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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6063773 - 09/05/13 03:51 AM

The effects of CO on the diffraction pattern seems rather complicated. As I have not the means to include diffraction theory in my RoT algorithm I have to keep things simpler. There are so many factors influencing resolving doubles involved that I have for many of them resort to the probability approach.
I have now an idea how to realize different CO sizes for my 140mm refractor (details later) and will first also check the assumption that some CO makes splitting of close doubles easier and that larger CO makes resolving of unequal doubles more difficult. Nest I will also check the effects of CO size on the brightness of the rings and the relation of ring brightness and magnitudes of secondaries. Will take some time.
Wilfried


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3c_273
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6064828 - 09/05/13 04:56 PM

Fred,

Thanks for your lucid comments regarding the resolving ability of a telescope on a given double star.

Last night was especially good, and I decided to go for some very close pairs. I'll just list the raw results:

ES 187: 18:23:45.03 +51:38:55 (J2000). Double. Sp: G5
Sep: 2.6, PA: 206, Year: 2008 9.32-9.43

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:13:21
A lovely pair of almost equally bright stars.

HU 940: 19:5:30.85 +33:52:21 (J2000). Double. Sp: F8
Sep: 0.5, PA: 194, Year: 2010 9.18-9.78

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:22:19
A faint oval star, elongated in the WDS pa.

A 260: 18:57:34.06 +32:9:20 (J2000). Double. Sp: A0
Sep: 0.9, PA: 246, Year: 2010 9.17-9.60

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:27:10
Elongated in the expected direction.

A 703: 19:7:12.16 +44:50:30 (J2000). Double. Sp: G0
Sep: 0.6, PA: 189, Year: 2009 9.01-9.28

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:36:49
Not split or elongated.

HU 1300: 19:20:12.77 +34:10:52 (J2000). Double. Sp: A3
Sep: 0.7, PA: 182, Year: 2010 8.92-9.56

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:46:50
Elongated, not split, in the 2.8.

BU 1132: 19:43:14.78 +27:1:6 (J2000). Double. Sp: B8
Sep: 0.6, PA: 214, Year: 2007 9.35-9.94

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:59:35
Barely elongated in the 2.8. It looks single in the 6.


Note that I use 9, 6, and 2.8mm orthoscopics (the 2.8 is the Takahashi, which has a built in barlow) for this work. At under an arc second, I really don't split anything, but I can elongate it. The very high power (725x) of the 2.8 is sometimes (not often) the only way I can get a position angle on a close pair, like BU 1132. As to the telescope itself, I've not had it on an optical bench, but it's bokeh inside and outside of focus is close to identical, better than in my C-11. It has a refractor like "snap" as a star comes into focus. So while I can't give you a strehl ratio for it, it seems to have pretty good optics.

Note that I couldn't even elongate A 703, but if the star magnitudes are accurate, it should have been plain.

If you're still with me after all of that dull text, now I can define "untheoretical". It's when in theory I should easily split a star in good seeing, but I don't. Other "hidden variables" are at work that I don't quite understand.

Bill Hartkopf, of the USNO, confirms that they are trying to get the APASS magnitudes into the WDS. Their budget for this is quite limited, and they are suffering under the sequester, so don't expect anything soon. In the mean time, you never know what you might find when you look at a pair, although Night Assistant does show what other catalogs list about a given star. This helps me know what not to observe.

Getting an accurate algorithm for predicting the resolvability of a given pair with a given telescope is a daunting task at best. Look at the complexity of Brad Schafefer's algorithm for limiting magnitude of a telescope:

http://adsabs.harvard.edu/full/1990PASP..102..212S

I suspect that the one for binary resolvability will be more convoluted. Wilfried is doing us a valuable service.


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fred1871
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Reged: 03/22/09

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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6065307 - 09/05/13 10:11 PM

Tom, your list of observed pairs will no doubt provide some new data for Wilfried, as some of these appear to be near-enough to being "limit" observations. That's useful.

Your comments about your telescope and observing details are of interest. Commercial SCTs do vary a fair bit in optical quality, but some are surprisingly good. I've used a number of C8s in the past, and looked through others; the best one had very good optics, immaculate star images, essentially zero image shift in focusing, etc. It had refractor-like focus snap too.

