Nils,
Regarding the html code, here are some changes to make to adjust for your
experience:

1) if (form1.Teletype[0].checked) {
var DS=0.21*D;
var FL=Math.pow(0.96,2);
}
if (form1.Teletype[1].checked) {
var DS=0.00;
var FL=Math.pow(0.99,2);
}
if (form1.Teletype[2].checked) {
var DS=0.35*D;
var FL=Math.pow(0.98*0.96,2);

First is reflector. I changed to enhanced coatings for my own scope, and the
obstruction % on personal scope.
Second is refractor. Changed to best multi-coatings.
Third is contemporary SCT. Changed to typical current coatings.
These were all inaccurate in the on-line calculators.


2) FO = Math.pow(0.99,6); // EYEPIECE (4 COATED AIR-GLASS SURFACES)

Changed to 6 air-to-glass surfaces for more typical widefield. This could be
changed for a specific eyepiece if desired by changing the number after the
comma.

3) DE = 7*Math.exp(-AG*AG/20000); // DIAM EYE PUPIL IN MM

This is the age-dependent diameter decrease in pupil diameter that predicts a
certain pupil diameter at a particular age. Adjusting the starting point (7mm)
may result in a more realistic level IF you know your exact pupil diameter. My
own dark-adapted pupil was never as large as 7, and currently stands at 4.5mm.
Altering this changes an assumption and changes limiting magnitude calculations.
I experimented to find a realisting starting point for my pupil diamter than
works in the calculator. I was NOT adjusting the age asumption, which might nt
be a valid one.

4) FS=1.0; // OBSERVER'S SENSITIVITY

This figure can be adjusted for the individual. I recommend playing with the
figure if you consistently reach deeper or not as deep as the predictions in a
variety of scopes.

5) // CALCULATE SKY BRIGHTNESS
if (MZ>=(7-K)) {
BS=54; // BEST POSS. SKY BRIGHTNESS
FS=Math.pow(10,0.4*(7-K-MZ)); // & GOOD EYESIGHT
}
else {
XX=0.2*(8.68-K-MZ); // FS ASSUMED = 1

Here, pupil diameter and observer sensitivity are assumed. These figures can be
adjusted as well.

6) M=M+(EX-6)*0.16; // EMPIRICAL EXPERIENCE CORRECTION

Here an adjustment can be made for experience in the end factor if you think the
difference is too great or too little.


I think the purpose for a limiting magnitude calculator is to predict the limit
for a particular observer on a particular scope on a particular night.

There is no harm in playing around with the assumptions IF it leads to a more
accurate prediction. I have a fair amount of experience using a 5" Maksutov, 8"
SCT and 12.5" newtonian, and know the average darkness of the site where I
observe. I find that I can predict my limit +/-0.2 magnitudes on all three
scopes (as verified using limiting magnitude charts from Roger Clark and Brian
Skiff and photomeric charts of M14, NGC206 and a few others), which means to me
that the adjustments I made in the code made the calculator more accurate for me
in my circumstances.

Robert's calculator cleans up some of the errors in the Bogen one and is a
little friendlier.

Here is a chart for people on this site to use for a limit determination:
http://www.geocities.com/SiliconValley/Garage/9531/lim_mag.htm

Don Pensack
Los Angeles








--- In bigdob@yahoogroups.com, "Nils Olof Carlin" <nilsolof.carlin@...> wrote:
>
> Don,
>
> >
> > Wasn't that a 1989 article?
>
> You're right - I copied the wrong year from somewhere I can't recall now.
> >
> >>and I don't know the details of
> >> either Bogen's or Robert's calculators and their algorithms.
> >
> > Look at the html code behind this page:
> > http://www.go.ednet.ns.ca/~larry/astro/maglimit.html
> > In IE or Opera, Click View, then Source.
>
> While this is possible, I just don't think it's fair to the user to force
> her to dissect the Javascript code to find out what the calculator actually
> does. This criticism applies equally well to Robert's version - including
> the undocumented sensitivity adjustment mentioned in a recent message.
> I'd much prefer to read about the algorithms used, in order to understand
> what factors are considered (or left out).
> >
> >
> >>So,
> >> considering the quirk (or worse) in Schaefer's application of the
> >> sensitivity equation (of Knoll et al) that I described, I wonder how your
> >> "modifying the pupil diameter calculator to correspond with my own
> >> reality"
> >> was done.
> >
> > It is a simple matter to modify the html code and then see the resultant
> > pupil diameter. This definitely adjusts the results. I experimented to
> > find the correct results for pupil diameter and sensitivity.
>
> Simple or not, I had hoped you would have answered my question and told us
> what modifications you did.
> >
> >>Perhaps like what Steven just described, artificially adjusting
> >>the retinal sensitivity by entering a downsized eye pupil value?
> >
> > Actually, sensitivity can be adjusted separately in the code.
>
> Of course you could do any modification to please yourself, but undocumented
> modifications puts any other potential user at a serious disadvantage - and
> yourself, if you (like Steven) don't understand the true consequences. In
> the 1990 paper, Schaefer uses established scientific practice in giving a
> detailed account of the model he uses, and I have done my best to do the
> same, but by not giving away the details, meaningful discussion is
> essentially made impossible. (With reservations for possible discussion in
> the S&T article (BTW could anyone scan it for me? if so, please reply off
> list), which is not readily available (the 1990 paper is available on the
> net!).
> >
> >> I don't know the details of how the Knoll data were obtained (or even how
> >> his equation fits the experimental data), but Blackwell defines his
> >> threshold as "corresponding to a probability of 50 percent, due allowance
> >> having been made for chance success ". If Knoll's data are comparable in
> >> this respect, the limiting magnitude for careful star observation with no
> >> time limit, where the object is seen only intermittently, should be
> >> (numerically) significantly higher.
> >
> > A reviewer in CN cited Schaefer's results dependent on a "visible 10% of
> > the time" as a "threshold" observation. I'm not certain whether that is
> > true or not.
>
> I think you refer to a discussion that you initiated 2 years ago in the
> reflectors forum - but it is not so. I think it should be clearly understood
> that for basic light sensitivity, Schaefer uses the equation by Knoll et al,
> based on experimental sensitivity data - *not* data from actual star
> observations. The sensitivity of the eye (including binocular vision, a
> feature that the calculators have left out) of an individual observer will
> differ to some extent, and if allowance is desirable for this (which it is),
> it should be done explicitly.
>
> http://www.sciencecartoonsplus.com/pages/gallery.php
> I think Robert should be more explicit in step 2 <grin>
>
> Nils Olof
>