Putting the "Rule of Thumb" to test
Posted 27 January 2013 - 05:00 PM
PS: BU1190 is HIP 28187 and should be therefore not so difficult to locate
Posted 27 January 2013 - 08:52 PM
My past experience with a 7-inch refractor would suggest these pairs, faint and very close, would be a good challenge for that - a bit larger than Norme's 15cm, and with no CO and higher transmission otherwise, a further small bonus in light grasp. And for dim pairs, avoiding averted vision with its reduced resolution matters for close pairs; wider ones allow a.v. to give benefits.
More thoughts - historical examples - in the next note.
Posted 27 January 2013 - 10:29 PM
Wilfried, since BU1190 is HIP 28187, maybe I can find it. Thanks.
I agree, something weird seems to be happening around mag 10, companions are getting fainter and we tend to use more averted vision with less resolution, maybe.
Posted 27 January 2013 - 10:41 PM
One who struck me as of particular interest was Ercole Dembowski (1812-1881), where Lewis gives averages for Dembowski's use of his 7.4-inch (~19 cm) Merz refractor. Dembowski had a particular interest in a revision of Struve's Dorpat Catalogue, and was regarded in his time as an exceptional observer for accuracy. After his death his observations, from various sources, were collected and published, the editing being done by Otto Struve and GV Schiaparelli.
To the point - the average for nearly equal faint pairs of Dembowski, 7.4-inch scope, was mags 9.3 and 9.5 at 0.7". Remarkable with a Dawes' Limit of 0.62". This is a stand-out result when compared to the other observers Lewis lists with telescopes under 25cm. SW Burnham with his 6-inch has 0.9" for mag averages of 8.6 and 9.8 (though somewhat less equal pairs, increasing the difficulty). Burnham with 9.4-inches (~24cm) has 0.6" at mags 8.3 and 8.8 averages. W Struve and Secchi, similar apertures (9.6-inch), manage 0.5" with stars 8.4 and 8.7 (Struve) and 8.4 and 8.9 (Secchi).
For 9th magnitude pairs (average 9.6 and 10.0) at 0.9" we have to go to Hough with the Chicago 18.5-inch (47cm) Clark refractor. Yes, it might be under-achievement with that size of telescope - because Schiaparelli with 19.2-inches (48.8cm) manages 0.4" at mags 9.2 and 9.5. The Dawes Limit for 19.2-inches is 0.23".
For now, not much point looking at what various observers did with 28-inch and 36-inch telescopes. Scary close with faint stuff.
One could of course scale these results for different apertures to get a rough idea of what might be possible - or follow Treanor's example, graph the results - but this time for faint pairs, instead of unequal pairs. Both approaches could be informative.
Potential problems? most likely the photometry of the pairs. To eliminate that would require re-doing Lewis - find the original publications, identify the closest faint pairs, then get modern photometry (Tycho, etc) for these. In the case of doubles that appear not to have changed much, or binaries with high-graded orbits, identification would provide a check on separation measures as well.
The above is a "thinking aloud" exercise. I feel the old observations (yes, all done with refractors) might throw some more light on the faint pairs issue after further analysis. So the above notes are offered for others to also consider in relation to the (multiple) vexed issues of the limits of resolving doubles of various types.
Posted 28 January 2013 - 03:00 AM
Ups - what are we talking about? May be we should indicate the WDS# for clarification: For ADS 2705 I found WDS03428+3016 AG72,ADS2705 6.4" +10.52/10.78mag.
Norme, very well done in splitting ADS 2705 - at only 1.0" and listed magnitudes 9.24 and 9.95 this is a tribute to your eyesight and observing skills...
Obviously you and Norme mean A2705 with WDS05307+1154 and for this one I get with CO 0,28 a req. ap of 153mm.
Fred thanks for the necessary hints to make this evident.
Posted 28 January 2013 - 06:06 AM
So I'd agree, we should do best to use WDS designations - my long-time preference anyway - that gives us the RA and Dec and the Discoverer designation - the standard way of referring to doubles. And it should avoid confusions.
I'll now look at the lists again and see if I need to revise any comments. I've been trying to stay with "A" not "ADS" designations.