I'm a little surprised you don't get splits below 1 arc second. I've seen splits below that on fairly equal pairs - say at 0.8" - with a couple of C8s, and not only the best one. My C9.25 gave a "discs-kissing/round, but just about touching", effect on Zeta Bootis recently; that's at Dawes for the aperture (0.5"), and a good C8 should do similar things at around 0.6", unless the stars are too faint to see well. Try mag 6 or 7 pairs, and see what happens.

Orthoscopic eyepieces can be very good for doubles, especially with uneven pairs, because they can have very low light scatter. I saw a striking version of this with my 140mm refractor, when observing Saturn - a 5mm Ortho showed faint moons clearly, very close to the planet, that were hard or impossible to make out with a 5mm Nagler T6. The Nagler is a very good eyepiece, but has noticeably more scatter on bright objects. I wouldn't use it for trying to see The Pup near Sirius.

I see a rather big gap between your two higher-power eyepieces. 6mm at 333x; 2.8mm at 725x. Seeing will often limit the ability to use 725x; I'd suggest getting an eyepiece in between, that might do pretty nearly everything 725x can do, and do it with a bit less power, and keeping visibility of dim stars easier. 725x could be overkill for an 8-inch a lot of the time. Perhaps a 4mm Ortho, to give 500x? It's 50% more power than the 6mm, and could be usable more often, and might make fainter stars easier to see well.

And I did some follow-up on one of your doubles. A703, which you couldn't resolve, may as you suggest have wrong photometry. Here I'm suspecting that it's simply a copying error in WDS.

I had a look at the 1996 WDS to see what mags were given before Tycho mags were imported. That gave 8.3 and 10.6, where the new listing is almost even. Visual observers in the early 20th century would notice that large difference between the stars, whether or not their assigned mags were accurate. The combined brightness (seen as a single star, unresolved) would be similar, whether the mags were the new version or the old, so that doesn't help.

Next I went to Hipparcos. Useful. It lists a double, separation 0.57", primary magnitude around 8.4 (Hipp 8.34; Tycho 8.44) and a delta-m of 1.99 (let's call it 2.0). That fits well with the older data too.
So it looks as if the current listing has wrong magnitudes copied in - the old (1996) listing is closer to accurate. On Hipparcos/Tycho, 8.4 and 10.4 would be a fair call.

That would also explain why you couldn't see elongation - it's not only still very close, but the big magnitude difference will hide any elongation effect from the dim secondary. Otherwise, you might have had a result similar to BU 1132 with delta-m of 0.6.


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WRAK
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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6065541 - 09/06/13 03:02 AM

Thanks Tom, will go through your list and give you feedback with the calculated RoT value for your observations.
Appreciate Schaefers work very much and used his approach for estimating the influence of light pollution on TML for the TML check in my RoT algorithm. Schaefers approach is a fully analytical one in contrary to my statistical one. His algorithm uses a row of multiplication of factors and I use separated modules for the factors and add or substract then the values of the results of the modules and hope so to reduce the range of errors. Interesting result of Schaefers algorithm is that the comparison with about 350 TML observations gives a rather large standard deviation and I have therefore concluded that the simple formula 2.7+5*LOG10(D_mm) is good enough for a statistical approach with a rather similar standard deviation.
Wilfried


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3c_273
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6066239 - 09/06/13 02:11 PM

Fred,

Again, thanks for the concise, detailed analysis.

I had another good night (rare for around here [near Washington DC], where we often go for a month or so without clear skies) and learned of a few more complications.

Again I went for close pairs. Again, I had a very difficult time with them. I'd go from my 25mm finding eyepiece directly to the 2.8 or 6. I could elongate a pair like HO_457:

HO 457: 20:24:21.08 +29:22:55 (J2000). Sp: A0
Sep: 2.0", PA: 60 degrees, Year: 2008. 9.08-9.18mv.

but I couldn't split it. A pair of 9mv stars (substantiated by the UCAC4, which lists them as 8.7mv) should have been easy. When I switched to the 12.5, there they were! Two ~1.5" blobs 2" apart, looking like a tiny shimmering peanut.

