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

Question

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?

Answer 1

If this is an achromatic refractor, you won't want to use an eyepiece that yields more than about 240X. As a general rule, 60X per inch of aperture is the max you can get assuming the optics are well made and collimated. You'll find that there aren't that many nights where you can get a steady and sharp view at that magnification, so a more practical high power eyepiece would be one that yields 150 to 180X. That is enough to see fine details on Jupiter and Saturn, and when it's near opposition, Mars. To find your magnification with any given eyepiece, divide the focal length of the objective lens by the focal length of the eyepiece. If I'm not mistaken, the telescope you own has a focal length of 1,000mm. Therefore a 5mm eyepiece would give you 200X, a 7mm eyepiece 143X. One good line of eyepieces for you to consider are the Burgess/TMB super planetary eyepieces, which are very good for their price. I took delivery of one recently and was very impressed with the lack of false color, flat field, light transmission and sharpness. The generous eye relief and 60 degree apparent field of view make observing with glasses on very enjoyable, so if you wear glasses these are well worth buying. They come in focal lengths from 9 to 2.5mm.

Answer 2

i agree with geoff g. this is a good scope. the usual rule of thumb applies, 2x per mm of aperture.

if at all possible, please try borrowing some higher power eyepieces for a test drive. good eyepieces cost $$$, but are a sound investment.

Answer 3

I'd get a 15mm eyepiece instead of the 5mm the 5 might be a bit too powerful for the amount of aperture you have.  A 25mm 15mm 10mm and a barlow would give you a nice range of power options

Answer 4

I owned a very similar telescope for a while, and found that the maximum magnification I could use was 250x (4 mm), though 167x (6 mm) and 200x (5 mm) worked better most of the time. I'd recommend getting something around 8 mm (125x) plus a good quality Barlow lens, which will double both your eyepieces' magnification. This is a really nice telescope, and I'm sorry I sold mine.

Answer 5

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.

Answer 6

Hi, I have a 102mm refractor, but the focal length is only 820 compared to your 1000mm which gives you a higher magnification than mine. Having said that I have had mine up to 290x with a 2.8mm EP many times, but usually use the 3.6mm EP which gives me 225x. It is a Takahashi FS-102 so I can push it further if I need to (which I don't)

Bearing in mind the increase in focal length these would give you 360x or 275x respectively. However the Celestron scopes are generally good optically, (having had one myself) so without knowing your seeing conditions I would suggest a 5mm lens, but would not suggest using a barlow on it as that would be too much magnification for the scope to handle. 5mm will give you 200x which is about the best you can reasonably expect in average conditions.

In fact I would not recommend a barlow tube on a 10mm, but go for a 18mm which with a barlow will give 9mm, and for the high magnification buy a good 5mm eyepiece.

Hope this helps, I am a refractor fan myself.