Author Topic: Optical experiment with a 102 mm achromatic objective  (Read 273 times)

John Pfister

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Re: Optical experiment with a 102 mm achromatic objective
« Reply #15 on: January 31, 2018, 07:45:24 AM »
Philip,

yes, I mixed up the ratios of stopped down 200mm. Still, the point being that in same aperture, the greater f/ratio helps with seeing induced focus error.

Neil English wrote about that: http://neilenglish.n...-stable-images/

I'm not sure how to weigh the aperture diameter vs. seeing error and focus depth vs. seeing error. as seeing error has both lateral shift and defocus...
It also depends on the type of seeing, too, but it must be better to have greater focus depth on the same aperture.

Cheers!

juskemenbo

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Re: Optical experiment with a 102 mm achromatic objective
« Reply #16 on: January 31, 2018, 01:02:25 PM »
Thanks for clarifying Aleksandar. I think I might investigate this a little further. I also had the impression that the brightness in the stopped down Schief was a bit higher than in the refractor. Strange, since I eliminated internal reflexions in the lens by using the gel and especially since the Schief has 2 mirrors, one correction lens and a diagonal prism in the optical train. But the difference, if there was any, was quite subtle. When I compared the chief to several 8" SCT's in the past, the Schief always lost as far as image brightness was concerned, even though it has no central obstruction. I think it has to do with the high reflectivity coatings of modern SCT's versus the standard aluminum coatings of the Schief.

Seth Mamidi

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Re: Optical experiment with a 102 mm achromatic objective
« Reply #17 on: February 03, 2018, 09:00:15 AM »
One explanation for the possibly slightly brighter image in the Schief. I did not put a mask on the primary mirror, but instead I've put a stop at the correction lens, so maybe I had an effective opening of perhaps a bit more than 10 cm...

Michael Washington

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Re: Optical experiment with a 102 mm achromatic objective
« Reply #18 on: February 09, 2018, 10:30:53 AM »
I'm not sure what you mean by "stability". You're dealing here with two issues.

(1) Smaller apertures have more "stable" images because of the size of atmospheric thermal cells. The Schief is a 200 mm aperture and the cells are usually between 3 and 4 inches (about 75 to 100 mm to round it off). The bigger the aperture the more "dancing" the image will exhibit.

(2) Depth of focus simply means that an image will remain within the 1/4 wave Rayleigh limit over a certain range of focal shift. The definition of the depth of focus (DoF) is the amount of focal aberration corresponding to 1/4 OPD =± λ/(2Nsin<sup>2</sup>U<sub>m</sub>), where λ = wavelength (usually 0.00055 mm) , N is the ref. index of the medium (in this case the air N= 1.0), and sin<sup>2</sup>U<sub>m</sub>is the sine of the angle subtended by the marginal (peripheral) ray and the paraxial focal distance. The expression can be re-written in terms of thefocal ratio, F#,as OPD =±2λ(F#)<sup>2</sup>. Doubling the focal ratio quadruples the DoF distance. If you go from f/10 to f/20, doubling the focal ratio the DoF tolerance will increase 2<sup>2</sup> or four-fold. In case of an f/10, the depth of focus OPD tolerance =±0.11 mm. For anf/20 it's 0.44 mm.

Clearly, the slower the system the more it's going to stay "in focus", thereby appearing more stable. If this is coupled with an aperture not exceeding 4 inches or so the image will also be more stable because of the thermal cell size of small air pockets, so the performance will be judged "superior" to that of a bigger, and/or faster system.

Mladen