A couple of days ago I received an eyepiece I've been mulling the purchase of for a while - the 17mm Nikon NAV-HW. I love the 12.5mm so much, I decided that I wanted to standardize on this line for those two focal lengths, and potentially replace the 17mm ES92 I have with the NAV-HW. A factor in the the decision was also the inclusion of the NAV-HW's 14mm EiC barlow. There are several objects, most notably M33, M81, and M101, which I feel would benefit from something between 17mm and 12.5mm, hence convertibility to a 14mm eyepiece being an attractive feature of the NAV-HW.
I had two moonless nights in a row of superb seeing and reasonably good transparency to try out the Nikon and compare it with the ES92. My thoughts are below.
Unboxing Impressions
The 17mm NAV-HW feels considerably heavier than the 12.5mm despite having a similar physical size. It's more compact than the 17 ES92 and actually feels more dense and almost heavier because of it, even though objectively it's lighter by 3/4 of a pound. This gives it a very solid feel.
Given some issues I saw with the eye lens of the 12.5mm (which is still with Nikon for inspection), I decided to do a bright light test with the Nikon NAV-HW as well. I did not see the same kind of machine marks present with this eyepiece that I did with the 12.5mm, but I did see the same semi-hazy appearance when then light was shining at an oblique angle to the lens, and some imperfections around the periphery. Given this does not seem to dramatically impact the performance of the 12.5mm, I decided not to worry about it with the 17mm.
Inspecting the field lens shows interior surfaces that are not blackened like in most other eyepieces, but upon asking around and doing some research, it appears this is normal and not something to be concerned with. The most striking thing was the field lens itself. It was hard to tell it was there at all. Most eyepieces exhibit deep purple/blue/green reflections, but this lens hardly showed any reflections. The coatings made the lens seem almost invisible.
The included EiC barlow has a solid heavy feel to it and beautifully threads onto the end of the eyepeice. The barlow itself is threaded to accept 2" filters, and the three Tele Vue filters I have thread into it more smoothly than my actual Tele Vue eyepeices! Nikon's machining is incredibly precise and accurate. I get the same feeling when threading on the 12.5mm's EiC barlow. Mechanically these are top shelf eyepieces.
Night 1
First Impressions
The first night was the steadiest seeing I think I've ever seen. Targets included M33, M31/32/110, M13, M15, M27, M42, M43, and M37. All observations were done with my 15" F/4.5 dob with Paracorr 2.
By the time I could muster the ability to peel myself away from Jupiter, M31 had risen high enough to be my first target.
The first look through the Nikon NAV-HW was sheer disappointment. The field was badly vignetted. I had known about the fact that the 17mm NAV-HW doesn't have an optimum setting in the P2, but it was close enough that I wasn't necessarily worried about residual coma. What I wasn't expecting is enough vignetting to make only the central ~65-70% of the field seem usable. Light falloff was detectable starting at 70% and dropping 3-4 magnitudes by the time it hit the field stop, which was nearly invisible due to the vignetting.
Taking the eyepiece out of the P2 and putting it directly into the focuser, totally resolved the problem. The field was brilliant from edge to edge, with the exception of comatic stars.
In contrast, the 17 ES92 has perfectly even field illumination from edge to edge. There are no detectable vignetting issues at all, and the 17 ES92's optimum P2 setting is somewhere between E & F.
I then converted the NAV-HW to 14mm mode and put the Paracorr back in. This resolved the vignetting issue and the field was just as open and accessible as the 12.5mm Nikon. Even a bit more so since the EiC seems to add about half a millimeter of effective eye relief.
Facing some disappoint at the deal-breaking vignetting, I decided to make the most of it and see what else I could discern with it.
Star clusters - M13, M15, and M37
Normally 17mm and 14mm are not the focal lengths I do glob hunting with, but given how steady the seeing was, I felt like doing a star tightness test between the Nikon and Explore Scientific would be useful. The lower the magnification, the more you will see errors manifest in the eyepiece itself, since you're not magnifying errors in the telescope's focal plane as much. Errors in the eyepiece should be proportionally more pronounced as a result.
Both M13 and M15 were jaw-dropping in the Nikon. I have never seen such small, perfect little poinpoint stars. When switching to the ES92, the stars were also very small and pinpoint, but not to the degree that they were in the Nikon. The real critical test came with observations of M15. The tight, bright patch of stars in the core was a nice challenge for both eyepieces. The Nikon NAV-HW did a better job of splitting out individual stars in this patch than the ES92 did. Stars were tighter in general, and that patch was slightly more resolved in the Nikon.
Despite the vignetting around the periphery, the central view was astonishing.
Later in the evening, M37 showed similar results. Tighter, more well defined defined stars.
Extended objects - M31, M33, M27, M42
For this test, I was trying to pick apart differences in contrast from the ES92. In comparing the 12.5mm NAV-HW to the Docter, and the 10mm mode to the 10 Ethos, I found that the Nikon slightly edged out both in terms of brightness and what details could be seen. I had always felt the 17 ES92 had outstanding transmission, so I was interested in seeing where the Nikon landed.
