Starman1
Vendor (EyepiecesEtc.com)
Reged: 06/24/03
Posts: 17639
Loc: Los Angeles
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Bill,
You may have a Scholarly aversion to the use of the extra glass of a Paracorr, but the roughly 3% light loss is not noticeable to a human eye--any human eye. In fact, fainter stars are visible nearly everywhere in the field WITH it than are visible WITHOUT it because they are so much better focused.
In theory, the extra glass surfaces, because they cannot be perfect, will add light scatter and wavefront deformation.
In practice, the benefits radically outweigh the debits.
I used "Paracorr" generically. There are other coma correctors that do not add magnification. My comment implied that edge-of-field evaluation of an eyepiece without coma correction (in short focal length scopes) can be inaccurate.
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Don Pensack
www.EyepiecesEtc.com
12.5" Teeter/Zambuto, 5" Maksutov
Sustaining Lifetime IDA member
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ausastronomer
Pooh-Bah
Reged: 06/30/03
Posts: 1479
Loc: Kiama NSW (Australia)
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Quote:
John,
Good point. But I'm at the opposite end of this in that I don't like a Parracor in my optical train since I don't like the extra magnification plus perceive the light loss from the extra glass. So my "grain of salt" relative to reviews goes to those that use it to base the review on.
-Bill
Bill,
It depends what I am doing, as to whether or not I use the paracorr in the scope or not. I do a lot of viewing nights running sky tours at public outreach events. In this situation the aim is to provide as aesthetically pleasing a view as possible, hence I use the paracorr, as it does provide a "cleaner" view edge to edge. Light throughput is not a concern as generally we are observing the brighter targets. When I am observing in respect of my own objectives, on my own or with 1 or 2 friends, I don't use the paracorr. In these situations I like the ability to go as deep as possible, hence my preferred option is the Pentax XW's without the paracorr. FWIW the 14mm and 20mm XW's are my preferred option in the 18"/F4.5 with or without the paracorr. They still do very well without it, in this scope and in the 20"/F5 and the 25"/F5. In my 10"/F5 the 14 is very good, but not as good as in the larger newtonians, the 20 is barely tolerable, but I still like it for its contrast and throughput. In my friends TOA 130 they again perform superbly.
Clear Skies
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John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession #1333 (fully loaded with OMI optics)
14"/F4.5 SDM #33 (fully loaded with Zambuto Optics)
10"/F5.3 SDM #50 (fully loaded with Suchting Optics)
Pentax XW's, Nagler T4's, T5's, DELOS & ETHOS. TV Paracorr, 1.8X TV Barlow and 2.5X TV Powermate
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ausastronomer
Pooh-Bah
Reged: 06/30/03
Posts: 1479
Loc: Kiama NSW (Australia)
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Quote:
Bill,
You may have a Scholarly aversion to the use of the extra glass of a Paracorr, but the roughly 3% light loss is not noticeable to a human eye--any human eye. In fact, fainter stars are visible nearly everywhere in the field WITH it than are visible WITHOUT it because they are so much better focused.
In theory, the extra glass surfaces, because they cannot be perfect, will add light scatter and wavefront deformation.
In practice, the benefits radically outweigh the debits.
I used "Paracorr" generically. There are other coma correctors that do not add magnification. My comment implied that edge-of-field evaluation of an eyepiece without coma correction (in short focal length scopes) can be inaccurate.
Don,
This is very true. From 25% off axis you see more stars with the paracorr than without. The reason being the scope is diffraction limited for a larger distance off axis and the airy disk is smaller. The brightness of the airy disk is relative to the square of it's size. Hence as the airy disk expands because of coma it gets dimmer and you dont see it.
However, when I want to go deep and hunt specific targets I generally go without the paracorr as I believe the on axis performance is "fractionally" reduced with it in.
