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Eyepieces for Takahashi 106N

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#1 Dennissloan



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Posted 12 April 2021 - 02:10 PM

Which eyepieces to purchase in viewing the Milky Way this summer with Takahashi 106N with Takahashi 2 in diagonal. Considering 21 Ethos, 31 Nagler or 41 Panoptic? Or something else

#2 213Cobra


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Posted 14 April 2021 - 03:48 AM

I have a 106ED I use visually. There are some caveats to consider since the focusers are different:


1/ Unless you have an unusually large pupil, the exit pupil with the Panoptic 41 will be too large.


2/ The 31mm Nagler and the 21mm Ethos both have their field stops almost 0.4" above the shoulder, which means your FSQ-106N might not have enough in-travel to bring these eyepieces to focus. I will ask a friend who has a 106N whether he's successfully used either with his scope. Removing the Camera Angle Adjuster might get you there but I just can't assure it offhand.


3/ On the other hand, the Panoptic 35 delivers a nice 7mm exit pupil and its field stop is well placed for a limited-travel focuser. Alternately, the Panoptic 27 will give you a very comfortable and useful exit pupil of 5.4mm.


4/ From there, the Nagler 17T4 has a neutrally-placed field stop that will not prove incompatible, and it will yield a 3.42mm exit pupil, also very comfortable and useful.


If I could be sure about the Nagler 31 being compatible with the focuser travel, I would consider that + a Nagler 22 or Panoptic 24. But until I can confirm the in-travel of the 106N, I'd recommend a Pan 35 or 27 + a Nagler 17.



Edited by 213Cobra, 14 April 2021 - 12:43 PM.

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#3 Erik Bakker

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Posted 14 April 2021 - 04:26 AM

Great advice from Phil!


+1 waytogo.gif

#4 Jon Isaacs

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Posted 14 April 2021 - 05:27 AM

To add to what Phil wrote:


Based on my experiences my eyepieces and the very similar TeleVue NP-101, the 31mm Nagler is the standout eyepiece with the 21 mm Ethos and 41 mm Panoptic having their place as queens to 31 mm Nagler as king.


The 35 mm Panoptic is very good too but definitely second fiddle to the 31mm Nagler.  Depending on your dark adapted pupil, you will probably give up some effective aperture using the 41mm Panoptic but it may not be significant.  Another issue is whether Dennis needs to wear eyeglasses for astigmatism.. 


Looking for back focus specs for the FSQ 106 N, I found this:




If I'm reading it correctly,  the back focus is 120mm.  I've measured the optical insertion length of a few diagonals, the TV Everbrite is the shortest at 101mm, there maybe shorter. The 31 mm Nagler and 21 mm Ethos use up 9.6 mm so that leaves just 8mm for adapters and spare back focus.


This supports Phil's concerns. If you could borrow a 31mm Nagler or another eyepiece with a known back focus length, then it could be determined if the 31 mm Nagler and 21 mm Ethos would work.


Another eyepiece worth considering is the APM 30mm UFF. I recently purchased one to provide a light weight alternative to the 21mm Ethos, 31mm Nagler and 35 mm and 41mm Panoptics. It's 547 grams compared to 725 grams for the 35 mm Panoptic and 950 grams-1020 grams for the other three.


It has quite a bit of eye relief, spec'd at 22mm, it's parfocal with the 35 mm Panoptic so it adds about 4mm to the back focus making it 124mm.


It's a sharp eyepiece in my fast scopes, I've spent quite a bit of time with in Newtonians operating at F/4.7- F/5 with a Paracorr 2 as well as the NP-101.  I measured the field stop at 36.3 mm so the TFOV is identical to the 21 mm Ethos and slightly narrower than the 35 mm Panoptic.


Another eyepiece to consider is the Explore Scientific 30mm 82 degree. It adds about 3mm to the back focus, you'd have 123 mm to play with. I have limited experience with this eyepiece. In my coma corrected Newtonians, it shows some off-axis astigmatism, it's not as sharp across the field as the other 4 mentioned.


I think the 35 mm Panoptic is the safe choice and an excellent view, the 30 mm UFF is affordable, safe choice and the 31mm Nagler and 21mm Ethos require research.


The 20 mm APM 100 degree might be a player, I haven't used one but I think the 30 mm plus focal focal lengths are where you want to be. The 21mm Ethos is a companion to rather than a replacement for the 31mm Nagler-35 mm Panoptic.



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#5 Joe G

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Posted 14 April 2021 - 11:28 AM

I have the old FSQ 106.


What Phil said above is true.  With my 31mm Nagler and ES 20mm the focuser needs to be racked all the way in.  It is hard to tell if it just reaches focus or is slightly out.  It is close but not comforting to not be able to rack through focus.


The ES 30mm does reach focus with room to spare.  While not the Nagler it is still a nice eyepiece.

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