Eyepiece letters

[Zul]

Hello everyone i want to ask what is the meaning of the letters witch are on my eyepieces for example SR4mm , H20mm? Also i want to ask is there any way to know what is the eyepiece field of view?

Thanks all.

[Capricorn]

SR = Special Ramsden or Super Ramsden or Symetric Ramsdem.

Take your choice what you want "S" to denote, seems to be a lot of different opinions around.

H = Huygens.

Not sure what the specifics of the individual designs are. SR's seem to have 3 lens in them, think it is 3 elements in 3 groups. No idea about the H's. However they both seem to be the inexpensive end of the market and of dubious quality and performance.

Info from: Telescope Eyepiece Fundamentals

In general each design has a defined field of view, plossl's = 52 deg, seems that planetary's are 55 deg. What H and SR's are I do not know, it will be somewhere just locating seems a problem.

[John]

Huygenians and S Ramsdens (I've seen the "S" defined as meaning a number of different things) have an apparent field of view of around 40 or 45 degrees. As Capricorn says they are basic eyepieces, OK to get started but designs like plossls, orthoscopics and proprietary ones like BST Explorers will deliver more satisfying views when you are ready to upgrade.

[Zul]

Thanks

[kenmyers]

I have an SR and plenty of huygens for both the .965" and 1.25"... The field of view is VERY POOR in the .965" but not all that bad in the normal 1.25".

I have been given a 25mm Kelner for the .965" and it see's two full moons of view - squared.

I have no idea how to work out field of view... does anyone?

[John]

Ideally you need to know the apparent field of view of the eyepiece to work out the true field of view when used in a particular scope.

Assuming that the .965" 25mm Kellner eyepiece has an apparent field of view of 40 degrees, used in a 900mm focal length scope it gives a magnification of 36x. Divide the apparent field of view by the magnification given (40 / 36) and you get a figure of 1.11 which is the true field of view in degrees. Just over 2 times the diameter of the full moon.

[Astro Imp]

Find a star on or near the equator time how long it takes to travel across your EP in seconds, divide by 240. This gives the true field in degrees.

[kenmyers]

Thank you John and Thank you Astroimp. This is a great help and gives me something else to do tonight!

clear skies! yey!