Upon reflection, I think what's going on is that with the higher powers, the diffraction pattern is, of course, being magnified. It no longer looks like a point source, and they appear like wiggling blobs (seeing was ~Pickering 4) of extended light. A tiny blob with an integrated 8.7mv requires a bit of averted vision to make out. Let's see now. Using the 6, with my -6 diopter myopic eye, that works out to about 350x. This means the pair looks like a pair of 9 arc minute blobs 12 arc minutes apart. The resolution of the human eye is at best one arc minute, and once you start using averted vision, it drops precipitously. High powers on faint stars cause me to use averted vision, and thus I see blobs. When I look directly at it, what I see are dancing speckles, elongated in the correct pa. At the lower power, the stars are essentially points, only 4 arc minutes apart, and the light is more concentrated by a factor of 4. No more averted vision is needed, and Viola! I see the pair.

The vagaries our vision, and the atmosphere are going to make an accurate algorithm for binary splitability fairly complex, with terms for seeing and eyesight acuity (both rather subjective) being part of them.

------------------------------------------------------------------------------

Wilfried,

I see you're collecting visual observations, so here's a URL that has all of mine:

http://mainsequence.org/html/obs/Double_Stars.html

Bear in mind as you use these that Burnham (or was it Aitken?) said the first thousand or so observations by an observer weren't to be trusted.


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fred1871
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Reged: 03/22/09

Loc: Australia
Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6066784 - 09/06/13 08:18 PM

Tom, I get better seeing a lot of the time than you appear to have; though the last couple of years I've had far too often cloudy nights.

But Pickering 5 is about where I start observing double stars. Below that, maybe some wide ones at low power; but they don't look good at Pickering 4, and tight ones are too mushy/difficult.

Even in good seeing, it's possible to over-magnify - resulting in images that don't help seeing what's there. Your description of "blobs" can be a result of too much power as well as poor seeing conditions.

Your example, HO 457, is informative - highest powers, elongated (presumably, smeared images); less power (160x?), two stars, separated. I've had similar experience - one of the fainter pairs I've seen as two stars, not just elongated, is STF 1620 in Virgo - listed mags 9.1 and 10.35, at 2.2" - and seen as two stars at 160x with 140mm.

I tend to start with moderate magnifications, then work up as needed. I find a lot of fairly tight doubles are split at 160x with the 140mm refractor; likewise the C9.25 at 180x. The tighter and more uneven ones can benefit from higher powers, but often enough I find pairs visible at 160x-230x that are harder to see at 285x and much harder at 400x - the extra power is counter-productive.

And that holds for a lot of fainter pairs - there's a balance point, between where the eye has enough separation to see, and where the light gets spread so much that stars are too dim. As Schaefer's work showed, you get benefits going from low to medium power in seeing fainter; but although his graphs plateau at a certain point, meaning no further benefit, I find in the case of close doubles that there's some loss rather than a plateau. A bit less power can make some pairs easier to see.


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WRAK
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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6067529 - 09/07/13 10:23 AM

Tom, assuming 0.34 CO and no light pollution the RoT algorithm gives the following results:

Quote:

...

ES 187: 18:23:45.03 +51:38:55 (J2000). Double. Sp: G5
Sep: 2.6, PA: 206, Year: 2008 9.32-9.43

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:13:21
A lovely pair of almost equally bright stars.

-> Proposed Aperture 77mm - no limit observation, just warming up

HU 940: 19:5:30.85 +33:52:21 (J2000). Double. Sp: F8
Sep: 0.5, PA: 194, Year: 2010 9.18-9.78

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:22:19
A faint oval star, elongated in the WDS pa.

-> Proposed Aperture 263mm - 200mm would be outside the double standard deviation. Was the elongation clear enough to count this as valid limit observation for resolving this double?


A 260: 18:57:34.06 +32:9:20 (J2000). Double. Sp: A0
Sep: 0.9, PA: 246, Year: 2010 9.17-9.60

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:27:10
Elongated in the expected direction.

