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ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

TS-1 approximate focal length confirmed (1 Viewer)


Well-known member
Got a TS-1 yesterday. I managed to attach a William Optics SWAN 15 mm 72° by removing the 1,25" empty barrel, and, with considerable caution to not tighten it, cross-thread the eyepiece into the TS-1 body. The same eyepiece displayed a disturbing curvature of field in the Svbony 406P ED 80 mm scope.
Mounted to the Kowa, there is field curvature but it is only prominent in the outer 20 percent or so of the field.

Then I compared the magnification with the Nikon ED50A with a 27x Wide MC, an eyepiece that also has a 72° AFOV, and the ED82 30x Wide DS
(72°, too).
Both Nikons are sharp to the very edge with their EPs.

  • Magnification compared to the ED82A 30x Wide DS very similar, but perhaps a fraction smaller in the Kowa but visibly more than the small Nikon, suggesting 29x mag.
  • The actual FOV was a tiny bit wider with the Kowa/WO combo than the big Nikon's, suggesting a slightly lesser magnification, if both eyepiece's AFOV are up to specs. 29x?
  • I estimate the exit pupil size to 2.1 millimetres which would suggest a magnification just below 29x.
With all these numbers at hand, the focal length of the TS-1 should be around 430 mm and aperture f/7.1, so a relatively slow scope. Indirectly, this is also confirmed by the very decent function of the WO eyepiece. In the very slow Celestron C90, I can't recall seeing any curvature of field with this eyepiece, and the very fast Svbony which shares the focal length with the Kowa but with an aperture <5.4 displays quite bad curvature with the WO.

There seems to be very difficult to find other records of the TS-7's focal length, and the record somewhere stating 700 mm is obviously completely off the mark.
It is in the 430 mm range, say 420-440 and I think that would cover measurement errors.

By the way, the image is quite decent in daylight. I found myself searching for the perfect focus until I understood it's my own astigmatism. With me using a contact lens, the exquisite sharpness should be evident. I have boosted it both 2.5x and 6x and it's not too shabby at all. There are horrible spikes from streetlamps. The weather is gloomy so colours and contrast are not possible to assess. It is a perfectly usable scope. Unfortunately I can't have it permanently on my balcony due to humidity and temperature variations.
I paid €92 with a 25x EP (though it's more likely a 20x), and it came with the proprietary caps, that are the screw-in type and made with aluminium, with extraordinary quality. The scope was accompanied by the typical Velbon tripod of its heyday.
A truly horrible tripod.

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