A Premium Imaging Alternative: The SharpStar 100mm Quadruplet Astrograph

With its 3.5-inch-long dew shield retracted the 100QII is 19 inches long. Tube rings with a slotted carrying handle and Vixen dovetail bar are standard. Credit: Alan Dyer

Plus:   Superb optics, with flat field for imaging; excellent fittings and finish

Minus: 2-inch eyepieces and diagonal will not reach focus; no filter drawer

Summary: The SharpStar 100QII is a superb f/5.8 astrograph providing a wide, flat field over a full-frame sensor.

Who Is It For? Photographers looking for an astrograph with the specs and price to compete with models from Takahashi, TeleVue, Sky-Watcher, and StellarVue.

The new 100QII refractor from SharpStar is an astrograph – an instrument first and foremost for imaging – and to that end it performs beautifully. The 100QII is natively an f/5.8 system, yielding a focal length of 580mm, using what appears to be a triplet front objective and a built-in single-element rear flattener. The glass types are not specified.

In my tests, the 100QII’s optical design yielded star images near-perfect to the corners of a full-frame (24x36mm) sensor, with only 0.5 stop of light falloff at the corners and no haloes of false color on bright stars.

A single raw unprocessed image of Comet Atlas (C/2019 Y4) shows the low level of corner vignetting over a full-frame sensor (above) and, in the closeup (below), the small amount of off-axis aberration at the very corners of the frame using the 100Q’s built-in field flattener for f/5.8. Credit: Alan Dyer

The mechanics were also excellent. The 3.2-inch focuser is a smooth rack-and-pinion design with a precise 10:1 dual-speed adjustment and a firm lock.

However, unlike the SharpStar 76mm and SharpStar 140mm refractors I also tested on loan from All-StarTelescope.com, the 100QII’s focuser does not rotate independently from the tube. There is, however, a smooth camera angle adjuster at the back of the focuser for rotating and framing the camera. There is no provision for inserting filters.

While the 100QII comes with 2- and 1.25-inch visual backs, many eyepiece/star diagonal combinations did not reach focus. The focuser includes a camera angle adjuster and tilting adjustments. Credit: Alan Dyer

With the focuser racked in all the way, the 100Q’s focal plane fell 97mm behind the rear surface of the 2-inch adapter. The focuser racks out over a travel of 40mm. This combination should allow most DSLRs and other cameras to reach focus. I had no problem when using my 2-inch camera adapter with a length of 40mm from focuser to T-ring.

The 100QII is described as having an optional tapered camera adapter that goes from the M82 thread of the focuser’s draw tube to a male M48 thread suitable for T-rings and many camera adapters. My test scope didn’t come with that accessory, but I would recommend it to ensure that the camera sits at the optimal distance from the 100QII’s internal field flattener while providing a solid attachment less prone to tilting than a slide-in camera adapter.

The main drawback of the 100QII is that, despite coming with a 2-inch visual back and a 1.25-inch adapter, its use as a visual instrument is limited. None of the 2-inch eyepieces I tried would reach focus when using a 2-inch diagonal. The focuser just wouldn’t rack in far enough. However, dual-barrel 2-inch/1.25-inch eyepieces, such as the TeleVue 13mm and 8mm Ethos, and the Baader Morpheus 13mm, were able to reach focus.

Even using a 1.25-inch star diagonal with the 100Q’s included step-down adapter proved a problem. None of my 1.25-inch eyepieces would reach focus when using a premium 1.25-inch mirror diagonal. Only with a smaller, and cheaper, prism diagonal was the light path short enough to allow all 1.25-inch eyepieces to reach focus.

Including 2-inch and 1.25-inch adapters that have a thinner profile with the scope, or shortening the tube by ½- to 1-inch while increasing the focuser’s length of travel, would alleviate the problem.

The SharpStar 100QII is a superb 4-inch astrograph that will compete with the best from the big names, at an attractive price. But don’t buy it for visual observing.

MSRP: $2500

Website: http://www.sharpstar-optics.com

About Alan Dyer

Alan Dyer is an astrophotographer and astronomy author based in Alberta, Canada. His website at www.amazingsky.com has galleries of his images, plus links to his product review blog posts, video tutorials, and ebooks on astrophotography.

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