Understanding Magnitude Systems in Extragalactic Imaging

In summary, the conversation discusses different magnitude systems, specifically the AB and Johnson systems. The speaker has been using values calibrated to the AB system in their experiment and has noticed smaller magnitudes than expected. They question whether their results are wrong or if they have been affected by the AB system. The response highlights that there is a small difference between the two systems and that the filters being used may not perfectly match either system. Additionally, the use of photographic emulsions in determining magnitudes may not be accurate for CCD imaging. The conversation concludes by advising to be cautious of highly accurate magnitude figures unless the calibration process is thoroughly described.
  • #1
ian2012
80
0
I didn't realize there are different magnitude systems.

I have been doing an experiment on CCD extragalactic imaging and apparently I have been using values of magnitudes calibrated to the AB magnitude system. I noticed that my magnitudes were oddly smaller (numerically) that expected. Are my results wrong or have I been fooled by the AB system?
 
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  • #2
The difference between AB and Johnson magnitudes is pretty small.
The filters you are using are unlikely to perfect match the response curve of either magnitude system
The magnitudes were based on photographic emulsions which don't match the response of your CCD
The atmopshere varies

Basically - be suspicous of highly accurate magnitude figures unless the experimenter describes the calibration in detail
 

1. What is the purpose of understanding magnitude systems in extragalactic imaging?

The purpose of understanding magnitude systems in extragalactic imaging is to accurately measure and compare the brightness of celestial objects in the distant universe. This is important for studying the properties and evolution of galaxies, as well as for determining the distances and positions of objects in space.

2. What are the different magnitude systems used in extragalactic imaging?

The two most commonly used magnitude systems in extragalactic imaging are the apparent magnitude system and the absolute magnitude system. The apparent magnitude system measures the brightness of an object as it appears from Earth, while the absolute magnitude system measures the intrinsic brightness of an object without the effects of distance.

3. How are magnitudes calculated in extragalactic imaging?

Magnitudes in extragalactic imaging are calculated using a logarithmic scale, where a difference of 5 magnitudes represents a difference in brightness of 100 times. The formula used is: m = -2.5log10(Fobj/Fref), where m is the magnitude, Fobj is the flux of the object, and Fref is the flux of a reference object.

4. What are the limitations of using magnitude systems in extragalactic imaging?

One limitation is that different filters and instruments may yield slightly different magnitudes for the same object. Additionally, magnitudes do not take into account the distribution of light within an object, so two objects with the same magnitude may have different appearances. Finally, magnitudes do not provide information about the size or physical properties of an object.

5. How do we convert between different magnitude systems in extragalactic imaging?

To convert between different magnitude systems, we can use conversion equations or tables that take into account the differences in flux measurements for different filters and instruments. It is important to note that these conversions are only approximate and may vary depending on the specific characteristics of the objects being studied.

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