Added comment - I've now revised my previous post to delete discussion of a pair I mis-identified.
Suggestions I made there, regarding scaling of telescopes relative to each other with different sizes, I'll repeat at another time when I've worked through some new examples.
Posted 28 January 2013 - 07:27 AM
Fred, I am curious why I was able to get down to a 9.9 mag companion (a distinct tiny speck fluctuating in and out, but not difficult) where some of the great observers topped out at lesser magnitudes at similar separations. I did have exceptional seeing and some moon light.
Posted 28 January 2013 - 08:31 AM
... curious why I was able to get down to a 9.9 mag companion ...
Norme - this does not seem so difficult. My own so far best result in this area was J781 in Cyg. Jonckheere advertised this double with 3" +9.4/9.4mag but the latest WDS entry is 3" +11.4/11.5mag. While I do not fully buy the WDS data (nearby +10.81mag TYC2669-00673-1 seemed a bit brighter than J781) this was a positive resolution with a 140mm refractor even if already with averted vision.
Posted 28 January 2013 - 07:14 PM
I think the data used by Lewis , which gives averages for the individual observers, is biased by selection. First, it deals with pre-1914 work. So later observers, like Jonckheere, who increasingly worked on finding faint doubles, as the brighter ones were largely discovered already (so they thought), are not included.
In the earlier era there was far more interest in the bright to middling pairs. These appeared to provide plenty of examples, were more accessible to moderate telescopes, and easier to measure than faint pairs. Also, if you assumed that on average faint means further away, then fainter pairs (on average) were less likely to show orbital motion. So were less interesting.
Some early observers did list a lot of dimmer fairly wide pairs, John Herschel in particular. But the Struves, Burnham etc were plenty busy enough without checking through 10th magnitude and fainter stars for new pairs. Aitken did add a lot of fainter pairs, especially when he took to using Lick Observatory's 36-inch refractor, rather than the 12-inch he'd used for some surveys.
The Southern Hemisphere observers added a lot of fainter pairs after the large refractors (26-28-inch) became available in South Africa. But that was in the later 1920s, continuing into the 1950s. Earlier work in the south had been mostly with more modest equipment - Russell's Sydney Observatory group (11.4 and 7.25-inch refractors) 1870s-90s, Innes (mostly a 9-inch refractor) early 20th century. There was a brief period when TJJ See took the Lowell 24-inch refractor to Mexico, which resulted in some more southern discoveries, but See didn't spend long enough in Mexico to add huge numbers of new pairs. Many of his southern doubles are faint companions to fairly bright stars (from Lewis - m8.0 and 11.5 at 0.9"; mag 7.4 and 12.6 at 1.4", as averages).
The emphasis on (dim) red dwarf binaries is post-Lewis, likewise most of the work of Jonckheere, plus the Nice (France) surveys by Couteau et al which found bright new pairs as well as a lot of faint ones, especially with the 30-inch refractor.
But if you look at Lewis's averages for observers with the Lick 36-inch you'll find dim stars; and that's true to some extent with the 18-inch and larger scopes as well. One example - Hough with the Chicago 18.5-inch is listed for uneven pairs (averages), mags 7.1 and 11.5 at 1.1", and for the very unequal, 6.6 and 12.2 at 1.9".
Typically, even with the biggest telescopes, there was simply less interest in finding faint companions to faint primaries. Lewis (28-inch), Burnham and Aitken (36-inch), do have dim very close pairs to their credit. These are around mag 9.0 to 9.5, at 0.2"-0.25". Hough with the 18.5-inch did average mags 9.6 and 10.0 at 0.9", but that's still within the magnitude 10 limit and it's about as faint as anyone went in that period with less aperture than Lewis on the Greenwich 28-inch. "Close" rather than "how faint" was the watchword for the bigger scopes; with allowance for dim companions to brighter stars - Aitken is listed, for very uneven pairs, 36-inch again, as mags 7.5 and 13.7 (!!) at 2.5"; Burnham, same scope, 5.7 and 12.3 at 0.9" (!!!). Don't try this at home.