Clear Skies
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John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession #1333 (fully loaded with OMI optics)
14"/F4.5 SDM #33 (fully loaded with Zambuto Optics)
10"/F5.3 SDM #50 (fully loaded with Suchting Optics)
Pentax XW's, Nagler T4's, T5's, DELOS & ETHOS. TV Paracorr, 1.8X TV Barlow and 2.5X TV Powermate
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BillP
Postmaster
Reged: 11/26/06
Posts: 9093
Loc: Vienna, VA
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Quote:
Bill,
You may have a Scholarly aversion to the use of the extra glass of a Paracorr, but the roughly 3% light loss is not noticeable to a human eye--any human eye. In fact, fainter stars are visible nearly everywhere in the field WITH it than are visible WITHOUT it because they are so much better focused.
In theory, the extra glass surfaces, because they cannot be perfect, will add light scatter and wavefront deformation.
In practice, the benefits radically outweigh the debits.
I used "Paracorr" generically. There are other coma correctors that do not add magnification. My comment implied that edge-of-field evaluation of an eyepiece without coma correction (in short focal length scopes) can be inaccurate.
Don,
Hello. Point taken on the off-axis area. But I'm very much an on-axis person who focuses there very intently. Peripheral vision for me is just there for context, not study. Items of interest go to on-axis always. And as far as human eye's, all humans (and the complete optical trains) are different...I don't know what else to say. Whether I can see lower light thresholds than normal I don't know. While in A.F. it was quite apparent several of us had much better acuity than others even though everyone had "normal" vision (pilots). Plus on test charts my vision comes in at better than 20/10 for acuity - don't know if that would impact light detection as well.
But my comment comes from comparison and use of Parracor in my telescope. I was not intentionally looking for thruput differences, but readily noticed them while viewing a very familar cluster to me. After that I did specific testing for it and it was quite clear. Stars visible only with averted vision under Parracor were visible with direct vision without Parracor.
I tend to "test" every scholarly piece of information given me because when the rubber hits the road, more variables usually come into play and the outcomes become different that predicted at times.
-Bill
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Bill
The KING: Any Eyepiece in the Focuser
The SUBJECTS: XT10i Dob---Meade 2080 SCT---TSA-102 S APO---Onyx 80EDF APO---SkyLight 60mm F15m---P.S.T.
If you want to make an apple pie from scratch, you must first create the universe. - Carl Sagan
Edited by BillP (04/25/07 12:58 PM)
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Starman1
Vendor (EyepiecesEtc.com)
Reged: 06/24/03
Posts: 17639
Loc: Los Angeles
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Quote:
I tend to "test" every scholarly piece of information given me because when the rubber hits the road, more variables usually come into play and the outcomes become different that predicted at times.
-Bill
A good philosophy.
Studies done in labs, and results garnered by hundreds of thousands of observations by the AAVSO show that humans cannot reliably (better than chance) detect differences of less than 0.1 magnitude, about a 10% difference in lumens.
The curve shows that some individuals might be able to reliably detect differences approaching 8%.
This difference (<0.1 magnitude) would be difficult to detect, even if doing averted vision limiting magnitude tests on clusters where you had reliable magnitudes for each member (like in Clark's book on Visual Astronomy).
Tests done on Paracorrs show a transmission around 97+% on a consistant basis.
That you see a difference I don't doubt. That it is due to transmission I do. My guess is that the difference is probably more due to light scatter and wavefront modification than it is due to transmission. That doesn't make it any less real, but:
1.the cause is not likely to be transmission solely, though this could be a part.
2.the difference would have to amount to 8% or so to be visible at all.
Your f/4.7 dob has a coma-free central zone of only 1.8mm (a Paracorr expands this to 10.8mm). At the image scale of your 10" scope, that's 2.88' per mm.
So the coma-free field of your scope is a trace over 5'(over 30' with a Paracorr). That's certainly enough, in a driven scope, to view all the planets in the central coma-free zone.
But not a lot of clusters, or larger galaxies.
And if the scope is undriven, the likelihood is that the object isn't being held in the coma-free zone, since the passage across this zone only takes 20 seconds.
So even if you view at/near the center, a lot of objects will be outside the coma-free zone, where a coma corrector would make a difference.