-> Proposed Aperture 158mm - should have been no elongation but a clear split or at least a rod with magnification x200 or higher


A 703: 19:7:12.16 +44:50:30 (J2000). Double. Sp: G0
Sep: 0.6, PA: 189, Year: 2009 9.01-9.28

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:36:49
Not split or elongated.

-> Proposed Aperture 214mm - should have been not plain but at least possible with 200mm if the advertised data would be correct

HU 1300: 19:20:12.77 +34:10:52 (J2000). Double. Sp: A3
Sep: 0.7, PA: 182, Year: 2010 8.92-9.56

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:46:50
Elongated, not split, in the 2.8.

-> Proposed Aperture 194mm - 200mm aperture should show at least a rod

BU 1132: 19:43:14.78 +27:1:6 (J2000). Double. Sp: B8
Sep: 0.6, PA: 214, Year: 2007 9.35-9.94

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 4, Time: 21:59:35
Barely elongated in the 2.8. It looks single in the 6.

-> Proposed Aperture 228mm - 200mm are on the lower side of the standard deviation means still a fair chance with very good seeing

...




The high magnifications you use puzzle me a bit because in my experience the image quality gets drastic worse with magnification more than double the aperture in mm - but in my locations seeing is rarely better than Pickering ~7 and most times worse.
As all observations have been for rather equal bright stars only the first component of my RoT model is active meaning Dawes modified with CO for decreased size of the Airy disk. I wonder if 0.34 CO is already too high for any benefit from this effect. This could be an explanation why you get elongations instead of rods or even splits.
Thanks for the link to your observations - impressive. Problem for me is how to find obervations on the aperture limit.
Regarding HO457 - here the proposed aperture is 77mm, so why you could not resolve it with the C8 remains unclear.
Wilfried


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kcolter
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Re: How good are reflectors for resolving binaries? new [Re: WRAK]
      #6067682 - 09/07/13 11:53 AM

I'm a deep sky observer, but enjoy the challenge of double stars. I've seen the companion to Sirius many times in a 20 inch Dob with Zambuto mirror, including this morning for the first time this fall. As to aperture masks, when I have used them on large Dobs, I've used off axis aperture masks which eliminate central obstruction (at least I thought they did.) I've read Roland's comments about the disadvantage reflectors typically have compared to refractor objectives with regard to more light scattering on mirrors from dust. I think that's true. 20 inches of dust scatters more light than 6 inches of dust, and most Dob mirrors are probably dirtier than most refractor objectives. Big Dobs collect more scattered light, and for lack of a better term, more "scintillated light" when seeing is less than optimal.

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3c_273
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Re: How good are reflectors for resolving binaries? new [Re: kcolter]
      #6068322 - 09/07/13 06:28 PM

Fred,

Congratulations on your excellent seeing. Visiting Australia in your winter and seeing such things as M22, M8, 47 Tuc, or Omega Cen near the zenith in a good telescope is on my "bucket list". One of these months, after I retire!

A note on the higher powers. There's not a single guide that recommends powers higher than 50x per inch, so to increase this by ~50% should be madness. William Herschel, on the other hand reported using 1,000x on his 6" speculum mirror telescope (with a single element lens, mind you!), so I thought I'd give it a try. After having used it for a while, it's too much power for the vast majority of cases. Given the radius of the Airy disk (Center of disk to first minimum) is about 0.6 arc seconds, stars will appear to be 87" across when viewed from perfect skies with my 2.8mm. Let the skies do their usual wiggling, and the C-8 not be in total thermal equilibrium with the outside air, and the disk only expands. Thus my use of the term "blobs" to describe a star's appearance, especially with averted vision.

Quote:

A bit less power can make some pairs easier to see.




This is my observing experience as well. There are few doubles that I think are "best in the 2.8.". Occasionally, there are. I still haven't figured out why this is so.

Wilfried,

According to Celestron, the CO on my C-8 is 0.31. I don't know if this makes much of a difference in your calculations.

I've noticed over the years that the published magnitudes of a star in the WDS are often inaccurate. Just last night I saw my first "L(ewis)" star, L 36:

L 36: 21:45:57.78 +25:32:26 (J2000). Double. Sp: U
Sep: 4.6, PA: 294, Year: 2010 9.5-10.0

Telescope: C-8, Observatory: Little Tycho, Date: 2013 Sep 7, Time: 0:15:36
An "L" star that exists! Mirabile dictu! It's 2mv fainter than its WDS mv. WDS
data: 9.5 - 10.0mv, U, 294 pa, 4.6", 2010.

