Celestia Forums

Let me point out that the AutoMag scheme that I incorporated for the 1.2.5 version some time ago, is becoming increasingly useful once more and more stars are "in". While Automag is incorporated in the Linux menue, in Windows you may just use CTRL Y to toggle it on|off and play with it.

In a nutshell, AutoMag roughly keeps the /density/ of stars in the window constant while zooming from large FOV = 120deg down to small FOV's by means of SHIFT Mouse L .
Gives a nice 3d effect. I am going to check in soon some slight tuning of the AutoMag, that was possible once the large 780k star data were available...

Byr Fridger

Oh what a "joy", the box lost my identity again;-).

The previous mail was me, of course...

This system is /exclusively/ for /fast/ writers. Thinking is immediately punished by loss of identity;-)

Above, I forgot to mention that the AutoMag emulates the action of a /telescope/ where the density of stars in the FOV also stays roughly constant, if the magnification is increased and hence the FOV is decreased. Dimmer stars come into view with increasing magnification, but lots of stars are also moving out of the focal plane since the effective FOV decreases...

Bye Fridger

I hate to act der besserwisser, but a telescope does unfortunately not work like this! Increasing the magnification power does not improve the sensitivity of a telescope (i.e. you do not see fainter stars). Thus the number of stars increases with the field of view (fov) for a given telescope aperture, because a larger part of the sky is covered (the focal plane has nothing to do with it). I'm sure you find this obvious after some thought.

Your "AutoMag" feature is useful, not because it emulates the action of a telescope, but because it keeps the field of view conveniently populated, not too crowded and not too empty.

/Alexis

PS I agree that the "login time" is too short in this forum.

alexis wrote:I hate to act der besserwisser, but a telescope does unfortunately not work like this! Increasing the magnification power does not improve the sensitivity of a telescope (i.e. you do not see fainter stars). Thus the number of stars increases with the field of view (fov) for a given telescope aperture, because a larger part of the sky is covered (the focal plane has nothing to do with it). I'm sure you find this obvious after some thought.

Your "AutoMag" feature is useful, not because it emulates the action of a telescope, but because it keeps the field of view conveniently populated, not too crowded and not too empty.

/Alexis

PS I agree that the "login time" is too short in this forum.

Sorry but you do, if you remember about the different effects of magnification on point sources and extended light sources like sky background. The point sources (stars) of course remain unaffected as you say, but the /contrast/ is increased since the sky background gets darker!

Bye Fridger

Again, thinking a moment about Alexis statement was enough to be punished with identity loss by this inpatient machine;-)

Bye Fridger

I see you remain unconvinced. Well, the claim is easy to put to test: take your favourite telescope, point it in a random direction in the night sky with a low power eye-piece (say 50x). Count the stars. Do the same with a high power eye-piece (say 300x). Are the number of stars the same? I think you will find they are not. The reason is simple: the telescopic image of a star is not a point but a finite size point spread function (PSF) that also scales with the magnification, so the contrast remains constant. In addition, think of the implications if one could observe arbitrary faint stars by just increasing the image scale!

/Alexis

PS Just an interesting historic detail: I once developed a similar AutoMag function I used in an ascii 2D planetarium program of mine. Then it was extremely important to keep the number of stars per on-screen ascii-character to a reasonable level, because the number of characters on a display is very limited, and it rapidly got very messy with crowded starfields.

alexis wrote:I see you remain unconvinced. Well, the claim is easy to put to test: take your favourite telescope, point it in a random direction in the night sky with a low power eye-piece (say 50x). Count the stars. Do the same with a high power eye-piece (say 300x). Are the number of stars the same? I think you will find they are not. The reason is simple: the telescopic image of a star is not a point but a finite size point spread function (PSF) that also scales with the magnification, so the contrast remains constant. In addition, think of the implications if one could observe arbitrary faint stars by just increasing the image scale!

