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WRAK
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Reged: 02/18/12

First experiments with CO mask new [Re: 3c_273]
      #6073209 - 09/10/13 01:15 PM

Last night conditions (cloudy sky with clear spots inbetween) did not allow a regular session so I had time at hand to give my setup a first try.
Polaris was my first target to check the effects on the diffraction pattern for a single star (neglecting the faint companion). Seeing was with Pickering ~5 rather mediocre so the diffraction pattern was not stable but the obvious expected effect of enhancing the brightness of the first diffraction ring was evident especially for 0.3 and 0.35.

Next target was Epsilon Lyr with x75 magnification. Zero CO showed nice dented rods, 0.1 CO made the image a tad more crisp with hints of dark space between the star disks. Not much change with 0.15 and 0.2. 0.25 CO delivered the same impression as zero CO, 0.3 CO deleted the dents in the rods and 0,35 made the rods a tad fat like elongations.

Same target but with x280 magnification: Zero CO showed clear separated star disks with a rather faint first diffraction ring and 0.1 CO again made the image appear a tad crisper. 0.15 and 0.2 did not change much and 0.25 was similar to zero CO with first diffraction ring a bit brighter. 0.3 CO made the image less crisp and the first diffraction rings got bright enough to disturb the dark space between the star disks. 0.35 CO even more of this effect to a degree of making the image more impressive in terms of diffraction rings but less precise in terms of resolution.

Regarding TML CO 0.1-0.25 did not show any obvious influence (on some very faint stars in between the double-double, at least one with +12.676mag according to AAVSO) but 0.3 and 0.35 CO did.
This are only first findings to be confirmed but it seems that the positive influence of CO regarding size are restricted so smaller CO values less than 0.25 and degradation of resolution begins with ~0.3 CO.
Wilfried


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Asbytec
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Reged: 08/08/07

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Re: First experiments with CO mask new [Re: WRAK]
      #6074289 - 09/11/13 01:24 AM

Wilfried, your observations are interesting. My experience with a ~30% co (only) seems to suggest a push from 0.77" arc to closer to 0.70" arc Dawes split. I am still waiting to see if that's actually the case. Need a good target and a good night.

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azure1961p
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Reged: 01/17/09

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Re: First experiments with CO mask new [Re: Asbytec]
      #6074377 - 09/11/13 03:59 AM

I'm curious how the indent or notch of a Dawes split would "delete" when the CO became large as the notch in theory should have deepened as the spurious discs appeared smaller due to light going into the rings. I would wonder if the CO mask was now obstructing the zone of good or better definition in the center of the objective elements .

Pete


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WRAK
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Reged: 02/18/12

Re: First experiments with CO mask new [Re: azure1961p]
      #6074401 - 09/11/13 04:44 AM

Quote:

I'm curious how the indent or notch of a Dawes split would "delete" when the CO became large ...



I think that at low power the first diffraction ring cannot be seen but only the central disks - therefore notched. With increased CO the first diffraction ring gets brighter and "fill in" the gap therefore eliminating the notches. Further experiments will show if this assumption could be right.
Wilfried


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WRAK
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Reged: 02/18/12

Re: First experiments with CO mask new [Re: Asbytec]
      #6074404 - 09/11/13 04:48 AM

Quote:

Wilfried, your observations are interesting. My experience with a ~30% co (only) seems to suggest a push from 0.77" arc to closer to 0.70" arc Dawes split...




This would be correct from the theoretical point of view and can still be true in reality as the brightening of the first diffraction ring may be without side effects for close doubles. To check the effects of CO on close doubles I need a target near or slightly below Dawes - may be next time.
Wilfried


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WRAK
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Reged: 02/18/12

Re: First experiments with CO mask new [Re: WRAK]
      #6074475 - 09/11/13 06:59 AM

Did some research on Eps Lyr - the faint stars I have seen between Eps1 and Eps2 are the components E and F (also named SHJ277) but the given magnitudes are with +11.71 and +11.2 not this faint as I found with AAVSO. I would especially doubt the second value.
Wilfried


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Asbytec
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Re: First experiments with CO mask new [Re: WRAK]
      #6074512 - 09/11/13 07:34 AM

Oh, I see, you were at low power describing disappearing notches. I missed that. Yea, I am very curious to see how close a split can be and still call a dark space...at high power. Hopefully soon.

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azure1961p
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Re: First experiments with CO mask new [Re: Asbytec]
      #6074563 - 09/11/13 08:30 AM

At 280x however you are but at low power of course and you are claiming lower resolution . I understand how the first diffraction ring can swell into the gap separating it from the spurious disc particularly in 5 Pickering. Still Lords claims a benefit on near equal doubles with a 72% CO. Im guessing the focus at the edge of your objective lens elements is not the same as the masked off center. Perhaps its a refractor thing. Lords did it with his 12 1/2" f/7 reflector - a whopping CO over 9"!!!


Pete


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WRAK
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Reged: 02/18/12

Re: First experiments with CO mask new [Re: azure1961p]
      #6074583 - 09/11/13 08:45 AM

Pete, I am talking about x75 - x280 was different, see above.
With CO 0.9 you see nearly nothing besides diffraction rings - I know it because I have done it.
Wilfried


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Asbytec
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Re: First experiments with CO mask new [Re: WRAK]
      #6076137 - 09/12/13 12:01 AM Attachment (12 downloads)

I always found this interesting...

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WRAK
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Reged: 02/18/12

Re: First experiments with CO mask new [Re: Asbytec]
      #6077419 - 09/12/13 05:01 PM

Norme - great graphical representation of the influence of CO on the energy distribution. Matches perfect with what I have seen with CO values >0.4. What is the source?
Wilfried


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Asbytec
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Re: First experiments with CO mask new [Re: WRAK]
      #6077815 - 09/12/13 09:49 PM

Wilfred, someone attached that to a thread long ago, I grabbed it because it was interesting. It shows the diffraction ring only pattern at 90% you mentioned, and it even shows the decrease in the first minimum.

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azure1961p
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Re: First experiments with CO mask new [Re: Asbytec]
      #6077824 - 09/12/13 09:59 PM

Yes it does Norme. That's actually a fantastic graphic.

Ok currently elongating a pair at .021". I'm at 250x of course or it wouldn't be possible.

Pete


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Asbytec
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Re: First experiments with CO mask new [Re: azure1961p]
      #6078073 - 09/13/13 12:27 AM

I dunno, Pete. There are so many factors involved. First, it's close or beyond what appears to be an empirical limits at 0.5R. That limit might be pushed depending on conditions and personal ability to note any minor elongation - if it exists in such a tight PSF at all.

That's my question. When in theory do two points merge into a single and indistinguishable PSF? I say define that limit and try to observe to it. Damn the conditions, try it.

Someone with the right conditions, CO, and personal skill might be able to press a few hundredths of an arc second off empirical observations provided that separation does not exceed the hard limit of the indistinguishable PSF. That limit may well lay right at or slightly below 0.5R. It does with extended objects, but does it apply to two point sources, as well?

Edited by Asbytec (09/13/13 12:28 AM)


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fred1871
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Reged: 03/22/09

Loc: Australia
Re: First experiments with CO mask new [Re: Asbytec]
      #6078177 - 09/13/13 02:58 AM

All interesting stuff, and there's the reminder that the observer's ability to recognise (see) what's there is the final link in the chain - and why there are different limits even when it's the same telescope on the same night on the same object.

The animated diffraction patterns item, posted above, is useful, but I'd love to see it as a set of diagrams, which would allow studying the effect at each level of CO.

But finally it is, as you say Norme, what level of overlapping diffraction patterns, merged into some kind of image - near 0.5R or some other fixed point - can be seen elongated rather than round by the observer. And which observers can be outstanding in visual acuity, by consistently doing better than other careful and experienced observers, without getting into false positive territory?


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Asbytec
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Re: First experiments with CO mask new [Re: fred1871]
      #6078244 - 09/13/13 05:49 AM

I dunno, Fred, how to answer that. One has to respect the experienced and expert observers and take what they say seriously. Anytime we observe hard and deep right at the limits of every variable present we risk false positives, our reputation, and our own integrity. No pain, no gain.

