What is the highest power eyepiece i should get for my 102mm refractor telescope?

[JohnJ79]

I have a Celestron 102mm Refractor telescope (C102-HD). It came with a 25mm eyepiece. I want to add 1 or 2 higher power eyepieces to it. I am wondering what is the highest power eyepiece i could add to it and still get quality views through it?

[Gnomon]

The highest practical magnification you can get from a 102mm objective lens is twice that, 204 magnifications. Let's round that to 200 magnifications.

The eyepiece to achieve this depends on the focal length of your telescope, which you haven't told us. Checking on the web, it appears to be 1000mm.

Magnification = Objective focal length / Eyepiece focal length.

So to achieve maximum magnification, you need an eyepiece with a focal length of 5 mm. You'll find, though, that the image is disappointingly dim at this mag, so I would recommend that you first get one with half that magnification (focal length = 10mm). This will give you high magnification but a good bright image.

[Vinnie]

I believe that the C102HD is an f10 achro. The general "rule" of approximation of max usable mag is indeed around 200x for any 102mm aperture, but there are varying factors that come into play. One of the most important limiting factors is the actual viewing conditions that you have at any given time at your location. Another is the optics of the scope and the EP that you choose.

In relation to the latter, several things will happen as you push up to 200x or more on this scope. Your FOV will start to narrow very significantly, CA (chromatic aberration) will increase especially on planets, and your eye relief will drop down to almost nil. This is assuming that you are looking at, say, a basic 5mm Plossl EP to yield 200x.

There are a couple of options. One is to move to a corrected Erfle type deign EP that delivers good eye relief and a relatively wide field. A Baader Hyperion 5mm would be a good example.

Given that you only have the original 25mm EP which will be a plossl, a better budget option at this time would be a 10mm plossl and a 2x barlow. This will give you the equivalent of an extra 12mm and 5mm EP, without significant loss of eye relief at 200x with the 10mm barlowed, and a little less CA. Later if this works for you, you may consider moving to more expensive EP's and adding a "minus V" or "fringe killer" type filter

HTH

Vin