/Alexis

Well, while the following arguments are certainly not
necessary in order to justify my AutoMag scheme, it may be just
interesting to discuss this issue in some greater depth;-):

An excellent and standard reference on these matters is the book
"Visual Astronomy of the Deep Sky", written by the physicist Roger.N. Clark, Ph.D.

Since you do not seem to believe me (well, I usually work on construction sites;-)),
let me quote from page 49 of that book. The chapter is called
"The Faintest star Visible in a Telescope".

"The eye's fundamental limit is around 50 to 150 photons of green light
arriving over a several second period, corresponding to a star as faint
as 8.5m." (t00fri:) Normally, depending on the observer's location,
one may only see stars of 5m - 7m with the naked eye. "Magnification does not
change the brightness of a point source in the telescope, but it does decrease
the surface brightness of the background (in agreement with my previous statement above)
and reduces the field of view so other stars do not interfere. Therefore, the
fundamental limits of the eye can be reached when a telescope is used. Here it really
is possible to see the equivalent of 8.5m stars naked-eye" (clearly your above
extrapolation to infinitely faint star detection is nonsensical)!

"A star image is actually a diffraction disk, but it is so small that, if faint,
the disk is a point to the eye at any reasonable magnification at all"
(t00fri: this of course invalidates your PSF argument above...
we construction workers call this the "correspondence principle", connecting smoothly
between the laws governing the wave regime of light and geometrical optics)

"The fundamental magnitude limit M_t of a telescope is given by

M_t = M_e + 2.5 log_10 (D^2 t/D_e^2),

where M_e is the eye limiting magnitude (8.5 at best), D=telescope diameter,
D_e is the eye diameter (7.5mm), and t=telescope transmission factor ~0.7.

The surface brightness M_b of the sky or an extended object is darkened by the
/telescope magnification m/ and transmission factor t as follows

M_b = -2.5 log_10 (D^2 t/m^2 D_e^2)

This is why the magnification helps to detect faint stars when the sky is bright,
or even under dark country skies compared to when /low power is used/!"


And then in that book you may inspect detailed diagrams about actual
measurements of the effect of telescope magnification
on the faintest visible stars etc...


Bye Fridger

t00fri wrote:Again, thinking a moment about Alexis statement was enough to be punished with identity loss by this inpatient machine;-)

Bye Fridger

I think the problem lies between the chair and the keyboard hehe j/k....Were you using windows at the time?

Rassilon wrote:

t00fri wrote:Again, thinking a moment about Alexis statement was enough to be punished with identity loss by this inpatient machine;-)

Bye Fridger

I think the problem lies between the chair and the keyboard hehe j/k....Were you using windows at the time?

While the attentive reader is asking immediately:

What is located between t00fri's chair and his keyboard, h?, h?

Bye Fridger

touch?

t00fri wrote:"A star image is actually a diffraction disk, but it is so small that, if faint, the disk is a point to the eye at any reasonable magnification at all"

This is misleading! Only very small telescopes (D < 5cm) do generally produce diffraction limited images of stars. A typical good site has a seeing disc of 2 arcseconds (an excellent site under good conditions 1 arcsecond) and the resolution of the eye is about 1 arcminute (reference). Thus the eye resolves the PSF already at 30x-60x magnification. Rarely, if ever, are telescopes used with a magnification as low as 30x! I conclude that the eye almost always resolves the seeing disc under normal telescope use, and that my PSF argument therefore remains valid.

t00fri wrote:M_t = M_e + 2.5 log_10 (D^2 t/D_e^2)

This in itself tells you that the fundamental limiting magnitude is independent of the magnification, and is the other argument that your statement that "the number of stars seen through a telescope is independent of the fov" is basically wrong:

Assume there are N_lim stars in the field of view of magnitude brighter than M_t, but you only see N_0 < N_lim stars due to a bright sky. Then by increasing the angular magnification m times, you will see N_1 < N_lim/m^2 stars (N_lim/m^2 is the the number of stars brighter than M_t in the new fov, because the new fov space angle is m^2 times smaller than the original). Now you see why N_1 cannot equal N_0 for arbitrary m, because for every finite N_lim and positive N_0, there is an m such that N_0 > N_lim/m^2 > N_1. QED

t00fri wrote:And then in that book you may inspect detailed diagrams about actual measurements of the effect of telescope magnification on the faintest visible stars etc...

