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Is this the 600mm focal length, f7.5? If so the 28mm will give a bit over 21x. A 2x Barlow or Powermate will give 42x. A 4x Powermate will give 85x. (4x Powermate is 2 to match eyepiece). This is a very expensive option to give only 85x.
You want about 120x min so 600/120=5. You need 5mm effective focal length. Either a 5mm eyepiece or 10mm with 2x Barlow. You dont need a 2 Barlow for 10mm as the eyepiece will only be 1.25. 9-10mm Plossls are cheap as are 2x 1.25 Barlow. Anything from GSO (or BINTEL rebranded) will give good results.
If you need to wear glasses when observing then you need longer eye relief. There are other options in that case but for the cost of a Powermate you can get at least a couple of good eyepieces.
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