# Thread: Triplet or Doublet Lens Refractors?

1. ## Triplet or Doublet Lens Refractors?

Hi guys what is the difference between a triplet and a doublet objective lens refractor besides the hefty price?

2. Originally Posted by admin
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. 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.

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. Hi Kirk,

Welcome to Astronomy Forum.

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