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TS-Optics 6" f/5.9 2.5" R&P Focuser (Rich Field)


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The following is derived from notes taken during "first light" over three days towards the end of May 2020. After the initial "tests" the telescope was configured for AP and has been ever since. 

The telescope arrived well packed and double boxed from Munich within 4 days of placing the order via DPD, my initial consideration that this thing is big, even when compared to my 127mm f9.4. The manufacturer’s specification maintains that the all up mass is 8.5kg, on my digital bathroom scales the actual mass is approaching 9.2kg.

 The supplied vixen format extruded aluminium dovetail was regarded as somewhat inadequate for the mass of the telescope as a consequence it was replaced with a Losmandy type that I had to hand.

The overall indication is that it is a solid well made telescope, looking at the front element in daylight exhibited virtually no reflection whatsoever, a normal indication of good quality high transmission coatings. Using a torch (flashlight) to examine the internals revealed extensive baffling along the first third of the tube mimicking the perceived steep light cone.  The very front of the focuser draw tube however was only partially blackened, striking me as somewhat incongruent considering the obvious quality and attention to detail exhibited elsewhere in the build. However in retrospect I consider this is an indication of the manufacturing process for this particular element, where I am assuming, rightly or wrongly the draw tube was stood on its end during the anodising process.

The focuser apart from the slight criticism mentioned above is a very solid 2.5” hybrid affair of the Rack and Pinion/ Crayford variety and is smooth in operation with little perceived backlash especially so when utilising the 1:11 built in micro drive. The focus tube terminates in a 68mm thread on which is supplied a 2” rotary lock with a locking thumb screw, it is also comes replete with a standard reducing bush with a brass compression ring. The back focus with the supplied rotary lock is quoted by the manufacturer as being 149mm.

Anticipating the tubes arrival I had borrowed a laser collimator from a former colleague and fellow astronomer as it was my intention to check the accuracy of the focusers collimation with respect to the objective and the accuracy of the focusers rotation about the tube. I set the tube up with a homemade baking parchment “target” at the objective end, replete with rubber band fixings, the coherent light fell on the centre of the target. Rotating the focuser about the tube indicated a slight deviation from centre (approximately 0.5mm RMS) but I considered that it was well with the limitation of my somewhat crude experiment.

The collar on which the focuser rotates has a series of capturing bronze set screws equidistant about its periphery, which I consider could be used to adjust the rotational accuracy if required.  The thumb screw that locks the rotating collar can be slightly awkward to tighten dependent upon the particular aspect of rotation with relation to the focus knobs.

The retractable dew shield is lined internally and made of metal; it is fixed in place using a single thumbscrew. I consider the weight of the dew shield is considerable as it droops slightly when fully extended; if I was designing similar I would have opted for three (3) thumbscrews equidistant about its circumference.

The telescope is supplied with a plastic plug by way of dust protection for the focuser and a metal end cap for the objective. The metal end cap is “fitted” utilising with what appears to be stick on felt strips. These felt strips molted fine dust on to the objective during initial use, however that was removed quite easily with a blower brush.

Although the tube was bought primarily for NB utilising an OSC via duo and quad band filters it is also my intention to take the tube and mount to a local dark sky sight for visual. Obviously being an achromatic there were some concerns over possible chromatic aberration on bright objects. Although this class of instrument is specifically orientated around low power wide field deep sky work when used in its pure optical configuration.

 Various reports in several on-line fora have indicated somewhat juxtaposed considerations as to the colour correction of this telescope and its various incarnations over the years. These opinions have ranged from no perceptible CA to diabolical. Obviously such observations and subsequent conclusions are to a certain extent subjective and are based upon the experience of the observer; as are mine.

The telescope arrived at the end of May 2020 subsequently the opportunity for true dark skies were somewhat limited at 53N. However my initial impression of the telescope was somewhat disappointing. Using  a 40mm Celestron Plossl via a star diagonal ( Revelation Dielectric Quartz) on the area around Deneb the scope demonstrated significant field curvature at both intra and extra focal positions and obvious colour separation on the elongated stars away from the centre of field.

 Having previously checked by laser the squareness of the objective to the focuser any possible objective “tilt” was discounted as a possible cause.  Also the symmetrical nature of the aberration(s) about the centre further indicated no objective tilt. Exchanging the Celestron 40mm Plossl for a Televue 40mm FL of the same design resulted in no perceivable field curvature to the aperture stop. No colour separation or elongated stars. M81/82 were obvious even with a lack of true astronomical darkness. (Bortle 5)

Although this telescope could not be construed as being perfectly suited for planetary observation the tube was swung to Jupiter, although not ideally placed it was considered that Jupiter would provide an indication of any chromatic aberration that will be demonstratable when viewing bright high contrast objects. After centring the magnification was ramped using a Meade 4.7mm UWA (190 X). The scope demonstrated some colour separation that was marginally more pronounced along the vertical (Y axis), although not unnoticeable in “X”. Switching to a OVL “Nirvana” 4mm (224 X) effectively eliminated any colour spread in “X” and reduced significantly its overall perception.

The Moon exhibited a very barely perceptible colour fringe on the illuminated limb during “wide field” observation. Increasing magnification whilst sweeping the terminator exhibited some tenuous blue/yellow colour fringing  dependent upon eye alignment whilst using a 4mm and 7mm OVL Nirvana, although present the colour aberrations did not distract from the view.

In conclusion; I consider that the TS 152mm f5.9 Achromatic is a capable telescope for visual use, the caveat being that it is operated within its design “rich field” parameters. Due to the comparatively fast optical configuration and achromatic design it is very unforgiving of narrow field (possibly inferior) eyepieces. Its mass requires a substantial mount, of an EQ6 class or greater for anything other than casual observation.  It is also considered, that for some the mass and awkwardness of the telescope may be too much if not installed on a permanent setup; although the supplied handle does assist greatly when mounting/de-mounting the tube. But overall, a fairly good telescope with good contrast views that I am looking forward to using at a dark sky site.

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