Brief observations on some wide angle 2” EPs inc Siberia 50mm Erfle

[markse68]

I should preface this by stating these are just some observations from me- I’ve only been in this hobby this past half year or so and am no eyepiece or optical expert- I have very little experience of eyepieces, particularly high end ones and  I’ll probably even use the wrong terms. And this is based on about an hour comparing these eps in daylight on terrestrial targets. Eyepieces compared in my 8.75” f7.3 dob.

I finally yesterday finished major upgrade to my dob which included fitting a 2” moonlite focuser. Over the past 6 months I’ve been collecting a few 2” eyepieces in preparation for this momentous event! 

So today I had first chance to try them and compare. From left- Tal 25mm Plossl (1.25” reference ep), mystery military ep of unknown fl, the intriguing Siberia Erfle 50mm (available from ENS), TV WF 32mm, TV WF 40mm

My target was the HSBC building, 1.3miles from me and useful for comparing  actual field of view by counting visible window panes across the afov. This is the actual view through the Tal 25mm plossl. Mine has an unusually (for a plossl) large afov of 60deg.

Mystery military  EP

Been dying to try this ep since I picked it up on eBay for £20 as it seemed to have a huge afov- similar to a Nagler! Early observations suggested it  had an orthoscopic characteristic with very low rectilinear distortion but how did it perform in my scope? Well I like it a lot. It seems to compare in magnification to the Tal plossl so I think it’s a 25mm too. The field of view is huge, the image very sharp in the centre and colours neutral. There is no noticeable rectilinear distortion- the frames of the windows appeared perfectly straight across the field of view though I’m not sure how important that is in astronomy 🤔. Downsides were it loses focus right at the edges but it wasn’t the worst offender for that. It also shows a colour fringe at the very edge but that’s so far out it’s not offensive. In terms of actual fov (counting windows) it matches the TV WF 32mm. In terms of ergonomics this was the second best- it was very comfortable to view through and eye placement was fairly easy though it was possible to get blackouts if you moved too far off axis

Siberia Erfle 50mm

I stumbled across this on eBay and purchased it from ENS for what seemed a reasonable price given it’s new old stock and quite unusual- possibly manufactured by Tal but branded Siberia. It’s a very physically long ep but remarkably light for its size- significantly lighter than the TV WF 40mm! I’d read others commenting that they couldn’t reach focus with it in their scopes and it turned out it needs 15mm in-focus to reach its focal plane. So I made sure when designing my upgrades to allow 20mm in-focus from the primary focal plane. This ep has afov comparable to a TV plossl, so about 50-55degrees I think. That makes it the narrowest afov on test. It has a slight but noticeable yellow caste to the image- a vintage tone. It has a lot of rectilinear distortion (pincushion) though comparable to the Tal plossl. It is the worst on test for edge loss of focus. Doesn’t sound very good... But it has the easiest eye placement of the eps on test! And it has the same actual fov as the TV WF 40mm, yet is much lighter than that lens. And it’s available to buy new! (very limited stocks though I believe). I think I’d rather carry this around with me than the TV WF 40 (which is a lump!) but I need to use them both properly first to evaluate.

TV WF 32 and 40mm

these shared common characteristics. They have the same afov of more than the Tal plossl but less than the mystery military ep- so maybe 70deg? They have a lot of pincushion distortion but are pretty sharp to the field stop. They were also the worst for eye placement with the 40mm being the worst. Seeing the whole field sharp was really tricky on the 40mm but I wonder if that will improve at night with larger dark adapted pupils? I think this has something to do with exit pupil size.

Mine is quite a slow scope so I’m not sure coma would come into this (I’ve not knowingly seen coma yet anyway) but will update if this should become apparent under the stars.

Mark

 

 

 

 

 

 

 

Edited November 24, 2019 by markse68
Formatting

[dannybgoode]

Nice comparison Mark. So would you recommend the Siberia or is it one to steer clear of?

[markse68]

Too early to say really- I think if you have a fancy wide-field wide angle ep already  then only if you’re into curiosities, but if you’re after a cheapish very wide angle ep probably similar to say a 55mm plossl but with very ergonomic eye placement then it might be worth a punt? Steve at ENS says he uses them for outreach as they’re so user friendly and you’re not going to worry too much about kids mucky fingers on the lens 😉 It’s the widest view ep I have (tying with the TV WF 40mm but I’m not lugging that around- beautiful though it is- so I’ll be using it for the time being 👍

[John]

Very interesting report Mark.

I used to have 3 of the Tele Vue Widefields - 15mm, 19mm and 24mm but these were in the 1.25" fitting. These were modified erfles I believe. Long out of production now though and replaced with the Panoptics which themeselves have now been around for a long time too !

The TV Widefields were specced as 65 degree AFoV eyepieces.

Interesting to read about the other eyepieces too - I've owned a couple of the Tal 25mm plossls but not the others you report on.

[markse68]

Thanks John, yes I’m keeping my eyes peeled for the other members of the TV WF family but they don’t seem to turn up very often these days- particularly the shorter focal lengths. I don’t think the Tal is the greatest plossl optically but I love its wide afov and find myself reaching for it every time I get out with the scope- it’s a favourite.  I have a feeling the military ep will be another 😉

Edited November 25, 2019 by markse68

[John]

Interesting that you assess the AFoV of your TAL 25mm plossl at around 60 degrees. I've owned 3 of them over the years and they all had a different size field stop and therefore a different AFoV. The widest of mine was around 55 degrees I think and the narrowest around 48 degrees. All 3 performed pretty well.

 

[Stardust]

Scope looks nice and you finally got to use the siberia. If those issues show up during the day I'm dubious of what its performance will be like on the night sky...

[markse68]

Still evaluating it tbh- just had a nice moon session and it was quite fine. The slight sepia cast wasn’t offensive and the image was sharp. I did get a very slight green fringe at the edge of the lunar disk but not horrendous. The moon went quite egg shaped as it moved toward the edge of frame but no worse than the TV WFs. 

On bright stars there’s definitely some slight coma (edit- astigmatism- little seagull stars) at the edge of field but I was also seeing that with the mil ep- not so the TV WFs.

The slight loss of focus at the edges was more noticeable in daytime than at night- most objects you’d be looking at would fit in the central area I guess so unlikely to be an issue- it didn’t bother me anyway- didn’t notice it unless looking for it.

It’s still the most comfortable eye placement wise of these eps but I think I prefer the wider afov of the mil ep. 

I think it’s a good ep- not great and not without minor issues but quite useable and sharp and it’s got the wide fov.

Mark

Edited December 10, 2019 by markse68

[Stardust]

Great to here of some real world use, sounds quite reasonable  for the money. Easy eye placement can be a real help when doing outreach.