I've tried to view faint planetaries and galaxies without the Paracorr, but one thing I've found is that focus makes fainter details visible, and I simply have a harder time deciding where focus is when I view without the Paracorr. Sure, I could move the scope to center a star, focus, then move it back to the object. But I find it so much easier to focus on any star, anywhere in the field, with the Paracorr in place than without it. And the focus tightness on even galaxies makes the image detail visibility so much greater.
I'll have to try a limiting magnitude check on the stars around M57 (where I have accurate magnitudes to 19) to see if a difference is noticeable. M14 is a good test, too (the horizontal branch of the cluster is 17.1), and NGC206 in M31 (about a half dozen giants visible in the 12.5"). These tests will be good for ultimate stellar limiting magnitude.
I've already performed the before/after test on some large galaxies (like M81) and nebulae, and I see more details with the Paracorr in place.
I'll report back on stars.
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Don Pensack
www.EyepiecesEtc.com
12.5" Teeter/Zambuto, 5" Maksutov
Sustaining Lifetime IDA member
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sixela
Postmaster
Reged: 12/23/04
Posts: 13991
Loc: Boechout, Belgium
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Quote:
Studies done in labs, and results garnered by hundreds of thousands of observations by the AAVSO show that humans cannot reliably (better than chance) detect differences of less than 0.1 magnitude, about a 10% difference in lumens.
That's for relatively bright objects with a 10% brightness difference using day (photopic) vision. It's hazardous to extrapolate that to objects at the threshold of visibility using night vision.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Skywatcher 130mm f/5 BlackLine (finder, widefield scope and solar continuum scope)
Sumerian 250mm f/4.8 Alkaid (as travelscope without platform and on Tom O. platform above).
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Starman1
Vendor (EyepiecesEtc.com)
Reged: 06/24/03
Posts: 17639
Loc: Los Angeles
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Quote:
Quote:
Studies done in labs, and results garnered by hundreds of thousands of observations by the AAVSO show that humans cannot reliably (better than chance) detect differences of less than 0.1 magnitude, about a 10% difference in lumens.
That's for relatively bright objects with a 10% brightness difference using day (photopic) vision. It's hazardous to extrapolate that to objects at the threshold of visibility using night vision.
Well, the AAVSO figures are for nighttime observations, I believe.
Plus, further studies I've read indicate the nighttime detection of brightness differences may be substantially less sensitive than photopic judgments.
I certainly don't hear a lot of people complaining about a 30% illumination drop off at the edge of the field in their newtonians.
I was being charitable to say that someone could see an 8% difference. It's far more likely that 20% might be a scotopic minimum detectability level.
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Don Pensack
www.EyepiecesEtc.com
12.5" Teeter/Zambuto, 5" Maksutov
Sustaining Lifetime IDA member
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sixela
Postmaster
Reged: 12/23/04
Posts: 13991
Loc: Boechout, Belgium
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Quote:
Well, the AAVSO figures are for nighttime observations, I believe.
But IRC for objects bright enough to excite photopic vision.
Plus, further studies I've read indicate the nighttime detection of brightness differences may be substantially less sensitive than photopic judgments.
Quote:
I certainly don't hear a lot of people complaining about a 30% illumination drop off at the edge of the field in their newtonians.
It's one thing to ask "which is brighter" about two objects that you can see. Certainly for extended objects, that demands a lot of difference. It's another thing to see something in one setup and to only divine it in another.
I'm still sitting on the fence about this.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Skywatcher 130mm f/5 BlackLine (finder, widefield scope and solar continuum scope)
Sumerian 250mm f/4.8 Alkaid (as travelscope without platform and on Tom O. platform above).
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Starman1
Vendor (EyepiecesEtc.com)
Reged: 06/24/03
Posts: 17639
Loc: Los Angeles
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My argument boils down to:
The eye's sensitivity to small brightness differences is too low for a small difference in light transmission (as with a couple extra lenses in an eyepiece) to be visible.
That small differences in detectability occur is without doubt.
The causes of the differences in detectability lie elsewhere than in small brightness differences.
What are the differences other than brightness that determine differences in detection (of details or simply visibility)?
Some possibilities:
--surface polish, i.e.wavefront distortion. An eyepiece is a part of the optical train. Wavelength errors are important here, just as in the mirrors.