It's 2mv off. The stars I've listed, that you kindly did your calculations for (thanks!) might well be subject to the same inaccuracies, thereby throwing them off.

KColter:

In my experience, mirrors are not more dirty than lenses, but lenses have one advantage. From the point of view of an observer, all dust is seen in silhouette, whereas dust on a mirror is reflective, scattering light to unwanted places.


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fred1871
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Reged: 03/22/09

Loc: Australia
Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6068577 - 09/07/13 09:30 PM

Tom, seeing here varies a great deal, as anywhere. Some areas of Australia are as bad as you say your locale is. Where I am has some very good seeing nights, but mostly they're middling, as expected. I get fairly dark skies for a populated area because I'm near the northern edge of the city, and as well a lot of the streetlights are downward-reflector style so there's less stray light. Average seeing where I am is slightly less good than I experienced in 6 months in San Diego, Ca, some years ago.

From your comments: There's not a single guide that recommends powers higher than 50x per inch
For general observing, that might be the case. The better guides point out that close double stars are often viewed with higher powers. And the double star specialists go further.

Couteau spells it out (others do too) : that the "resolving magnification" (which makes the size of the first ring equal to the limit of visual acuity, supposed to be 1 arc-minute) is equal to the radius of the aperture in millimetres. However - to see the diffraction image in detail (as in observing a close binary) "one must use a higher magnification, called the useful magnification, which is usually three to four times and may be up to five times the resolving magnification".

The numbers? - for a C8 the resolving magnification is 100x, but the useful magnification is in the 300x to 500x range. For a 140mm refractor (mine and Wilfried's), the useful magnification is 210x to 350x. In each case, up to a bit over 60x per inch.

Although Couteau suggests that higher than five times the resolving magnification results in "the edges of the image [becoming] badly defined and the eye cannot transmit information", this I'm inclined to think is more often true with larger telescopes where seeing conditions are setting the limit; and Couteau worked mainly with 20-inch and 30-inch refractors.

Various 19th century observers who used smaller telescopes were inclined to use higher powers than 60x per inch for the closest pairs. But I will say that I find 400x on a 140mm refractor (5.5-inch) the useful limit generally - at 570x I find I'm confirming what I saw at 400x, not getting new information. And most of the closest doubles are seen with 285x or 333x.

Christopher Taylor is similar - his use of 820x on 12.5-inches is ~65x per inch.

William Herschel was much inclined to over-magnify. I've not studied whether this was necessary or merely a way of exhausting the possibilities in his case.

You shouldn't be too surprised at finding a Lewis pair that exists. The problem as usual is likely to be photometry, where the WDS might still have very old eye-estimates of magnitude, because newer photometry has not yet been incorporated. Lewis used the Greenwich 28-inch refractor so I expect that stars that are rather dim in smallish scopes (such as a C8) were fairly bright in the 28-inch. It's about a 2 1/2 magnitude gain. That's similar to the gain in going from a 3-inch telescope to a C11. Noticeable.

Different sources of photometry will have different accuracy levels and different degrees of consistency. Jonckheere is one of the worst, and there are a lot of his doubles. Lewis didn't discover many; mainly re-measuring for changes, and orbit calculations.

L 36 - shows quite nicely on DSS plates; and appears probably dimmer than listed, as you say. ~2 mags in visual could be correct.

Scatter in mirrors versus lenses - mirrors tend to higher scatter than lenses even when both are clean. There's discussion of this in various works on optics; and it turns up in the discussions, here on CN, of mirror versus prism diagonals as well.


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azure1961p
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Re: How good are reflectors for resolving binaries? new [Re: fred1871]
      #6068746 - 09/08/13 12:14 AM

Tom,

The 50x per inch is a good rule but its not gospel. Part of the reason that rule came about was as a reflexive counter claim to manufactures boasting absurdly high magnification capabilities on instruments of exceedingly small size and just as often poorly fabricated. The fifty power per inch is a fine generalization though - so long as exceptions are observed...

