Subject: Re: SCT, 1/4 wave & 33% obstruction Date: Thu, 20 May 1999 12:53:03 -0700 From: Dan McKenna To: tomdemary@my-dejanews.com tomdemary@my-dejanews.com wrote: > In article <37430C97.2DA3AE12@as.arizona.edu>, > Dan McKenna wrote: > > > > > > tomdemary@my-dejanews.com wrote: > > > > > Has anyone analyzed obstruction vs. seeing? Bad seeing degrades > > > the incoming wavefronts, distorting, advancing, retarding portions > > > of the wavefront. If a telescope has a large obstruction which > blocks > > > the center of the incoming wavefront, then the portion of the wave- > > > front which contributes to the image might have a poorer spacial > > > correlation than if the center of the wavefront was not masked off. > > > > > > > The image quality is a function of D/r0 where D is the telescope > diameter > > and r0 is the Fried parameter. > As best I can figure out, you are only addressing aperture vs seeing, > relating the Fried coherence diameter to seeing. I don't see how this > relates to unobstructed aperture vs obstructed aperture for > the same seeing conditions. The effect of blocking the center portion > of the wavefront will certainly depend on the spacial coherence > of the incoming wave. Ok, I tend to be confusing lets break it down and put it back together. A 10 inch at 0.5 microns has a point spread function with a .42 arc second full width half max. ( so the program tells me) If you imaged two point sources ,no atmosphere, and ideal optics, with a .42 arc second separation the contrast from the peak to the valley between the two objects would be about 50%. Now lets expand this idea to lines. If I have my numbers right Mars varies from about 12 arc seconds to 25 arc seconds in diameter. I will now compare the observed contrast for a given number of lines across mars at say 20 arc seconds in diameter. The lines consists of a sine wave target that in one cycle varies in contrast from 0 to 100 % with a sine function and repeats across the diameter of 20 arc seconds.We will do two cases one with a clear aperture and one with a 30% obstruction like a secondary, I will leave out the spider. So, for a input contrast of 100% as a function of spatial frequency here are the results , from Oslo6 ( optics program). column 1 cycles per arc sec column 2 sine wave lines across 20 arc sec column 3 contrast 0% obs column 4 contrast 30% obs 1 2 3 4 0.26 5.2 84 77 0.53 10 69 56 1 20 40 34 1.6 32 15 17 2.17 43 2.5 2.8 So the obstructed reduces the contrast at lower spatial frequencies and does a little better at higher. And now the atmosphere: The output from the wave front simulation is not at the same intervals as the optical program so the numbers are a little different For seeing of 0.25 arc seconds, I have measured 0.28 on Mauna Kea no joke ! about 90% of it was near or in the dome. (C.F.H.T.) sine lines across 20 arc seconds contrast 4.9 97 9.3 89 19.7 70 34 41 44 26 for 1 arc second seeing 4.9 70 9.3 33 19.7 3 34 0 44 0 for 2 arc second seeing 4.9 33 9.3 3 and above 0 Putting it together requires the input contrast* telescope contrast* atmosphere You can multiply it out if you want, as you might see from the numbers at least, the difference in performance is dominated by the seeing. These numbers represent the long exposure contrast i.e. greater than 1 minute exposure. As any one knows who has spent any time looking at various seeing conditions the short exposure seeing can vary by a factor of at least 3 or more over a fraction of a second. Its only a hobby Dan