Telescopes
Page 5 - Star Diagonals and Barlow Lenses

by Larry McNish
(all photos and diagrams by the author)
Page last updated May 4, 2007


Light Paths in a telescope, with a Barlow Lens, and with a Star Diagonal

For basic information on Telescopes - go to Page 1.
For more information on Telescope Mounting Systems - go to Page 2.
For more information on Telescope Accessories - go to Page 3.
For more Technical Information on Telescopes - go to Page 4.




Barlow Lenses

Barlow Lenses
  • Incorporate a "diverging" lens that "widens" the beam of light making the apparent focal length of the primary longer.
  • Effectively increases the "power" of an eyepiece
  • They come in different magnifications:
  • 2.5 x top
  • 2 x bottom
  • although they reposition the eyepiece further from the telescope I found very little refocusing was necessary


Star Diagonals

1 1/4 inch Star Diagonal
  • flips one axis of the light beam 90°, then, since you have to look "down" instead of "straight into" the eyepiece, you are providing another 90° of flip for a total of 180° (i.e. complete inversion of one axis)
  • provides a more comfortable viewing angle especially for objects high in the sky
  • may be constructed with a prism (using multi-coated glass surfaces like good eyepieces), or
  • may be constructed using a flat "front-surface" mirror polished to 1/10th of the wavelength of light
1 1/4 inch Star Diagonal - Light Path Increase
  • repositions the eyepiece further from the "normal" location in the back of the telescope
  • requires refocusing the telescope to compensate (which means moving the primary mirror on a Schmidt Cassegrain Telescope)
  • this effectively increases the focal length of an SCT
  • For an 8 inch SCT with focal length of 2032 mm the increase is 3.232 in. or 82.1 mm or 4% for the specific diagonal shown
  • This will increase the magnification thereby decreasing the field of view of eyepieces
2 inch Star Diagonal
  • For telescopes with a 2 inch "back" or focuser and used for 2 inch eyepieces
  • can accommodate 1 1/4 inch eyepieces with an adapter
  • Star Diagonals are normally used with Schmidt-Cassegrain telescopes and may be used with refracting scopes that have enough focuser travel
  • Not normally used with Newtonian or Dobsonian telescope because the focuser will not travel far enough
2 inch Star Diagonal - Light Path Increase
  • repositions the eyepiece further from the "normal" location in the back of the telescope
  • requires refocusing the telescope to compensate (which means moving the primary mirror on a Schmidt Cassegrain Telescope)
  • this effectively increases the focal length of an SCT
  • For an 8 inch SCT with focal length of 2032 mm the increase is 4.712 in or 119.7 mm or 5.9% for the specific diagonal shown
  • This will increase the magnification thereby decreasing the field of view of eyepieces
2 inch to 1 1/4 inch adapter
  • allows 1 1/4 inch eyepieces to be used in the 2 inch Star Diagonal
  • adds a small distance to the light path (the thickness of its eyepiece support ring)



Possible Mounting Configurations of Barlow lenses and Star Diagonals

The images and comments below are relative to a Schmidt-Cassegrain telescope (as shown)
or a refracting telescope (not shown). Many of these configurations are not possible
on a Newtonian or Dobsonian telescope because the focuser will not travel far enough
to bring objects to a focus.
True upright image - i.e. a binocular view
  Configuration Image
The eyepiece is placed directly into the telescope.

This makes it difficult to look through the eyepiece when the telescope is pointing to objects higher up in the sky - the eyepiece is low down and you half to get down and look upwards to see through the scope.
The up-down direction is inverted, and the left-right direction is reversed.
The Star Diagonal is placed into the telescope and the eyepiece is placed vertically into the diagonal.

This is a much more comfortable viewing position for astronomical objects, but if the telescope is mounted on a fairly tall tripod, you have to be tall or get on a step stool to look down into the eyepiece for objects near the horizon.
The up-down direction is restored, but the left-right direction is still reversed (as viewed from the back of the telescope).
As above, but the Star Diagonal has been rotated 90° to the right.

This is a common position for viewing objects near the horizon when the scope is mounted on a fairly tall tripod, or if children need to be able to look through the scope.
The left-right direction is restored, but the up-down direction is still inverted (as viewed from the right side of the telescope).
As above, but the Star Diagonal has been rotated 90° to the left.
The left-right direction is restored, but the up-down direction is still inverted.

(Yes - it's the same as the one above, regardless of whether the eyepiece is on the right or left)
The Barlow lens is placed into the telescope and the eyepiece is placed directly into the Barlow.