Unfortunately this book is not easily available to me... but it sounds crazy. When we estimate limiting magnitudes in planned observations, only telescope aperture, transmission (including detector sensitivity), sky emission (per arcsec^2), seeing, and exposure time is considered. Never magnification. Magnification (or, image scale) is only important for detector considerations (saturation or thermal noise of detector, for example. Infact, due to thermal noise it is usually better to use a smaller image scale (larger fov) to increase the limiting magnitude!).

Clear skies,
Alexis

/Alexis:

Mathematical induction proofs and blindly extrapolating arguments to infinity
(like your "counter argument" above) are not at all the issue in this
discussion.

We talk about a /compensation/ of about 2-3.5m of illuminated diffuse sky
background by means of telescope magnification effects. Not more not
less. I think most experienced amateur astronomers (with telescopes so
small that such effects do matter) know about this (Yes, I do observe the sky
with telescopes since I was a child). The above-mentioned book devotes
a large chapter to this important issue that you seem to deny entirely.

Suppose I want to see the central star (~14m) in
the ring nebula of Lyra (M57), sure I use a very big magnification
(besides averted vision), in order to suppress the diffuse light from
the background sky and nebulosity relative to the star's light. If I
want to see as many /faint/ stars as possible in M11 (Scutum) or M13, clearly
high mag does the job best.

And so on...

My essential mistake was to try and make the effect of my AutoMag
scheme intuitive in terms of the above telescope features, which do hold
only over a /limited range/ of magnitudes (2-3.5m) and not for ever,
of course!

On the other hand, 2-3.5m is a huge effect in practice...


Bye Fridger

t00fri wrote:Mathematical induction proofs and blindly extrapolating arguments to infinity (like your "counter argument" above) are not at all the issue in this discussion.

Often, driving matters to extremes helps clarify the logic.

t00fri wrote:(Yes, I do observe the sky with telescopes since I am a child).

Not only children observe! (Sorry, couldn't help myself )

t00fri wrote:The above-mentioned book devotes a large chapter to this important issue that you seem to deny entirely.

I consider myself an experienced amateur astronomer, and I am aware of the fact that sometimes greater magnification helps seeing faint details. Yes, there is a small effect, probably due to some brain processing/eye physiology effects. But you don't see four times as many stars per square arc minute by increasing the angular magnification by a factor of two, for instance. And it doesn't change that I find the statement

t00fri wrote:AutoMag emulates the action of a /telescope/ where the density of stars in the FOV also stays roughly constant

misleading, because to the casual reader it would seem that magnification and not telescope aperture is of importance to sensitivity, which is also a very common misconception. I'll tell you what my experience with amateur telescopes / magnification is. If I point the telescope into a random part of the sky at low magnification, I always find stars in the fov. If I on the other hand point the telescope with high magnification, I usually see very few or no stars at all. Now, that's an easy experiment to do next time you're at the telescope, and I guarantee you'll find similar results.

t00fri wrote:On the other hand, 2-3.5m is a huge effect in practice...

Yes, it corresponds to an increase of the signal with a factor of 6 to 25. I'm very skeptical. Does herr dr. Clark claim this sensitivity increase in his book? Averted vision, for instance, only helps half a magnitude and gives generally a greater effect than increasing the magnification (in my experience).

/Alexis

/Alexis:

alexis wrote:I hate to act der besserwisser, but a telescope does unfortunately not work like this! Increasing the magnification power does not improve the sensitivity of a telescope (i.e. you do not see fainter stars).

