# How do I obtain the brightness of a star in the night sky?

I am planning to conduct a research which involves observing the brightness of a star. So far, I haven't been able to obtain much information as to how I may go forward with the observation process. I'm willing to buy a telescope and other equipment. What telescopes are ideal for this and what other equipment must I require to obtain the apparent brightness value of the star?

• Just one star? What do you hope to learn from this? Apr 14 '18 at 10:14
• @JamesK I plan to model its brightness over a period of time. Apr 14 '18 at 12:52
• Which star? What do you hope to learn from this? Apr 14 '18 at 13:03
• photo.stackexchange.com/questions/92387/… is my question about something a little different, but may be helpful. Remember that stars can have different brightness at different spectral frequencies (eg, visual magnitude vs photographic magnitude).
– user21
Apr 15 '18 at 14:05

I don't know about the telescope, but I know that you will need the telescope that has CCD technology to be able to convert from counts/second (as brightness) that CCD detects to quantitative values easily.

For the observational procedure, here is the standard regardless of what kind telescope you use:

1. Take a picture of your object (let's name it ImObj).
2. Take a picture of a standard star (let's name it ImStd) or stars (i.e., stars that have real brightness in a database, let's call it MagStdReal).
3. Take pictures for calibration (let's name it ImCal). If CCD telescope, these images include dark and flat images.
4. Use ImCal to calibrate both ImObj and ImStd (i.e., removing noise and etc.).
5. Convert image brightness of calibrated ImStd and ImObj to quantitative values (let's call it MagStdFake and MagObjFake). Note that you can use any arbitrary zero reference point for MagStdFake and MagObjFake, but they use the same reference.
6. Find conversion equation to MagStdReal = f(MagStdFake). The simplest form of f() is just linear shift: MagStdReal = a + MagStdFake.
7. Apply MagObjReal = f(MagObjFake). And now you get to do some science.

Also note that, a telescope filters certain wavelengths with certain response function. So, you have to observe everything in one setting of telescope. You cannot mix images or products of different setting of a telescope. Only MagObjReal or MagStdReal those are telescope insensitive (ideally).

I provide very short and conceptual procedure to give you the idea. There are a bunch of details that are important. If you use keywords in the list procedure here and search in Google, you should get more relevant hits to each topic.

If the target star varies by half a magnitude or more, it's possible to estimate its magnitude visually by comparing it to nearby reference stars of similar magnitude. Sky at Night, Sky and Telescope, and the Royal Astronomical Society of Canada have articles explaining the procedure.

The American Association of Variable Star Observers (AAVSO) provides a chart plotting tool and comprehensive observing manuals for visual, CCD, DSLR, and other methods.

You probably want to find how bright the star is when calibrated against stars of known brightness. You take a picture of the region around the star, then compare the brightness of the star with the brightness of stars of known brightness in the same field.

A standard telescope with camera set-up. Then either a good catalogue or a system such as photometry pipeline to compare the brightness of the star to others of known brightness.