Diffraction pattern study is not the same as say, lunar or planetary observation where contrasts in an extended image must be preserved and often no higher a power value than 40x per inch and more commonly - as you know - 25x per inch. Here the 50x rule is even a little liberal and optimistic. Stars however having no such extended nature we can hope to see, leave us with the effects and artifacts of the instrument itself as representations of the stars through their resultant diffraction patterns produced. These patterns haven't the needs of contrasts that might benefit the Great Red Spot for example. Its a different kind of observing and one a little more specialized than many guidebooks and such would contemplate.

Using 80x to 100x per inch is a relatively tall order still however and a lot is dependent on the seeing as you'd guess but also the robustness of the stellar diffraction pattern to shine through at these unusually small exit pupils. In practice something like 60x to 70x per inch might be a more useful rule of thumb with averegish conditions. Too, small refractors can hit the higher power per inch envelope sooner than medium apertures and certainly large or huge scopes. With my 8" and good seeing under. 600x is fine and simply easier on the eyes when it gets down to the finer details. Stretching it to 800x has yet to be done successfully but I can't blame the scope as atmospheric refraction has been the culprit even on very good nights. It might literally need 10/10 seeing.

Other instances where 100x per inch (or more)!is beneficial is in small high surface brightness planetaries. Stephen James Omeara using 900x with a 5" refractor is a case in point. Too, extremely small discs like the Galilean moons can show details better in medium to larger telescopes when the magnification is as high as 70x per inch. It'd be grossly excessive on Jupiter but Ganymede at a mere 1.7" benefits nicely - of course when the seeing obliges. Ideally contrast is better at say 25x per inch but that doesn't apply here in application because the ability for the eye to discern such incredibly small pale shadings on a virtual dot is too difficult and so the penalty of higher magnification can be put aside here for the sake of creating a large enough image so it can subtend enough area to begin to reveal itself at all.


Pete

Edited by azure1961p (09/08/13 12:28 AM)


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WRAK
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Re: How good are reflectors for resolving binaries? new [Re: 3c_273]
      #6068927 - 09/08/13 05:25 AM

Quote:

...Wilfried,
According to Celestron, the CO on my C-8 is 0.31. I don't know if this makes much of a difference in your calculations...



Difference is only a few mm in proposed aperture as all examples are rather equal bright and therefore the influence of CO is limited to the change in size of the Airy disk.
But I assume 0.31 refers to the size of the secondary mirror and there are some additional mm for the mounting.
Wilfried


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brianb11213
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Re: How good are reflectors for resolving binaries? new [Re: azure1961p]
      #6068935 - 09/08/13 05:47 AM

Quote:

The 50x per inch is a good rule but its not gospel. Part of the reason that rule came about was as a reflexive counter claim to manufactures boasting absurdly high magnification capabilities on instruments of exceedingly small size and just as often poorly fabricated. The fifty power per inch is a fine generalization though - so long as exceptions are observed...

Diffraction pattern study is not the same as say, lunar or planetary observation where contrasts in an extended image must be preserved and often no higher a power value than 40x per inch and more commonly - as you know - 25x per inch. Here the 50x rule is even a little liberal and optimistic. Stars however having no such extended nature we can hope to see, leave us with the effects and artifacts of the instrument itself as representations of the stars through their resultant diffraction patterns produced. These patterns haven't the needs of contrasts that might benefit the Great Red Spot for example.



Yeah ... and (providing the seeing cooperates) as much power as is available is probably going to be helpful when trying to measure double stars using a bifilar micrometer.

Max useful power also depends on the acuity of the eye, my old eyes are starting to drift a bit now but when I was in my early 20s I found that planetary detail was better seen at 20D than 25D. Even on tiny discs. 40 years later I need around 30D & have more issues with low contrast details as a consequence ... Not that this has any relevance to double star work, or using the diffraction pattern to collimate a scope under steady seeing conditions. I reckon 80D - 100D is a reasonable "limit" (assuming the optics are impeccable as well as the seeing - and providing you have a tracking mount!)

