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Others have steered you down the right path.
5mm gives you 180X which depending on your prevailing local seeing conditions you may not get use on a fair number of nights. I had very good local seeing on the Central Coast and could have used 180X at least 9 nights out of 10. But that's pretty rare for most topographical conditions.
A 7mm eyepiece gives you 128X which to be honest I don't think is quite enough.
That leaves you IMO trying to find something decent at the 6mm focal length. Unfortunately pentax don't make a 6mm XW but a 6mm Televue Delos would easily be your best option. You could also go for a 6mm Nagler T6 or hunt around for a 2nd hand 6mm TV Radian. Other options are 6mm Clave if you can find one or 6mm UO ortho or 6mm UO HD ortho or the Baader 6mm genuine ortho. There are also a number of high quality 6mm japanese plossls you could raise 2nd hand. There are quite a few options 2nd hand where you will get an excellent eyepiece for not a lot of money. The new options are great eyepieces but not cheap. The cheap new 6mm eyepieces IMO aren't as good as some of the older stuff in the lower price brackets.
Cheers,
John B
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