Eyepiece for TS 50mm RACI finder?

[Ags]

I am looking for the ideal eyepiece for my new 50mm RACI finder. I am currently using a Super 25, which I have tried combining with a barlow nosepiece to reduce exit pupil (making it an effective 17mm), although the eye relief is very long with the barlow. Another thing I have noticed is that I can only get sharp focus with my glasses on - possibly the huge exit pupils bring out my astigmatism?

The infinity focal plane of the RACI is 5mm above the base of the helical focuser, so eyepiece choice is limited, although I can use the revelation barlow nosepiece to raise the focal plane for other eyepieces.

I am looking for an eyepiece with smaller exit pupil than the Super 25, with a wide field. It's also got to be light and cheap and barlow well. The center has to be sharp and the edges only OK (remember the RACI is f4.2). The eye relief needs to be sufficient for glasses. I'm looking for a true field of around 5° (a 21mm Plossl would give this).

In terms of the quality of view I am after, the Super 25 unbarlowed seems to offer acceptable views, although given the exit pupil size I expect my eye pupil is stopping the finder down to f6.

One EP that might match my criteria is the Svbony 23mm 62° (barlowed), but I know very little about that beastie, except that it is ridiculously cheap and apparently has a plastic aspheric lens.

Edited May 2, 2021 by Ags

[Voyager 3]

What is your budget ? I've always thought that the 20mm explore scientific 68° is a perfect match with a 50mm finder . If you find gold in your garden's tree , a 16mm Nagler T5 can't be beat .

[Ags]

I am looking more at the budget end of eyepieces. The ES 20/68 would be pretty good, but I am hoping find a gem in the eyepiece bargain bin!

[Ags]

@Voyager 3 you've got me dreaming about maybe picking up the ES 14/82° I have been thinking about for years. That would still give 5.5° true field, 15x magnification, and 14mm will give better exit pupil than 16mm.... but would it reach focus 🙄

Edited May 2, 2021 by Ags

[Louis D]

14 hours ago, Ags said:

One EP that might match my criteria is the Svbony 23mm 62° (barlowed), but I know very little about that beastie, except that it is ridiculously cheap and apparently has a plastic aspheric lens.

I use a pair of them 3x Barlowed in my binoviewers to good effect.  With the eye cup pulled off, there's enough room for eyeglasses.  Since the top is an engineered plastic, there's little risk of scratching eyeglasses on it.

Even at f/6 in my AT72ED field flattened refractor, the images are pretty decent and should meet your criterion of sharp in the middle and softer at the edges.  Just don't try to ever clean the eye lens because it is a fairly soft plastic and is prone to damage.  As such, always wear eyeglasses when using it and you'll be fine.

Here's a comparison of it with other eyepieces in that focal length range from my collection taken with a cell phone camera through the eyepiece:

Edited May 2, 2021 by Louis D

[Ags]

I have been trying out various eyepieces with the finder. I can reach focus with a Vixen SLV 6mm, ES 6.7mm and Vixen NPL 20 and 30mm. The only eyepiece that couldn't quite reach infinity focus was a Speer WALER 10mm. I have been quite liking the wide field offered by the Super 25, so the Svbony unbarlowed might work well for me.

I am toying with using this as a basic travel scope. The views are ok, especially at low magnification,  and it might actually be the right instrument for M31 or the Veil. The ES 6.7mm and Svbony 23mm would be all the travel eyepieces I need.

[Ags]

Decided not to go for the svbony in the end. The idea of a disposable eyepiece just feels wrong to me, enough junk in the  world. I have decided on getting an ES 16mm 68 degrees instead - it is more money but the optics are of unquestionable quality. it will give 5 degrees FOV and just under 4mm exit pupil so stars will be brighter and the sky background might be a bit darker. The Super 25 gives a 6mm exit pupil which is way too big for a very light polluted observing spot.

Also the 16/68 will be a spiritual successor to the Hyperion 17mm I once owned and enjoyed (at f13).

Edited June 5, 2021 by Ags

[Louis D]

Another option would have been to try and locate a used 18mm Meade 5000 UWA in 1.25" barrel to get the widest field possible at about the same exit pupil as the 16mm ES-68.

[Ags]

Well... just to conclude the thread I got an ES 20/68 in the end. Should come next week!

[Voyager 3]

Congrats ! Inform us how it turns out to you .