113 replies to this topic

[areyoukiddingme]

I've tried with GSO, ES, Paracorr 1, Paracorr 2, and a whacky Skywatcher that I took to with a file. . . 

 

The ES doesn't reach focus for me, necessitating Portaball surgery, so that's out. But even inserting the eyepieces into the CC and looking for vignetting does seem to suggest that it's better than the P1 and P2. In the P2, during day time, i see the vignetting effect while looking through the 17HW.

 

As far as vignetting, the GSO is the best.

[CrazyPanda]

Have you tried the HW with a GSO or ES coma corrector? There seems to be something about TV optical design that makes eye placement an issue with certain products. I experience(d) that for years - Radians, NT1s & 4s, 31T5, Delos, Delites <13mm. Yet other makes/designs may not present thise issues, or not so acutely anyway. 

I haven't. I draw the line at a dedicated coma corrector for one eyepiece

[25585]

I haven't. I draw the line at a dedicated coma corrector for one eyepiece

That is a shame, when it might be the easiest solution. For such an exotic and expensive eyepieces, a relatively inexpensive dedicated accessory is not overkill. 
 

Otherwise ask Tele Vue what you can do....

[CrazyPanda]

A lot of it has to do with space. I'm very efficient with it without a lot of extra room to spare

 

 

The best solution to the vignetting for me is the ~0.5" thick barlow that takes up very little space in the case when attached to the eyepiece. If the 14mm focal length wasn't useful to me, then yes I might resort to a dedicated CC for the eyepiece and design the layout around it. Or I might sell the 17 HW and stick with the 17 ES92.

 

But since I'm 50/50 on whether 14mm is more useful than 17mm in my scope, I'm happy to just convert to 14mm instead. If I ever do want 17mm, I can just unscrew the barlow and live with some mild vignetting. Or keep the barlow in, take the Paracorr out, and have an effective focal length of 16mm with no coma correction. The lack of coma correction won't be a problem if I'm looking at targets like M33, M101, and M42.

 

So I have enough flexibility that I don't feel the need for a separate coma corrector for this eyepiece.

 

I spent more time analyzing the source of the vignetting tonight. I defocused a star and pushed it near the edge of the field. Something starts cutting the star's defocused disc off starting around 50% of the way to the edge. By the time the star is pushed to the edge of the field, only about 10% of the area of the defocused disk remains. In comparison, without the Paracorr, over 50% of the disk remains. The 21 Ethos with Paracorr is similar - over 50% of the disk remains, and it doesn't start getting cut off until about 75% of the way to the edge.

 

I also cut a small paper tab out and taped it over the secondary mirror such that it protruded in from the edge about half an inch. I could see its silhouette in the defocused star. In the 21 Ethos, as I moved the star further off-axis, more of the paper tab was revealed, until the star started getting cut-off by something that WASN'T the edge of the secondary mirror (as it started cutting off the paper tab silhouette as well). When I tried this with the 17 NAV-HW, I was only able to reveal about half of the silhouette that I was able to reveal in the 21 Ethos, before the star started getting cut off (and the tab silhouette along with it).

 

So it would seem that whatever is intercepting the light of the star is happening well before the edge of the secondary mirror, else I would be able to reveal more and more of the silhouette of the paper tab that's intruding in from the edge of the secondary mirror. So the secondary mirror size is not the issue.

Edited by CrazyPanda, 01 March 2024 - 09:52 PM.

[Starman1]

A lot of it has to do with space. I'm very efficient with it without a lot of extra room to spare

Storm-IM2700-deepsky.png

The best solution to the vignetting for me is the ~0.5" thick barlow that takes up very little space in the case when attached to the eyepiece. If the 14mm focal length wasn't useful to me, then yes I might resort to a dedicated CC for the eyepiece and design the layout around it. Or I might sell the 17 HW and stick with the 17 ES92.

But since I'm 50/50 on whether 14mm is more useful than 17mm in my scope, I'm happy to just convert to 14mm instead. If I ever do want 17mm, I can just unscrew the barlow and live with some mild vignetting. Or keep the barlow in, take the Paracorr out, and have an effective focal length of 16mm with no coma correction. The lack of coma correction won't be a problem if I'm looking at targets like M33, M101, and M42.

So I have enough flexibility that I don't feel the need for a separate coma corrector for this eyepiece.

