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Equipment Discussions >> Eyepieces

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JustaBoy
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Re: Takahashi Abbe series new [Re: The Ardent]
      #6010169 - 08/06/13 07:30 PM

Tak needs your money...


-Chuck


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Sarkikos
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Re: Takahashi Abbe series new [Re: jrbarnett]
      #6010187 - 08/06/13 07:38 PM

Jim,

Quote:

Sure, but the magic of any eyepiece isn't transmission. The eye can't detect less than a 10% delta in throughput at peak visual wavelength, all else being equal. The magic of an eyepiece is in ensuring that as much of the transmitted light as possible winds up exactly where it is supposed to be in the focused image. That's where things like polish quality and baffling make a difference, and with respect to baffling Takahashi hasn't always been particularly careful. The 5mm LE in particular was a poor design in this regard. In addition, the Takahashi eyepieces I've owned (sets of MC Orthos and LEs) haven't been outstanding performers, but rather mid-pack performers on par with lower cost eyepieces like Televue Plossl and Celestron Ultimas. At least these aren't over-the-top in price so trying them out won't represent much risk even if they do turn out to be ordinary performers.




I've never owned a Takahashi eyepiece or even used one. I own eight Brandons, including two bino pairs, and have used them often. They do have a certain magic, especially when binoviewing planets or the Moon.

But I've seen for myself that the Brandons' light transmission is not the best. They were bested by Sterling Plossls, as well as by XW's. I would not put a Brandon in the focuser if I'm trying to bag a faint fuzzy toward the LM of my scope. Sometimes the magic works and sometimes it doesn't.

Mike


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Ain Soph Aur
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Re: Takahashi Abbe series new [Re: Sarkikos]
      #6010362 - 08/06/13 09:25 PM

I believe all these discussions regarding the new Tak Abbe's are a bit premature and verging on rank speculation until....

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SteveC
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Re: Takahashi Abbe series new [Re: dscarpa]
      #6010402 - 08/06/13 09:52 PM

Quote:

Ancortes has the Tak Abbes in new product announcement in Astromart. They'll be $153 plus shipping with the 9, 18 and 32 mid August the rest in September. They say these will be a revival of the older Tak orthos. Does anyone know how good those are? Davuid




I agree with Jim Barnett. I owned the complete set of MC orthos, they weren't any better than the old UO VT orthos. At $150 ea, the new Tak ortthos better have something special going on, because there's a lot of competition out there - both new and used. I'd wait until the reviews are in before investing, or at most, just try one before buying a set.


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Fomalhaut
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Reged: 08/16/08

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Re: Takahashi Abbe series new [Re: Starman1]
      #6010941 - 08/07/13 07:28 AM

Quote:

--the eye's response is logarithmic--a ten times increase in measured brightness is seen as a doubling in perceived brightness.




Don,

Why should this be mathematically linked to the arbitrary fact that we earthlings are using the decimal system?
Then for the LGMs in M31 with a total of twelve fingers and toes each, a twelve times increase in measured brightness will have the same doubling effect?
And what about those creatures on a certain planet in M101 with an individual total of six fingers and toes only, which has caused them to making use of the hexal system???
Hence, I think you must be wrong on this one...

OTOH, if we based that above mentioned doubling on the (universally "existent") natural logarithm with e=2.71828... as its basis => could this bring us a bit closer to the universal function of increasing perceived brightness in dependence on measured brightness? => Can anybody out there actually provide a mathematically or physically well founded opinion on this?

Chris


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Sasa
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Reged: 11/03/10

Loc: Ricany, Czech Republic
Re: Takahashi Abbe series new [Re: Starman1]
      #6010986 - 08/07/13 08:13 AM

Quote:



So let's talk threshold. How is a threshold of visibility perceived? Well, it turns out it's not a visible/invisible dichotomy, it's visible 100% of the time, visible 50% of the time, etc. A true threshold is often referred to as "visible 10% of the time with averted vision" (see the work of Bradley Schaefer), which means "invisible 90% of the time with averted vision".
In fact, the sliding degree of visibility would make a cutoff point very very hard to determine.