Quote:

Other instances where 100x per inch (or more)!is beneficial is in small high surface brightness planetaries. Stephen James Omeara using 900x with a 5" refractor is a case in point. Too, extremely small discs like the Galilean moons can show details better in medium to larger telescopes when the magnification is as high as 70x per inch.



Experiences obviously differ: I've found with planetary nebulae the best magnification is never more than 25D: as for the Galilean satellites, I've never seen clear surface detail but what I've suspected has been low contrast & going over 25D - 30D kills it.

Sidgwick suggests as a practical upper limit to magnification, 140xsqrt(D) - which, as it happens, is approximately 50D for an 8 inch aperture, rather more than that for smaller scopes and rather less for larger. (70D for 4", 35D for 16") Given that seeing doesn't improve with aperture, this sort of makes sense. However, if you want to use a power much over 20D, your eyes are going to need to be pretty free of floaters.


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azure1961p
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Re: How good are reflectors for resolving binaries? new [Re: brianb11213]
      #6069143 - 09/08/13 09:42 AM

Brian,

Binoviewers have an uncanny efficiency that a lot of folks appreciate. A common comment from observers with binos is they can see the same details as mono but at lower magnifications. You didn't mention binos so I thought Id throw it out there. Right now my binos are too too high powered due to my ocular choices as its still new to me. Its is a nice things when both eyes see the same object - there's this plainness of definition thsts truly refreshing. So to that end you may find you can enjoy the lower power efficiency once again.

Pete

Edited by azure1961p (09/08/13 09:44 AM)


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brianb11213
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Re: How good are reflectors for resolving binaries? new [Re: azure1961p]
      #6069178 - 09/08/13 10:02 AM

Quote:

Binoviewers have an uncanny efficiency that a lot of folks appreciate. A common comment from observers with binos is they can see the same details as mono but at lower magnifications.



Again, other people's experience may differ from yours.

I tried a binoviewer & apart from the obvious reduction in "floaters" I had a couple of issues when using them - eye strain & a loss of light transmission which was roughly equivalent to a whole magnitude. I never saw any increase in ability to resolve fine detail or low contrast detail when using a BV at any magnification. Proper binoculars are OKish (though there can still be some eye strain) but I am far happier using one eye per objective.


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WRAK
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Mask for testing effects of CO new [Re: brianb11213]
      #6070975 - 09/09/13 10:24 AM Attachment (5 downloads)

First prototype is ready consisting of a foam ring over the dew cap and an aluminium bar with a thin tube in the center to accept CO plug-in's. The metal part still needs black painting to avoid potential reflections

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WRAK
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Re: Mask for testing effects of CO new [Re: WRAK]
      #6070978 - 09/09/13 10:28 AM Attachment (7 downloads)

The CO plug-in is simply a black disk on a stick to be applied on the tube in the center. I will make such plug-in's in sizes from 0.1 to 0.35 (or larger if needed) for analysis of the influence of different CO values on diffraction pattern and resolution of binaries

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WRAK
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Re: Mask for testing effects of CO new [Re: WRAK]
      #6070982 - 09/09/13 10:31 AM Attachment (7 downloads)

Finally the applied 0.35 CO on the 140mm scope. At least at daylight there is no difference to see between zero CO and 0.35 CO. Will take some time to get reliable results but this setup seems good enough to study the effects of CO for resolving doubles.
Wilfried


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Erik Bakker
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Re: Mask for testing effects of CO new [Re: WRAK]
      #6071547 - 09/09/13 03:24 PM

Did a lot of similar experiments in the 80's. Interesting to learn how your experiment evolves. Applying a CO to a refractor is different from having a scope with a very different design where the CO is actually part of the optical design. Keep us posted!

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3c_273
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Re: Mask for testing effects of CO new [Re: Erik Bakker]
      #6071708 - 09/09/13 05:11 PM

Size of the secondary in a C-8 Ultima.

Today I tried to measure the diameter of my secondary and and its mount in the corrector plate of my C-8. I got 69mm. Plus or minus a mm, at best. This gives, assuming the corrector is 203.2mm in diameter (I didn't measure that), a 34% obstruction.

Wilfried, so you, and not Celestron, were closer to right.


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