I spent more time analyzing the source of the vignetting tonight. I defocused a star and pushed it near the edge of the field. Something starts cutting the star's defocused disc off starting around 50% of the way to the edge. By the time the star is pushed to the edge of the field, only about 10% of the area of the defocused disk remains. In comparison, without the Paracorr, over 50% of the disk remains. The 21 Ethos with Paracorr is similar - over 50% of the disk remains, and it doesn't start getting cut off until about 75% of the way to the edge.

I also cut a small paper tab out and taped it over the secondary mirror such that it protruded in from the edge about half an inch. I could see its silhouette in the defocused star. In the 21 Ethos, as I moved the star further off-axis, more of the paper tab was revealed, until the star started getting cut-off by something that WASN'T the edge of the secondary mirror (as it started cutting off the paper tab silhouette as well). When I tried this with the 17 NAV-HW, I was only able to reveal about half of the silhouette that I was able to reveal in the 21 Ethos, before the star started getting cut off (and the tab silhouette along with it).

So it would seem that whatever is intercepting the light of the star is happening well before the edge of the secondary mirror, else I would be able to reveal more and more of the silhouette of the paper tab that's intruding in from the edge of the secondary mirror. So the secondary mirror size is not the issue.

I explained why it is getting cut off by the Paracorr a few posts back. It's related to where the focal plane is in the eyepiece and the inability to get close enough to the Paracorr lens.
Attach a 48mm to 2.4" adapter to the Paracorr top and remove the tunable top and attach the eyepiece directly and it's possible you could reduce the vignetting by optimizing the distance between lens and eyepiece.
Likely not eliminate, just reduce.

[25585]

Would an earlier Paracorr work?

[Starman1]

Would an earlier Paracorr work?

The earlier version of the Paracorr is shorter, so might vignette less.

But the 17mm Nikon would be even farther from its best correction, because its focal plane would require an additional 8mm of infocusing of the focuser to get to focus, meaning the eyepiece would be 8mm away from the best coma correction on the lens, and that would be outside the +/- tolerances on correction.

You see, the in-most setting on the Paracorr 1 is the equivalent to setting B on the Paracorr 2, and the 17mm Nikon needs a setting 5mm below A on the Paracorr 2.

[CrazyPanda]

I explained why it is getting cut off by the Paracorr a few posts back. It's related to where the focal plane is in the eyepiece and the inability to get close enough to the Paracorr lens.
Attach a 48mm to 2.4" adapter to the Paracorr top and remove the tunable top and attach the eyepiece directly and it's possible you could reduce the vignetting by optimizing the distance between lens and eyepiece.
Likely not eliminate, just reduce.

The 17HW seems to bottom out on the safety ring at the bottom of the tunable top rather than the shoulder of the HW stopping at the top of the tunable top. I don't think you can get the 17HW any closer without cutting off the bottom threads of the eyepiece itself. I also couldn't imagine a couple millimeters making a difference. If anything if I extend the paracorr all the way out, the vignetting in the 17HW improves. Coma correction obviously gets worse, but the vignetting seems less severe. It's a strange interplay between the two optics.

[Starman1]

That means the vignetting is not related to the distance from the front lens in the Paracorr, which I assumed.

You're right--a strange interaction.

[25585]

I wonder if the 17mm would not vignette with a GSO or Baader CC?  I’m a P2 fan but it might be worth spending $100 on a GSO and figuring out what spacer you need to get the proper Correction with the Nikon 17.  I’m just throwing idea out there. Personally it would be a deal breaker for me which is a shame.  I couldn’t afford the Nikon anyway.  I think the Baader might come with a tunable top now as well... not sure cause I have a P2 and it works with all my junk.  There’s also the HRCC.  Sorry if no real solution offered just a shame you have such an eyepiece you like and it doesn’t work well for you.  Best wishes 

 

k

 

edit - sorry didn’t read all the replies and see someone already mentioned the other coma correctors

Perhaps Siebert could make a CC that would not vignette for the 17 HW, and would be tailored to that eyepiece. Such might be lighter as well, not being a multi-eyepiece CC.