I've found that the sky varies in transparency, and the variation exceeds, by a large margin, the small degree of separation between invisible and visible 10% of the time with averted vision that you are dealing with when you talk about light transmission. With the same scope, I have seen wide variations from night to night in the degree of visibility of certain features of extended objects, and with the same eyepieces.

At best, we are probably talking about the difference between seeing something 30% of the time with averted vision versus 10% of the time with averted vision. Good luck with that. First you have to actually find a threshold object or feature, and that won't be easy. One suggestion might be the galaxy cluster Abell 2151 in Hercules. There are a vast number of galaxies in that group and they vary significantly in magnitude.
Chances are, if you use a map to guide you to the galaxies in the cluster, that you might find one that meets the criterion of being a "threshold" object. Then, you could begin to compare different eyepieces on that threshold object.
Of course, the object would have to be at the same altitude for every eyepiece in order to provide similar extinction.
And it would have to be on the same night, at the same site, or the comparisons could be skewed by transparency changes. Of course, that presumes there wouldn't be momentary fluctuations in transparency during the test, so absolutely no clouds, dust, smog, or water vapor could be allowed in the atmosphere.

I don't think you will be able to get reliable, repeatable, data. It certainly would be worth a try. The hard part will be selecting targets.




Don, I think that you are right. There is definitely and "enhancement" at the threshold as Thomas and you are mentioning due to statistical fluctuations of incoming light. But it would be hard to actually see it.

I estimated that ten percent in telescope/eyepiece transmission could translate to about 50% change in time needed to spend by eyepiece in order to give the object a chance to pass the threshold (if the detection is defined as 10% of time above threshold). There are people that operate even lower (for example at 5% time threshold, the difference increase to about 70% - but still much smaller than factor of 3 in your estimate - change from 10% to 30%). For me it is hard to say if that would be detectable at the eyepiece, I also doubt because, as you stated, the conditions during night are changing probably more. But what can be done, instead of finding the right values of the threshold, is how the difficulty of seeing near-to-threshold object changes through 0.95D aperture mask. This would be too valuable and may be easier to do, since it does not require so much time as finding real change in threshold.


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BillP
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Re: Takahashi Abbe series new [Re: Starman1]
      #6011219 - 08/07/13 10:45 AM

Quote:

So there is ample evidence to suggest that a brightness difference of less than 10% is going to yield experimental results that do not differ from random chance.




And to beat the dead horse more... Translated, "ample evidence to suggest" means "I hypothesize", which of course does not mean truth, fact, or anything of the sort. The various controlled experiments which yield the 10% approximation are completely different from a threshold observation case and are not extensible at all to this scenario, so not applicable since those studies (the ones I have read anyway) do not model the threshold observation capability. In one study related to threshold, the human eye can register as little as a single photon, but getting it through to the perception system requires around 9 photons.


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Starman1
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Re: Takahashi Abbe series new [Re: Fomalhaut]
      #6011311 - 08/07/13 11:47 AM

Quote:

Quote:

--the eye's response is logarithmic--a ten times increase in measured brightness is seen as a doubling in perceived brightness.




Don,

Why should this be mathematically linked to the arbitrary fact that we earthlings are using the decimal system?
Then for the LGMs in M31 with a total of twelve fingers and toes each, a twelve times increase in measured brightness will have the same doubling effect?
And what about those creatures on a certain planet in M101 with an individual total of six fingers and toes only, which has caused them to making use of the hexal system???
Hence, I think you must be wrong on this one...

OTOH, if we based that above mentioned doubling on the (universally "existent") natural logarithm with e=2.71828... as its basis => could this bring us a bit closer to the universal function of increasing perceived brightness in dependence on measured brightness? => Can anybody out there actually provide a mathematically or physically well founded opinion on this?

Chris



It wasn't meant to be accurate, merely an example of a spread between the response of the eye in terms of perceived brightness and the increase in lumens.
It's more likely to be similar to how we perceive sound.
The exact log of the response of our eye I don't know, but its veracity is well known, which is why we have the ability to perceive light levels over a trillion-to-one range without damage to the eye.
How it will relate to the magnitude scale, itself a logarithmic scale, I also don't know.


