28mm EP - Edmund Optics RKE or Tak MC Erfle?

[parallaxerr]

I'm on the hunt for a 28mm eyepiece to fill a (probably psychological) "hole" in my collection, between 24 & 32mm. A 28mm EP would give me an exit pupil in my current scope of ~3.3mm, which I have found in previous scopes to be favourable for DSO viewing in my local skies.

Well, there's plenty of 28mm EP's out there you may say, but here's the clincher; I want to stick with the 1.25" format to avoid the associated on-cost of 2" accessories and filters. Lots of searching has resulted in the EO RKE Erfle and TAK MC Erfle being the only readily available options.

Whilst the TAK has an extra 15° of AFoV, I read that edge correction is poor. Conversely, the EO RKE has only 45° AFoV but is supposedly sharp to the edge and is said to have a particular floating effect that is "magical".

The TAK comes in at circa £210 with the RKE @ £72.

Has anyone compared these eyepieces to make a subjective comparison?

Are there any other 28mm 1.25" options I'm missing? A 28mm plossl would be ideal, but no luck finding one.

Cheers all,

Jon

 

Edited October 21, 2020 by parallaxerr

[John]

What scope will this be used in Jon ?

The Erfle design is not generally known for being sharp right across the field in faster focal ratios, regardless of what the adverts say.

There is the OVL 27mm flat field but again it's not too well corrected in fast scopes. Decent in F/10 ones though:

https://www.harrisontelescopes.co.uk/acatalog/skywatcher-extraflat-27mm-eyepiece-1.25.html?gclid=EAIaIQobChMIrY-mtIfG7AIVFu3tCh12_wnPEAQYAiABEgJRUfD_BwE

 

 

 

Edited October 21, 2020 by John

[parallaxerr]

Hi John,

It's to go in the F8 achro.

Hadn't seen the OVL EP as I was searching 28mm but 27 is close enough, thanks for the link. In fact 27mm nails the exit pupil target, I was just splitting the gap at 28mm.

Do you have any experience of the OVL 27mm?

[John]

I had one for a while. It worked well in my slower scopes - F/8 and F/9. In faster ones it still worked of course but showed some edge of field astigmatism. Quite a nicely made eyepiece I thought. Similar in build quality to the Nirvana's and William Optics ranges.

Might be worth a "wanted" advert if you are not in a hurry. You might be able to pick one up for quite a bit less than the retail price.

[parallaxerr]

Sounds good John, reviews seem mixed but nothing really bad and mostly liked. Certainy in no rush judging by the weather forecast!!!

[martinl]

Used, you may be able to find a Pentax 28mm XL (55 degrees). They are supposed to be nice, but I haven’t tried one (loved the 7mm XL I had, though).

New, I’d be tempted to go with the RKE. They have a cult following for that special “floating” effect and would be fun to try out. 

[parallaxerr]

That Pentax looks interesting Martin, never had one. Looks like a wanted ad would be required there.

The reports of the RKE floating effect are certainly making me think hard about this one!

[Ruud]

The floating effect comes from the tapered top of the housing. The 28mm RKE has a lot of eye relief, and from the proper viewing distance you only see a slim rim of housing because you are looking alongside the sloping top.

So the image fills the eye lens while the eye lens seems to hang in front of you, almost as if not supported at all.

This effect is not unique to the 28mm RKE. When you use a Morpheus without eye cup you get a similar illusion. Should you use the 28mm RKE with an eye cup, I'm sure the floating image effect would disappear.

I was amazed of the image quality the 28mm RKE delivers in the f/4.2 Edmund Asroscan. Being only a three element design the Rank Kellner design ought to be a very popular budget eyepiece. Unfortunately, the price for a 28mm RKE is $85.

[parallaxerr]

Thanks Ruud, that's how I understand it. One negative I'm picking up on is the effort required to hold the specific position to achieve the effect, which may be less than ideal for longer sessions.

[Ruud]

Yes, that happens with long eye relief eyepieces. Some people find it difficult to keep their eye centred on the exit pupil. Tactile feedback from a long enough eye cup usually helps.

Still, I think an eyepiece like the RKE should be priced under £50. It is after all a simple three element 45° design.

But it still is an appealing eyepiece! Maybe you can find one on the used market.

 

[John]

54 minutes ago, parallaxerr said:

Thanks Ruud, that's how I understand it. One negative I'm picking up on is the effort required to hold the specific position to achieve the effect, which may be less than ideal for longer sessions.

I'm not a fan of "floating" my eye over the top of an eyepiece personally. I like a nice eye cup to rest my eye socket against and also to keep stray light off the eye lens.

I had that problem with the ES 92 12mm and the Tele Vue 32mm plossl before I fitted the eye cup extender to it.

These are personal preferences though. Some folks seek out these characteristics.

I nearly mentioned the Pentax XW28 in my earlier post but I'm aware that they are like hens teeth to find and do have some mild issues of their own. Probably better than an erfle though.

 

 

 

Edited October 21, 2020 by John

[Louis D]

28mm RKE in a field flattened AT72ED at f/6 compared to some others in my collection:

[parallaxerr]

Thanks for the info @Louis D, I've seen your posts like this on CN and they're very helpful!
 