Well, your reputation as a "Besserwisser" has definitely been growing
recently;-).

Your "jokes" about my (1 minute lasting) typo: "since I /am/ a child"
and "herr Dr. Clark" (note he is british) fit in well and are neither
particularly intelligent nor amusing...

Since I am running out of spare time to continue this discussion, let
me just give you the following URL's for /actual/ observations of the
discussed effects (1) and background reading concerning Clarks
investigations (2):

(1) http://clarkvision.com/visastro/m51-mag/index.html

(2) http://clarkvision.com/visastro/omva1/index.html

---!!---
In particular, have a look in (1) at the lower drawings and the
animation of M51 by the author Roger Clark as function of
increasing telescope magnification! One does not only see more details
of the spiral arms appear, but note the increasing number of faint
stars popping up with increasing magnification .
---!!---

These are careful, actual observations at a 12.5 inch telescope by
Clark which perfectly match my own findings and those of many other
experienced amateurs. Of course, this increasing sensitivity is bound
to stop at some critical magnification, as is physically obvious and
discussed in detail in (1),(2) and the book.

Let me end by noting that e.g. the library of my lab (one of the world's
leading accelerator labs in particle physics) and that of the "Bergedorfer
Sternwarte"/Hamburg (remember where the Schmidt corrector plate came from;-))
both have Clark's book in their shelfs. It is highly acclaimed by all
reviewers (even at amazon.com;-)) and published by "Cambridge
University Press" after all...But our young genius /Alexis puts away
with these complex matters by means of 1 minute simplistic mathematical "no-go proofs" ...

Clear Skies

Fridger

t00fri wrote:Your "jokes" about my (1 minute lasting) typo: "since I /am/ a child" and "herr Dr. Clark" (note he is british) fit in well and are neither particularly intelligent nor amusing...

In contrast to your's, examplified in an earlier post in this thread? Well, I'm sorry you don't share my sense of humour. I didn't mean to offend you.

Thanks for the links! I found them very interesting and useful. Clark's book may be a standard book on the subject, but when I said it was not easily available to me, I meant it. I made a library search, and of all public and university libraries in Sweden, there is only one single copy in Lund. And unfortunately it is not available even from Amazon, probably it's out of print.

t00fri wrote:One does not only see more details of the spiral arms appear, but note the increasing number of faint stars popping up with increasing magnification.

Yes, I noted, the number of stars popping up was very dramatic, increasing from one visible star to 10 by doubling the magnification. But this was a drawing, and in reality I doubt the effect is this dramatic. In addition, only stars in the direct vicinity of M51 were considered. What I say is: some increase in sensitivity is expected with magnification during visual observations (from experience; probably due to brain processing / eye physiology), but this increase is insignificant in comparison to the number of stars you loose by decreasing the fov.

I gladly admit that your line of thought was more sophisticated than I first anticipated by your original statement, by including second-order effects such as our eyes improved ability to see faint things when magnified. But this doesn't change the fact that I find your original statement misleading, for reasons I've explained in this thread.

As for your last remark, I didn't read it.

/Alexis

/Alexis:

Let's just "push the reset button";-)

In fact, I enjoyed your crisp argumentation as well as your very
first astronomical publication in 1982...

Bye Fridger

Thanks! Next time you need a stubborn besserwisser, you know where to find him I always enjoy learning new things, and the work you refered to (like Clark's) took into depth things I've casually thought about. So, I thank you for taking the time to argue with me...

/Alexis

alexis wrote:Thanks! Next time you need a stubborn besserwisser, you know where to find him ;-)

Agreed;-)

alexis wrote: I always enjoy learning new things, and the work you refered to (like Clark's) took into depth things I've casually thought about. So, I thank you for taking the time to argue with me...

/Alexis

My pleasure...and 255 hits in the thread, can't have been too boring either;-)

Bye Fridger