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JustaBoy
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Re: Takahashi Abbe series new [Re: Starman1]
      #6011360 - 08/07/13 12:10 PM

Log 2.512?

http://www.stargazing.net/david/constel/magnitude.html

-Chuck


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dscarpa
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Re: Takahashi Abbe series new [Re: edif300]
      #6011410 - 08/07/13 12:32 PM

Thanks for the info Jim. Think I'll stick with my 18 BCO and 9 Kasai HC for now. David

Edited by dscarpa (08/07/13 12:33 PM)


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Starman1
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Re: Takahashi Abbe series new [Re: BillP]
      #6011438 - 08/07/13 12:47 PM

Quote:

Quote:

So there is ample evidence to suggest that a brightness difference of less than 10% is going to yield experimental results that do not differ from random chance.




And to beat the dead horse more... Translated, "ample evidence to suggest" means "I hypothesize", which of course does not mean truth, fact, or anything of the sort. The various controlled experiments which yield the 10% approximation are completely different from a threshold observation case and are not extensible at all to this scenario, so not applicable since those studies (the ones I have read anyway) do not model the threshold observation capability. In one study related to threshold, the human eye can register as little as a single photon, but getting it through to the perception system requires around 9 photons.




There are, in the retina, random firings of nerves that result in apparent points of light being visible.

The retina, under severely reduced light input, not only loses its resolution, but also the reliability of seeing a light input that is real. I haven't read the tests wherein the response of the retina to light can be measured to be a mere handful of photons (one article I read said 150 photons, but the principle is the same), but I've had long conversations with an optometrist friend who is very interested in vision and the experiments regarding vision. He can talk for hours (and has) about the visual apparatus and how it works chemically. He says the tests with peripheral vision detectability of light (off and on) gets unreliable when the lights get fainter than a certain brightness, resembling random chance of being correct at fairly high levels of brightness. partially due to the low resolution of the peripheral vision and the "desire to see something" in the eye/brain combination. That last comment got me thinking about threshold observations and their repeatable natures.

The 10% brightness differences to which I was referring were studies done in low light where the people had, probably, mesopic vision. The only studies I've read that were fully scotopic dealt with spurious visibility of red and green and what happened in the brain to introduce the spurious colors.

But when you take into account:
--AAVSO reports on reliability of observations
--lab studies on perception of brightness difference
I would say we do have ample evidence to suggest that 10% brightness difference might be a statistically relevant minimum for detection of brightness difference.

That may not hold true for an individual (I would be surprised if there was not a spread of accuracy around that point), and how it relates to seeing a brightness difference at the threshold of vision is probably, I admit, an extrapolation that is dangerous to make. Plus, the random firings of retinal cells might make detection of a stellar object unreliable.

You don't really faint fuzzies observe very long before you realize that you start to see things that may or may not be there. [In my own case, there are either hundreds of thousands of objects *just* beyond the reach of my vision or my eyes tend to fill in the blanks when looking for objects that are threshold objects. More likely the latter, I think.]

But, we are talking the reliability of the threshold detection of anything that is not consistently visible. And the way the eye works means that repeatability is likely to diminish the fainter the threshold. The "sliding scale of visibility" at the edge of vision, for stellar objects, covers a fairly wide range. Schaefer's work suggests a difference of a small percentage of visibility may represent the better part of a full magnitude.
For stellar objects. For extended objects? Who knows?

And what is a truly threshold observation?
I've seen Barnard's loop in Orion and the Veil nebula, through filters, using my naked eye. They were consistently visible, not only 10% of the time with averted vision, but 100% of the time with direct vision, yet such observations are written about as indicative of threshold observations. Obviously not, I'd say.

If you look at the sizes of extended objects at, say, the magnitude 22 isophote versus the magnitude 25 isophote, it is obvious that the edges of many deep sky objects are only determined by setting an arbitrary cutoff of magnitude. More revealing, I think, is the range of magnitudes
covered by the edges of extended objects. Brightness gradients have a lot to do with visibility. Which is why determining what is a threshold object to even test vision in the field is going to be very difficult. What if one observer sees the core 10% of the time, and another observer sees the core a larger percentage of the time and the outer edges of the object 10% of the time? Could there be a range in detection covering several magnitudes when we are talking true threshold observations?