This makes it difficult to look through the eyepiece when the telescope is pointing to objects higher up in the sky - the eyepiece is very low down and you half to get down and look upwards to see through the scope.
Same as for an eyepiece without a Barlow
A Barlow, then a Star Diagonal, then the eyepiece.
Same as for a Star Diagonal without the Barlow.
A Barlow, then a Star Diagonal, then the eyepiece.

The Star Diagonal has been rotated 90° to the right.
Same as for a Star Diagonal 90° right without the Barlow.
A Barlow, then a Star Diagonal, then the eyepiece.

The Star Diagonal has been rotated 90° to the left.
Same as for a Star Diagonal 90° left without the Barlow.
A Star Diagonal, then a Barlow, then the eyepiece.

Note: Often the Barlow does not "fit" in this configuration, i.e. its barrel will not fit entirely into the diagonal.

You also have to be really tall to be able to look down into the eyepiece in this configuration.

Therefore this configuration is not recommended.
Same as for a Star Diagonal without the Barlow.
A Star Diagonal, then a Barlow, then the eyepiece.

The Star Diagonal has been rotated 90° to the right.

Note: Often the Barlow does not "fit" in this configuration, i.e. its barrel will not fit entirely into the diagonal.

This configuration is not recommended since the weight of the Barlow and the eyepiece puts a lot of torque on the Star Diagonal.
Same as for a Star Diagonal 90° right without the Barlow.
A Star Diagonal, then a Barlow, then the eyepiece.

The Star Diagonal has been rotated 90° to the left.

Note: Often the Barlow does not "fit" in this configuration, i.e. its barrel will not fit entirely into the diagonal.

This configuration is not recommended since the weight of the Barlow and the eyepiece puts a lot of torque on the Star Diagonal.
Same as for a Star Diagonal 90° left without the Barlow.
A Star Diagonal, then the eyepiece.

The Star Diagonal has been rotated 45° to the right.
The view looking directly in from the right side.
A Star Diagonal, then the eyepiece.

The Star Diagonal has been rotated 45° to the left.
The view looking directly in from the left side.
Two Star Diagonals.

Can you use two Star Diagonals to make the image "correct"? - Yes.


The first Star Diagonal restores the left-right direction, sending the image to the right side (so you have to stand at the right side of the scope to see it correctly). The 2nd Star Diagonal restores the up-down direction, sending the image upwards (so you have to look down into the eyepiece).

Requires significant focuser travel and increases the focal length of the SCT by the combination of the light path length of both diagonals.


Correct - the view from the right side of the scope facing the assembly and looking down into the eyepiece.


Incorrect - the view from the back of the telescope looking down into the eyepiece.
Two Star Diagonals.

As above but rotated 45° downwards.
The view from the right side of the scope facing the assembly and looking down into the eyepiece.


The view from the back of the telescope looking down into the eyepiece.
Two Star Diagonals.

As above but rotated 90° downwards.
The view from the right side of the scope facing the assembly and looking into the eyepiece.
Two Star Diagonals.

An alternate configuration with the eyepiece at the "top" of the assembly.
The view from the right side of the scope facing the assembly and looking into the eyepiece.



Other Accessories to Correct the Image

Accessory Description


Porro Prism




Reverses both Up-Down and Left-Right directions so that an astronomical telescope can be used for terrestrial objects and gives a "correct" view.

However, This makes it difficult to look through the eyepiece when the telescope is pointing to objects higher up in the sky - the eyepiece is very low down and you half to get down and look upwards to see through the scope.

Increases the light path length therefore requires refocusing and increases the magnification thereby decreasing the field of view of eyepieces.

You can't use a Star Diagonal for a more convenient viewing angle because the diagonal will flip one of the directions again defeating the purpose of the Porro Prism.


45° Erecting Prism Diagonal




Reverses both Up-Down and Left-Right directions so that an astronomical telescope can be used for terrestrial objects and gives a "correct" view.

Also provides a convenient viewing angle (rather than looking "straight through" or "looking upwards").

Increases the light path length a similar amount to a right-angle star diagonal, therefore requires refocusing and increases the magnification thereby decreasing the field of view of eyepieces.

This is the best accessory to allow using an astronomical telescope as a "spotting scope" to view terrestrial objects during the daytime. However, the extra glass and the manufacture of the prism may affect the quality of astronomical views at night.

Can also be used with a Barlow providing the Barlow is inserted into the back of the telescope first.

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