Planewave CDK 12.5 Strehl Test and CCDI Analysis

Planewave Instruments CDK 12.5 Telescope

View showing visual back, EFA focuser control box, and SBIG ST-2000XM camera


I.  Optical Analysis:  Strehl and Wavefront Error Testing

Test Date:  Septermber 19th, 2010
Skies:  Clear
Seeing:  6 out of 10, with some atmospheric turbulence present
Equipment:  Planewave Instruments CDK 12.5 @ f/8, 2544mm focal length
Skies:  Clear
Seeing:  6 out of 10, with some interference in atmospheric steadiness from jet-stream
Transparency:  7 out of 10
Temperature:  90 degrees F
Dewpoint:  64 degrees
Wind:  Calm
Test camera:  Lumenera SKYnyx 2-2C webcam with Baader IR/UV cutoff filter @ 320 x 240 image size, 41 fps capture rate for 1000 frames each
Mount:  Astro-Physics AP-1200GTOCP3


Strehl Test Summary:
The Planewave CDK 12.5 tested out with a good Strehl value of 0.90, as measured mid-bandwidth in the Green (approximately 550 nm wavelength). 
This corresponds to a P-V wavefront error of just under 1/6th wave at the camera chip.  Factory ray trace analysis indicates the theoretical maximum wavefront accuracy at the focal plane/CCD chip is 1/6th wave with a central obstruction of 42%.  Considering that a P-V value of 1/4 wave is a diffraction-limited performance level, just under 1/6th wave is a very good result for the CDK 12.5,  and better than diffraction-limited in optical performance.  This means that in all types of atmospheric seeing conditions no matter how good, the telescope's performance will only be limited by the atmospheric seeing itself and never limited by the telescope's optics.
Reference Chart for Strehl Values and P-V Wavefront Errors.  Chart courtesy of R.F. Royce Precision Optical Components:

Commonly Encountered Wavefront Relationships

P-V  Fraction P-V  Decimal Marechal RMS* Strehl Ratio Comments
1/3 .333 .094 .71  
1/4 .250 .071 .82 Rayleigh Limit
1/5 .200 .057 .88  
1/6 .167 .047 .92 Good
1/7 .143 .041 .94 Very Good
1/8 .125 .036 .95  Excellent
1/9 .111 .032 .960 (.96)  Excellent
1/10 .100 .028 .969 (.97)  Excellent
1/11 .091 .026 .974  
1/12 .024 .024 .978 "Super Apo"

Strehl Test Results:

Very good Strehl test for this optical design with large central obstruction, despite poor atmospheric turbulence conditions.  Note that all Zernicke defects have been left added in, and the Strehl was still able to reach 0.901.  Very good results for a compound mirror system.

The Roddier software interferogram results above indicate very good glass figure of the optics, with no significant under or over-correction.

II.  CCDI Results

Camera:  ST-2000XM w/AO-8
Ambient Temp:  90 degrees F
Image Scale:  0.60 arc-sec/pixel

The FWHM values were swelled a little by poor atmospheric turbulence conditions.  Note the very low aspect ratios, indicating very low optical axis tip/tilt.  All values are very low and should produce excellent imaging results with round stars and good contrast.


III.  Test Image Field at Full Resolution of 0.60 arc-sec/pixel

Note the nice round stars edge to edge, indicating near-perfect optical alignment.  Focus was a little soft due to atmospheric turbulence, but subsequent frames taken in better seeing with better focus later that night still confirmed sharp, nicely round stars field wide.


III.  Sample Image of the globular cluster M53 with the Planewave CDK 12.5

Globular star cluster Messier 53 (M53, NGC 5024) is one of the more outlying globulars, being about 60,000 light years away from the Galactic center, and almost the same distance (about 58,000 light years) from out Solar system. At this distance, its apparent angular diameter of 13' corresponds to a linear diameter of roughly 220 light years. It is rapidly approaching us at a velocity given by Mallas as 112 km/s, by Harris as 79 km/s. M53 has a bright compact central nucleus of about 2' in diameter, although its stars are not very concentrated toward the center when compared to other globulars, and a gradually decreasing density profile to the outer edges.

 (Text info courtesy of SEDS).

Image Details:


IV.  Astro-Physics 27TVPH Focal Reducer Testing:
The AP 27TVPH focal reducer was tested on 12/12/2010.  This focal reducer may be useful in some cases with SBIG ST-series cameras to help relieve severe image scale and provide a wider field of view.  The reduction factor is not great....0.78x.  But this can still provide for better looking images on small-chip cameras and help hide defects due to severe image scale, especially when atmospherics are not the best.  Below is a test frame of the open cluster M52 from the Planewave CDK 12.5 with the focal reducer mounted in the optical path.  The focal reducer produced very acceptably round stars edge-to-edge across the entire field:


Image Details:


Wade Van Arsdale
Page last updated December 13th, 2010
Little Rock, AR., USA