Impossible??

[Tropo-Bob]

I saw the following advert on Amazon. IMO, there is no way that a 38mm 1.25" EP can have a 52degree field of view.

However, I would love it if was true. Any thoughts on how this could possibly be true?


NEEWER 1.25" Wide Angle Telescope Eyepiece, 52 Degree Wide Apparent Field Lens with 38mm Focal, Multiple Coated HD Optical Glass 4 Elements Standard 1.25"

[refractordude]

Hello there Tropo-Bob


How about a link, thanks.

[Tropo-Bob]

Link:

https://www.amazon.com.au/s?k=telescope+eyepiece&page=2&crid=P81QAUI1NA2P&qid=1720873525&sprefix=telescope+e%2Caps%2C279&ref=sr_pg_2

[leon]

Maybe, maybe not but i wouldn't buy anything from Amazon, ever.

Leon :thumbsup:

[Stefan Buda]

Looks like they even invented a new type of glass to make the impossible become possible: "HD Optical Glass"
I have designed a few eyepieces in the past and the longest focal length I was able to fit into a 1.25" barrel was 32mm with 45 degree field of view.
A quick calculation shows that at 38mm focal length the field of view drops to about 40 degrees, but, allowing some field distortion, it can be increased to 42 degrees. I think 52 degrees on the barrel looks much better than 42, for marketing, and most buyers won't be able to measure it anyway.

[SkyWatch]

The maximum field stop for a 1 1/4" (32mm) eyepiece is around 27mm- and this dictates the maximum possible FOV - and therefore the maximum AFOV as well.
Televue note that their 32mm Plossl with a 50 degree AFOV and a 27mm field stop gives the maximum true FOV possible for a 1 1/4" (32mm) diameter eyepiece.
A 38mm fl eyepiece with a 52 degree AFOV would need a field stop of around 37mm diameter- which would mean an outside diameter for the eyepiece of around 41mm or 1.6"... This is quite difficult to achieve with a 1 1/4" eyepiece. ;)

Sorry Tropo-Bob, but as you suspected it isn't possible.

(BTW, I loved the blurb for the Neewer eyepiece in question which states that it "Comes with a standard 1.25" interface..." I suspect that the person who put together this ad simply has no idea.)

[refractordude]

Hello there Leon


Amazon and Walmart have the best return policy. Both pay for return shipping within 30 days. However, you have to be careful the seller is not a vendor. The item must be shipped to you from Amazon/Walmart.

[Stefan Buda]

A 27mm field stop in a 1.25" barrel would need to be very close to the front end of the barrel in order to avoid vignetting in a normal Newtonian scope.
So, even Televue are pushing it a bit because the field stop of their 32mm Plossl appears to be quite a bit back from the front end of the barrel.

[Tropo-Bob]

Thanks guys, good answers.





Leon,
About 2 years ago, I brought a new, 3.5mm XW Pentax from Amazon for $175.00. That was an incredibly good deal.

Today, I ordered a 15mm Vixen Plossl for $52.30. That is also a low price compared to anywhere else.

[OzEclipse]

Hi Bob

Longer focal length eyepieces, dobs and middle age eyes don't play very well together. They are a young observers eyepiece.

For example, if you use a 12" f5 and a 40mm eyepiece at around age 50, only half the light enters your eyes and you get the light transmission/object brightness equivalent to an 8" scope.

Not an issue if you are using a f10 SCT but more of an issue the faster the f ratio of your scope.

Joe