Used acronyms: NEML=Naked Eye Magnitude Limit, SQM=Sky Quality Meter, TML=Telescope Magnitude Limit, CO=Central Obstruction, delta_m=difference in magnitudes between double star components, RoT=Rule of Thumb, pD_mm=proposed D_mm for resolving a binary (ident with earlier used pA=proposed Aperture), D_mm=Diameter (of scope) in mm, UCAC4=USNO CCD Astrograph Catalog 4th edition
150mm MCT f/13, 31% CO
"People say I'm in denial. I disagree."
Quote:It sounds like your application of central obstructions is covering out the better part of your objective. You probably have zones of inaccuracies that the CO brings to clear definition by way of the degrading effect it has on the diffraction pattern. You may have a clunker of a refractor. At any rate I did a 50% CO obstruction on my reflector and aside from the ghastly bright rings the spurious discs of Epsilon Lyrae were superb.You may find your tests are more fruitful by masking off the outter inch or so of your refractor.Lords would do no less.Pete
Quote:Pete, I am not sure what your point is - the higher CO the better resolution?
I also do not understand your reference to Lord - I don't know of any experiments of Lord with size of CO. But in his Rule of Thumb algorithm for resolving unequal double stars he uses CO as parameter besides aperture, seeing, separation and delta_m. For example his result for BU163 0.9" +7.31/8.88mag for a 140mm refractor with fair seeing (Pickering 5) is "eXeedingly Difficult" and with a 140mm reflector with 0.4 CO it is "UNresolvable".
I also do not understand your argument with 0.5 CO on your 200mm reflector - aperture rules and you have still an photon sampling surface equivalent to an 173mm refractor so why should the spurious disk not be still rather crisp. The relevant question here is only if 0.5 CO enhances your chance for resolving doubles compared with your standard CO. If you feel this is the case then you should stay with 0.5 CO and never look back.
Finally - I am not in possession of truth and look only for obvious evidence and I am so far rather satisfied with my findings: small CO enhances somewhat the resolution (especially for equal doubles) and large CO leads somewhat to degradation (especially for unequal doubles). That above of this CO effect aperture still rules at least to some degree is anyway not in discussion - a 200mm scope with 0.5 CO will certainly resolve more doubles than a 60mm scope with zero CO.
Quote:...refractor has the equivalent of TDE. And there can be zones in the central part of an objective...
Quote:True enough Fred however more than one refractor has improved its image due to stopping down the aperture and as a result of course increasing FL.I am aware of TDEs being often a reflector issue though. Still the falling apart of the image at a 40% CO still seems peculiar. I'm getting Normes take here - optimized for high end frequency not low contrast and a trade off of peak intensity but how does that explain the pattern falling apart like that on doubles? I can see the large CO modifying the pattern but not destroying high frequency contrast like the classic Dawes notch and such.Its as though there's this smoothing taking place .As an aside, I'm glad you are getting a ten inch F/7. Its actually got a nice edge over the 8" I've seen in side by sides. More so than Id have guessed even.Pete
Quote:...previous ideas suggested there's a linear relationship of CO to Dm factors - increased CO helps equal pairs by making the disc smaller; then CO is unhelpful as Dm increases from about 1-magnitude.
...made the companion dimmer, and larger CO sizes made it harder and harder to see.
In all cases, it appears that large CO of 0.35-0.40 is less good than moderate CO in the 0.15 to 0.25 range.
Quote:... This seems to suggest the central disc is getting larger above 0.25D and that should not be correct...
Quote:... I posted a qoute from Lord some time back stating his advantageous use of a 72% CO which was something like 9" covering a center on his 12" reflector... Its be a fine thing to see what ROT supports his CO claim.Pete
Quote:... I am having a hard time understanding what it means when you say the diffraction pattern "begins to fill up the gap" turning a possible split back into a rod formation...
Quote:...this is simply a question of magnification - if you have a split with a not this high magnification you see only the spurious disk and not the diffraction rings or may be the first diffraction ring directly attached to the spurious disk.
Quote:... I'm now looking again at Wilfried's way of putting a CO in front of his refractor - those two bars, and not overly thin at that - they'll have a further diffraction effect, and a non-symmetrical one. That will change the diffraction pattern in a different way from the central disc, and the effect will be superimposed on the effect of the CO disc...
Quote:... So, you're using just enough magnification to suit your acuity?
Quote:Conclusion: CO value of relevance for resolving faint equal binaries - in this case 140mm aperture with CO 0.4 equivalent to 80mm aperture with zero CO.
Quote:For a large obstruction seeing has to be very very good for it to be in any way beneficial in splitting a binary. This is a known but little discussed difficulty of large obstructions. When seeing is mediocre, turbulence will scramble all that extra energy in the rings along with the central spot to yield a mess.