Quote:The MTF is one of the best ways to depict what happens with a CO. I don't know of a better way to demonstrate the CO effect in a graphical presentation
Quote:How to understand and interpret the MTF is described in Suiter as well as countless sites dealing with photography.
Quote:To fully understand the MTF ...
Quote:Joe,Contrast is not just the difference between the brightest and darkest part of an image.I think what people are referring to above has to do with the properties of the human visual system. It is not a linear system, so the mean level of brightness, the size of the exit pupil, magnification and many other things come into the picture.It is a complicated problem. In your reading get hold of the definition of intensity, contrast (there are several) and gamma, and explore the visual system. That is if you want to.You might start here:http://en.wikipedia.org/wiki/Contrast_(vision)Glenn
Orange Tube C8, Tasco 7te-5, 4" Criterion Dynascope, Jason/Towa 313
Nikon Action Extreme & Pentax WPii Binoculars
At my club, everything one could want up to 18"
20 inch f8 Chief Ed Jones designed and a 17 f9 Zanbuto Wide Band Chief Mike Jones designed .Both unobstructed telescopes .
Norme 150mm MCT f/13, 31% CO Yellow Zone "People say I'm in denial. I disagree."
Quote:So what's my point? SCT owners should worry less--a lot less--about the size of the secondary mirrors, and a lot MORE about cooling and collimation. Because, when everything is right, even the run of the mill scope does pretty darn well with image quality. If the larger secondary obstruction really damages the images (and I'm sure it does some)it's a pittance compared to other factors.
Quote:Quote:So what's my point? SCT owners should worry less--a lot less--about the size of the secondary mirrors, and a lot MORE about cooling and collimation. Because, when everything is right, even the run of the mill scope does pretty darn well with image quality. If the larger secondary obstruction really damages the images (and I'm sure it does some)it's a pittance compared to other factors. Amen Don!
Cactus Patch Observatory / 14" LX200
"The four points of the compass be logic, knowledge, wisdom, and the unknown. Some do bow in that final direction. Others advance upon it. To bow before the one is to lose sight of the three."
1987 Meade LX3 2120 10" SCT on hypertuned EQ6 Pro Baader Self-Mod Canon XS_CLS Clip Filter DSI color_Orion ST80 Coulter 10" Compact Odessey Dob http://www.astrobin.com/users/shawnhar
Quote:How we observe it is, apparently, non linear. But, with average vision, normal color perception, and normal contrast sensitivity most can expect to see pretty much the same thing.
Quote:One can't argue the physics of the MTF. However, execution of an optical system, combined with the ability of a system to approach it's potential is 'what it's all about'.As for CO, IMHO it plays a bigger role in consistency of high mag performance than anything else. When seeing isn't ideal, larger obstructions seem to play a synergistic role in limiting a system's performance (ie, limiting it's potetial).
12.5" Home built F-4.8 Eq Newt. Lightholder Optics mirror 12'x12' roll-off roof observatory 6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount. 4.5" Orion Starblast My equipment philosophy... If it ain't broke, fix it anyway.
Quote:For example, the Dawes limit for a 150mm clear aperture is 0.77" arc. My 150mm obstructed aperture has split 7 Tau at (as reported) 0.74" arc with a clear fall off in contrast between both stars just as MTF theory predicts (preliminary, anyway, higher spacial frequency above that of an unobstructed aperture.)
Quote:MTF applies to extended objects only. MTF does NOT describe point sources.
BTW The Dawes limit is off the MTF chart anyway.
Quote: neither statement is correct. Raleigh is 0.82 and Dawes ~ 0.97. The MTF describes diffraction of exactly that: point sources (either singularly or in infinite numbers.)
Quote:Here's a rough scale of things, even though both obstructed and unobstructed are normalized to 1. Really, obstructed is compressed horizontally by a factor of (1 - CO^2).
Quote:The MTF in the last 3% of the frequency range (Dawes limit) rapidly approaches zero and is equal to the MTF of an unobstructed aperture. There is no increase or decrease of contrast in that range that could be attributed to aperture obstruction.
Quote:Optics is a complicated subject filled with approximations, conventions, regions of validity, criteria and a host of special conditions.
...in some contexts it makes no sense as in coherent imaging, significant phase shifts...
The real MTF is a measured data set...
For astigmatism (or any aberration with an angular component like coma) however you have to add orientation data...
Dawes criteria is normally described for an unobstructed scope and contains the 1.22 factor we all grew up with. For obstructed scopes the distance to the first Airy minimum is somewhat smaller and becomes essentially unity at 50% obstruction.
Quote:The figure of 1.0 wave you quote is the aberration coefficient of balanced spherical Ai=As(p^4-p^2) for a strehl of 0.8. But the same term is 0.25 for spherical alone Ai=As*p^4.
Quote: As I understand it, amateur apertures are, for all intents and purposes, coherent in a range near peak sensitivity. And since phase is responsible for intensity distribution, it is at least somewhat accounted for.
Quote:But, in closing, I do have to say that observations in better than average seeing are pretty much consistent with what MTF suggests, if imperfectly. Namely, reduced diffraction effects improving my CO ratio from 37% to ~28% at full aperture (yea, I did the unthinkable...:)), and splitting stars /reported/ at below Dawes with noticeable contrast between the peaks. No immediately apparent improvement in planetary contrast, however, there may have been some. All of this is pretty much consistent. I am not an MTF crusader, but I do understand what it's telling us.
Quote:Quote: My other scopes have been dobs 6,8and 10 inch. From my expereance optical quality is number one, much more than telescope type. A SCT with perfect optics and a 30% central obstruction can never give better that 1/4 wave performance. Not that 1/4 wave performance is bad. That said, you will never find a SCT, or any other design with perfect optics.
Quote: My other scopes have been dobs 6,8and 10 inch. From my expereance optical quality is number one, much more than telescope type.
StarStructure 24" f/3.3 Kennedy | ServoCAT/Argo Navis Astro-Physics 175EDF | AP1100GTO | ATS Portable Pier Psalm 19.1 - A Psalm of David. "The heavens declare the glory of God, and the sky above proclaims his handiwork".
Quote:Quote: My other scopes have been dobs 6,8and 10 inch. From my expereance optical quality is number one, much more than telescope type.
A SCT with perfect optics and a 30% central obstruction can never give better that 1/4 wave performance. Not that 1/4 wave performance is bad. That said, you will never find a SCT, or any other design with perfect optics.
Compustar C14 Leo Henzl's Custom C8(VP Sales Celestron) Celestar C8 6" Refractor Adv. GT mount 6" F5 Omni XLT Newt. LXD-75 F4 Imaging SN8 Meade 8" F6 Newtonian Dynamax DX6 Criterion RV-6 ETX-90 Astro Meade 2045 4" SCT B&L 4000 Vixen/Celestron 80mm F11 JC Penny 60mm AZ/ALT Refractor 1963 60mm 15-60 B&L Spotter Binos 25x100 8X40 20x80 Arcturus Bino-Viewer