Posted 29 January 2013 - 04:12 AM
Posted 29 January 2013 - 07:20 AM
It's good that we have forums like this to discuss astro matters. It allows a range of people with different areas of knowledge to contribute. Each of us can bring something useful to the group discussions, and we all benefit. I offer what I can.
Having said that, I'll say I'm finding the RoT issue more complex than I'd previously thought, and I didn't expect it to be simple.
I'll now have to get my SCT sorted out so I can do some experimenting with CO effects again. And, unlike much of my past SCT observing, push the limits. Except for faint pairs the C9.25 (235mm) didn't really do better than the 140mm refractor. Wilfried has likewise commented he prefers, for doubles, his 140mm refractor to his C9.25. So the next step is trying tougher doubles to see what the particular limits are for the SCT.
Posted 29 January 2013 - 07:54 AM
Since it will take some time to get a handle on all these questions I upload the current state of the ROT model (filled with Orion doubles as a starter for CO 0.28 and NEML of 4.5) to give the opportunity to check the CO-implementation against own observations.
Posted 29 January 2013 - 08:18 AM
For example, modified Dawes in my scope is right at 0.71" arc. Best I have done so far is 0.74" arc (as reported), right in the middle of theoretical and modified Dawes. STT 517 was too tight at 0.67" arc (but I will look again tonight.)
But, for an inherently aberrant aperture (Strehl of 0.94) to match or even exceed calculated Dawes means it's important. Most scopes (including SCTs, and especially refractors) seem to be optically this good or better these days. Well, IMO...
As for overachieving, I hate to fail. I will observe something until I am reasonably sure I saw it, even if that takes a hour. But, I've toned it back for the sake of getting a confirmed and readily visible disc. For example, even though I know where 42 Ori companion is, it was not easy nor clean. Success, maybe, but probably not good useful data. And, yes, you simply must screen the outlying observations, the most errant data points. That's smart data collection.
Posted 29 January 2013 - 10:47 AM
I also revisited STT 517 AB in what I hoped was good seeing. It was not, about 6/10. So nothing new to report there: elongation with a pinch. But, I do have to ask a question. Even though this one is very tight, why is it I very occasionally get a hint of dark space? I would not call it a split based on the elongated and pinched appearance predominating, however there are those curious times when it does look separated. Can moderate seeing cause this effect?
I did try BU1048, but failed to split it even in over performing mode. BU1048 is one of those doubles seemingly easy enough with 2" sec sep, but I could not split it. The rolling first ring gave tantalizing hints that attracted my attention, but no faint speck ever materialized.
I did not attempt many of the much fainter doubles on the Orion list. With the weather, seeing, and a few other reasons, I went with some of the brighter 8th and 9th mag pairs. The ones I thought might have the best chance of success.
Seeing was 6/10 as Orion hoovered over a nearby apartment complex. BU 1190 was higher in the sky and seeing was a little better at around 7/10 with a brief period of very calm skies after a patch of cloudiness passed. NELM was right at 4, maybe slightly better.
Wilfried, I have one more night before I put my scope in storage for my vacation to the US. If you have a couple of doubles near the zenith, maybe in Gemini, you'd like some reports on, please post them. I will try to get them for you.
Attached is my Orion list. Green highlight means a distinct companion was readily seen or a dark space separated the pair. Red means I failed to achieve a clean, distinct split. The blue ones were on my "to do" list.
At a glance, what I thought was interesting was I managed a couple tight ones above the 150 Ra limit, but failed a couple of wider pairs at or just below that limit. It's a small sample, for sure, but interesting none the less.
Posted 29 January 2013 - 04:02 PM
I have uploaded a list of doubles in Gemini of interest for any double star observer with a small telescope based on an intermediate state of my current RoT model for CO = 0.28 and NEML = 4.5 sorted by required aperture for a 50% split probability. I have marked the for you probably most interesting range from 140-160mm required aperture in yellow. Any observation report out of these range would be of interest to me.
Posted 29 January 2013 - 11:12 PM
Posted 30 January 2013 - 06:59 AM
Posted 30 January 2013 - 07:02 AM
Posted 30 January 2013 - 07:10 AM
... BU 1190 was higher in the sky and seeing was a little better at around 7/10 with a brief period of very calm skies after a patch of cloudiness passed. NELM was right at 4, maybe slightly better...
