I would not swap the 20mm for a 25. I would keep the 20 and add a 32. I will explain why.
I am going to outline a strategy for you. This approach applies to any telescope. The only change is whether you have a 1.25" or 2" diagonal/focuser.
Watch the Video - First, some background on selecting an eyepiece -
This is a very general video discussion of eyepieces and why there are a variety of designs
https://www.youtube.com/watch?v=m7u9Q5hV7yc
EYEPIECE STRATEGY SUMMARY
* One or two low power wide view eyepieces
* One or two midrange eyepieces
* Two to four high power eyepieces
* Or, Zoom plus barlow to cover mid range and high power
* Planning to use a barlow can save you money
You have a 130 mm / 1000 mm FL scope.
A general guideline is that you double the aperture in mm to get the maximum usable magnification you can expect under excellent conditions.
130 X 2 = 260X - not a limit or a guarantee, just a target.
FL scope / target power = FL eyepiece =
1000 / 260 = 3.84 mm. Call it 4 mm for convenience.
Assuming your scope has a 1.25" focuser, the best low power/widest view eyepiece would be a 32 mm Plossl. That will give you 31.25X and a 1.6 degree field of view, FOV. That is a good low power and wide view for this scope. It will help you when star hopping and when you want the widest view possible.
So we have established the low power, 32 mm, and the high power, 4 mm. Now we fill in between.
Focal length scope ( FLS) / Focal Length eyepiece ( FLE) = Magnification
31.25x = 32 mm Plossl - Add this one
50X = 20 mm ( you have)
100X = 10 mm ( you have)
143X = 7 mm
The above mags should work almost any night.
The below mags will be usable under good conditions. You may choose to barlow for these rather than buying more eyepieces.
180X = 5.5 mm
220X = 4.5 mm
250x = 4 mm ( top mag for this example)
Nothing rigid about these mag targets, I am just using them as an illustration of a spread of options. You might choose different focal lengths/ magnifications based on the line of eyepieces you like or the magnifications you want to hit. The point is that you want to work toward a spread of magnifications, as budget allows. This lets you use the right mag for any given target to get the best view.
Nothing to stop you from trying to go higher than 250X, and on certain targets, like the Moon, you may be able to go higher on exceptional nights. A lot depends on transparency and "seeing".
What is SEEING and why it can be bad. How it will limit how high you can go. This is not a problem with your telescope or your eyepiece.
http://www.skyandtelescope.com/astro...ng-the-seeing/
http://www.damianpeach.com/seeing1.htm
http://www.damianpeach.com/pickering.htm
The alternative method to multiple eyepieces is to use a zoom eyepiece. This is my primary approach. An 8-24 mm zoom will give you 41X to 125X and everything in between.
You could put the zoom in a 2X barlow for 82X to 250X. Going over 250x for that scope will be a very rare night, BUT it might be fun to try it on the moon on an exceptional night using a 3X barlow.
The 32 mm Plossl +zoom + barlow is typically the lowest cost approach to covering the entire range of your scope. It has become my primary approach for all of my scopes.
* I never expected the zoom eyepiece to become my primary eyepiece, but it has.
* With a zoom, the eyepiece seems to disappear as you just move in and out at will, no swapping, no thinking about eyepiece changes
* Watching double stars split as I rotate the barrel is wonderful
* One filter serves over a wide range of magnifications, no screwing and unscrewing to try other eyepieces
* Moving smoothly from and between small changes in magnification helps when seeing is not the best
* I am always working at the optimum magnification for this target.
* Sharing the view with others is easier, especially in my manual tracking Dob - I hand it over at low mag so it stays in the view longer. They zoom back in to whatever magnification works best for them.
* My eyepiece case has been greatly simplified
* Kids love the zoom
So, eyepieces are a tool that you will want to add over time. Like having a spread of wrenches/spanners in your toolbox. Above is just a strategy for where and how to add them.
If you were in the USA I would provide a list of sources and eyepiece lines but I am not certain what is available locally.
I hope you find this helpful.
Ed