I want to use 2 in eyepieces with my SCT and/or Mak. Will there be vignetting from the OTA itself?
October 19, 2010
Vignetting is normal in any real optical system. Some light must be blocked because of off-axis distortions or aberrations from the optics themselves. Glare and reflections from inside and outside the OTA are another reason field stops or baffles are designed into telescopes and other optics. No one wants bright blobs of light to appear and interfere with the objects they are trying to view.
When combining various components for observing with your scope, determining the degree of vignetting under the actual focused spacing without ray tracing is difficult.
However, a comparison of the exit diameters of the baffle tubes of SCTs and Maks to the diameter of the field lenses in typical 2 in eyepieces is a ready way to assess vignetting. (The field lens is the eyepiece lens at the base of the ocular, the lens farthest from your eye when the eyepiece is in use.) Often an eyepiece’s field lens diameter is the same as the limiting field stop for the eyepiece design. The field stop literally defines the edge of the field of view, removing aberrations and glare while at the same time blocking or vignetting the light.
By definition, a 2 in eyepiece is only 50 mm across at its base. In reality, field stops in these oculars are smaller, sometimes much smaller than the 2 in barrel diameter. A longer focal length, wide-angle design like Celestron’s 32 mm Axiom LX has a field lens 46 mm in diameter. A standard Plössl or Kellner 2 in design of the same focal length might have a 40 mm field lens. Shorter focal lengths of any design will have even smaller field lenses, sometimes as small as only a few millimeters.
Here are the approximate diameters of the back of the baffle tube for Celestron SCTs. Two Maks are also shown for comparison.
When combining various components for observing with your scope, determining the degree of vignetting under the actual focused spacing without ray tracing is difficult.
However, a comparison of the exit diameters of the baffle tubes of SCTs and Maks to the diameter of the field lenses in typical 2 in eyepieces is a ready way to assess vignetting. (The field lens is the eyepiece lens at the base of the ocular, the lens farthest from your eye when the eyepiece is in use.) Often an eyepiece’s field lens diameter is the same as the limiting field stop for the eyepiece design. The field stop literally defines the edge of the field of view, removing aberrations and glare while at the same time blocking or vignetting the light.
By definition, a 2 in eyepiece is only 50 mm across at its base. In reality, field stops in these oculars are smaller, sometimes much smaller than the 2 in barrel diameter. A longer focal length, wide-angle design like Celestron’s 32 mm Axiom LX has a field lens 46 mm in diameter. A standard Plössl or Kellner 2 in design of the same focal length might have a 40 mm field lens. Shorter focal lengths of any design will have even smaller field lenses, sometimes as small as only a few millimeters.
Here are the approximate diameters of the back of the baffle tube for Celestron SCTs. Two Maks are also shown for comparison.
| OTA | Baffle opening |
|
C14 |
54 mm |
|
C11 |
54 mm |
|
C925 |
46 mm |
|
C8 |
37 mm |
|
C6 |
27 mm |
|
C5 |
27 mm |
|
C127 Mak |
27 mm |
|
C90 Mak |
15 mm |
C14s through C11s should have no problems with vignetting. Vignetting will probably occur for the widest angle, longest focal length 2 in oculars with the C8’s 37 mm opening. The smaller OTAs will have vignetting with all but the shortest focal length 2 in eyepieces.
Updated 12/27/13
