# Thread: Calculating Magnification of Telescope from Eyepiece and Focal Length

1. ## Calculating Magnification of Telescope from Eyepiece and Focal Length

Here I will describe how to calculate the magnification of of your telescope from the eyepiece and and telescope focal Length.

To calculate:

Magnification = Focal length of the telescope / Focal length of the eyepiece

so for example:

Magnification = 900 mm / 9 mm

Magnification = 100X

The telescope with an eyepiece of 9mm would magnify the object (eg moon) you are viewing 100X.

Download an EXCEL file that will automatically calculate every possible eyepiece for your telescope and the resulting magnifications:

http://www.astronomyforum.net/downlo...?do=file&id=18

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3. Hi
This is my first go at this forum
I have a 8in Dob (skywatcher) wich I am told is good for 400x
under good conditions.
Do you think I should get a 3mm eye peice
And where would I get one in Australia

4. hey 7715

that would be 333x and you would have to be under extrem good conditions to see really good.

on a good nite i up it too 300x on jupiter but like i said u need exteme conditions.

but for a good eyeview a 5mm would be ok,, 1000/5 =200x
i have a 12.5 in a barlow and i push it to 240x .
but with good eps around 150x to 180x you get great views thats for sure..

5. ## The Following User Says Thank You to ozzmozizz For This Useful Post:

7715gra (07-05-2009)

6. Originally Posted by 7715gra
Hi
This is my first go at this forum
I have a 8in Dob (skywatcher) wich I am told is good for 400x
under good conditions.
Do you think I should get a 3mm eye peice
And where would I get one in Australia
Hi 7715 ...Welcome aboard the forums ... I have a 8in. sct with a focal length of 2000mm and use my 5.5 on the moon only ...It will give me 363 power , but conditions have to be great ...Normaly i use a focal reducer for a shorter focal length and a wider field of view ...By the way , meade says my scope has a theoretical power of 600 but i havent been able to pull it off ..

7. ## The Following User Says Thank You to roverich For This Useful Post:

7715gra (07-05-2009)

8. I sometime use the focal lenght of the objective lens or mirror divided by the eyepiece focal lenght in "inches". A 25mm eyepiece is "1" inch. 12.5 is roughly a "1/2 inch. I find I can do this in my head sometimes easier then millimeters. Certainly mm are accurate and I'm in no disagreement.

9. if i built a telescope with a focal length of 6 m, what focal length should the eyepeice have to give a magnification of 950 times? (hypothetically)

10. Welcome aubrie

You would need about a 6.5 mm eyepiece.
- A 6 meter focal length? Wow.

11. ## The Following User Says Thank You to GriffinBF For This Useful Post:

12. Hi Aubrie ...Welcome aboard the forums ...

13. I want to help
If you build a telescope with a focal length of 1.3 m, what focal length should the eyepiece have to give a magnification of 100 times?

14. A 13mm eyepiece.

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