Results 1 to 3 of 3

Thread: AFOV question

  1. #1
    mitaccio's Avatar
    mitaccio is offline Junior Member
    Points: 7,626, Level: 61
    Level completed: 26%, Points required for next Level: 224
    Overall activity: 0%
    Achievements:
    10 Days registeredFirst 1000 Experience Points365 Days+ Registered Achievement!750 Days+ Registered Achievement!1000 Days+ Registered Achievement!
    Join Date
    Feb 2009
    Posts
    24
    Points
    7,626
    Level
    61
    Thanks
    0
    Thanked 2x 2 Posts

    Default AFOV question



    I want to understand the value of say a Naegler (sp?) eyepiece. I have a scope with a 1200mm focal lenght. I'd like to get an eyepiece that will give me a really wide view of the sky. I know that the higher the focal length of the eyepiece the higher the magnification. And that to find the magnification I divide the focal length of the scope by the focal length of the eyepiece. But the FOV. I divide the magnification by the FOV? So if I have a 1200 mm scope with a 25 mm ep, I get 48x. If the eyepiece is 50mm FOV, I roughly have 1 degree of Actual FOV? Seems narrow.

    With a 1200 mm FL scope, what would be a good eyepiece setting to get in a wide sky view?

  2. #2
    Vinnie's Avatar
    Vinnie is offline Guest
    Points: 24,260, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    Got three Friends20+ Friends Achievement!First 1000 Experience Points5+ Referrals Achievement!100+ Threads Achievement!
    Join Date
    Sep 2008
    Location
    Queensland, Australia
    Posts
    3,474
    Points
    24,260
    Level
    100
    Thanks
    41
    Thanked 682x 362 Posts

    Default

    So what you are looking at is the TFOV (True Field of View) which is the actual width of the section of sky that appears in your view as related to the AFOV (apparent field of view, or sometimes called angular field of view) which is the angular width of that section of sky as it is distributed across the EP.

    There are three different methods available to calculate TFOV

    In the first, you need to calculate the magnification of the EP which is the FL (Focal Length) of the scope divided by the EPFL (Eye Piece Focal Length), which also demonstrates that the longer the EPFL the lower the mag

    Now by dividing the AFOV as specified by the EP manufacturer by the mag you will get a value for the TFOV

    This method is not particularly accurate, and could have an error as much as 10%, but does serve as a quick comparison of EPs and does also serve to demonstrate that for any given EP, shorter scope FLs give wider fields, or if you like, for any given scope lower mags give wider fields.

    The second method is to bring the EPs field stop into play. The field stop is the diameter of the aperture of the bottom lens in the EP, or if you like, the aperture that lets the light into the EP. Some manufacturers specify this, but its easy enough to measure anyhow.

    In this case the TFOV is the (Diameter of the field stop/focal length of the scope) x 57.3 where 57.3 is the rounded off value of a radian in degrees ie 180/Pi

    This method is more accurate, say an error of no more than about 2%, but again demonstrates that for any given EP, shorter scope FLs give wider TFOVs, or also since the longer the EPFL the wider the field stop, also shows that lower mags give wider TFOVs

    The third, and most accurate method (and I include this mainly just out of interest) is by Star Drift.

    Here you need to focus your scope on a known star (you need to know its Dec) and then move the scope so that the star drifts from one edge of the FOV, through the centre and to the other edge (ie along the diameter of the field of view) with the scope stationary. By timing this drift say 5 or 10 times then taking an average time for its drift you can use this equation

    TFOV = [(drift time) x cos(dec of star) x 360] / 86,164 where 86,164 is the number of seconds in a day

    .................................................. ........................................

    To get the widest TFOV possible with a given FL of scope you need to go to the lowest practical mag with the widest AFOV EP available.

    I can't afford Naglers, not many of us can, but if your scope accepts 2" EPs there are various EPs available at around 70° AFOV at budget prices

    So say a 38mm 70° AFOV 2" ep will give a TFOV of roughly 2.1° in your scope, or a 32mm would give about 1.9°

  3. #3
    bilbowski77's Avatar
    bilbowski77 is offline White Dwarf
    Points: 7,409, Level: 60
    Level completed: 30%, Points required for next Level: 141
    Overall activity: 0%
    Achievements:
    First 1000 Experience Points365 Days+ Registered Achievement!750 Days+ Registered Achievement!1000 Days+ Registered Achievement!2 Posts Achievement
    Join Date
    Feb 2009
    Location
    Denver CO
    Posts
    29
    Points
    7,409
    Level
    60
    Thanks
    32
    Thanked 4x 2 Posts

    Default

    Very good information, Vinnie, thanks. I am looking into some more EP's soon, but no Naglers for me, too rich for my blood.
    Lat 39.8
    Long -105

 

 

Similar Threads

  1. Afocal and AFOV
    By pederv in forum Astrophotography Forum
    Replies: 5
    Last Post: 02-02-2011, 10:53 PM
  2. is afov over rated?
    By finman in forum Telescope Eyepieces Forum
    Replies: 8
    Last Post: 01-01-2011, 06:12 PM
  3. AFOV (Apparent Field of View)
    By Sean O'Dwyer in forum General Astronomy Forum
    Replies: 2
    Last Post: 07-07-2004, 03:10 AM
  4. AFOV
    By Mike Thomas in forum Amateur Astronomy Forum
    Replies: 10
    Last Post: 07-01-2004, 03:59 PM
  5. AFOV vs Aperture Poll
    By Tony Flanders in forum Amateur Astronomy Forum
    Replies: 34
    Last Post: 01-04-2004, 12:41 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBulletin® Version 4.2.0
Powered by vBulletin®
All times are GMT. The time now is 10:26 AM.