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
Roberto Botero, Kent, UK
Quote:Fred, while your impression that the C925 might be compared with a 155mm refractor in terms of performance for resolving doubles may be correct this relation is not the topic of the graph above. This graph illustrates how utterly wrong the RoT algorithm works for CO 0.35 or larger. In the short term this means that the RoT only works for CO up to 0.35 - this is no problem for most scopes but I will try to correct this if possible.Regarding obstruction of C925 - specs give 85mm for the secondary mirror but including mounting the CO is exactly 90mm. This means 90/235 = 0.383 CO. But I fear there is more to consider with SCTs (in comparison with other types of reflectors - as for example the Schaefer TML calculator does). Ed Wiley is happy to get with his C11 results down to Rayleigh with video - I assume then this would be impossible for visual observing.Back to the topic RoT: I have far too few limit observations in this aperture range and it will be difficult to get much more because you can not reduce the aperture without getting absurd high CO - and when you can not reduce the aperture you never know if an observation is actually on the limit.Only possibility would be a large Newton with small CO as a start and masks for reducing aperture as well as increasing CO to maintain a given ratio - may be a bit cumbersome but it could be done.Wilfried
150mm MCT f/13, 31% CO
"People say I'm in denial. I disagree."
Quote:... The first list, pairs around 1.0" (about Rayleigh for 140mm), are examples of doing better than I'd expected. The delta-m figures for these range from 1.6 to 3.0. And, yes, the dm1.6 pair was the (relatively) easiest; the pairs around dm 2.5-3.0 noticeably a lot harder, even in Pickering 8 (about as good as it got). 55 Leo 6.0, 9.0 1.1" dm3.0 -> pA 158mm15 Boo (KUI 66) 5.44, 8.43 1.0" dm3.0 -> pA 169mmBU 13 Ori 7.57, 9.22 1.0" dm1.65 -> pA 149mmHEI 670 Ori 5.97, 8.36 1.1" dm2.4 -> pA 137mmBU 314 Lep 5.9, 7.5 0.9" dm1.6 -> pA 145mm42 Ori (DA 4) 4.6, 7.5 1.0" dm2.9 -> pA 152mmUpsilon Gru (BU 773) 5.70, 8.24 0.9" dm2.54 -> pA 170mmBU 603 Leo 5.97, 8.53 1.0" dm2.6 -> pA 155mm-> According to the pA numbers 15 Boo and Ups Gru should have been the most difficult but all observations are within a reasonable pA range...Second list, doubles where the 2ndry star falls on or about the first bright ring. Again, culled after the event from a couple of years' observing. The standout here is a dm 2.85 pair (BU 1190); otherwise, dm is generally 2.0 to 2.5. Without the diffraction ring I'd expect to have found doubles split with dm 3.5 to 4 at this separation, because these are wider (1.3"-1.6") than the first list (0.9"-1.1"). near or on 1st bright ring BU 1190 Ori 6.95, 9.81 1.4" dm2.85 -> pA 148mmSTT 145 Gem 7.3, 9.9 1.5" dm2.6 -> pA 138mmBU 91 Tau 8.1, 10.6 1.5" dm2.5 -> pA 150mm47 Tau (BU 547) 5.05, 7.32 1.2" dm2.27 -> pA 124mmTheta Gru 4.45, 6.60 1.5" dm2.15 -> pA 99mmBU 281 Psc 7.4, 9.4 1.6" dm2.0 -> 110mmBAS 3 Tau 6.16, 8.2 1.3" dm2.0 -> 120mmBU 1089 Oph 6.57, 8.98 1.5" dm2.4 -> 116mm-> most of these pairs should have been rather easy and be possible with smaller aperturesThe easiest of these were, unsurprisingly, BAS 3 and BU 281 (160x); the hardest was BU 1190 (285x). ...I also have examples of pairs wider than the first diffraction ring, with larger delta-m. The second dark interspace for 140mm is ~1.83"; the 2nd bright diffraction ring centred at 2.2". Here are some pairs separated 1.7" to 2.0", with delta-m of 3 or more. A 146 Vir 7.31, 10.77 1.7" dm3.5 -> pA 164mmRST 690 Cir 6.7, 9.8 2.0" dm3.1 -> pA 121mmBU 15 Ori 7.44, 11.6 2.0" dm4.15 -> pA 188mmKui 17 Tau 4.26, 7.85 1.8" dm3.6 -> pA 118mmSTF 562 Tau 6.82, 9.94 1.9" dm3.1 -> pA 129mm-> A146 might have been a bit of a challenge but certainly doable and BU15 is an extraordinary observation with 140mm - congratulation. The other pairs should have been rather easy These required varying magnifications, typically 230x-285x, though when seeing was best some were visible at 160x. ...
Quote:I've had the experience of observing a double star that another experienced observer could not see, same time, same telescope, same eyepiece. The two of us looking alternately. To me the companion was fairly obvious; to him it was invisible, then after "look in this position" it was "maybe I can see it". So we have the observer factor as well. Some observers will need a bigger telescope, or higher power, or more practice on doubles.
Meade 305 ƒ/10.4 LX200 • Royce 250 ƒ/19 Dall Kirkham • AstroTech 250 ƒ/8 RC • AstroTech 250 ƒ/4 "RFT" Newtonian • Meade 80 ƒ/6 ED APO • TEC 140 ƒ/7 APO • Orion 180 ƒ/15 Mak Cass ... Denkmeier II binoviewer - Nikon 8x60 binocular
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