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."
Celestron C8 XLT on CG5 8inch EQ mounted newt Parks 8inch f/6 90mm mak
Quote:Fortunately, there is a solution...LOCK the primary in place at the exact point it should be separated from the secondary, then install a rack & pinion focuser.
My binos: 6x30, 7x35, 10x70, 15x60, 20x80 My scopes: 12.5" Telekit, 8" SCT, 4"f5 TV, 3"f11 Vixen ARCO OBSERVATORY 7200'
Quote: My other scopes have been dobs 6,8and 10 inch. From my expereance optical quality is number one, much more than telescope type.
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.
150mm MCT f/13, 31% CO
"People say I'm in denial. I disagree."
Quote:Quote:Fortunately, there is a solution...LOCK the primary in place at the exact point it should be separated from the secondary, then install a rack & pinion focuser.Mike I haven't followed the CO thread really carefully but your statement above seems to be a common belief among many SCT owners... the problem is, how can the correct spacing be determined easily without a heck of alot of testing? I'd guess you're proposing something like a Roddier test? Or, using the intra/extra focal patterns at different backfocus distances? All of this sound very time consuming and frankly may not be conclusive. My SCT images are often mushy, probably due to poor thermals and maybe collimation. I'd need to deal with all of that before even attempting what you're suggesting, yes?Loren
Quote: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:I routinely see very difficult detail in my 32% and 34% obstructed SCTs. Both have excellent optics, and both are excellent performers.
Quote:Bottom line... .25 SA is worse than a 30% obstructiion..
Quote:The 1/4 wave in Suiter is the traditional terminology going back to Rayleigh.
"A telescope is either good or cheap, not both" - but sometimes you get lucky!
Ed "You have to grow old, you don't have to grow up."
Quote:What's interesting to me is the idea an obstructed scope has a smaller Airy disc that is a bit brighter than it would normally be if it's intensity were spread over 1.22 Lambda/D. This, in my understanding, is what differentiates CO diffraction and spherical aberration. This is what causes an obstructed scope to exceed a perfect unobstructed aperture on very small scales (near the radius of the Airy disc.) And it is what allows, under very minimal induced aberrations, to actually exceed Raleigh and Dawes limits by about half an arc second.
Astronomy educator/Sidewalk astronomer
Owner of Astronomy Delight franchise
18 inch f4.42 Dob on eq platform w ST120 f/5 finder
12 inch Zhumell Dob
8 inch f/6.9 home made Dob with Seevers optics
William Optics red 10th Anniversary 80mm FD
C8 XLT on Vixen GPDX
26lb eyepiece box
Cernan Space Center astronomer
Member of Northwest Suburban Astronomers
Quote:Practically speaking, from my experience as a casual observer with MCT, SCT and achromatic refractor, I wish an instrument's intrinsic limitations were the only obstacle to a good view. Usually seeing (high level and local) is the most significant barrier to obtaining a detailed view. I've been careful to accumulate each of my scopes on a modest budget. If one keeps that factor in mind, the views through a sub $300 Maksutov or circa $400 C8 are astounding value, and show me far more than the 6" APO that I don't and won't own. They are also easy to mount for visual use. That these instruments are often considered as a "runner up" is a shame - a well tuned catadioptric scope with good optics can be a powerful tool. Rather like my MGB providing an excellent and fun drive, despite it not being an E Type Jaguar...
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"
Quote:I do not know how to bias focus for mid range contrast - maybe smallest blur?
Quote: I'm saying it's a complex system and not easily explained or described.
Quote:This does though ignore the brightness/scale advantage that the larger aperture always has, which can offset a little contrast loss simply because the image for a given magnification is much brighter. I personally find that brightness makes the lowest contrast detail easier to see, especially as the magnification gets higher and the exit pupil in the smaller aperture gets below about 1mm.