Orion 21mm Stratus vs. TeleVue Nagler Type 4: Some Impressions
The Stratus currently costs $119. It has six focal length options, including (in mm) 3.5, 5.0, 8.0, 13, 17, and 21. The literature lists it as having a 68 degree apparent field of view. Eye relief is listed at 20mm. Its weight, even when barlowed, was not enough to shift the altitude orientation of my dob. The main barrel of the eyepiece is 1.25” in diameter, though Orion suggests that it could be used in a 2” focuser as well.
I have been into visual astronomy for the past eight years, though my real learning curve has taken off in the past two years. The curve took off with the purchase of a 16” Meade Starfinder. The Starfinder (1835 mm focal length) used in this review has little original equipment. Among the highlights of its modifications are a well-aligned JMI-DX1 focuser, a modified sling mirror cell, a professionally refigured primary mirror (it used to be dogmeat, with a Strehl ratio of approx. 0.7, and was approx. 0.4 wave PV before refiguring!), and fan cooling system.
After finally getting good optics through refiguring, I decided that my $49 plossls weren’t bad, but the narrow fov and tight eye relief at shorter focal lengths was a real problem. So, I took a chance and purchased a 21mm Orion Stratus. After all, they were cheaper than other “high end” long ER/wider field eyepieces! The eyepiece worked well compared to what I had grown accustomed. But, I really wanted to compare it to premium eyepieces, such as Nagler Type 4s. I chose the Type 4 because of its eye relief, focal lengths similar to the 21 Stratus and barlowed 21 Stratus (i.e., 22mm/21mm and 12mm/10.5mm), its being a wide-field EP, and MOST IMPORTANT…a very trusting friend was willing to loan me his Type 4s.
The Review Session.
The 4:00 am temperature was sub-zero with light winds. My skies were reasonably dark (mag 5.5 at zenith), and had a window of seeing and transparency in the 3/5(average) range in the clear sky clock. My instrument of choice was my 16" f/4.5 Meade Starfinder. Both the scope and the eyepieces were allowed to cool down all night to avoid tube currents, etc. I chose the f/4.5 (instead of other scopes) because it would be the most taxing of any of my scopes for the eyepieces.
In the house: coatings.
When inspecting the 22mm Nagler and the 21mm Stratus at my *warm* kitchen table, I noted very little difference in the reflectivity of coatings when held under standard indoor lighting. There may have been a slight edge in favor of the Nagler, but I cannot be sure.
Field of view.
I do not have enough background to judge exactly what the fields of view were in the two eyepieces, but as expected, the Nagler was wider. I would expect this because of the reported 68 degree apparent field in the Stratus, compared to the 84 degree in the Nagler. Since the 22mm Nagler and unbarlowed Stratus resulted in similar magnifications (i.e., 83x vs. 87x, respectively), the wider apparent field in the Nagler resulted in a wider true field. The Stratus’s field of view was much wider than anything that I have grown accustomed to in the past. But, was it 68 degrees? Someone more adept at testing needs to establish this.
When placing bright targets just outside of the field of view, the Nagler gave little indication that the object was present. There was no flaring, glare, etc. This was not true with the Stratus. While I got virtually no reflection when bright targets were in the fov, I got a nice little flare when placing very bright objects (e.g., Saturn) just outside the field. The flare was not as prominent as seen in other, less expensive eyepieces, but more prominent than the Nagler. Upon closer review at the kitchen table, I saw that the inside of the Nagler and its barrel were a flatter black than the Stratus. The blackened Stratus interior had an oily appearance. I am going to try to eliminate this reflective substance at a later date.
The Nagler's stars were a little sharper outside of the middle 70% of the diameter of field in the Stratus. However, if I wasn't specifically looking for sharpness off axis (say in the double cluster), the dropoff in sharpness was subtle enough that I wouldn't have noticed. The eyepieces had similar sharpness on-axis. I tried focusing the off-axis stars in both eyepieces in order to observe what happened on-axis. For both EPs, focusing on-axis also brought me to as close to optimal off-axis focus as I could get (note, I'm using a JMI DX-1 WITHOUT a micro focus control). If seeing had been 4/5 or 5/5 instead of 3/5, there might be a difference in on-axis sharpness. However, I have yet to see steady enough skies to know for sure.
In order to avoid buying more eyepieces (in deference to the permissions statement posted by the CFO), I wanted to see how the Stratus performed in a TeleVue 1.25” 2x Barlow (174x). In order to judge the quality of the image from the barlowed Stratus, I compared it to a 12mm Nagler Type 4 (152x). First, let me say that the views of Saturn in the Nagler and the barlowed Stratus were breathtaking. I had five confirmed moons in the FOV for each EP, and the TV had sharper moons. But, details on Saturn and its rings were more pronounced in the Stratus than in the Nagler. I am unsure, but it seems plausible that an extra 22x might bring out slightly more planetary detail. With the barlowed Stratus, the Cassini division was distinct, sharp, simply beautiful. In the Nagler, the Cassini division was also clearly visible, but the edges of the dark division were softer than in the barlowed Stratus.
I also viewed the trapezium, but did not do the side-by-side, as it was low in the west/southwest just above the roof of my house. Even in a not-so-optimal sky position, the barlowed Orion split elements A through F of the trapezium. Admittedly, it was a tough split on E and F due to the low altitude and heat from my house, but still encouraging.
When looking at Saturn, I noted that both the barlowed Stratus and the 12mm Nagler gave a postcard appearance. What I mean by this is that the detailed, beautiful planet was superimposed on a velvety black background… (when I wasn’t breathing on the eyepiece). Using the 22mm Nagler and the unbarlowed Stratus, I noted a similarly dark background when viewing star clusters. On a similar note, I detected no differences in image brightness when comparing the Stratus (barlowed and not) to the 12mm and 22mm Type 4s, respectively.
Ease of use.
Being new to widefield eyepieces, I did not realize that finding the “sweet spot” for looking through an eyepiece could be tricky. Indeed, it was for me with the Naglers. The Naglers (both 12mm and 22mm) were difficult to get my eye in the correct position without some trial and error. The Stratus was very intuitive, in that there were no problems finding the sweet spot.
My comparisons lead me to highly recommend the trial of the 21mm Stratus eyepiece. Its biggest drawbacks were the outside field flaring, narrower fov, and slight softening of stars beyond the central 70% of the field of view in an f/4.5 newtonian reflector. After factoring these limitations into my subjective opinion, I have concluded that my 21mm (alone and barlowed) is a solid performer compared to similar focal length Nagler Type 4s. A more experienced observer under better seeing conditions may find other relative strengths for the Naglers, but I was very pleasantly surprised by the Stratus. It fit my bill very nicely for a 60-70 degree eyepiece that held up well in a fast scope…and that pleased the CFO! Would I advise someone to sell off one or two Naglers and buy a set of Stratus eyepieces? No. But, if someone doesn’t already have Naglers and wishes to keep costs down, would I advise them to check out the 21mm Stratus? You bet! I liked it enough that Santa is bringing me a 13mm Stratus!
Clear skies to all.
Click here for more about the Nagler 22. -Ed.
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