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

["Nils Olof Carlin" <nilsolof.carlin@swipnet.se>]

Alan,

Thanks a lot for that text from Brian Skiff, I find it very enlightening,
it must have appeared only days before I got connected to the internet.

> 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. 

Let me stress that Roger N Clark has done us amateurs a tremendous
service by pointing out the importance of high magnifications on deep-sky
objects (he is not alone, but I think clearer than others). This is valid
of course, but (I claim) he made a mistake in interpreting Blackwell´s data
that lead to some overestimation of optimum detection magnification. If it
had really been badly off, I am sure he would have found out himself. 

> ....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 agree, but I believe this is because the darker background permits better
dark adaptation. My plots show that this fails for images magnified to >1
degree apparent size. Clark sometimes calculates magnifications to 6
degrees!
> 
> 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).

The Blackwell data cover visual contrast thresholds for backgrounds of 3.8
to 23.8 mag/sq arcsec=msa (B.-s table VIII; from daylight to dark sky[=22
msa according to B Skiff]). Averted vision is chosen by the volunteers for
darker backgrounds than 19 - 19.5 msa, and here is the most interesting
region for dark sky observing. Some measurements go as far as 27 msa, but
regrettably none are made with totally dark background.

> 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.

Briefly: on page 11 Clark says:"...the optimum magnified visual angle
occurs when the first derivative (the slope) of each curve in Figure 2.6 is
equal to 
-1." This is a double error: The curves are for constant background, but
the whole data set should have been plotted for constant contrast, for this
to apply. Also, using -1 instead of -2 (which would have been correct, if
the curves had been for constant contrast), Clark falsely finds a maximum
that he calls the OMVA.
 
I have made a simple visual photometer with which I compare the sky to a
white surface lit by a green LED, and adjust the current till their
brightnesses match. This gives a reading of current, and of sky background
in arbitrary units, and I can compare sky brightnesses at the different
sites I visit. What I need is a way to calibrate it: If I knew the Milky
Way brightness in the cygnus star cloud (I live at 55.5 deg N!), averaged
over about 1 degree, and the brightness in star-poor areas in lyra-draco
nearby, I and anyone else could do an absolute calibration. Who can help
me? Brian Skiff?

I would really like to share this idea, if I could get the calibration
data!
 
> 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.  

Maybe, but I find those mechanisms very unlikely: the consumption of
activated rhodopsin is obviously very dependent on the light level, and the
time constants involved must be highly light dependent. After all, if you
go from dark to daylight, it takes much less than a second to lose your
dark adaptation!
> 
Regards,

Nils Olof