Widefield eyepiece for 127 Mak

[Froglord]

I know the Mak will never be truly widefield, but I'm looking for a nice 1.25" eyepiece to maximise the field, with a budget of maybe £100-120. Also I need good eye relief for glasses, which I'm estimating at 17mm absolute minimum, and preferably nearer 20mm.

Assuming a focal length of 1500mm and an aperture of 120mm (based on reports that it's not a full 127mm), this gives a focal ratio of f/12.5.

I'm pondering these:

• Stellalyra 24mm UFF 65°, giving 63x and a 1.92mm exit pupil.
• ES 26mm 62°, giving 58x and a 2.08mm exit pupil.
• ES 30mm 52°, giving 50x and a 2.40mm exit pupil.

Has anyone used these in a slow scope like mine?

Any other options I might consider? (No, not the Panoptic 24mm - it costs more than my scope!).

[Coco]

18mm & 25mm BST   £110 for both.
https://www.firstlightoptics.com/bst-starguider-eyepieces.html

I've used both in a 127 Mak and they are superb.

[Louis D]

Just about anything look good at f/12.  I would just get a 32mm Plossl for now.

If you want to go truly wide on that Mak, and you don't mind a bit of vignetting, you can fit a 2" visual back and use a 2" diagonal and 2" eyepieces with one.  That's how the US versions are sold.

Here's a through the eyepiece comparison of the true field of view when bumping up from a widest field 1.25" eyepiece to a widest field 2" eyepiece using this strategy:

Notice how much more true field is visible.  Yes, the edges are 65% as bright as the center, but it's a trade off I'm willing to live with.

[Don Pensack]

An alternative for the Mak is the 40mm 1.25" Plössl.

The exit pupil is larger (brighter image).

The magnification is lower (~39x), and the eye relief is a bit longer than a 32mm Plössl.

Apparent field will be 40-43°, about that of an Abbe orthoscopic.

**note: your "127mm Maksutov actually has a clear aperture of 121mm and a focal length with 1.25" diagonal of 1540mm, making the scope f/12.7.

That makes the exit pupil of a 40mm only 3.1mm, so a 40mm will be about the brightest image you can get.

True field will be 1.00°

If the visual back is shortened (the threaded portion on the visual back is removed), the focal length shortens to 1518mm and maximum true field increases.

The UFF 24mm actually gives you a bit more true field due to its larger field stop--1.03°, though 64x as a lowest power is a bit high for that aperture.

 

So I'll agree with Louis--a 32mm Plössl is a good compromise--48x and a 1° field.

Edited May 4, 2023 by Don Pensack

[John]

The Vixen NPL 30mm that the OP already has is a decent low power eyepiece and gets 95% of the way there in terms of true field. Cost = zero 🙂

[Louis D]

5 hours ago, John said:

The Vixen NPL 30mm that the OP already has is a decent low power eyepiece and gets 95% of the way there in terms of true field. Cost = zero 🙂

Missed that in the OP's sig.  So, short of going the 2" route I suggested, the real question is, what does the OP hope to improve upon versus the NPL 30mm?  Wider apparent field of view at slightly wider true field of view with the 24mm APM UFF, but with a smaller exit pupil; or narrower apparent field of view at slightly wider true field of view, but with a larger exit pupil?  Only the OP can answer that.

[Froglord]

Thanks for all the replies, everyone!

@Louis D I've thought about upgrading the visual back, but that would mean a lot more expense: a new visual back, a 2" diagonal/prism and 2" eyepieces. I wouldn't say 'never', but I've decided not to for now. The wider field sounds great, but the drop in brightness puts me off a bit.

As for the 30mm NPL, I like it optically, but find the eye placement really problematic. As a long focal length plossl, it should have lots of eye relief, but the lens is so recessed that I can't see the full field (of only 50°) without mashing my glasses hard into my eye socket! Even with glasses off and the cup raised it still doesn't sit right.

As a replacement for the NPL, I was hoping to find something with a slightly wider field (usable field, with glasses on) and better eye relief. I was assuming I'd be able to get something optically better too, but it sounds like there may be no significant difference in such a slow scope.

[Louis D]

As you can tell from my eyepiece AFOV image above, the APM UFF 24mm performs quite well at f/12, has a 63° measured AFOV, a 65° effective AFOV, a 27.5mm measured field stop (as big as you can go, with a bit of field stop fuzziness), and 17mm of measured, usable eye relief.  It is quite usable with eyeglasses and sharp to the edge at f/12 (and probably f/10 as well).

I already had 2" diagonals and eyepieces, so I just needed to get the MCT to SCT thread adapter and an SCT 2" visual back for my 127 Mak.  I think those two together were under $50 pre-inflation.

The loss of illumination is surprisingly hard to notice visually with 2" eyepieces.  The astonishingly wider true field of view is not.  The only annoying artifact is when a bright star or planet passes the edge of the 27mm rear port.  It starts reflecting off the rear baffle and creates an oval artifact in the field of view.  I keep meaning to flock that baffle tube to see if I can minimize or eliminate this.

You could try to hunt down an 80s or 90s vintage 30mm/32mm Plossl with the eye lens mounted nearly flush to the top of the eyepiece.  Apparently, enough non-eyeglass wearing folks complained about it being hard to hold the exit pupil in them, so practically the entire marketplace recessed the eye lenses of their longer focal length Plossls.  I can just use my 32mm GSO Plossl with glasses, but not my 26mm Sirius Plossl.

There are a bunch of 80s/90s/early 2000s vintage, Japanese made 1.25" Konigs in the 24mm to 32mm range that come up occasionally in the classifieds, at least here in the US.  They also had their eye lenses mounted almost flush to the top of the housing, and so should be usable with eyeglasses.  They typically had a 60° to 65° AFOV.  They perform fine at f/10.

You could also keep an eye out for the discontinued Meade 5000 Plossl line.  They had a 60° AFOV and are basically the same as the overpriced ES-62 line.  All had their eye lenses mounted flush to the top, have a removable twist-up eye cup (with a bit of work), and should have enough eye relief in the 26mm version for eyeglass wearers.  I have the 40mm 2" version, and it is a really nice eyepiece at f/12.  The outer 50% of the field is fuzzy at f/6, so not recommended in faster scopes.

Not widest field, but still a nice, cheap eyepiece at f/12 (probably f/10 as well) is the SVBONY 68° Ultra Wide Angle 20mm.  It has enough eye relief to be usable with eyeglasses and is basically sharp to the edge at f/12.  It has a 23.7mm measured field stop.  It does better than the Orion SWA as can be seen below.  I use a pair in my Arcturus binoviewer for widest field viewing.