Long focal length Plossl eyepieces.

[Grump Martian]

For low power and widefield viewing I use a 22mm 82° and 32mm 68° eyepieces. Using them is very comfortable with no kidney bean effects. I have though often though about longer focal length Plossl eyepieces. Televue have a 55mm 50° Plossl . Stellalyra have a 50mm 48° Plossl. I have an unbranded 40mm Plossl which is differcult to use to be honest. I wonder how owners of the above Plossls get on with them. What are the views like?

Edited January 2 by Grump Martian

[JTEC]

You might find this relevant and interesting  … interesting site in several ways imv

http://www.scopeviews.co.uk/My Favourite Eyepieces.htm

[John]

I've owned a Vixen "silvertop" 2 inch 50mm plossl in the past. It's performance was decent but the eye relief was long so I needed to "hover" my eye well above the top of the eyepiece, not something that I'm fond of doing. The AFoV of that one was stated as 50 degrees but it may have been a degree or two less in reality. 

I used this one with a C8 SCT I had at that time. I think I decided that I liked the views that 40mm SWA eyepieces produced better (darker background sky ?) and they showed the same amount of true field.

 

[Mr Spock]

For the widest field possible I use a 42mm LVW. However my favourite is the 30mm UFF; lovely to look through, sharp, and with a flat field of view. At 70° it's fairly wide too.

[Louis D]

Unless you're using a super slow scope like an f/15 Mak and want a large exit pupil for narrow band nebula filters, I can't think of a particularly good reason to have a long focal length Plossl.

[Second Time Around]

I use a 40mm Plossl with my Quark at f/26 for h-alpha solar when seeing is poor.  I also use a 55/67mm afocally with my night vision device.  The latter makes a huge difference to the brightness of nebulae.  But these are quite specialised uses.

[John]

Celestron used to market the Japanese made Ultima eyepieces in long focal lengths. This was a 5 element modified plossl design I think. Here are the 45mm, 60mm and 80mm from that range, all in the 2 inch fitting:

 

Edited January 2 by John

[Second Time Around]

Very interesting, John.  I'd love to get hold of an 80mm.  The reduced apparent FOV wouldn't be so much of a problem with my night vision device as it's limited to 40 degrees anyway.

Anyone got one to sell by any chance?

[John]

11 minutes ago, Second Time Around said:

....Anyone got one to sell by any chance?

Not me I'm afraid Steve - I've owned the Ultima's up to 35mm FL but none of the "big boys" 🙂

[Louis D]

They are highly sought after in the US, show up infrequently, and command high prices for what is a basically a Plossl.  I think Gary Russell or Harry Siebert could cobble together something similar if asked.

[John]

1 hour ago, Louis D said:

They are highly sought after in the US, show up infrequently, and command high prices for what is a basically a Plossl.  I think Gary Russell or Harry Siebert could cobble together something similar if asked.

The original Ultimas, Orion Ultrascopics and Parks Gold Series are highly favoured but some members here too as I recall.

I believe they were made by Masuyama in Japan.   

Edited January 3 by John

[Nik271]

Wow, that's 80mm focal length eyepiece in the photo! My calculator shows that it's maximal apparent field of view will be 2 arctan(5/16) or about 35 degrees, and this is assuming a 50mm field stop. 

[John]

5 minutes ago, Nik271 said:

Wow, that's 80mm focal length eyepiece in the photo! My calculator shows that it's maximal apparent field of view will be 2 arctan(5/16) or about 35 degrees, and this is assuming a 50mm field stop. 

That sounds right. The 60mm and 80mm's are very niche eyepieces !

 

[Second Time Around]

On 03/01/2024 at 14:48, Louis D said:

They are highly sought after in the US, show up infrequently, and command high prices for what is a basically a Plossl.  I think Gary Russell or Harry Siebert could cobble together something similar if asked.

Thanks, Louis.

Harry Siebert quoted me $300 for a 67mm Plossl.  That was 2 or 3 years ago too.  This is the maximum focal length with a 2 inch focuser to get a 40 degree FOV that night vision devices have. 

Unfortunately, Gary Russell doesn't sell to the UK.  It may be that, like some other US companies, he doesn't have an export license.

Edited January 31 by Second Time Around

[Louis D]

2 hours ago, Second Time Around said:

Unfortunately, Gary Russell doesn't sell to the UK.  It may be that, like some other US companies, he doesn't have an export license.

It's probably the whole change to making the international seller collect VAT instead of the courier/buyer collecting/paying VAT.  It creates a huge amount of paperwork to collect it for each and every possible taxing jurisdiction and then remit it.  I think that's what Don P.  ran into with his recently shuttered store.  He only had to collect sales tax for California sales because he didn't exceed $500,000 in sales to any other state.  That really simplifies paperwork.

I also know Gary is 80 miles from the nearest post office from which to get the proper export paperwork to fill out and file, so he doesn't want to deal with that headache.

The US market is large enough that most sole proprietorships don't feel like they're giving up much in sales to stick with US-only sales.

[Don Pensack]

5 hours ago, Nik271 said:

Wow, that's 80mm focal length eyepiece in the photo! My calculator shows that it's maximal apparent field of view will be 2 arctan(5/16) or about 35 degrees, and this is assuming a 50mm field stop. 

Without distortion, the apparent field at 80mm and a 46mm field stop would be 32.1°.  Field stops in simple eyepieces don't exceed 46-46.5mm in a 2" eyepiece.

