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Review of 15 inch Obsession with Torus optics.
By Eric Jensen
My name is Eric Jensen. I am an amateur astronomer living in Norway. In April 2002 I received a 15 inch Obsession with optics from Torus (now Optical Mechanics).
Unfortunately, the aquisition did not end up being quite what I had hoped for. What follows are my experiences.
Bright stars showed many spikes, these were so long that I could not even see the four spider vane diffraction spikes, they were lost among all the others. On the planets, the spikes translated into lost contrast, Jupiter and Saturn had a large halo around them.
The star test showed the classical symptom of a turned-down edge (TDE), with completely smeared out diffraction rings inside of focus, and clear concentric rings outside. I estimated the TDE to be about 1/2 inch wide by masking the edge by that amount and seeing that the spikes then more or less went away.
The numbers stated on the interferometric test certificate indicated a mirror of excellent quality:
Peak-to-valley wavefront (P-V): 0.197 wavelengths
RMS: 0.026 wavelengths
Strehl ratio: 0.973
All of the above at 633 nm reference wavelength.
Obsession/Torus asked me to send the mirror back. TDE was confirmed, and the mirror was refigured by Torus. The expenses were covered by Obsession/Torus.
A new interferometric test certificate was issued after the refigure. The numbers stated were even better now:
P-V: 0.122 wavelengths
RMS: 0.014 wavelengths
Strehl ratio: 0.993
According to these numbers, the mirror should have been of an outstanding quality.
Unfortunately, the image was still not satisfactory. There were still spikes, although these were now shorter. By masking off, I estimated the TDE to be about 1/4 inch wide now.
Note that I had been using our astronomy club's 18 inch Starsplitter with excellent optics for about 4 years by this time. I knew what large, high-quality optics should show. It had never shown such spikes. In other words, it was a reference mirror to compare mine to. Also, for 2 years I had owned a 10 inch Starmaster EL with superb optics. Nor did it ever show any spikes.
This time, I had the mirror tested by another optical company. The numbers arrived at by this optician, who used Foucault and Ronchi tests, were:
P-V: 1/3 wavelength
RMS: 1/8 wavelength
Reference wavelength was now 550nm, which is slightly more stringent than light at 633nm. This does not, however, account for the large difference between the results arrived at by the two companies.
The optician at the independent company company did not calculate Strehl ratio because according to this individual the formula used to do so was not applicable in this case. The formula works best for diffraction-limited mirrors.
The mirror was found to have an optically rough surface, was overcorrected throughout, and had a central depression not quite covered by my secondary mirror. The TDE was confirmed and estimated to be approximately 1/4 inch wide.
See the comparison images below. The mirror to the left is the Torus mirror. The one to the right is a sample optic of similar diameter and focal ratio, provided by the company performing the evaluation. The comparison is included to show contrast against the condition of the Torus mirror. The mirror to the right is representative of the kind of quality that Obsession led me to believe I was purchasing.
The above image (Foucault test), shows quite well the difference in surface condition between the two mirrors.
The ronchigrams above show the difference in edge condition between the two. The Torus (left) has lines that brake inward at the very top and bottom. This indicates the TDE. Note also the central depression in the Torus.
In the end, I decided to have a second independent optician refigure the mirror. This optician also confirmed that the mirror was overcorrected, had a rough surface and a TDE.
Fortunately, this last refigure was a success. The mirror is now very good an I am enjoying nice, high-contrast views.
I showed the independent test results to both Obsession and Torus. I wanted the cost of refigure, coating and transport to be covered.
After a few e-mails back and forth, Obsession agreed to cover all costs - except shipping from Norway to the U.S. - appr. $270.00 - which I had to pay myself. This is despite the fact that shipment to the U.S. was necessary in order to get the mirror repaired.
It is yet to be explained how a mirror with the faults described could be provided with such excellent interferometric test results - twice.
On the plus side, the telescope structure is mechanically well made, looks attractive and has smooth movements in both axes. I am enjoying it very much with my present mirror - twice refigured.
Vev-ansvarlig: Hilde Birch
Sist endret: 8. oktober 2002.