Question on aperture and magnitude.

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In summary: Ratio of the square of aperture diameters= (100/6)^2= 10,000So difference in magnitude=-2.5Log(base10)(10,000)=-500
  • #1
garyd
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Homework Statement



A telescope with aperture 10 times the human eye could see stars down to magnitude?

Homework Equations



I was not given an equation with the question but I have found the following but I do not know if it is correct.

M=Log(2.51188)*(A2/A1)^2

The Attempt at a Solution



A1= aperture of human eye-7mm
A2= 10* 7 (10>human eye)

Log(2.51188)*(70/7)= 40 (to 2 s.f.)

I am finding the concept of stellar magnitudes difficult to grasp so any help would be appreciated.
 
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  • #2
garyd said:

Homework Statement



A telescope with aperture 10 times the human eye could see stars down to magnitude?

Homework Equations



I was not given an equation with the question but I have found the following but I do not know if it is correct.

M=Log(2.51188)*(A2/A1)^2

The Attempt at a Solution



A1= aperture of human eye-7mm
A2= 10* 7 (10>human eye)

Log(2.51188)*(70/7)= 40 (to 2 s.f.)

I am finding the concept of stellar magnitudes difficult to grasp so any help would be appreciated.

Yeah, the magnitude system is pretty stupid and annoying :biggrin:. You have the right concept for this question, just the wrong equation. Since you said you found magnitudes confusing, I'll try to explain:

What you actually measure with a detector (e.g. retina, photographic plate, CCD chip, or some other kind of sensor) is what astronomers call flux, F. The flux of a source is the amount of light energy arriving from that source per second, and per square centimetre of area at the observer. In other words, it's the power per unit area, measured in watts/metre2.

Magnitude is basically a logarithmic scale for flux, because human vision tends to work logarithmically. I.e. what appear to be "steps in brightness" by the same amount to the human eye are actually increases in flux by factors of ~10. The only tricky part is that the scale is reversed (smaller magnitudes correspond to larger fluxes). The equation for the difference in apparent magnitude in terms of the flux ratio is

m1 - m2 = -2.5log10(F1/F2)

As you can see, you don't need to know the individual fluxes in order to find the magnitude difference. All you need to know is their ratio. So what you have to figure out is the ratio of the flux seen by the eye to the flux seen by the telescope. As you correctly stated, the flux scales with the square of the aperture diameter. This is because the flux increases linearly with the collecting area of the aperture, and area depends on the square of the diameter. This makes sense. Flux multiplied by area gives you the total power detected. Increase the number of square metres, and you increase the power. You can get the flux ratio from the ratio of the squares of the aperture diameters, just as you were doing. Note: it would probably be better to use the letter D for aperture diameter, since A usually means area in this context.

EDIT: Another need to know (just assumed to be common knowledge) is that humans can see down to a visual magnitude of around 6, under ideal conditions (clear, dark skies and no light pollution). If the magnitude is any larger, the star is too faint to see.
 
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  • #3
Thank you for your help, I have been finding this topic to be very counterintuitive (at least for me anyway). Ok, let's see if I have made any progress!

Ratio of the square of aperture diameters= (70/7)^2= 100

So difference in magnitude=-2.5Log(base10)(100)=-5

Human eye can see magnitude 6.

Would that mean that the answer to my question is- Magnitude 11, or am I still as confused as ever? Again, thanks for your help.
 
  • #4
garyd said:
Thank you for your help, I have been finding this topic to be very counterintuitive (at least for me anyway). Ok, let's see if I have made any progress!

Ratio of the square of aperture diameters= (70/7)^2= 100

So difference in magnitude=-2.5Log(base10)(100)=-5

Human eye can see magnitude 6.

Would that mean that the answer to my question is- Magnitude 11, or am I still as confused as ever? Again, thanks for your help.

Yeah, exactly. The equation tells you that the object will always look 5 visual magnitudes brighter when seen through a telescope, so the object with the faintest flux you can see is going to be an object that is 5 visual magnitudes fainter than the faintest one you can see with the naked eye. Hence you go from 6 to 11.
 
  • #5
I had the ratio backwards for *this particular application*, and I sort of "explained away" a negative sign in the previous post. Let's get the math to match what I said in the previous post. The flux ratio you're considering should be 1/100. The proper way to do this is like so:

Let F2 be the faintest flux you can see with the naked eye. It corresponds to an apparent visual magnitude of m2 = 6.

Since the telescope has 100 times the collecting area as your eye, a star with a flux of F2/100 will produce the same total power as the star with flux F2 did in your eye. Therefore, the faintest source for the telescope has flux F1 = F2/100, or F1/F2 = 1/100.

So we have:

m1 - m2 = -2.5log(0.01) = -2.5*(-2) = 5

m1 - m2 = 5

m1 = 5 + m2 = 5 + 6 = 11.
 
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FAQ: Question on aperture and magnitude.

What is aperture and why is it important in photography?

Aperture refers to the opening in a camera's lens that controls the amount of light that enters the camera. It is important in photography because it directly affects the exposure of the image, as well as the depth of field.

How is aperture measured?

Aperture is typically measured in f-stops, which represent the ratio of the lens' focal length to the diameter of the aperture. The smaller the f-stop number, the larger the aperture and vice versa.

How does aperture affect the sharpness of an image?

The aperture size directly affects the depth of field, or the area of the image that appears in focus. A larger aperture (smaller f-stop number) will result in a shallower depth of field, while a smaller aperture (larger f-stop number) will result in a deeper depth of field.

Can aperture be adjusted manually on a camera?

Yes, many cameras allow for manual adjustment of aperture through the use of aperture priority or manual mode. This gives the photographer more control over the exposure and depth of field of their images.

Is there a specific aperture that is best for all types of photography?

No, the best aperture to use depends on the desired effect and the type of photography being done. For example, a smaller aperture may be preferable for landscape photography to achieve a deeper depth of field, while a larger aperture may be better for portrait photography to create a shallow depth of field and blur the background.

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