Desirable though apochromatic refractors may be for astrophotographers, they can be very expensive, especially once you pass 3.5-inch apertures. However, Teleskop Service
has released a new quadruplet apo with an aperture of 4 inches, claiming a very flat field of view at an appealing price.
This instrument is supplied with tube rings in a well-constructed aluminium flight case, but the choice of dovetail bar is up to the owner. We used a Losmandy-style dovetail bar for this review. Similarly, although an industry standard finderscope shoe is included, you will need to purchase a finderscope separately.
The telescope tube is unbaffled but flocked in black to reduce unwanted reflections and increase contrast. Likewise the dual-speed rack and pinion focuser’s tube is internally ridged with a deep matt black finish to reduce reflections. As well as the now standard 2-inch eyepiece holder with 1.25-inch eyepiece adaptor (both of which make use of anti-marring brass collars) the focus tube has a removable end section.
Unscrewing the 2-inch eyepiece holder reveals a 68mm female thread, which you could use to screw in an adaptor with a male T thread and thus attach imaging equipment. The advantage of a screw-fit connection for your astrophotography gear is that there is far less risk of tilt or flexure in comparison with a push-fit connection, leading to more uniform star shapes across the field of view.
Twin dual-element optics
The optical elements of the Imaging Star 100Q are arranged in two groups of fully multicoated lenses, comprising a doublet primary lens with an ED (extra-low dispersion) glass element at the front and a two-element correcting lens just in the front of the focuser. The multicoating was blemish-free and evenly applied.
The optics produce a focal length of 580mm (f/5.8) resulting in a generous field of view 2º and 11 minutes wide by 1º and 27 minutes deep when used with a camera with a typical APS C sized sensor DSLR cameras. To put this into some perspective, this field of view would be sufficient to capture the Witch’s Broom Nebula and Pickering’s Triangle, or the Rosette Nebula, in a single frame.
Although this telescope is designed primarily for imaging use, we were curious to see how well it performed as an observing instrument, so we added a 2-inch dielectric star diagonal with 17mm and 8mm eyepieces. A star test showed an excellent extra-focus Airy disc, but a rather muddy intra-focus disc indicating some over-correction, although this did not seem to spoil our observations, with star shapes remaining excellent to over 90 per cent of the field of view. We enjoyed excellent views of the Pleiades open cluster in Taurus, open cluster M35 in Gemini and the Orion Nebula, but the highlight was the beautiful double star Almach with comet C/2014 Q2 Lovejoy in the same field of view. They were separated by just 49.5 arcminutes through our 17mm eyepiece.
Moving on to imaging, we attached our APS C sized one shot colour CCD camera to the telescope, along with a 60mm finder-guider to perform our autoguiding. We captured a series of images of the Triangulum Galaxy, the Orion Nebula and the Rosette Nebula despite the rather murky sky conditions during the review period, and these confirmed that the Imaging Star 100Q does indeed produce a very flat field of view right into the corners of the frame when using a APS C size sensor, although very close scrutiny of the stars showed the tiniest hint of pinched optics. The rack and pinion focuser supported our imaging camera securely with no slippage and it was easy to achieve focus using the 10:1 reduction microdrive and a Bahtinov mask.
The TS Imaging Star 100Q should certainly be on the shortlist of any astrophotographer looking for a refractor that provides a very flat field of view straight out of the box.
Dispensing with the triple trouble
The TS Imaging Star 100Q has, as its name implies, been specifically designed with astro imaging in mind. All astrophotographers want their telescopes to have well colour-corrected optics and a flat field of view. Apochromatic refractors commonly have a triplet lens designed to produce a colour-fringe free view by ensuring that red, green and blue light are all brought to focus at the same distance from the lens.
However, triplet refractors still suffer from field curvature showing elongated stars in the corners of the image frame. External field flatteners can correct this distortion but spacing of the camera sensor becomes very critical for these to work correctly.
The Imaging Star 100Q resolves these issues by implementing a quadruplet type design in which four lenses (one of which is made from FPL53 Ohara extra-low dispersion glass) are used to correct both chromatic aberration and field curvature. In this quadruplet design, there are two elements at the front of the telescope and two at the rear. Spacing issues are obviated completely because the correct spacing of 150mm is achieved automatically when the image is in focus.
Rack and Pinion Focuser - A substantial focuser is a prerequisite for an imaging telescope and the 3-inch rack and pinion focuser is certainly that. The focuser is very smooth in operation with 46mm of travel and a 10:1 reduction drive, yet it was capable of supporting the weight of our imaging equipment with no slippage.
Retractable Dew Shield - The telescope has a retractable dew shield that also impedes stray light. The interior of the shield is finished in a black flock material to reduce reflections and once extended by 100mm it can be locked in position by a single thumbscrew. The overall fit could have been a little tighter.
CNC Tube Rings - The supplied pair of CNC-machined tube rings have secure hinges and easy to lock fastening knobs. Threaded holes on both top and bottom surfaces allow for the attachment of a dovetail bar and other accessories.
Finderscope Shoe - A substantial finderscope shoe is attached to the focuser. This shoe held both our Synta-manufactured finderscope and our Altair Astro finder-guider very firmly, despite having just a single thumbscrew for tightening. A wide range of other finder devices, including some red-dot variants, will also fit this standard shoe.
Aluminium Flight Case - The telescope is supplied in an aluminium flight case with a tightly sculptured interior that includes cut-outs for the two tube rings. This holds the telescope snugly, giving a high degree of protection. Externally there are two lockable clasps and a comfortable carrying handle.