TMB Planet Hunter 80

This is a very, very well made refractor particularly suited for solar observing and imaging. The objective was designed by the late Thomas Back and defined to be an “enhanced achromat”. It uses Ohara S-PMH53 and S-LAH51 glasses for optimum color correction. The scope was initially sold under the brand Burgess Optical and later by the German firm Astro-Professional, with minor differences in the OTA. My one belongs to this latter production and I purchased it from an English amateur because as far as I know the Planet Hunter never circulated in Italy.

5-prol

The scope has a nominal diameter of 80 mm and an effective aperture of 77 mm with 900 mm focal length (f/11.7). The tube is white, well finished, provided of a Taiwanese-made Crayford focuser with 1:10 reduction which is very useful for focusing at high powers and during high resolution imaging.

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Compared side-by-side with a TS 80 mm f/7 refractor (FPL-51 doublet) the Planet Hunter showed better diffraction images and, particularly, much less color; actually the PH can be defined more as a semi-apochromatic refractor than an achromat.

The most striking difference between the two can be seen in the near UV that is in calcium imaging, K and H lines. This is the chromosphere in the calcium line as it appears on the screen during imaging with the TS 80ED:

TS80ED full

quite blurred indeed, while below is about the same region in the Planet Hunter after processing

2018-09-17-1023-RB-CaH-AR

which shows reverse granulation and plage network in good detail, of course within the limits of a 80 mm aperture.

The scope is therefore really well corrected from spherical aberration at short wavelengths. Images in the H alpha are also good but not significantly better than in my 150 mm achromat reduced to 104 mm (f/7.2).

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