Let’s face it, astrophotography is an expensive hobby. But occasionally a piece of equipment comes along that is both affordable and excellent; the Rokinon (also sold by Samyang) 135mm f/2 lens is one such example. Camera lenses make a great entry point into astrophotography since they are small enough to be used on lightweight, portable, affordable star trackers (such as the Vixen Polarie or Sky-Watcher Star Adventurer) and can be used with both DSLRs and astrophotography-specific cameras.
How I Use It for Astrophotography
Some clever people have designed 3D-printed rigs specifically for this lens to allow easy attachment to a number of dovetail types or focusing with a ZWO EAF focuser belt drive attachment. Two examples are from Astrodymium and Astronetics. I use an older version of the Astronetics, although the new design looks great. I attach either a filter drawer or filter wheel to the lens using a Nikon to M42 converter (the Rokinon lens is also sold with Canon, Sony, and several other manufacturer connections) and then attach my ZWO ASI294MC Pro camera to it. I have also used it with my Nikon DSLRs. The whole package fits very nicely on any telescope mount, like my Sky-Watcher Star Adventurer.
This lens has the tightest stars I have seen in any astrophotography optic despite its modest price point. With my ZWO ASI294MC Pro camera I regularly get focus at a FWHM (full-width at half maximum) of 2.1 pixels or less. Having tight stars can make a big difference in how an image looks and how easily you can see the target object. In the example below of the North America Nebula, the DSLR image (right) with the Rokinon lens turned out better than the astrophotography camera image (left) with a more basic lens.
I have also found this lens to be extremely stable with temperature changes; I have imaged multiple nights in a row without having to re-focus.
An important factor in selecting an optic for astrophotography is chromatic aberration. This is caused by the red, green, and blue light not being focused to the same point, which creates blue halos (most often) around stars. My less expensive camera lenses, and even higher-cost doublet and some triplet telescopes, have some chromatic aberration. Not so with the Rokinon; even around very bright stars, there is little to no halo (and I suspect that the small halos I do occasionally get have more to do with the filter I’m using than the lens).
A common issue with fast optics like this lens is field curvature, which causes stars to display coma or other odd shapes toward the edges. This effect is present in the Rokinon 135mm, although it starts far enough out from the center to still allow for good astrophotos. The vast majority of targets are relatively small at a 135mm focal length anyways, even the larger nebula complexes. Stopping down only one stop to f/2.8 helps to reduce this effect. The image below shows the corners, edges, and center of a 20s exposure of the Rho Ophiuchi complex taken with my Nikon D5300 (APS-C) at f/2.8.
With an astrophotography camera attached, one must pay more attention to sensor tilt, which is more of a problem with these cameras than DSLRs. This effect is exhibited with my ZWO ASI294MC Pro despite its smaller 4/3-size sensor but it can be fixed with a tilt plate adapter. Some strategic cropping may still be necessary.
For the astrophotographer on a budget or anyone looking to assemble a good wide-field imaging rig, the Rokinon 135mm f/2 lens is a must-have. At a modest price point, its performance is excellent. It is fully manual, making it easy to use with astrophotography cameras that cannot electronically change the aperture. You can expect great results!