After doing extensive back and forth between the two eyepieces, I was having a hard time separating them. Part of the problem was the vignetting in the NAV-HW. It no doubt skewing any results as I found it so distracting.
I started doing comparisons against M42, and there were some moments that I felt the ES92 was better, and some the Nikon was better, but after noticing a persistent darkening of the view, I looked at my secondary and found it had fogged up, so I disregarded all observations I made of M42 for the night. I decided to revisit the following night since I knew it would be clear.
Night 2
After sleeping on it, I was mostly convinced the Nikon would have to go back, but I wanted to give it some more shakedown time, and concentrate more on the 14mm mode this night. I didn't do much comparison between 17mm mode and the 17 ES92, but did go back to M42 once it had risen.
14mm mode on M33 is a sight to behold in that eyepiece. It PERFECTLY frames M33 (well, what's visible from my 21.0 skies anyway) and just pulls you right into the view. It's exactly what I was hoping to get from the 14mm mode of the Nikon. Perceived contrast was better than using a 17mm eyepiece, and brighter than using a 12.5mm eyepiece, and had better framing. The view of M33 in 14mm mode alone had me considering keeping the Nikon just as a 14mm eyepiece.
To further test that theory, I did some math and found that 14mm mode without a Paracorr is very similar to 17mm mode with a Paracorr (122x/3.1mm exit pupil vs 116x/3.3mm exit pupil). When looking at stars, I found coma was not as bad in 14mm mode + no paracorr as it was in 17mm mode + no paracorr. It was tolerable, and since I was primarily looking at galaxies and nebulae with a 17mm focal length, I felt like this might be a possibility. But long term my plan is an F/3 or shorter big dob, and that would throw that possibility out the window, so this would be a short-term work-around only.
To do more testing with the Nikon in 14mm mode + Paracorr, I decided to thread on the O3 filter and aim at the Veil nebula. Generally this is a LOT lower of an exit pupil than I use with an O3 filter, but I wanted to try it anyway, and I'm glad I did. Just wow! I gave my observing eye an extra 10 minutes to adapt to this new darker level, and just like the view of M33, I was totally immersed in the Veil. Despite the dimness of the view from the O3 and that exit pupil, the Veil just popped. I could see all kinds of structure and I felt like I was floating there in space looking at it.
Later in the night, after M42 had risen (and this time with my dew heater on!), I went back to a more critical comparison between the NAV-HW in 17mm mode + Paracorr, and 17 ES92 + Paracorr. I had gotten somewhat used to the vignetting and was even starting to convince myself it wasn't that bad. Being less distracted by it, I felt like I could do some good comparisons between the two eyepieces as long as I stayed focused on the central ~70 degrees of the Nikon.
After about 90 minutes of back and forth comparison, I came to the same conclusion I did between the 12.5mm Nikon and the Docter - the view in the Nikon is just brighter. I could trace nebulosity in M42 out a bit further in the Nikon than the ES92, and the view overall was just more vibrant. That said, due to my light pollution levels, it also appeared a bit more washed out. The contrast in the ES92 was *perceived* to be higher because it was a bit darker, and I felt like certain structures showed more easily in the ES92. This was a situation that each eyepiece presented M42 *differently*, not necessarily better or worse. The differences were also so subtle it was hardly worth worrying about. The most notable difference was with M43 - the "comma" shape was better defined in the Nikon - another thing I had observed with the 12.5mm version against the Docter.
Unfortunately, the brighter view in the center of the eyepiece was more than erased by the dramatic vignetting in the rest of the field. The Nikon would make a fantastic 17mm Delos, but it's simply not a 17mm 100 degree eyepiece in the P2.
Conclusions
The summary is that stars in the Nikon NAV-HW are incredible. Tighter and sharper than the 17 ES92. For the part of the field that is fully illuminated, the Nikon is brighter than the ES92 (though this effect would be better utilized from a dark sky). The Nikon in 14mm mode is incredible. I wish I had the opportunity to try it against M81 and M101, but they are not visible from my observing location this time of year.
Unfortunately the vignetting with the P2 is too severe to effectively use the Nikon in 17mm mode, and for anyone who has a dob and a Paracorr 2 who is considering getting the 17 NAV-HW, I strongly recommend you don't. The vignetting isn't widely reported and had I seen reports of it, I would have avoided the 17mm Nikon. Simply put: the 17mm Nikon is very, very incompatible with the P2. It's not just that it doesn't have a perfectly optimum coma setting in the P2, it's that the vignetting makes it unusable. Do not use a 17 NAV-HW in a P2! You have been warned!.
I'm currently on the fence about just returning it and staying with the 17 ES92, or keeping it and using it as a 14mm eyepiece only while also still keeping the 17 ES92. The latter option would make it a really expensive "filler" eyepiece since I wouldn't be using it for general purpose observing. I still prefer the 12.5mm and 10mm focal lengths for general purpose DSO observing, and 17mm and up for some of the bigger targets.
Were I to return it, I would definitely miss the 14mm focal length + huge, easily accessible field. But I would also potentially miss some of the other gear I could get for the same cost.
Edited by CrazyPanda, 08 October 2021 - 10:51 AM.
