"I have been paddling in the shallows of a great ocean of knowledge." - Sir Isaac Newton * * 15" F4.55 Starsplitter Dob & a Denk II binoviewer * * http://peaceofsky.wordpress.com/ Pacheco State Park Fremont Peak
-DannyMy warehouseMy Channel
Quote:In my experience, the difference is huge.
AS of NSW
AS of Hunter Valley
Fact: I like reflectors and big ones. All the refractors I own are either finderscopes, binoculars, or riflescopes.
18"/F4.5 Obsession #1333 fully optioned (OMI Optics)
14"/F4.5 SDM #33 fully optioned (Zambuto Optics)
10"/F5.3 SDM #50 fully optioned (Suchting Optics)
Pentax XW's, T4 & T5 Naglers , UO HD orthos, 6mm Delos, 8mm Radian, 8.5mm Pentax XF, 13mm Ethos. 12mm Celestron Illum Astrometric eyepiece. Rainbow Optics Spectroscopic Eyepiece Set. Orion Optics UK 250 lpi Ronchi grating eyepiece. 2" Astronomiks UHC, OIII & H-Beta filters, 2" DGM Optics NPB filter. 2" Variable Polariser. 2" Hoya colour filters.
Leupold 10x42 & Pentax 16x60 binos.
150mm MCT f/13, 31% CO
"People say I'm in denial. I disagree."
Quote:Here's some more to read! The short answer is, apparently, *yes*, there can be, if all the other aspects that affect optics are properly handled, and the seeing cooperates reasonably well.
Happy owner of-- A Mag 1, 12.5 inch Porta Ball A Dual Axis Equatorial Platform A PST Double Stack
Quote:I haven't spent all that much time with mirrors bigger than 15", so your comments about big mirrors are interesting. Our club has recently purchased an 18" Obsession, and it disappoints me to say that I am disappointed. On the other hand, our club also purchased a 14" dob, and after a club member refigured the (awful!) mirror it now performs as well as any. A real pleasure to observe with.Is the big 18" incapable of providing high power images that are as good as the 14", regardless of mirror figure? It may be so, but I won't believe it until I have the opportunity to see this for myself. I believe that the Obsession mirror is guaranteed to be "diffraction limited". There are those in our club who say that the 18" dob is a "light bucket", and not a "planetary scope". However, the same thing was said about the 14", and after that mirror was refigured there is no question that it is an excellent "planetary scope". So, for me the jury is still out on this question for bigger (> 14") mirrors, but I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.
Quote:I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.
Quote:yeah something barely detectable once in 7 years.
Quote:Image quality IMO is more dependent on seeing which we have no control over. The best optics and eyepieces still won't help bad seeing conditions. We're at the mercy of the atmosphere. The best views I've ever had were with telescopes of "average" (no apo's, all commercial mirrors) optical quality, 8" aperture and under but the atmosphere was very, very forgiving. 1/4 wave, 1/10 wave?.....................-Rob
Active instruments: 4.25" f/6 Newtonian on altazimuth Springfield mount with crutch tripod -- telescope for the handicapped. 12" f/6 Newtonian Telescope maker since 1958; always more to come? Well maybe just one more and it will be something very special Stellafane Awards: 1977 1st award for 4" f/6 refractor 1978 1st award for 4" f/4 Lurie Aplanat 1983 1st award for Newtonian smoking pipe For Sky and Telescope articles, please see Bio info.
Quote:We have 4 18"/F4.5 classic Obsessions in our 3RF arsenal of scopes. One of these has a .90 strehl mirror, the other 3 have mirrors with a strehl > .97. These mirrors have been interferometrically tested. On one occasion only in the 7 years we have had these scopes in our care, have I been able to pick the lower graded mirror apart from the others and that was a feature on Jupiter which was barely detectable; and that could have even been attributed to variable seeing as I changed scopes. That having been said that mirror is still a very good mirror. It's smooth with an excellent edge and slight undercorrection. In terms of peak to valley its worst point on the mirror face is 1/4.5 waves. That mirror is in fact the mirror in my scope and it can still push 1075X under favourable conditions. Would I have gained anything with 1 of the other mirrors, yeah something barely detectable once in 7 years.
Quote:I don't live anywhere near New York. In fact I have some premium optics, observe under Bortle I skies day in day out and haven't changed my opinion on this when it comes to larger optics.Cheers
My eyepieces are made from the waste product of exploding stars. 10XTi 102XLT ST80A(2" Focuser) XW: All; XO: 2.58 Televue: Naglers-T1 Smoothside-full set, 17T4,12T4,Ethos 17,4.7; plossels-40,32,20,17,&7.4mm; Pans-22,24mm; Delos-6,8,12,17.3mm ES100: 5.5,9*,14,20 ES82: full set ES68: 16,20,24,34 NLV: 5,9,10,15 Ortho: HD-7,9; OPS-9,12 Meade RG 7mm Other: Pentax 12.5K(.965), 10mm Parks Zoom: Nag3-6 *=on b/o DAS Dark Site
Quote:The worst mirror you referenced was a Strehl 0.90 which is already a pretty decent optic. Do you think you would see the difference between it and a 0.8 Strehl? Under excellent skies, I think it would stand out to all but the most casual of observers.dan
Quote:With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave).
Quote:True, under mediocre seeing, once in a great while, the seeing, by accident, becomes much better. And then you will thank your lucky stars that you have 1/10 wave mirror! Images become close to miraculous! Ed
Quote:I use a large enough scope that it is very rare for the seeing to actually exceed the resolution of the mirror.What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.
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.
Mike Lockwood - Owner, Lockwood Custom Optics. 20" F/3 MX Starmaster, 14.5" F/2.55 self-built Newt., nine self-built scopes, 4.25" to 30" http://www.Loptics.com/
Quote:It's not real complex.
The difference in the tightness of star images and detail resolved on objects IS noticeable even under somewhat bad seeing, and even on large instruments.
A good reflector of any focal ratio, properly equilibrated and collimated, small or large, should produce good images on most nights, and superb images frequently.
For any poor instrument, thermal conditions will vary, but chances are that infrequently, once in a great while, those conditions will help the poor mirror perform better than it actually is!
In my experience, any test claiming high Strehls should be taken with a grain of salt. Evaluation under the stars on a good number of nights, and comparisons with other instruments on the same night, under the same conditions, is a test method that I recommend for telescope owners.
Quote: I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans
Quote: Quote: I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans how did that effect your balance?
6'' Orion SkyQuest f/5 12.5" Astrotel truss dob f/3.3 20" Super FXQ Starmaster William Optics 80mm ZenithStar ED II 1850/2500 Observing sessions grand total for 2013, 49. So far in 2014, 11
Quote: Or do you mean does Don fall down now?
Quote: No, i think he was asking about his checking account since getting the Teeter.
Quote: Quote: Or do you mean does Don fall down now? No, i think he was asking about his checking account since getting the Teeter.
Quote:What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.