Hardware Test Last revised: 06/22/02 Pietro.org Astronomy

 Celestron Eyepieces Test Report 

This report looks at a selection of Celestron eyepieces as used on the Celestron 5" series of SCTs.  All eyepieces are sold by Celestron and are of the 1" barrel diameter size.  Manufacturer specifications and eyepiece test results from field measurements with a C5+ are presented.  Calculation of various types are made from these numbers.  To reflect typical usage all field measurements are given for the eyepiece through a star diagonal with and without the Celestron Reducer/Corrector.  Results are reported in table form.

Table Sections

The following explains what each section of the table means. 

Eyepiece Designation and Model #: Celestron eyepiece designation and model
Manufacturer's Specs Specifications from the Celestron catalog
Measured Field Stop (mm) The field stop was measured using a micrometer.  In all cases it was in front of the field lens.  This value is important in the calculation section of the table. 
Measured FOV (deg) Measured field-of-view using "star drift method" timings on a Celestron C5+.  First row is with a star diagonal alone and the second with the addition of the Celestron Reducer/Corrector.  The third row is the percentage increase in field due to the Reducer/Corrector.
Calculation Sections Each section here uses the above numbers to determine a quantity. Results are given for eyepiece through a star diagonal and with the addition of the Reducer/Corrector.
Image Scale (deg/mm) This is the image scale at the field stop.  If you were to put a reticle marked in millimeters at the field stop, the number reported would be the degrees spanned by each 1-mm on the reticle.
Telescope Focal Length (mm) This is the telescope focal length to the field stop.
Telescope Focal Ratio (f/#) F-stop of telescope with this eyepiece calculated from above focal length divided by 127-mm objective size of the 5" series of Celestron SCTs.  Objective size from NexStar5 specs and derived from Oct. 1993 Sky & Telescope C5 report.
Telescope Magnification (#x) Magnification of the eyepiece.  Calculated telescope focal length divided by manufacturer's reported eyepiece focal length.

The Table

Notes follow after the table for fields marked with an asterisk "*." 

Table: Celestron Eyepiece Specifications/Measurements and Calculated Values when Used on a C5/G5/NexStar Telescope

Eyepiece Designation and Model #: SMA 25-mm #93007-A Cross Hair Kellner 20-mm #93304 Ultima 12.5-mm #93352 Ultima 5-mm #93350
Manufacturer's Specs Focal Length (mm) 25 20 12.5 5
Apparent Field (deg) 52 43 51 50
Measured Field Stop (mm)   22.1* 13.9 10.8 3.8*
Measured FOV (deg) w/Diagonal 0.94 0.58* 0.44 0.18
+Reducer/Corrector 1.43 0.88 0.68 0.27
% Increase 52% 53% 55% 50%
Calculations Calculations Calculations Calculations Calculations Calculations
Image Scale (deg/mm) w/Diagonal 0.0425 0.0414* 0.0407 0.0474*
+Reducer/Corrector 0.0647 0.0633 0.0630 0.0711*
Telescope Focal Length (mm) w/Diagonal 1347 1385* 1406 1210*
+Reducer/Corrector 885 905 910 806*
Telescope Focal Ratio (f/#) w/Diagonal 10.6 10.9* 11.1 9.5*
+Reducer/Corrector 7.0 7.1 7.2 6.3*
Telescope Magnification (#x) w/Diagonal 53.9 69.2 112.5 241.9
+Reducer/Corrector 35.4 45.3 72.8 161.3
Eyepiece Designation and Model #: SMA 25-mm #93007-A Cross Hair Kellner 20-mm #93304 Ultima 12.5-mm #93352 Ultima 5-mm #93350

"*" Notes: 

SMA 25-mm field stop on tested unit did not have a consistent rim distance from the edge of the barrel.  It gave the impression of being off-center.  This is the eyepiece that came with the telescope.

Cross hair Kellner 20-mm  field-of-view without Reducer/Corrector was not measured but derived from the Reducer/Corrector reading.  This will be addressed in a future update.

Ultima 5-mm eyepiece field stop is small and close to the field lens.  Value given is a a ballpark estimate.  All quantities derived from this measure are also flagged with an "*".  On the model tested black manufacturing spots (flat black barrel paint?) were evident on internal glass surfaces.  Viewing with a 6x loupe revealed the edges around the glass elements to be ragged (paint again?).  Construction quality does not match that of the Ultima 12.5-mm at all.  

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