Is Explore Scientific 92 deg critical for limited in-focus?

[HenrikBA]

Now I've read so much good about the Explore Scientific 17mm 92 degree, so I'd really like to get one, if I can use it with a 2" 45 degree erecting prism. I use a TeleVue NP101 refractor mainly for birding; it's a brilliant telescope, but it has very limited in-focus, probably due to the Petzval design. My preferred eyepiece is the Nikon NAV-HW 17mm, which I can't use with my 45 degrees prism. I bought a 90 degree APM prism with very short optical length, but even that was not possible to use with the Nikon NAV-HW, until I made a modification to move further down the eyepiece. Also the APM 20mm 100 degree eyepiece can't focus at infinity with my two 45 degree erecting prisms.

 

As 45 degree is often nicer to use for terrestrial observation, I'd really like to get a 17mm super wide angle eyepiece to use with my prism, so I'd like to hear if anyone in this forum knows, if the ES 17mm is less critical to limited in-focus than the Nikon NAV-HW (which I can't use either with the 14mm ec) and the APM 20mm? Most other eyepieces I have will focus all right with my 45 degree erecting prism.

 

Henrik.

[HenrikBA]

If anyone has an ES 17mm for sale, I might be interested.

[rl]

The ES17/92 weighs more than a kilogram. Will the scope plus 45 degree diagonal support it without balance problems? It's a great eyepiece on big scopes but can be an embarrassment on small ones. 

[John]

I have an ES 17 / 92. It is a very heavy eyepiece. It reaches focus about 9mm further outwards than the Ethos 17mm does, if that is any help.

 

[Louis D]

The 17mm ES-92 focuses pretty much right at the shoulder like many other ES and Pentax eyepieces.  That matches up with what @Johnsays above because the 17mm Ethos focuses about 10mm above its shoulder.

Weirdly enough, the 12mm ES-92 focuses some distance away from its shoulder, though I'm not sure which way.  I had kind of expected them to be close to parfocal.  Thus, it might not work with your diagonal like the 17mm probably will.

[HenrikBA]

Thanks a lot for your comments, I think I can balance the scope by moving my long dovetail forward in my Manfrotto videohead. The Nikon is very heavy too, there's still some headroom there. I was told in another forum that the ES 17mm 92 deg focuses around 13 mm further out than the Nikon NAV-HW 17mm, so I think it might just about be possible with my 45 deg erect prism.

[Louis D]

I just checked focus position, and the 12mm ES-92 focuses 2mm further out than the 17mm ES-92.  Interestingly enough, the 12mm ES-92 is parfocal with the 30mm ES-82 and 40mm Meade 5000 SWA (ES-68).  I had thought it would be the 17mm ES-92 that would be parfocal with them.

[Don Pensack]

The 17mm ES 92 has its focal plane at the shoulder of the eyepiece.  If the 12mm focuses 2mm farther out, its focal plane must be 2mm lower in the eyepiece barrel.

It's not surprising a 20mm APM won't come to focus, as its focal plane is nearly 10mm above the shoulder and it requires a lot more in-travel of the focuser.

Since a prism eats up a lot of back focus, you need to stick to eyepieces with lower focal planes.

[Louis D]

53 minutes ago, Don Pensack said:

The 17mm ES 92 has its focal plane at the shoulder of the eyepiece.

I'll have to try focusing the moon on a piece of white paper across the 2" eyepiece holder and then stick the 17mm ES-92 to see if it really does focus at the shoulder, within a millimeter let's say.  I had thought the 30mm ES-82 and 40mm Meade 5000 SWA focused at the shoulder up until a couple of nights ago.  Now, it appears they might focus 2mm below it.

[Don Pensack]

https://www.explorescientific.it/out/media/50c37e2d0427ea14408f9fe6735dd29c.pdf

Eyepiece spec chart for ES.

Focal planes above the shoulder are labeled minus because you have to move the focuser in.

Focal planes below the shoulder are labeled plus because you have to move the focuser out.

[John]

10 hours ago, Don Pensack said:

https://www.explorescientific.it/out/media/50c37e2d0427ea14408f9fe6735dd29c.pdf

Eyepiece spec chart for ES.

Focal planes above the shoulder are labeled minus because you have to move the focuser in.

Focal planes below the shoulder are labeled plus because you have to move the focuser out.

Thanks Don - very interesting !

Shame that the precise field stop position is not shown for the 12mm 92. The 17mm says 0.00 so I guess thats right at the shoulder of the eyepiece. The 12mm is likely to be around 2mm above that ?

Wow ! - 12 lens elements for the ES 120 degree 9mm

No wonder they are expensive !

[McNewt]

Interesting thread.  Following.

I have an 80mm refractor with limited in-focus.  All of my eyepieces will reach focus with a Baader clicklock diagonal, but some just barely.   This made me scratch my head when looking for a 2" Herschel prism as these tend to add a longer optical path than the Baader diagnoal.  On searching for a solution I noticed that the Baader Morpheus eyepieces supports both 1.25" and 2" fittings with the same barrel so in 2" "mode" these will sit lower in the diagonal potentially gaining back some of the lost in-focus travel.    I'm adding the Explore Scientific 92 degree eyepieces to the list of "problem solvers".  Though, for the time being I cannot justify starting on a new eyepiece set.   

[John]

If you use a hybrid barrel eyepiece like the Morpheus in 2 inch mode you need to be careful that the 1.25 inch section of the barrel does not contact the mirror or prism in your diagonal.

 

[Louis D]

@Don Pensack Interesting.  I was going by the markings on my refractor's focuser.  Perhaps they're not in millimeters.  I'll have to measure them against a metric ruler to see if they really are.  It could be that the two marks difference between the 12mm and 17mm ES-92 were actually 3mm because the 12mm was exactly parfocal with both the 30mm ES-82 (mushroom top) and the 40mm Meade 5000 SWA (ES-68).

[HenrikBA]

Thanks for your comments, I've now got the ES 17mm 92 degree, and it fouses with no problems in my 45 degree prism in the TV-NP101, still several mm of headroom. Very nice eyepiece, weight is no problem at all on TV-NP101.