F900 is the focal length of your telescope. (achieves focus in 900mm) D=4.5 means you have a 4.5 inch mirror An H15mm is a Huygens 15mm eyepiece. Very old and, by now, poor design. magnification is fl of scope/ fl of eyepiece
900/15= 60x power. 4.5in mirror = (50x per inch aperture maximum magnification) 225x max power (on a perfect night)
To get replacement eyepieces first you must measure the diameter of your focal tube. Eyepieces come in 3 sizes .965in (on cheaper scopes) 1.25in (most popular) and 2in.( mostly higher end scopes) If you have a .965in focal tube, you are limited in your choices. Antares makes some .965 plossl eyepieces also surplus shed has some inexpensive eyepieces. Or, if you want to spend the money , get a .965 to 1.25 adapter. This way you can buy 1.25in eyepieces and they will serve you well when you upgrade to a better telescope.
1.25 eyepieces are available all over the internet.
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