Last night our astronomy club decided to compare their 15" f/4.8 Tectron Dob against a nearly identical 14.5" f/5.1 Dob with a Galaxy mirror and an Astro-Physics 130-GT refractor. The club Tectron has a mirror that many members feel is inferior. It was tested and found to have a turned edge at the outer 0.3" - 0.4" averaging 1/4 - 1/2 wave and a zonal depression spanning the central ~3.5" dia. of approx 1/6 - 1/4 wave. Some slight astigmatism (~1/8 wave) and some surface roughness was noted. The specific test numbers are as follows:
TECTRON MIRROR TEST RESULTS
---------------------------
Description: 15" Mirror
Ross Null test @ 543 nm
Output Units: Waves
RMS: 0.113
Peak-Valley: 0.85
Strehl Ratio: 0.60
Defocus: Removed
Astigmatism: 0.15 54.6deg
Coma: 0.15 -59.3deg
Spherical: -1.09
Scale: 1.00
Wavelength: 543.8 nm
Data Sets: 10
---------------------------
The Galaxy mirror has never been tested, but I've never been thrilled with its performance above about ~150x. The image seems to break down, even in good seeing. I'm not a terribly experienced observer however, so I've never been able to say with confidence that it's the mirror's fault.
Seeing was mediocre last night. Transparency was pretty good. We pointed all three scopes at Polaris at about ~160x with Nagler eyepieces. This was the only magnification our eyepiece collection would permit on all three scopes at the same time. All scopes were set up before dusk and had hours to acclimate. However, temps were dropping by at least several degrees per hour leading up to the testing. Both mirrors are 2" thick and neither had fans running.
The Tectron displayed an out of focus outer diffraction ring with pronounced "spikiness" and some asymmetry. Turbulence prevented much analysis of the inner diffraction pattern but I did think it was a bit mottled which I assume could be the surface roughness.
Some thought the Galaxy mirror seemed a little more uniform inside the outer ring and a little less spiky on the outer diffraction ring. Others couldn't really see a difference. Any differences were certainly subtle.
The A-P's diffraction pattern was much smoother, seemingly perfectly round, with a very uniform and smooth outer ring. The inner diffraction rings were much more steady and easier to see. Overall the difference between the reflectors and the AP was huge compared to the differences between the two reflectors.
All observers felt that the Tectron showed a very pleasing IN-FOCUS image of the orion nebula at ~107x with a 17 Ethos. In-focus stars seemed pretty nice and round.
At the suggestion of the mirror tester, an aperture mask was cut from roofing paper to mask off the outer 0.5" of the mirror, covering the turned edge. No clear improvement was observed at the eyepiece, though we didn't look very hard for differences.
I found a few things surprising about these results: First, that a mirror with such a low Strehl could produce an image of the orion nebula that was everyone thought was hard to fault at 107x. Second, that a Galaxy mirror did not clearly outperform the Tectron of known low quality. Third, that the AP blew away a Galaxy mirror in the out of focus star image with much smoother outer ring and much more visible inner ring structure.
My first question is how much of a very good mirror's strehl might be "wasted" in typical New England seeing conditions? I've never been able to reconcile the immutable laws of physics stating the diffraction limit strehl of 0.80 with scope manufacturers' and consumers' obsession with getting as close as possible to strehl 1.00. People pay double for a strehl .98 objective compared to a .8 one. How visible is this difference in reality and under what conditions might it be INVISIBLE?
Is it plausible that the Galaxy mirror might be diffraction limited or better, but that some combination of the seeing and some thermal issues with the scope prevented any meaningful improvement in the image compared to the Tectron's .6 strehl? Or is it more likely the Galaxy mirror is a lemon and comparable in quality to the Tectron?
The answers to these questions will help the club determine how worthwhile it is to have the mirror refigured and what improvements we might reasonably expect with a top quality mirror.
Thanks for your input,
Dan
TECTRON MIRROR TEST RESULTS
---------------------------
Description: 15" Mirror
Ross Null test @ 543 nm
Output Units: Waves
RMS: 0.113
Peak-Valley: 0.85
Strehl Ratio: 0.60
Defocus: Removed
Astigmatism: 0.15 54.6deg
Coma: 0.15 -59.3deg
Spherical: -1.09
Scale: 1.00
Wavelength: 543.8 nm
Data Sets: 10
---------------------------
The Galaxy mirror has never been tested, but I've never been thrilled with its performance above about ~150x. The image seems to break down, even in good seeing. I'm not a terribly experienced observer however, so I've never been able to say with confidence that it's the mirror's fault.
Seeing was mediocre last night. Transparency was pretty good. We pointed all three scopes at Polaris at about ~160x with Nagler eyepieces. This was the only magnification our eyepiece collection would permit on all three scopes at the same time. All scopes were set up before dusk and had hours to acclimate. However, temps were dropping by at least several degrees per hour leading up to the testing. Both mirrors are 2" thick and neither had fans running.
The Tectron displayed an out of focus outer diffraction ring with pronounced "spikiness" and some asymmetry. Turbulence prevented much analysis of the inner diffraction pattern but I did think it was a bit mottled which I assume could be the surface roughness.
Some thought the Galaxy mirror seemed a little more uniform inside the outer ring and a little less spiky on the outer diffraction ring. Others couldn't really see a difference. Any differences were certainly subtle.
The A-P's diffraction pattern was much smoother, seemingly perfectly round, with a very uniform and smooth outer ring. The inner diffraction rings were much more steady and easier to see. Overall the difference between the reflectors and the AP was huge compared to the differences between the two reflectors.
All observers felt that the Tectron showed a very pleasing IN-FOCUS image of the orion nebula at ~107x with a 17 Ethos. In-focus stars seemed pretty nice and round.
At the suggestion of the mirror tester, an aperture mask was cut from roofing paper to mask off the outer 0.5" of the mirror, covering the turned edge. No clear improvement was observed at the eyepiece, though we didn't look very hard for differences.
I found a few things surprising about these results: First, that a mirror with such a low Strehl could produce an image of the orion nebula that was everyone thought was hard to fault at 107x. Second, that a Galaxy mirror did not clearly outperform the Tectron of known low quality. Third, that the AP blew away a Galaxy mirror in the out of focus star image with much smoother outer ring and much more visible inner ring structure.
My first question is how much of a very good mirror's strehl might be "wasted" in typical New England seeing conditions? I've never been able to reconcile the immutable laws of physics stating the diffraction limit strehl of 0.80 with scope manufacturers' and consumers' obsession with getting as close as possible to strehl 1.00. People pay double for a strehl .98 objective compared to a .8 one. How visible is this difference in reality and under what conditions might it be INVISIBLE?
Is it plausible that the Galaxy mirror might be diffraction limited or better, but that some combination of the seeing and some thermal issues with the scope prevented any meaningful improvement in the image compared to the Tectron's .6 strehl? Or is it more likely the Galaxy mirror is a lemon and comparable in quality to the Tectron?
The answers to these questions will help the club determine how worthwhile it is to have the mirror refigured and what improvements we might reasonably expect with a top quality mirror.
Thanks for your input,
Dan