Add a typical 4% distortion and you get about a 33-1/2° field.

[Louis D]

3 hours ago, Don Pensack said:

Without distortion, the apparent field at 80mm and a 46mm field stop would be 32.1°.  Field stops in simple eyepieces don't exceed 46-46.5mm in a 2" eyepiece.

Add a typical 4% distortion and you get about a 33-1/2° field.

Sure they do when they're above the shoulder.  There's some vignetting and lots of in-focus is required, but it does work.  I've got the bare upper section of a 12mm ES-92 with a 51mm physical field stop that actually measures in use to be 48.4mm, probably due to cutoff from the internal shoulder step.  Chromatic aberration is so bad toward the edge that I've never noticed vignetting.

[Mr Spock]

My 42mm LVW measures 47.50mm. Although it says 42mm and 72°, I find the focal length is only 42mm in the centre and is around 38mm on average - so I use that for my fov calculations, or around 3.7° actual in my 4" Tak. If you work back from the field stop it gives 3.678° so 38mm seems accurate enough.

[Highburymark]

On 02/01/2024 at 23:47, John said:

Celestron used to market the Japanese made Ultima eyepieces in long focal lengths. This was a 5 element modified plossl design I think. Here are the 45mm, 60mm and 80mm from that range, all in the 2 inch fitting:

 

Didn’t know about the 80mm - certainly never seen anything of that focal length here in Europe, apart from the 3” Masuyama 80mm. But that costs around twice an Ethos 21mm IIRC. I’m happy with the 67mm TeleVue which I use most nights with night vision. Did try it in a C8 once without night vision and it was pretty decent.

[Louis D]

I recall seeing one of the Celestron long FL lineup in a photography store glass case back in the mid-90s.  It was impressively large for the day, and expensive.

[Highburymark]

Then there was that bizarre Vixen 1.25” 100mm finder eyepiece, which I thought was an April fool when first pictured

[Don Pensack]

On 03/01/2024 at 17:42, Louis D said:

Sure they do when they're above the shoulder.  There's some vignetting and lots of in-focus is required, but it does work.  I've got the bare upper section of a 12mm ES-92 with a 51mm physical field stop that actually measures in use to be 48.4mm, probably due to cutoff from the internal shoulder step.  Chromatic aberration is so bad toward the edge that I've never noticed vignetting.

That's not a "simple" eyepiece, i.e. an all-positive design with the field stop below the field element.

Field stops in 2" all-positive designs max out at ~46.5mm.

What do you mean by "bare upper section?  Did you remove the field lens?

ES says the field stop int he 12mm ES92 is 19.6mm, i.e. the eyepiece has a "virtual" field stop that can't be measured by measuring an internal stop.

[vlaiv]

Out of interest, has anyone tried to make a plossl eyepiece?

It should not be too hard as it can be made in symmetrical configuration. I don't know exact prescription for each lens - but what about using available cemented doublets?

Quick search on AliExpress returned this hit:

42mm diameter 110mm focal length for about 10 euro with shipping.

Two of these spaced at say 50mm would give combined focal length of

42 + 42 - 50 / 42*42 = 1/f

f = 42*42 / 34 = ~52mm of combined focal length.

We can even aim for certain FL - say if I want 45mm of FL then I need to space lenses at 42^2 / 45 = 39.2 so 84 - D = 39.2 => D = 44.8mm

Let's do math again, I messed up - used diameter instead of focal length.

110 + 110 - 50 / 110*110 = 1/f

f = 110^2 / 170 = 71.17mm

It turns out that with these lenses we can get down to only 55mm (if we have 0 separation which itself is not possible) and not less!

Found it!

D32 F86 with distance of 7.6444mm

Anyway, you get the idea ...

 

 

Edited January 5 by vlaiv

[Zermelo]

2 hours ago, vlaiv said:

Out of interest, has anyone tried to make a plossl eyepiece?

 

[Louis D]

4 hours ago, Don Pensack said:

That's not a "simple" eyepiece, i.e. an all-positive design with the field stop below the field element.

Field stops in 2" all-positive designs max out at ~46.5mm.

What do you mean by "bare upper section?  Did you remove the field lens?

ES says the field stop int he 12mm ES92 is 19.6mm, i.e. the eyepiece has a "virtual" field stop that can't be measured by measuring an internal stop.

I recollect it came from Telescope Warehouse originally.  The owner, Bill Vorce, seems to get a lot of surplussed ES equipment.  This particular eyepiece (the 12mm ES-92) has no Smyth lens group, only the upper, positive section.  I'm guessing it was an abused return.  Without the lower section, it has a 28.5mm focal length with an exposed field stop below the lenses.  I removed the lower section's empty tube and added two step rings to adapt the original 2" nosepiece directly to the upper section.  This reduced the required in-focus from 40mm to 21mm.

It's third from the left in the group shot below.  In the second image, you can see it has such a wide true field of view that I had to add a second yardstick to measure it.  The slightly dark line near the left edge of the ES-92 29mm "full view" is where they push against each other.  None of my other eyepieces, even my 40mm Pentax XW, are this wide in TFOV.  If ES could reign in the extreme chromatic aberrations with a compensating prismatic, but zero power, lower element, it would make an awesome eyepiece.  Even as it is, I love scanning rich star fields with it.  It is what the Kasai Super Wide View 30mm 90° should have been.