5mm Eyepiece suggestions

[Mr Spock]

Until I can decide, I'll be using existing eyepieces with a 2" x2 Barlow I have. That consistently measures between x2.15 and x2.2 depending on what eyepiece its used with. I have both a 12mm and 10mm NLV; they give, with the Barlow, x323 and x387 - the 5mm would be x360.

It's a bit of extra faff I don't need, but I want to make the right choice. Who knows, it may give a different idea of what fl I might need.

Edit: just remembered I have an old Meade 140 Apo Barlow stashed away. Like my previous attempts with this Barlow I've measured it at x2.4. With the 12mm it gives x360...

[Ags]

I am happy with my SLV 6mm. Planning on getting the 2.5, 4, 5 and 20mm and a turret!

[Paz]

3 hours ago, Mr Spock said:

According to Vixen, the SLVs are 45° from 6mm downwards which doesn't match what other sites are saying. FOV isn't important anyway and I think this one is coming out on top regardless.

I haven't measured them but all the SLVs I have say 50 degrees on the barrel, I have them all except for the 9mm and 12mm.

[Fozzie]

No wonder I get confused.. the EP states 50, the vixen blurb says 45..

Still a lovely view irrespective!

[globular]

The old NVL line were 45 and 50 at different fl's.

But the SLV series seems to be 50 degrees throughout the range

If someone who has one can measure the field stop then we'd know for sure.

Edited November 15, 2020 by globular

[michael.h.f.wilkinson]

13 hours ago, globular said:

The old NVL line were 45 and 50 at different fl's.

But the SLV series seems to be 50 degrees throughout the range

If someone who has one can measure the field stop then we'd know for sure.

I have the 5, 9, and 15 mm SLVs, and compared the AFOV simply by holding pairs up to both eyes, pointing at an evenly illuminated white wall, and checking whether the image circles matched. The 9 mm and 15 mm match, but the 5 mm compared to either shows that the 5 mm has a slightly smaller FOV. Not something you would really notice in everyday use, but the 5 mm definitely has the smaller FOV. Therefore, I find it most likely that it has a 45 degree AFOV rather than the 50 degree AFOV of the 9 and 15 mm

[globular]

13 minutes ago, michael.h.f.wilkinson said:

the 5 mm definitely has the smaller FOV

Thanks Michael.

Shocking how so many companies think nothing of producing misleading information, often hiding behind very conveniently rounded figures.

[John]

37 minutes ago, globular said:

Thanks Michael.

Shocking how so many companies think nothing of producing misleading information, often hiding behind very conveniently rounded figures.

There are many instances of actually measured specs not quite matching claimed specs as well.

One brand whose specs do seem to be matched by reality consistently are Tele Vue. They also carry comprehensive and up to date data on their website:

https://www.televue.com/engine/TV3b_page.asp?id=214&plain=TRUE

 

 

[johnturley]

1 hour ago, michael.h.f.wilkinson said:

I have the 5, 9, and 15 mm SLVs, and compared the AFOV simply by holding pairs up to both eyes, pointing at an evenly illuminated white wall, and checking whether the image circles matched. The 9 mm and 15 mm match, but the 5 mm compared to either shows that the 5 mm has a slightly smaller FOV. Not something you would really notice in everyday use, but the 5 mm definitely has the smaller FOV. Therefore, I find it most likely that it has a 45 degree AFOV rather than the 50 degree AFOV of the 9 and 15 mm

I have the 4 mm SLV and the AFOV is definitely smaller than the stated 50 degrees, significantly smaller than that of some 50 degree Plossls that I have, and would also estimate AFOV to be about 45 degrees. 

John 

[globular]

If only someone, back in 1967 say, thought to write down some rules, an act of parliament perhaps, dealing with misrepresentation of information.
And maybe these rules would be refined and updated over time and even incorporated in a wider European framework?
And then they could be policed with the toothless authority of a regulator who serves the needs of business over the consumer?
🤔

....

It could be a lot worse, of course.
And thankfully we have some very fine retailers that do what they can despite poor information from some manufacturers.

[Mr Spock]

I used to have a 4mm NLV and that was definitely smaller then the 9mm I had. 45° v 50° seems accurate.

I won't mention the 42mm LVW 😜 When I hold that to my eye as mentioned above, the FOV is half way between a 65° LVW and an 82° Nagler regardless of what anyone says its FOV is... 

[Louis D]

Clearly, when production was moved to China between the NLV and SLV lines, the optical designs were not updated, but the marketing and eyepiece labeling groups got lazy (I'm being generous here to not call them dishonest).  It's not as bad as Meade calling their MWA line 100 degrees when they're way short of that.  Your very own Winston Churchill coined the term "terminological inexactitude" back in 1906 which sort of applies here.

[Don Pensack]

Vixen alas, doesn't state the field stop diameters anywhere in their literature.  Because these are negative/positive designs, a measurement of the internal iris will not give you accurate, effective, field stop diameters.

What will is doing a star timing of a star on the celestial equator and backing into the field stop diameter with:

FS = (TF x TFL) / 57.296  Where TFL is telescope focal length, TF is true field in decimal degrees.

 

As for the accuracy of quoted apparent fields in eyepieces, NO COMPANY is accurate, even TeleVue (though they're close).

Look at the measured apparent fields of the eyepieces in Ernest's tests:

http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p41976