If so, we aren't talking 10% differences here at all, but much larger figures. I wouldn't be surprised to discover that though we can detect brightness differences of 10%, plus or minus, that the differences we are really talking about as threshold observations cover much larger percentage changes in lumens.


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bremms
Carpal Tunnel
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Re: Takahashi Abbe series new [Re: dscarpa]
      #6011444 - 08/07/13 12:50 PM

Surface polish and coating quality will determine the scatter.
that will be the only real difference. That's where the Brandons, Clave's and Zeiss Abbe's are so good. My old Clave' 12mm had so little scattering. I didn't like the edge of field much at F6 in that one though.


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Astrojensen
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Re: Takahashi Abbe series new [Re: Starman1]
      #6011497 - 08/07/13 01:18 PM

Hi Don

Many interesting points to take into consideration! It seems I will need to do what I do best: Experiment with my telescope! Whether those experiments are useful for anything other than for my amusement is questionable, but I'll do it anyway. I'll make some masks that cover various percentages of the objective and observe some objects and see what I can see or not.


Clear skies!
Thomas, Denmark


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Sarkikos
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Re: Takahashi Abbe series new [Re: bremms]
      #6011552 - 08/07/13 01:51 PM

Quote:

Surface polish and coating quality will determine the scatter.
that will be the only real difference. That's where the Brandons, Clave's and Zeiss Abbe's are so good. My old Clave' 12mm had so little scattering. I didn't like the edge of field much at F6 in that one though.




The only real difference when observing which objects? Planet surface features can probably do just as well if not better with simple FC coatings such as on the Brandons. Dim nebulae and galaxies, not so much. For dim objects, decent FMC will trump simpler coatings. My simple coating eyepieces no longer go with me to the dark site.

Mike


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Sarkikos
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Re: Takahashi Abbe series new [Re: Astrojensen]
      #6011577 - 08/07/13 02:10 PM

Thomas,

Quote:

Many interesting points to take into consideration! It seems I will need to do what I do best: Experiment with my telescope! Whether those experiments are useful for anything other than for my amusement is questionable, but I'll do it anyway. I'll make some masks that cover various percentages of the objective and observe some objects and see what I can see or not.




What will stopping down the objective show? That the observer will be able to see fainter objects through larger aperture? I can make that prediction right now!

I think a more pertinent experiment would be to work on the other end of the instrument. Compare the perceived light transmission when observing threshold objects (however defined) through eyepieces that differ in level of coatings and/or number of elements.

Mike


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Sarkikos
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Re: Takahashi Abbe series new [Re: Starman1]
      #6011601 - 08/07/13 02:26 PM

Quote:

And what is a truly threshold observation?
I've seen Barnard's loop in Orion and the Veil nebula, through filters, using my naked eye. They were consistently visible, not only 10% of the time with averted vision, but 100% of the time with direct vision, yet such observations are written about as indicative of threshold observations. Obviously not, I'd say.




Whether an object is threshold or not will depend on the darkness of the site (among other factors). I've tried to see Barnard's Loop with my naked eye through a Lumicon H-Beta filter at my yellow zone site. It was definitely one of those objects which I'd have to say I might have seen, but I'm just not sure. I think that at this site, Barnard's Loop was either a threshold object or an impossible object. I'll have to wait for a very transparent night to determine that.

On the other hand, IME the Horsehead is a threshold object at my site. I need a night of excellent seeing to tease out the Horsehead with my 10" Dob at the yellow zone site. On such a night I can determine the comparative light transmission of eyepieces by seeing how well they pull the Horsehead out of the background. The eyepieces with FMC seem to do better than those with simpler coatings.