Norme, I think we may consider BU1190 and STT517 as positive limit observations. Congratulation on STT517 - even the modified Dawes limit for your scope would require 159mm aperture.
Posted 30 January 2013 - 10:40 AM
Okay, got a few tonight in Gemini. I began sampling ADS2525 and HEI 121. Both are dim stars in a sparse star field, star hopping took a lot of time. My neighbor's security lighting was reaping havoc with my view through the finder and my dark adaption (worse than the moon earlier this week...LOL.) I spent a good amount of time trying to find them, but was just not able.
BU 100: Try as I might, I failed to split this double. I even cheated and looked up the Rho and Theta for the companion and still failed. I could, however, see a dim star about 6" arc south of BU 100. I have not looked up that star's magnitude just yet, but I am curious about it. I did spend a lot of time with this first double of the evening, because at mag 11 I really needed to be fully dark adapted to have a chance. Still, no success. At 3.2" sep, it sits very near my 4th diffraction ring (~3.6" arc.)
STT 145: Split. Companion easily seen near PA 340 just outside the first ring at 263x (UO 12mm HD Ortho, 1.6x Barlow.)
A2450: Split. Faint companion easily seen at ~3" arc sep near PA 120 to 130 (south east, anyway.) However, it's PA is listed at 053. I suspect this is incorrect, but I just don't know. If it's correct, then I failed to split it and simply observed another star near PA 130. But, my gut tells me the listing is incorrect. That pair just looked and felt like a double. Stellarium, ironically, shows the companion more south of PA 90, too.
STT 171. Split - maybe. Okay, now we're getting into easier splits with Ra near 140, right? Wrong. This was a very difficult split. In fact, I had to cheat - look up the PA to find it. Afterward, however, I was able to observe the companion. But, it was not easy, not obvious, nor clearly a double star, it was truly difficult just to see the faint companion even fleetingly. So, I guess, NOT SPLIT. (LOL) Wow...very difficult.
Tonight, seeing varied from 6 to 7/10 with NELM of 4.4.
Wilfried, I wish I had gotten more. I just spent too much time early in the evening trying to find the dimmer stars and ran out of time later in the night.
I hope to keep tabs on your progress while I am traveling and wish you the best of luck.
EDIT: Wilfried, just ran across this. Sounds interesting.
Posted 30 January 2013 - 05:57 PM
PS: Have a nice trip
Posted 01 February 2013 - 03:40 AM
My current RoT model gives slightly larger errors in a broad range but does not freak out - certainly because it works with it's own data set and this is kind of compensation. The weakness of the RoT model is certainly the range with very faint secondaries. There is also some bias to recognise concerning the used data of the Lewis collection - Lord's alorithm does rather well with these observations and my RoT model rather not this good although I have adjusted the advertised magnitudes to the current state of knowledge.
Statistical evaluation: Standard deviation for Lord's algorithm is 25.4mm (error in relation to the used aperture) and correlation between actual aperture and predicted aperture is 0.947. Standard deviation for the current RoT model is 14.7mm and correlation is 0.963 - means in total slightly better values for the current RoT model.
After a creative break I will continue and attack the faint secondaries.
Posted 02 February 2013 - 02:59 AM
The various software packages I have do not always include standard (WDS) identifiers for doubles, even when they claim to, but give me single identifiers for SAO, or WDS, or HD, or just about anything. I don't want to waste time going through multiple identifying catalogs to get to the WDS standard designations (which means accepted discoverer, as well as position). I want a program that can give me the standard designation for each double, even if like your lists I get multiple extra identifiers as well - that's not a problem, and might at times help.
Posted 02 February 2013 - 03:55 AM
Posted 02 February 2013 - 05:29 AM
Concerning the cases with required apertures greater than used for the actual observation - these are quite often observations with apertures quite below the Dawes criterion and I do not allow the model to go below the (for reflectors modified) Dawes criterion. So this is a must have deviation and is not to be sonsidered as "error".