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Eyepiece experts.....do you know?

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#51 BillP

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Posted 20 August 2014 - 01:52 PM

.... is there anyone out there that can detect that 6 percent difference?  Really?

 

 

I would say everyone can, just depends on the circumstance.  So take the scenario where a faint star is at the threshold of vision at the point where a 3% brightness change will bring it under threshold.  Bet everyone will see that star no longer in the FOV.  So would imagine we can come up with a number of circumstances were 6% will be noticeable.  Certainly not all circumstances, but assuredly some. 

 

But what is always puzzling to me is the lack of concern over the 6% at the eyepiece.  If I am lax there, I guess I should not worry about the main objective that is 6% less that what I can get, or that one diagonal is 6% less than another?  It is a chain...it all adds up.  Remember...it only takes one weak link to break the chain.  One should not trivialize transmission performance at any place in the chain IMO.  As analogy, if one spends the money to get a high performance car, why would they ever want to put a low performance tire on it??  Geez...if 2 EPs have the same FL and AFOV and one has 6% less transmission than the other...who would want that retread! :lol:


Edited by BillP, 20 August 2014 - 01:54 PM.


#52 gnowellsct

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Posted 20 August 2014 - 02:17 PM

 

.... is there anyone out there that can detect that 6 percent difference?  Really?

 

 

I would say everyone can, just depends on the circumstance.  So take the scenario where a faint star is at the threshold of vision at the point where a 3% brightness change will bring it under threshold.  Bet everyone will see that star no longer in the FOV.  So would imagine we can come up with a number of circumstances were 6% will be noticeable.  Certainly not all circumstances, but assuredly some. 

 

But what is always puzzling to me is the lack of concern over the 6% at the eyepiece.  If I am lax there, I guess I should not worry about the main objective that is 6% less that what I can get, or that one diagonal is 6% less than another?  It is a chain...it all adds up.  Remember...it only takes one weak link to break the chain.  One should not trivialize transmission performance at any place in the chain IMO.  As analogy, if one spends the money to get a high performance car, why would they ever want to put a low performance tire on it??  Geez...if 2 EPs have the same FL and AFOV and one has 6% less transmission than the other...who would want that retread! :lol:

 

But note that the French magazine measured transmission in *magnitude* drop not %.  A 6% drop would be very small expressed in magnitude.   GN



#53 GlennLeDrew

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Posted 20 August 2014 - 03:25 PM

6% is about 0.06 magnitude.



#54 gnowellsct

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Posted 20 August 2014 - 03:33 PM

6% is about 0.06 magnitude.

 

Makes you wonder....



#55 Paul G

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Posted 20 August 2014 - 03:46 PM

 

IIRC, didn't tests show that one had transmission somewhere in the low 80 percent range?  That would be an appreciable difference compared to some modern eyepieces with 96-98% transmission.

 

 

 

 

On some German web site that used to get posted here fairly frequently, test data on transmission for the Meade UWA and the early 90s Naglers showed them in the high 80s--like 88 or 89%.   Some would argue that's not much compared to 96% for the 14XW against which I compared.  

 

More recently, just in the past week or so in the coatings test thread where some test results were posted, the Meade UWA showed at the bottom of the pack with the most reflective coatings.   Delos and XWs were rated 10-15 range; Meade UWA was 44.  That is a consistent finding in that reflections must be light that doesn't make it through the coatings.  

 

Greg N

 

 

Here are the results in a .doc file, I have a link to a web page format on a puter at home and will post it later:

 

www.astrosurf.com/gildup/Eyepiece.doc

 

The Meade UWA's were not fully multicoated, FWIW.

 

Edit: Here 'tis:  http://www.amateuras.../tips/tips3.htm


Edited by Paul G, 20 August 2014 - 03:53 PM.


#56 howard929

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Posted 20 August 2014 - 04:08 PM

At times debates between the differences of Naglers and ES 82's gets somewhat heated. No ES 82's tested. Too bad, that might have been informative.



#57 gnowellsct

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Posted 20 August 2014 - 04:13 PM

6% is about 0.06 magnitude.

Yes that is about 1/2 what the variable star observers say can be detected: I think they say one tenth magnitude.  But Bill P has a point.  If you have better coatings on the primary or on the diagonal and a 96% eyepiece instead of an 89% eyepiece then you are probably good to go for that one tenth of a magnitude.  G.


Edited by gnowellsct, 20 August 2014 - 04:14 PM.


#58 eklf

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Posted 21 August 2014 - 09:16 AM

This has been a very educational thread for me.

 

A final data point - what exactly does the 0.06 or 0.1 magnitude translate to in practical terms? In other words, if you take a typical open cluster what fraction of total stars will be rendered visible when limiting magnitude increases by 0.1? 

 

Some thoughts that come to mind - I suspect, like Don Pensack mentioned earlier, it is not going to be absolute number, but rather the fraction of time certain threshold stars are visible with averted vision. So i am left wondering if this difference is related only to stars visualised through averted vision, and that number of stars visible through direct vision won't change?

 

2) How does this 0.1 mag difference compare to differences in seeing?  Thus what is the range, in terms of limiting magnitude, between stars visible at excellent seeing vs average seeing?

 

Of course it also depends on the power, aperture as well type of open cluster - some clusters may have a narrow range where more stars may be visible.

 

As I write I am reminded of a limiting magnitude calculator somewhere on line that integrates multiple variables.  I will have to hunt it down and run some simulations.


Edited by eklf, 21 August 2014 - 09:17 AM.


#59 Brian Carter

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Posted 21 August 2014 - 09:31 AM

This has been a very educational thread for me.