By way of an update, I ordered the OVL 27mm Extra Flat as it seems to strike a nice balance between price and FoV, hopefully it will be comfortable enough to use for extended sessions. I'm hoping it will be of similar performance to that APM 24mm, nice and flat.

Edited October 24, 2020 by parallaxerr

[globular]

On 21/10/2020 at 19:49, John said:

I nearly mentioned the Pentax XW28

Is there a 28mm XW?  Or did you mean XL?

[John]

15 minutes ago, globular said:

Is there a 28mm XW?  Or did you mean XL?

Ooops !! - I meant XL. That focal length never appeared in the XW range, they went to a 2 inch 30mm instead. Sorry about the confusion !

 

[Don Pensack]

4 hours ago, parallaxerr said:

Thanks for the info @Louis D, I've seen your posts like this on CN and they're very helpful!
 

By way of an update, I ordered the OVL 27mm Extra Flat as it seems to strike a nice balance between price and FoV, hopefully it will be comfortable enough to use for extended sessions. I'm hoping it will be of similar performance to that APM 24mm, nice and flat.

JUST BE AWARE THAT THE 27MM IS A 53° APPARENT FIELD.  THE 24MM APM IS 65° (CLAIMED, 63° MEASURED).

[parallaxerr]

1 hour ago, Don Pensack said:

JUST BE AWARE THAT THE 27MM IS A 53° APPARENT FIELD.  THE 24MM APM IS 65° (CLAIMED, 63° MEASURED).

Hi Don, thanks for confirming 👍. I did realise that and saw it as middle ground between the Tak and RKE. 

Now, if someone could make a 28mm 68° 1.25" ep, I'd be most grateful 😂 

Edited October 24, 2020 by parallaxerr

[globular]

22 minutes ago, parallaxerr said:

Now, if someone could make a 28mm 68° 1.25" ep, I'd be most grateful 😂 

I know you're only joking - but there is some maths that shows why this can't happen.

Don will correct me if I've got this wrong; but there is a formula that links fov, fl and barrel size (actually field stop (fs)).

fov = (fs / fl) * (180/π)

so if you want fov=68 with a fl=28 then you need fs = (fov * fl) / (180/π) = 68 * 28 / 57.3 = 33.2

and a 33.2 field stop can't fit inside a 1.25" barrel, which is only about 28mm usable, without bad vignetting caused by the barrel.

[Louis D]

5 hours ago, Don Pensack said:

THE 24MM APM IS 65° (CLAIMED, 63° MEASURED).

True, but due to distortion, it has a 66° effective AFOV.  At 27.5mm, it has just about the largest possible effective field stop in a 1.25" eyepiece that doesn't require massive in-focus.  The edge does suffer a bit for it, though.

I prefer the 22mm AT AF70 in all ways over the 24mm APM UFF if a 2" focuser is available.  It's cheaper, wider (70° AFOV, 74° eAFOV), better at the edge, and has a slightly larger effective field stop of 28.4mm.  Eye relief and flatness of field is about the same for both.

[Don Pensack]

The AT AF70 22mm is no longer available, but the eyepiece is still available as the Astromania 22mm 70° SWA and several other brands (e.g. Omegon Redline, Telescope Service Expanse, etc.)

I think it's fairly inexpensive.  I clculate a 26.9mm field stop, though.  How did you get 28.4mm?

I like it too but it isn't compatible with binoviewers, and some people get the 24mm APMs for binoviewer use with glasses.

 

The 24mm APM UFF had a 63° apparent field in the flashlight test (+/-)

Most on-line retailers are claiming the field stop is 30.2mm, but it is not possible.

Even the engineering diagram shows something in the mid 27mm range.

27.3mm is more likely to be the correct figure.

I did some crude timings of stars and got 27.3mm +/- 0.1mm, which is close to your figure.

[Voyager 3]

On 22/10/2020 at 01:41, Louis D said:

28mm RKE in a field flattened AT72ED at f/6 compared to some others in my collection:

How many lenses does the Meade HD uses ? And how many does the APM uses ? The APM , pan , aspheric are quite different in brightness compared to the Meade HD etc . I think it will not cause a difference in night time viewing...

[Louis D]

8 hours ago, Voyager 3 said:

How many lenses does the Meade HD uses ? And how many does the APM uses ? The APM , pan , aspheric are quite different in brightness compared to the Meade HD etc . I think it will not cause a difference in night time viewing...

I wouldn't read too much into the absolute brightness of each eyepiece taken in isolation.  The camera was on auto exposure for each eyepiece, and the brightness of the room may have also changed as more or less light streamed in the nearby windows as the day wore on during testing.  What's more relevant is light falloff center to edge within each exposure for each eyepiece.  Notice how the RKE gets dim near the edge?  Notice how the Aspheric fuzzes out near the edge?  These are the relevant sorts of things to look for within each eyepiece's image.

The Meade HD-60s are purported to have 6 elements each.  Four in the upper, positive section, much like a 1-2-1 Konig and two in the lower, negative section, much like a Barlow or Smyth lens.  So, most likely 6 elements in 4 groups.  I can't find any definitive diagrams for you, though.

The 24mm APM UFF has 8 elements in 5 groups as seen below:

The Panoptic uses 6 elements in 4 groups as seen below:

The Aspheric is three elements as seen below: