(Fwd) Re: ATM barlows, magnification and also averted vision

["Alan Levin" <ailevin@ix.netcom.com>]

------- Forwarded Message Follows -------
From:          Self <Single-user mode>
To:            "Nils Olof Carlin" <nilsolof.carlin@swipnet.se>
Subject:       Re:  ATM barlows, magnification and also averted vision
Reply-to:      ailevin@ix.netcom.com
Date:          Sun, 22 Jun 1997 18:47:22

Nils Olof,

> Alan,
<mostly agreement snipped>
> > The place I disagree with you is in your implicit treatment of the
> > retina as an photographic device rather than a neural device. 
> 
> I don't think I implied anything such, I only pointed out that the retina
> *has* a finite resolution. Actually, for central seeing it is better than
> just the size and density of cones would indicate. For those whose retinas
> are not up to the full resolution that most of us enjoy, a bit higher power
> is needed.
> 
> > The detectablility of objects depends on a rather subtle  balance of the
> > contrast difference between the object and the background, and the
> > size of the object.  Thus, while contrast may decrease with more
> > magnification, detectability may increase. 
> 
> I believe you talk about averted vision here, and that is very much
> different from direct vision (that you use for planet observation).

Actually no, what I meant was that the detectability of a constrast
difference depends on both the difference in brightness between two
regions and the size (and to some extent the shape) of the regions. 
Because the neural processing is a cooperative phenomenon, many
neurons together become sensitive to smaller differences in contrast
over larger regions. In other words, though at higher magnification,
the light level for each rod is lower since the extended image on
the retina is larger, the object may become more detectable.

I also fear we are talking apples and organges somewhat (rods and
cones).  I have not read the original Blackwell papers, but
my sense from Clark's book is that they dealt primarily with black
and white perecption under low light conditions (rods).

> Trouble is, as far as I have found, Clark's calculations are not correct. 
> Very roughly, I found the best magnification to detect faint objects (with
> a given aperture) is high enough to make the background quite dark, but not
> more than makes the object 1 degree apparent size.

I'm not clear on what you mean here.  Have you found errors in Clark's
calculations and analysis, or in Blackwell's data reduction from his
experiments?  I took this rather to mean that Clark's conclusions do
not agree with your experience.

> 
> Best magnification(s) to see detail may be (include) more than this.
> 
> With an exit pupil of 0.7 mm the sky background is 5 magnitudes darker than
> at 7 mm, so a good sky of 21 mag/sq arcsec may seem 26 mag/sq arcsec, and
> against that background the sensitivity for averted vision can increase 2
> magnitudes.
> (averted vision is not useful for brighter backgrounds than about 19 mag/sq
> arcsec. Only snag is I don't (yet) know how to measure sky background in
> this unit.

You could post a question on this on s.a.a.  I am attaching a rather
long file from Brian Skiff that Tom Polakis posted to s.a.a a while
ago that discusses sky brightness theory and measurement in the
units you suggest.   If this turns out to be unintelligible as an 
attachment, let me know and I will send it as inline text.

<I have omitted this attachment for the post to ATM
below is a copy of the header if you want to look it up on dejanews:

From: polakis@indirect.com (Tom Polakis)
Newsgroups: sci.astro.amateur
Subject: Brian Skiff on Sky Brightness [~2000 words]
Date: 10 Feb 1996 00:04:39 GMT>

> My pet theory is (glad you brought this up) that the increase in
> sensitivity is due to a "dark" sky being bright enough to prevent good dark
> adaptation.
> 
> People seem to disbelieve this, but anyone can try this experiment:
> After good dark adaptation your usual way at your favorite dark sky site,
> take five minutes with your best observing eye covered, and look at the sky
> with the other. Then look with the covered eye and quickly compare the
> light sensitivities, you have a few seconds before the adaptations are back
> to similar levels.
> You may find that the covered eye has too much sensitivity for comfort, and
> you see lots of "detector noise", almost like in a night vision image
> intensifier in very low light.
> 
> Clear skies, 
> 
> Nils Olof

I think your pet theory is interesting,  particularly the timing for 
sensitivity of both eyes to become the same.  The fact that the time 
constant is seconds indicates that is it either a rapid neural adaptation or a 
change in iris dialation or some combination of  both.  Changes in 
photoreceptor chemistry take longer than that.  I think it is likely 
that it is a sort of gain adjustment at the retinal level in concert 
with your metaphor of detector noise.  

There would still be the issue of seeing detail in faint nebulae,
where the background level is well about dark sky and we are looking
for bright or knotty patches.  I thought Clark's example of M33 was
a good one, and my experience of late has been that using more
magnification I see more detail in it's structure.  In the continuum 
that would relate to seeing just slightly darker small rills on a 
brightly illuminated Moon.  I would expect magnification to help as 
more of the retina gets invloved in the effort to distinguish the 
contrast difference.

For me the value of Clark's book is that I am using more
magnification on many extended low surface brightness objects and
seeing more detail. 

Good Observing,
Alan