Mike


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Takahashi Abbe series new [Re: JustaBoy]
      #6011656 - 08/07/13 02:59 PM

Quote:

Log 2.512?

http://www.stargazing.net/david/constel/magnitude.html

-Chuck




Chuck,

2.512x more light was defined to be one stellar magnitude difference in order to simplify calculations with magnitudes:
100^(1/5) = 2.512 <=> 2.512^5 = 100
<=> 5 magnitudes difference means 100x more (or less) light

<=> 1st magnitude corresponds to 100x more light (measured photometrically) than 6th magnitude or 10'000x more than 11th magnitude. - This is extremely convenient for calculations, but OTOH here we have it again, our non-universal decimal system...

Chris


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Astrojensen
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Re: Takahashi Abbe series new [Re: Sarkikos]
      #6011783 - 08/07/13 04:18 PM

Quote:

What will stopping down the objective show?




This will allow me to precisely vary the used surface area of the objective, thus allowing me to try observing various objects with either 100% throughput (of the maximum possible, of course) and then, by using a carefully calculated aperture stop, use only 90%, or 80% or whatever, without introducing any other variables, since I will be using the same eyepiece, diagonal, scope, etc. The exit pupil will be getting smaller, and thus relative magnification higher, but it's the only way I can precisely vary the light throughput with the equipment available.

I already know that some of my eyepieces are better than others at revealing the faintest possible objects, but is that due to having very few elements, or something else? Can we really see a 10% brigthness difference on threshold objects, making it worthwile to go for minimal-glass eyepieces for that kind of target, or should we all insist on supreme polish on our eyepieces instead, since it's almost impossible to tell the difference between a well-polished and coated multi-glass eyepiece and a well-polished minimal-glass eyepiece? (the difference in light transmission between a multicoated multi-glass eyepiece and a minimal-glass one is much less than 10%, yet I can see a difference between my 9mm ES100 and my best 9mm ortho. I can NOT however, see a difference between my 9mm UO and my 9mm ES100!).

I do think it's possible to reliably detect a 10% difference in transmission, but I'll find out for sure, once I've done some experiments. It'll probably take a month or two, so don't all hold your breath.


Clear skies!
Thomas, Denmark


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Starman1
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Re: Takahashi Abbe series new [Re: Sarkikos]
      #6011839 - 08/07/13 04:49 PM

Quote:


On the other hand, IME the Horsehead is a threshold object at my site. I need a night of excellent seeing to tease out the Horsehead with my 10" Dob at the yellow zone site. On such a night I can determine the comparative light transmission of eyepieces by seeing how well they pull the Horsehead out of the background. The eyepieces with FMC seem to do better than those with simpler coatings.

Mike



Does that mean one eyepiece will see it 10% of the time and another 30% of the time, both with averted vision?
Or visible with direct vision?
Or do you mean it's just fainter in one than the other but still visible?

I've been seeing, recently, that objects I could not find, and which were not visible in my scope, became suddenly visible once I went over to a much larger scope (looking at the same object, of course), saw it, and returned to my scope to take a look for it.
It went from a "can't see it" object to "now I can see it" merely by seeing it in the larger scope.

This goes back to the 18th and 19th Centuries, when many astronomers noted that it always took a larger aperture to discover something than it took to observe it after it was discovered. I think that once you know something is there, you will often see it if it is within the realm of seeing in the aperture.

That even muddies the idea of "threshold" even more.


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MikeRatcliff
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Re: Takahashi Abbe series new [Re: Sarkikos]
      #6011956 - 08/07/13 05:49 PM

Quote:

Jeff & Leonard,

Quote:

For whatever reason you would seem to be missing the Third Way - putting tracking on the larger scope. It will change your world.




Eventually I intend to acquire a 14" Dob with tracking. But for now, I don't think it would be worthwhile to install a tracking mount on the 10". I don't want to put money and effort in that direction. That is not going to happen. It would be one more big gizmo to have to haul outside or to the dark site. I know several people who have an eq platform for their Dobs and they hardly ever bother to use the platform.

Besides, I've developed the ability to observe skillfully without tracking.

Mike




I was observing a small globular the other night with a 30 degree TMB supermono at 400x, and I was really glad that I had the equatorial table setup. I agree it is an extra hassle at times.
Mike


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