Results 1 to 4 of 4
Like Tree1Likes
  • 1 Post By kirk

Thread: Triplet or Doublet Lens Refractors?

  1. #1
    admin's Avatar
    admin is offline Administrator
    Points: 6,989,798, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 5.0%
    Achievements:
    First 1000 Experience PointsGot three Friends20+ Friends Achievement!5+ Referrals Achievement!100+ Threads Achievement!
    Join Date
    Mar 2007
    Location
    Sydney, Australia
    Posts
    13,481
    Points
    6,989,798
    Level
    100
    Thanks
    18,563
    Thanked 7,916x 3,807 Posts
    Blog Entries
    2

    Question Triplet or Doublet Lens Refractors?



    Hi guys what is the difference between a triplet and a doublet objective lens refractor besides the hefty price?
    Name: Gus OTAs: ED 100 PRO refractor, Orion ST80 (not the CF), 8" Dob stuck in Canada Mounts:HEQ5PRO Synscan mount, Manfrotto Tripod CAMS: Guidecam Philips SPC900 webcams (4), Canon unmodded-450D DSLR

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.


    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    |
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    | My Astronomy Blog

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.


  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 683x 362 Posts

    Default

    Quote Originally Posted by admin View Post
    Hi guys what is the difference between a triplet and a doublet objective lens refractor besides the hefty price?
    Eeermm, well a doublet has two lenses, and a triplet has three.

    I have a refractor that has four lenses but I'm not sure what to call it. Bruce? perhaps?

    ROTFLMAO. Gus man if no one comes up with a better way of explaining ED v Apo, Doublet v Triplet soon enough I'll write up an explanation for you.

    Actually definitions like ED and Apo can be rather subjective and I am personally interested to hear others interpretations and results at the EP

  3. #3
    kirk's Avatar
    kirk is offline Junior Member
    Points: 7,170, Level: 59
    Level completed: 10%, Points required for next Level: 180
    Overall activity: 0%
    Achievements:
    First 1000 Experience Points365 Days+ Registered Achievement!750 Days+ Registered Achievement!2 Posts Achievement1000 Days+ Registered Achievement!
    Join Date
    Mar 2009
    Location
    U.K.
    Posts
    12
    Points
    7,170
    Level
    59
    Thanks
    0
    Thanked 12x 4 Posts

    Default

    Trying to keep it simple:
    A lens, simple single one, will focus one wavelength (colour) at the desired focal plane. Other colours/wavelengths will be in focus at positions before or after the desired focal plane.

    A doublet lens, achromatic, is made up of 2 bits of glass, these are able to focus 2 wavelengths at the desired focal plane. Others will again be in focus before and after the desired focal plane but the magnitude of these differences is smaller then in the single lens. So we have 2 wavelengths at the required focal plane and the others not too far away. If the lens is designed to have Red and Blue at the focal plane then the yellow/green is pretty close. May be better to pick Orange and Blue.

    A triplet lens has 3 bits of glass and you will have guessed that it is able to focus 3 wavelengths at the same plane. Again other wavelengths will be focussed close to the desired focal plane and hopefully very close indeed.

    When designing the lens you decide on what wavelengths to have at the required focal plane. Read about a good scope, doublet, the other day in which the red wavelength was the one "left out" so there was a very slight red ring around stars, it was done like this deliberately but I cannot recall why.

    The problem is that just because there may be 3 bits of glass the design may not be too good. A good doublet can be better then a poor triplet. A good triplet should be very good.

    Simple lens theory and design assumes a "thin lens" in effect they have zero thickness and the optical plane of each lens is at the same place which they cannot be, a lens has a physical thickness.

    That takes care of the colour side except that there is also spherical abberations caused by the problem that a spherical surface is not the ideal shape so you get problems caused by using a spherical surface. These problems tend to be most significant for light passing through the edges of the lens.
    jasas1 likes this.

  4. The Following 8 Users Say Thank You to kirk For This Useful Post:

    1Brightstar (12-30-2010),admin (03-01-2009),jasas1 (12-08-2014),jgroub (07-26-2014),lordhong (11-01-2012),Orion118 (01-01-2011),Remy3D (12-21-2011),SkyWriter (02-26-2012)

  5. #4
    star drop's Avatar
    star drop is offline Guilty as Charged
    Points: 14,490, Level: 82
    Level completed: 98%, Points required for next Level: 10
    Overall activity: 0%
    Achievements:
    10 Days registeredFirst 1000 Experience PointsGot three Friends20+ Friends Achievement!365 Days+ Registered Achievement!
    Join Date
    Nov 2008
    Location
    .
    Posts
    2,265
    Points
    14,490
    Level
    82
    Thanks
    11
    Thanked 475x 211 Posts

    Default

    Hi Kirk,

    Welcome to Astronomy Forum.

 

 

Similar Threads

  1. The doublet of triplet with intruder [field: 0,34°x0,68°]
    By Danilo Pivato in forum Astrophotography Forum
    Replies: 1
    Last Post: 05-06-2009, 10:26 PM
  2. The doublet of triplet with intruder [field: 0,34°x0,68°]
    By Danilo Pivato in forum CCD Imaging Forum
    Replies: 0
    Last Post: 05-03-2009, 08:17 AM
  3. The doublet of triplet with intruder [field: 0,34°x0,68°]
    By Danilo Pivato in forum UK Astronomy Forum
    Replies: 0
    Last Post: 05-03-2009, 08:17 AM
  4. Megrez - Doublet or Triplet best for imaging?
    By Paul Buglass in forum UK Astronomy Forum
    Replies: 0
    Last Post: 06-27-2007, 08:19 PM
  5. Design advantages of Doublet vs Triplet?
    By Tom T. in forum Amateur Astronomy Forum
    Replies: 10
    Last Post: 12-02-2004, 12:45 AM

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 08:39 AM.