[Starman1]

I wonder if the 17mm would not vignette with a GSO or Baader CC?  I’m a P2 fan but it might be worth spending $100 on a GSO and figuring out what spacer you need to get the proper Correction with the Nikon 17.  I’m just throwing idea out there. Personally it would be a deal breaker for me which is a shame.  I couldn’t afford the Nikon anyway.  I think the Baader might come with a tunable top now as well... not sure cause I have a P2 and it works with all my junk.  There’s also the HRCC.  Sorry if no real solution offered just a shame you have such an eyepiece you like and it doesn’t work well for you.  Best wishes 

 

k

 

edit - sorry didn’t read all the replies and see someone already mentioned the other coma correctors

For those tuning in to this thread:

Baader MPCC--connects directly to 2" eyepieces but requires different spacers for each.  Uses a 1.25" to 2" adapter for 1.25" eyepieces; again with different spacers for different eyepieces.  Has to be moved from eyepiece to eyepiece on 2" eyepieces.

GSO CC--requires a spacer in the coma corrector between top and bottom for visual use--the spacer varies according to your lowest power or inmost focusing eyepiece.  Parfocalizing eyepieces makes it easier to use.  Otherwise, eyepieces must be slid in and out to get to focus.

Explore Scientific HRCC--has a fine thread helical top with a wide range of adjustment.  Requires 32mm of in focus from the scope's focal plane, so not usable in many scopes.  Easier to use when eyepieces are at least close to parfocal.

Tele Vue Paracorr Type II--has a helical top for adjustment for each eyepiece.  Requires 14mm of in focus from the scope's focal plane (usually not a problem).

[CrazyPanda]

Baader MPCC--connects directly to 2" eyepieces but requires different spacers for each.  Uses a 1.25" to 2" adapter for 1.25" eyepieces; again with different spacers for different eyepieces.  Has to be moved from eyepiece to eyepiece on 2" eyepieces.

 

Also worth noting that the MPCC is not great.

 

* https://farm3.static...a3c71a20e_b.jpg

* https://www.cloudyni...-1477533642.jpg

 

The 17 NAV-HW has incredibly sharp and tight stars. You'd basically destroy a significant optical advantage of a $900 eyepiece by pairing it with the MPCC since the MPCC adds on-axis spherical aberration.

[Procyon]

Have both 17mm eyepieces

 

For SCT's & Refractors. Both = sick

 

No idea what a CC is, lol, I'd still want one for a Dob though to be honest, tested it in someone's 20+" dob once at a Bortle 1-2 Zone, the long time owner said it bested his Ethos by showing him a fainter star in M42, he ordered both Nikons the following day, Ethos put up an amazing view and fight though, both solid eyepieces to have, the owner was Attilla Danko or Allan Rahill, from cleardarksky.com, curious to find out if he ever figured out a way to use it without any vignetting, he seemed comfortable using it that day, although he was playing with the Paracorr settings. If anyone knows them, they can ask them, great dude, maybe he figured it out.

 

Full lens of Caroline's Rose or M23 in one of these is jaw dropping

 

Nikon has the best transmission, tied or bested my previous 17.3mm Delos 

 

ES 17mm 92 puts up some awesome views, ultra sharp too.

 

So good, worth owning both, no need to move head viewing through them, just eye, zero blackouts or kidney beans in either, prefer to use both with eyeguards downwards

 

PS> Found his (Or possibly his friend's) notes from 2018 after he received his Nikon, it may or may not help someone.

 

I brought my Nikon HW 17 mm to OSP and had it tested in a Lockwood 28” at f/4.2 and a Kennedy 28” at f/4, both owners prefere it less the P2, and me to.

If one must has it at TV settings on the P2 at 56 mm ( barrel is 58 mm ) but who knows if TV is correct ?

 

Anyway one can mill off threads of barrel end and I has a own made P2 top less the stop ring down at 44 mm.
So I’m 2.5 mm from TV settings at a 58 mm barrel ( std )

I tested this EP at + 10 mm from TV settings in a CZ f/5. - SDM scope, and it was just great ( 10 mm off )

 

Its a awsome EP.

The Kennedy 28” owners was very impessed and used it for many hours on many objects. And did NOT perfere a CC on it.
No coma to speak off, I could see but very nice transmission.

Edited by Procyon, 18 May 2025 - 11:59 PM.

[faackanders2]

Which was designed first the P2 or 17E? Also is the HW more than 100° AFOV? As vignetting is said above to occur with the 21E, perhaps 17mm & longer 100° AFOV optics go beyond the easy capability of the P2?

 

As 17mm ES92 does not vignette, that 8° less AFOV might be making the difference.

102 AFOV