TMB Optical 152/1200mm OTA OPTICAL ANALYSIS

Wade Van Arsdale
Little Rock, AR., USA
March 23rd, 2010

TMB Optical 152 triplet apo refractor by Thomas M. Back, mounted on AP-1200GTO mount

 

Note:  with the untimely and unfortunate passing of Thomas M. Back, the brand-name TMB Optical triplet apo refractors using  Russian LZOS OK-4 glass are no longer available through Tom's company.  Tom was recognized as an authority on optical design and was respected worldwide for his expertise and outstanding contributions to the fields of astronomy and optical design.  Not only was Tom without peer in his field, he was also known for his kindness, humility, and sincerity to his many friends and those in the astronomy community whom he crossed paths with.  For his loyal customers and everyone who knew him or worked with him, he is and will continue to be greatly missed.


Rest in peace Tom.
Thomas M. Back, 1957-2007  

 

 I.  TMB 152 Optical Analysis:  Strehl and Wavefront Error Testing

Test Dates:  March 22-23, 2010
Skies:  Clear
Seeing:  6 out of 10, with some interference in atmospheric steadiness from jet-stream
Transparency:  9 out of 10
Temperature:  47 degrees F
Dewpoint:  44 degrees
Wind:  N, 5-10 mph
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, and resolution of 0.75 arc-sec/pixel
 Optical Analysis Software Used:  WinRoddier

The TMB-152 tested out with an excellent Strehl value of 0.970, as measured mid-bandwidth in the Green (approximately 550 nm wavelength).  This corresponds to a P-V wavefront error of approximately 1/10th wave at the camera chip, which is outstanding.  Considering that a P-V value of 1/4 wave is considered to be diffraction-limited, 1/10th wave is outstanding, and significantly beyond 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.  Overall, the TMB-152 tested out as an excellent performer both visually and for deep-sky imaging as well as for planetary webcamming.

A note about OTA cooldown with the TMB 152:
This OTA uses a phenolic type of compound for the tube instead of standard T6061 aluminum or other type of aluminum.  It is important to take the time necessary to cool down the optics and tube properly on this model.  To get the results shown below on the Strehl tests, I had to use forced cooling with a Lymax Cosmic One "cat cooler" fan system for internal heat dissipation inside the OTA.  I also pulled back the sliding dewcap to expose the lens elements to cooling better.  I ran the Lymax fan for 30 minutes (the maximum recommended by Lymax)  prior to testing the optics.  Without the fan, this model can take up to 2 hours to properly cool down.  Without the fan and with a cooldown period of 30 minutes, the Strehl in Green (reference bandwidth)  tested at 0.940.  After an equal time of 30 minutes on the Lymax fan for the second session, I was able to get the 0.970 Strehl in Green shown below.  These differences indicate that with this refractor it is crucial to allow a proper cooldown period for top performance of the optics.  Forced-cooling with a fan may also help speed up the cooldown times.

See Roddier tests below for each individual wavelength specifics.

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:

Green (reference):  Ronchigram (white test pattern below) shows almost perfect correction and an excellent Strehl value of 0.970, which corresponds to an outstanding wavefront error of only 1/10th wave.  Based on the data, this refractor should be an excellent planetary scope with very low levels of spherical aberration.
 Red:  Ronchigram shows some very slight undercorrection in the red wavelengths.  Slight undercorrection is usually not a problem because it tends to correct itself to some extent as the glass cools down to equilibrium with outdoor ambient temperature.  The Strehl value recorded is 0.940 which is very good and corresponds to 1/7th wave P-V.
 Blue:  Basically no under or overcorrection present in the blue.  A little bit of tube thermals present likely caused the rougher Ronchigram figure in the blue.  The Strehl value recorded is 0.950 which is excellent.   The correction in blue should stay close to this level as the scope further equalizes.

 

II.  Hi-resolution webcam images of Jupiter and Saturn with the TMB-152:

Camera:  Lumenera SKYnyx 2-2C webcam with Baader IR/UV cutoff filter

                                                   

 

III.  Collimation Testing

Equipment Used:
SBIG ST-2000XM CCD Camera.
SBIG CFW8 filter wheel.  All CCDI test frames shot through Custom Scientific (L-layer) clear filter with IR cutoff.
TMB Optical 152/1200mm OTA
Mount Used:  Astro-Physics AP1200GTO
Collimation method used:   Takahashi Collimator Scope with artificial light source.
CCD Inspector Software by CCDWare

Curvature, Tip/Tilt, and Collimation all very good and produced round stars across the entire FOV.
 Frames used for analysis.  FWHM shows non-optimal but workable seeing conditions for the tests.
 

Field curvature value of 5.9% is low on the 15mm image cone diameter provided by the CCD chip used for the test, indicating exceptional glass figuring quality and workmanship on the Russian LZOS objective lens glass.  No field flattener or reducer was used on the TMB for any of these tests.  You would however need an add-on field-flattener with APS-C (28mm) or larger-diameter image cone sizes).

IV.  High-Resolution Deep Sky Image Tests:  M12 Globular Cluster

(Note, this image was affected by atmospheric turbulence which softened the focus considerably.  I did not use processing tools to reduce star bloat in this image so we could examine a "worst-case" scenario that is encountered often in real-life imaging when higher-resolution focal lengths are used like this telescope delivers.  While this image does not allow proper visual evaluation of spherical aberration due to the poor atmospherics, it does still allow for chromatic aberration evaluation.  Focal point shift of the R, G, B color wavelengths and resultant color fringing on the stars from CA is non-existent in this image.  This telescope shows no visually detectable chromatic aberration at all).

Image Details:

Conclusion:
The TMB Optical 152/1200 triplet apo arrived here very well collimated and in excellent overall mechanical and cosmetic condition from the TMB Optical shop.  Its objective elements are made by LZOS in Russia, using OK4 extra-low dispersion glass for the ED center element.  The telescope showed outstanding control of chromatic aberration in the test imaging run, with very sharp focus and no star haloing, color fringing, or loss of focus in the blue bandwidth or any other color bandwidths.  The TMB-152 also showed almost nonexistent levels of spherical aberration on the visual star test.  This telescope has an excellent performance level for high-resolution CCD imaging.   It is also an excellent planetary and deep-sky visual scope that can take high magnification very well in good atmospheric seeing conditions.

 

Wade Van Arsdale
Little Rock, AR., USA
Latest edit:  Jan 1st, 2020