--tint, or hue
--spherical aberration
--chromatic aberration
--focus (as in spot diagram focus)
--light scatter (baffling, blackening, surface polish, inter-lens reflections, etc.)
--astigmatism (design, not the eye)
It's possible, even probable, that small differences in light transmission can, when added to other effects, make a difference in detectability of details or threshold of visibility, but I wouldn't expect it to be the major factor.
If lab measurements indicate much larger transmission differences between eyepieces (or accessories like a Paracorr), then transmission could become a more important factor.
I'd love to see those figures. But if a lab test shows a 3% difference between one and another, and there is a visible difference related to the detectability of faint features, I'd look elswhere than transmission for the cause.
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Don Pensack
www.EyepiecesEtc.com
12.5" Teeter/Zambuto, 5" Maksutov
Sustaining Lifetime IDA member
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sixela
Postmaster
Reged: 12/23/04
Posts: 13991
Loc: Boechout, Belgium
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Quote:
The causes of the differences in detectability lie elsewhere than in small brightness differences.
I'm still unconvinced. Until someone does studies on *detectability* of threshold objects rather than *comparison* between subjective brightness of two visible objects.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Skywatcher 130mm f/5 BlackLine (finder, widefield scope and solar continuum scope)
Sumerian 250mm f/4.8 Alkaid (as travelscope without platform and on Tom O. platform above).
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astroducky
sage
Reged: 10/02/06
Posts: 259
Loc: South East Asia
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Yes I would agree that there is a difference between detecting and comparing. Finding the magnitude difference is like the ability of the eye to discern the different grey shades ('brightness'). Some people are better to tell 2 close shades apart, others are not. However, seeing that colour (grey) and not seeing it is different than trying to tell the shade apart. This is like in music I guess. Some people are able to tell notes accurately but whether one can hear (detect) the lowest frequency or highest frequency sound is different than the ability to tell the notes apart. From what I observed with different eyepieces, although the transmission difference is *only* about 3 percent, it is possible to see faint stars/moons that are at the verge of detection in one other eyepiece than the other. It is because you either see it or do not see it and hence there is no need to tell or compare what 'shade' it is. Hence, I feel that there will be a difference in detecting an object at threshold even if the difference is small.
Just my 0.02 cents.
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Yang
12.5" Obsession "Obby" [2008 Paracorr/ AN/ Roundtable]
William Optics M88FD refractor on Microstar Deluxe
Televue, Baader and Pentax eyepieces.
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Svezda
scholastic sledgehammer
Reged: 06/01/07
Posts: 774
Loc: Texas
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Quote:
Yes I would agree that there is a difference between detecting and comparing. Finding the magnitude difference is like the ability of the eye to discern the different grey shades ('brightness'). Some people are better to tell 2 close shades apart, others are not. However, seeing that colour (grey) and not seeing it is different than trying to tell the shade apart. This is like in music I guess. Some people are able to tell notes accurately but whether one can hear (detect) the lowest frequency or highest frequency sound is different than the ability to tell the notes apart. From what I observed with different eyepieces, although the transmission difference is *only* about 3 percent, it is possible to see faint stars/moons that are at the verge of detection in one other eyepiece than the other. It is because you either see it or do not see it and hence there is no need to tell or compare what 'shade' it is. Hence, I feel that there will be a difference in detecting an object at threshold even if the difference is small.
Just my 0.02 cents.
These are very good points. There is a point at which a star will go 'on' for the observer, and the point when it happens might be after a very, very slight increase in transmission (say, after cleaning your mirror or lens, just as one example). I hadn't really thought of this before and had taken as more germane to this topic the idea that subtle <10% brightness differences between two objects can't easily be detected. I learned similar concepts in a cartography class in college about the idea that in making graduated symbol sizes on maps, making tinted or grayscale intensities for map legends, etc. /different enough/ that the average person could distinguish them sufficiently or rather actually /easily/ (based on similar visual perception research mentioned earlier). This is a completely different concept than determining the point at which an object can first be detected (like turning the knob on a dimmer light switch and seeing when you can first notice the light). Thanks for bringing this up - very interesting discussion.
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