Good seeing here minimizes one of those many variables and offers some hope when cooled and collimated - leaving skill and luck remaining. The best I've done so far is 0.57" arc (72 Peg data as reported.) That turns out to be 0.57/0.92" ~ 0.6R normalized or ~0.7R with CO factored in. It wasn't entirely difficult, and suggests one can go even deeper.

That also implies, with CO, one might elongate Chi Aql in a 6" obstructed depending on it's real sep. That's a real test of 0.5R and CO affects. So far - one attempt in less than ideal conditions - has been negative. But, wouldn't that be a nice ob to have under your belt if it were possible and people believed you? How many look at it's reported data and immediately write it off as too hard or impossible? (That was my point with Io in another thread.)

And 7 Tau seems to suggest there is room to go beyond Dawes, too. All of this provides some allure for doubles and the challenge they pose is motivating. It's amateur frontier observing pushing technology and skill. That's exciting as it is risky.


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fred1871
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Reged: 03/22/09

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Re: First experiments with CO mask new [Re: Asbytec]
      #6078423 - 09/13/13 09:38 AM

Norme, I agree about trying, to see what's possible. I hadn't thought I could see Zeta Bootis elongated, when it was down to 0.5R for my 140mm refractor, until I managed it. And I have no trouble believing some observers see hints of elongation, or something? ... at even closer (slightly closer) levels. Christopher Taylor's claimed best is ~0.4R with his 12.5-inch. Aitken's note on detecting doubles with the 12-inch at Lick goes down to ~0.45R...

The two doubles you mention after your success with 72 Peg are somewhat different types of critters - Chi Aquilae is at 0.5R for about a 7-inch scope based on separation (~0.4") - although delta-m of 1.2 will have some effect.

But 7 Tauri is wider, and more equal - mags 6.60 and 6.86 in the current WDS, 0.7" in 2012, and the ephemeris (gd 3 orbit) suggests 0.75" at present - so that's a 0.5R for about a 4-inch scope. I observed it last November, 140mm refractor, in rather unsteady seeing - elongation was visible at 285x, with star discs (peanut or notched effect) flickering in and out of visibility. That's at about 0.7R for 140mm. Some other doubles at about the same separation were clear flattened figure-8 pairs on nights with better seeing. At 0.6" - which is around 0.6R at 140mm, having a Rayleigh figure ~1.0" - I find elongation is clear enough if the seeing permits. It's consistently there at 400x; sometimes a bit less power will show elongation in the best conditions.

But below 0.6R things get noticeably tougher, and more demanding of steady atmosphere for allowing the certain identification of elongations, rather than uncertainty due to seeing variations. For me, 400x becomes necessary for definite identifications of elongation at this level.

Personally, I find it more interesting to try for the harder unequal pairs; but I'm happy to see others working on their limits for detectability of equal pairs. It's an interesting pursuit.


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: First experiments with CO mask new [Re: fred1871]
      #6078536 - 09/13/13 11:10 AM

Quote:

...
The animated diffraction patterns item, posted above, is useful, but I'd love to see it as a set of diagrams, which would allow studying the effect at each level of CO. ...



Fred, with the old fashioned but free Microsoft GIF Animator you can see the single planes in the preview mode - or you could cut out each frame and save it as non animated image.
Wilfried


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Asbytec
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Re: First experiments with CO mask new [Re: fred1871]
      #6079544 - 09/13/13 10:35 PM

Quote:

Personally, I find it more interesting to try for the harder unequal pairs...



Absolutely, I love the challenge they pose as well. They are interesting from that standpoint as well as interesting in optical theory of obstruction effects. This is one reason why I modified my scope, to press these babies, too.

Always a nice write up, Fred. As a newbie to doubles, I find them fascinating on many fronts, especially pushing the limits of man and machine while enjoying nature's beauty.


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azure1961p
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Re: First experiments with CO mask new [Re: Asbytec]
      #6079660 - 09/14/13 12:11 AM

For the record I was joking about the 250x showing .21". I didn't see any chortles or chuckles so I'm guessing it was taken without humour .

Pete

Edited by azure1961p (09/14/13 12:14 AM)


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