 

A final data point - what exactly does the 0.06 or 0.1 magnitude translate to in practical terms? In other words, if you take a typical open cluster what fraction of total stars will be rendered visible when limiting magnitude increases by 0.1? 

 

Some thoughts that come to mind - I suspect, like Don Pensack mentioned earlier, it is not going to be absolute number, but rather the fraction of time certain threshold stars are visible with averted vision. So i am left wondering if this difference is related only to stars visualised through averted vision, and that number of stars visible through direct vision won't change?

 

2) How does this 0.1 mag difference compare to differences in seeing?  Thus what is the range, in terms of limiting magnitude, between stars visible at excellent seeing vs average seeing?

 

Of course it also depends on the power, aperture as well type of open cluster - some clusters may have a narrow range where more stars may be visible.

 

As I write I am reminded of a limiting magnitude calculator somewhere on line that integrates multiple variables.  I will have to hunt it down and run some simulations.

 

1.  I would be doubtful of anyone who said they really noticed a .1 magnitude difference in brightness, at least without very careful study.  .1 magnitude is such a small change in brightness that it would be difficult to attribute it to the eyepiece. I suppose if you are going absolutely as deep as you can go, it might make a difference, but I certainly don't choose eyepiece based on that tiny amount of extra light.

 

2. Magnitude vs seeing:  Seeing and transparency will affect image brightness orders of magnitude more than the transmission.  I would argue that the .1 mag difference would be impossible to observe reliably unless seeing and transmission were more or less perfect.  Under average conditions you wouldn't know whether it was an eyepiece or a momentary flicker in the atmosphere above you. 



#60 Starman1

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Posted 21 August 2014 - 09:38 AM

And, scintillation, a part of seeing, results in momentary sharpening of focus for very faint stars, and this can result in significantly fainter stars suddenly becoming visible.  This can easily exceed several tenths of a magnitude.  The best example of this is the central star in M57, which, in some apertures, comes and goes using averted vision, even when the seeing is nearly perfect.

 

As for calculators created based on the works of Schaefer, here are a couple to play with.

Note: many of the Bogen calculators on line have errors: many have refractor and reflector results reversed, and many assume reflectivities as low as 88% for mirror surfaces.

The one to which I link has been corrected:

http://scopecity.com...-calculator.cfm

And here's one in graphical format that is easy to see when components are changed.

http://www.ilanga.co...ude/index.shtml



#61 JustaBoy

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Posted 21 August 2014 - 10:56 AM

Hi Don,

 

In playing around with the Scope City one, I find that there is still hope for me...

 

The older I get the better I'm gonna see!

 

 

Thanks,



#62 Starman1

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Posted 21 August 2014 - 11:02 AM

This is the one flaw with the calculator that everyone points out: the older you get, the fainter you see.

It is based on assumptions about pupil size and somewhat dubious.

Crumey has pointed out some of the flaws in the basic work (Blackwell's), so any such calculator should be taken with a healthy

sized grain of salt.

Plus, I believe the assumption is that the limit is visible with averted vision only ten percent of the time.

When i see something that little, I usually attribute it to problems with my vision, or "averted imagination"  :lol:



#63 JustaBoy

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Posted 21 August 2014 - 11:09 AM

"It is based on assumptions about pupil size and somewhat dubious."

 

 

The smaller the eye pupil the fainter the mag?

 

Would this somehow be based on the relative sizes of the eye pupil and the exit pupil being closer together?

 

Not in reality, of course, but in their programing?



#64 Brian Carter

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Posted 21 August 2014 - 11:14 AM

 "averted imagination"  :lol:

 

I have seen the central star in M57 dozens of times using this technique, it really works!

 

:p



#65 Starman1

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Posted 21 August 2014 - 03:53 PM

"It is based on assumptions about pupil size and somewhat dubious."

 

 

The smaller the eye pupil the fainter the mag?

 

Would this somehow be based on the relative sizes of the eye pupil and the exit pupil being closer together?

 

Not in reality, of course, but in their programing?

See the discussion of the Stiles-Crawford term here:

http://w1.411.telia....al/Schaefer.htm

(which is, by the way, not normally associated with rod vision at all, only cone vision)

 

and read the articles here:

http://adsbit.harvar...type=SCREEN_GIF

and here:

http://arxiv.org/abs/1405.4209 or  http://arxiv.org/pdf/1405.4209v1.pdf

 

You might also find this CN thread of interest:

http://www.cloudynig...hresholdlimits/

 

More discussion of Stiles-Crawford Effect (which may not be applicable at the limit):

http://en.wikipedia....Crawford_effect

 

This paper implies a finding consistent with anecdotal evidence--that of increased visual noise and a poorer contrast threshold with age:

http://www.journalof...t/10/10/15.full

 

I would argue that, without vision problems such as cataracts, declining visual sensitivity and increasing contrast threholds with age are just barely offset by the gain in limits from experience.  And, at some point, even experience will be overcome by age.  Which is why the age factor in the Bogan calculators is a questionable feature.  It's true that extraneous light outside the exit pupil will diminish as the pupil of the eye shrinks, but how this affects limiting magnitude is ?.  The effect is small, though, in the calculator, and only changes the limit by a small number of hundredths of a magnitude, so pretty much can be ignored except for the calculated pupil diameter with age, which, at least for this observer, are way off.


Edited by Starman1, 21 August 2014 - 05:43 PM.


#66 JustaBoy

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Posted 21 August 2014 - 04:17 PM

Thank you, Don!



#67 csrlice12

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Posted 21 August 2014 - 05:34 PM

That's one nice thing about getting older....every days a new day......and I still get that Wow feeling every time I use the scope, be it a new object or something I view a lot...








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