt BF diagonal of the 50THa generates some vignetting as the diameter of the blocking filter is small for a 750 mm focal length scope. For hires imaging of small chromospheric details this is no problem, but larger views become impossible. A better solution would be purchasing a BF1200 diagonal or an equivalent one, as I eventually did.

The results I have obtained so far are quite good and certainly better than those I had from my Quark alone. For example this is a chromosphere detail:

chromo luntmod 20180905

By replacing the BF diagonal with my Quark, that is by stacking two etalons, one can gain even more contrast on surface details (curiously the same happens if the original Quark BF is replaced by the Lunt diagonal…).

Compared to the more common PST mods, the one described above is fully reversible and the original LS50THa can be reassembled in one minute.

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