Inexpensive quick Newtonian astrographs are sport changers for astrophotographers. Most of those devices have a focal ratio within the neighborhood of f/4, permitting them to seize mild from deep-sky objects greater than six occasions quicker than the f/10 Schmidt-Cassegrain telescopes that dominate the market.

However with that quicker velocity comes an enormous drawback: a curved focal airplane and extreme coma, making pictures taken with such devices sharp within the heart and blurry across the edges. To fight this, producers provide varied Newtonian coma correctors, designed to alleviate (to various levels) this optical defect and permit customers to fill the total body of their CCD or CMOS cameras with sharp stars. Nevertheless, the effectiveness of this correction varies amongst designs, with some performing significantly better than others.

Right here, I consider a brand new sort of coma corrector: the Starizona Nexus 0.75x Newtonian Focal Reducer/Coma Corrector. As implied by the title, moderately than slowing the telescope — as typical coma correctors do — the Nexus addresses coma whereas additionally decreasing the focal ratio for a good quicker scope.

Correcting coma

With out correction, quick parabolic mirrors produce pictures which are sharp solely within the heart and degrade quickly as you progress off-axis. This isn’t on account of a nasty or faulty mirror, however is inherent within the Newtonian optical design. The quicker the mirror, the extra rapidly the picture degrades. For an f/4 or f/5 mirror, this equates to sharp stars solely within the central half of the sphere for a full-frame sensor, or about three-quarters of the body for a typical APS-C-sized chip.

Inserting a fancy lens system (a coma corrector) simply earlier than the point of interest — sometimes 2.2 inches (55 millimeters) in entrance of the sensor — can flatten out and switch these bird-shaped stars within the corners into sharp, spherical factors once more.

Over the previous decade, two designs have dominated the marketplace for this objective. The primary was Tele Vue Optics’ Paracorr, adopted extra lately by the Baader Planetarium Multi-Function Coma Corrector (MPCC). In my expertise with a number of f/3.6 to f/4 Newtonian astrographs, every of those works moderately properly; nevertheless, there are drawbacks to every design. Whereas these correctors create largely sharp fields, each nonetheless present a tiny quantity of residual coma within the corners of frames. Moreover, each the Paracorr and the primary model of the Baader MPCC amplify the picture within the strategy of flattening it, growing the focal size and f-number, thereby slowing the scope.

The newer Baader MPCC Mark II and Mark III have 1x magnification, in order to not change the sphere of view or focal ratio of your telescope. This can be a large benefit, as a quicker velocity is healthier for dim, diffuse deep-sky nebulae and galaxies. And though the Paracorr’s small 1.2x magnification is insignificant in comparison with different limiting elements, akin to skyglow or the variable high quality of the evening sky, the thought of placing within the optical path something that slows down the telescope’s velocity could be a bit unnerving.

However now, now we have a 3rd selection for coma correction, one which was unavailable just a few years in the past: the Nexus from Starizona. Not solely does this corrector produce pinpoint stars all the best way to the corners with an f/4 telescope, its 0.75x discount really will increase your subject by 25 p.c and makes your scope almost two occasions quicker, turning it into an f/3! The velocity improve and bigger subject alone had been sufficient to persuade me that this can be a superior product. And after you’ve seen the unimaginable stars this corrector produces, you, too, shall be satisfied.

I spent the higher a part of a 12 months evaluating this product with a wide range of devices underneath many alternative sky situations, from suburban Bortle 6 to world-class Bortle 1 darkish skies. The product carried out past all expectations and is now a everlasting resident in my astroimaging toolbox.

Discipline testing

The standard of the machining and match and end of the Nexus are spectacular. Even the black field the Nexus is available in is lined with form-fitted foam and gives the look of a high-quality product. Starizona offers a metallic screw-on cap for the 48mm threaded digicam finish of the Nexus and a black ABS plastic cowl for the lens finish.

The barrel of the Nexus is kind of lengthy in comparison with competing merchandise. This turns into obvious when inserting the digicam and corrector meeting into the drawtube of the telescope. What I discovered shocking is that even with the corrector put in, the point of interest on the drawtube is actually the identical. That is a formidable feat of optical engineering.

I examined the Nexus coma corrector with two totally different Newtonian astrographs: a GSO 8-inch f/3.9 and a 10-inch f/3.9. Each are completely mounted in roll-off roof observatories underneath reasonably darkish to pitch-black sky situations. I additionally examined the Nexus with two totally different cameras: a ZWO ASI071MC Professional APS-C shade CMOS (26mm diagonal) and an Atik 16200 monochrome digicam with a bigger APS-H sensor (35mm diagonal). I used customary off-the-shelf spacers from ZWO to get the required 55mm backspacing with the ASI071MC Professional, whereas the Atik 16200 and built-in filter wheel required a customized spacer from www.preciseparts.com. The ensuing focal ratio of the Nexus with these two devices turned a blistering f/2.9.

With each cameras, the primary check pictures that got here up on the pc display screen had been considerably of a shock. Stars within the corners had been each bit as sharp as within the heart of the body. I had by no means seen such perfection in my pictures. Additional shocking me, the Atik monochrome digicam was capable of seize sharp stars proper to the corners, even with the large-format chip. This implies Starizona’s declare of 28mm protection for correction is unquestionably on the conservative finish.

My closing check was to see whether or not all colours within the filter wheel within the monochrome digicam would come to the identical focus. Whereas utilizing each the Paracorr and MPCC, it had turn into behavior to refocus every time switching RGB filters, primarily as a result of the blue filter focus was so totally different from the purple and inexperienced. This isn’t the case with the Nexus! All three filters had been so near focus that I didn’t contact the main focus knob in any respect as soon as the shot was targeted for white mild.

The velocity improve was instantly noticeable on the primary pictures as properly. Even a five-minute subframe confirmed unimaginable element and faint nebulosity by no means seen with competing correctors in such a brief publicity. The bigger subject of view was similar to what I get with my Stellarvue SV80 refractor with its coma corrector, but with a lot sharper stars, larger general decision, and completely no chromatic aberrations. There was some noticeable mild falloff within the excessive corners; nevertheless, utilizing a flat from an LED panel took care of that with none issues. Taking the conventional one- to two-hour units resulted in a few of the most breathtaking deep-sky pictures I had ever produced. My thoughts was made up: This could be my major imaging setup for the instant future!

Very good efficiency

The Nexus 0.75x Newtonian Focal Reducer/Coma Corrector is a well-built and beautifully corrected lens. Testing with f/3.6 to f/4 Newtonian astrographs signifies superior efficiency and velocity in contrast with comparable merchandise which were in the marketplace longer.

In the event you’re at the moment utilizing a quick Newtonian on your astroimaging, take into account including this product to your optical arsenal. Though it’s a bit expensive in comparison with some competing merchandise, it is going to make attaining world-class outcomes along with your imaging setup routine.