# What is the apparent magnitude of the weakest stars we can see in one telescope on the ground with a diameter of 2.4m? [duplicate]

All I know is its main mirror diameter; 2.40 m

I also know that the eye, which its pupil is 5mm in diameter can see up to an apparent magnitude of 6.

How can I calculate the apparent magnitude that the telescope can see?

I have used the equation of

$$5⋅log(\frac{D_1}{D_0})$$ However, it gave the answer of 13.40, while the answer should be 19.40

• No, it's not. First of all, this is GROUND telescope, NOT a space one. Secondly, this doesn't have a transmission factor. Nov 30, 2018 at 13:12
• What do you know about the magnitude scale, and what have you tried so far? Nov 30, 2018 at 13:20
• the other question starts with the Hubble but the answer directs us to the general formula in wikipedia, also talking about the background contrast, so it definitely applies to the ground observations Nov 30, 2018 at 13:24
• It gives the wrong answer, the answer should be +19.4 mag while the one I got is 13.4. There's 6 magnitudes difference, that's why I asked Nov 30, 2018 at 13:33
• The question as it's currently written asks "How can I calculate the apparent magnitude that the telescope can see?", and the other question provides the answer, so I'm voting that this is a duplicate. However, if you reword your question to ask "I'm using this formula for my [details] telescope but I get +13.4 instead of +19.4 - where is my error?" it would be a different question and not a duplicate. And it would directly address the issue you're having :-) Dec 1, 2018 at 0:05