Whitepeak Observatory, Tacoma, WA

Roddier Tests: 6inch, 5inch and 80mm achros in green light

Below are the details of a testing exercise done upon a 152mm f/8 achromat (Zhumell brand). This scope is manufactured for Zhumell Optics by Chongqing-Deron Instrument Manufacture Co, LTD. a medium sized optical concern located in the People's Republic of China.

Zhumell 152/1200 achromatic refractor

Example images

The primary test method I used is by way of a software program which is based upon techniques of stellar image analysis developed by Claude and François Roddier while working at the University of Hawaii on high resolution astronomy. The software itself is open source and was developed by a team of four French amateur astronomers. Their pages devoted to this test program (and related information as to its use and theoretical basis) can be found starting here. Basically, the program requires two slightly defocused star images of specified angular diameter and representing the image equidistant inside and outside focus to be loaded into the program. Then, after parameters (scope and imager) are loaded and set, an analysis takes place which examines the image, pixel by pixel. Based upon Laplacian wavefront relationships, an evaluation of the wavefront of the telescope is produced, defined upon the 22 Zernike polynomials. (This is similiar in principle to the process used to calculate the required compensatory wavefront aberrations in adaptive optics.) See here for more detail on Zernike polynomials.

An operational "Cookbook" for the Roddier Optical Analysis program is available to members of the Roddier Yahoo group located if you are interested in doing this test on your telescope.

Here are the results of the first test protocol:





Here are jpegs of the fullsize fits startest images that were fed into the program. The target sar was Betelgeuse.The images were captured with a Neximage camera with 5.56um squeare pixel size as an short avi and then stacked without any further processing other than as specified by the program authors.

intrafocal extrafocal

These would seem to contain the relevant detail level neceessary for a reasonably accurate computation. The standard the software designer specifies is a ratio of between 10-15 pixel size/image size. These images meet that standard (5.56um and >60pixel diameter). Before running this test I had estimated (using the Aberrator star test evaluation program) about 1-1.5/10 wave astigmatism and 2/10 wave 5th order spherical aberration as the primary problems. This gave an estimation of ~0.05 rms or about 0.90 strehl.


The Roddier program also outputs a rather detailed and extensive list of aberration values. Here is the list from the first tests protocol:

Fichier =C:\astronomy\optics\6-inch-star\roddier-test-valeur-10435.txt
Saisie des paramètres
Zernike-Nom-RMS nm
2,Tilt (sin),-38.508157391656
5,Astig. (sin),4.33449536900963
6,Astig. (cos),9.59240711097016
7,Coma (sin),5.56220023960534
8,Coma (cos),-10.1921557655179
9,Trefoil (sin),4.76730428326879
10,Trefoil (cos),2.54327552447713
12,Sph. astig. (cos),-1.37821612795482
13,Sph. astig. (sin),0.732793039460621
14,Quad. astig. (cos),0.848030096330198
15,Quad. astig. (sin),2.05827435682236
22,Spher. (6° ord.),-1.97469093804219

Finally, here is a set of ronchi grams taken at the eyepiece @ f/8 with a 133line/mm grating:

The seeing was poor (>2"arc) at the time of the test and the images were somewhat underexposed which accounts for their "dirty" look. But what is important is the degree of parallelism of the lines. The ronchi test is more of a pass/fail type of examination; if significant curvature of the lines is not seen, then the optic can be assumed to be ~1/4 wave corrected at the wavefront. It is most sensitive to spherical aberration, zones and can indicate the presence of a turned edge quite reliably (none present here). For other forms of aberration it is much less useful. This was the first test I performed on this scope. Interestingly, the zone which results in the mild higher order aberration reflected in the Roddier test is detectable in the upper left image inside focus as a slight bulging of the lines just below center. Mild spherical aberration (overcorrection) is visible in slight inward bowing of the lines outside focus.

ADDENDUM: Retest of 6" and 5" and 80mm test results

I re-tested the 6" and also ran tests on a 5" f/9.3 Meade achromat and a 80mm f/11 meade DS-80 achromat upon Procyon. Images were taken with Neximage camera, ~1minute aquistion times, zero gamma and stacked without enhancement. A green filter (#58 wratten) was used to limit the waveband to ~500-600nm. An IR cut filter was also used. Summation of results in below panels:

6" second test

80mm test

127mm test

The results on the 127mm and 80mm are consistent with their visual performance record over the last two years.

If doing these tests on Chinese mass prodduction models like these, beware of image plane tilt induced by focus tube shift. This varies the collimation between inside/outside focus and will introduce errors of astigmatism and coma into the results. Take care to neutralize focus tube position for more acurate results. This was a problem that was not addressed in the first set of tests on the 6" scope.

I plan to re-run these tests in better seeing once spring arrives this year. I will update this page at that time, so check back!

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