Tecnosky 115 mm f/7 apo triplet

Another fine solar scope that I recently purchased in particular for Calcium imaging. The objective is a N-P-N triplet having the low dispersion lens made with FPL-51 Ohara glass in a thermally compensated cell. The white OTA has a beautiful opaque surface finish, is 70 cm long and weight 6 Kg without accessories. The R&P focuser has a 2.5″ inner diameter, the motion is smooth and regular all the way. Backfocus is 15 cm from the 2″ eyepiece holder end. The internal baffling is very efficient, and in general all parts of this refractor are really well made.

front
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The impressive row of baffles inside the focuser tube
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The mobile tube is driven by a rack and pinion mechanism that can be fully adjusted by the user to the desired tension. A micro 1:10 reduction is provided.

Removing the 2 inch eyepiece holder gives access to a M68 female thread. This is very useful because allows a direct connection – i.e. without additional adapters, of the Lunt LS50 PT module for H alpha imaging. With the focuser tube fully retracted the primary image of the Sun forms at the right distance from the module as in the donor scope and the original f/ratio is also preserved. In this configuration a suitable blocking filter shall be added at the camera side; based on its optical length a Barlow lens may be needed for extracting focus enough to reach the camera sensor. A full aperture front ERF is also recommended. Below is an example image I obtained with this setup:

The scope is enough corrected around @393 nm to allow captures of the chromosphere and of limb prominences at full aperture without the need to stop down the objective, although reducing the entrance pupil to 100 mm produces a further improvement in definition. For the below images I made use of straight-through Lunt Ca-K module and full aperture:

The scope is excellent for night time observing, of course, being designed for astrophotography as well as for high resolution. It shows a very good star test with clean, round and white diffraction rings, and no false colours around the Moon’s limb or the Sun. In side-by-side comparison with a 120ED doublet I eventually preferred the Tecnosky triplet in almost every aspect.

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