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Celestron eyepiece quality


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I have a Celestron NexStar 8SE and have found that using the supplied 25mm eyepiece gives substantially better quality than Celestron X-Cel LX 5mm and 9mm eyepieces. I appreciate that lower magnification is likely to give a clearer view but the X-Cel give very unclear views. Is this to be expected?

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  • random changed the title to Celestron eyepiece quality
40 minutes ago, random said:

I have a Celestron NexStar 8SE and have found that using the supplied 25mm eyepiece gives substantially better quality than Celestron X-Cel LX 5mm and 9mm eyepieces. I appreciate that lower magnification is likely to give a clearer view but the X-Cel give very unclear views. Is this to be expected?

You are not comparing like with like.  I have all the above-mentioned items, and find that the supplied 25mm Plossl is an adequate eyepiece.  The X-Cel LX are mid-priced eyepieces which perform quite well, but the C8 at f10 is not going to challenge any of these eyepieces.  The 5mm will give a magnification of x400  which will be too much magnification for a clear view  under average seeing conditions.

 

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Rather, it is understood that the images are sharper at the lower powers.  This is true of all telescopes.  As you go up in power, the telescope, the eyepieces, and whatever else is within the light-path, have to "work" harder to produce a satisfactory image.  The telescope itself must also be collimated, as precisely as possible for the higher and highest powers.

An 8" Schmidt has a focal-length of 2032mm...

2032mm √∑ 5mm = 406x

That's a bit ambitious, yet understandably desirable.  I'm a big fan of ramping up the power, myself.

Generally, an 8mm is suggested, and for the highest power, given seeing and other variables.  If you're not seeing reasonable sharpness with the 9mm as well, then I'd check the telescope's collimation.

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Remember that with a 25mm EP and the focal length of you scope (2032mm) that you are using approx 81 x magnification, whereas with the 9mm and 5mm you are getting 225x and 406x magnification. The sky conditions need to be good to get good views with higher magnifications like this, especially with the 5mm EP. If you used them in a shorter focal length scope you would probably get better views with the reduced magnification, so whilst they are not the best EP’s going they should be better quality that the standard 25mm EP supplied with the SCT, even though views might not suggest this in your scope. 

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Of course a 25mm eyepiece is going to show a more pleasing image in an f/10 scope than a much shorter focal length eyepiece.  The exit pupil (25/10 = 2.5mm) is much larger leading to a bright, sharp image.  In fact, 2mm to 2.5mm is just about the optimal exit pupil for any scope/eyepiece combo.

As @Alan64 says above, 8mm is about the shortest usable focal length in an f/10 scope because the exit pupil is 8/10=0.8mm and because the power is getting so high (2032/8=254x).

To compare apples to apples, obtain 9mm and 5mm Plossl eyepieces such as Celestron Omni Plossls.  They come in 9mm and 6mm focal lengths.  Compare these Plossls to the Celestron X-Cel LX eyepieces and report back on which set gives the better images.  At f/10, I would predict that they're going to be similar in image quality, but the Celestron X-Cel LXs will have much longer eye relief and a wider field of view making for a more relaxing and pleasing viewing experience.

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10 hours ago, random said:

I have a Celestron NexStar 8SE and have found that using the supplied 25mm eyepiece gives substantially better quality than Celestron X-Cel LX 5mm and 9mm eyepieces. I appreciate that lower magnification is likely to give a clearer view but the X-Cel give very unclear views. Is this to be expected?

As the others have pointed out, it's more to do with the magnification than anything else. Assuming you are in the UK I haven't had a night of decent seeing all this year, even when it's been clear there has been too much atmospheric distortion going on to ramp up the magnification, usually because of the jet stream / strong wind conditions higher in the atmosphere. Easiest way I find is to just look straight up with the naked eye, if the stars above seem to be twinkling then high magnification isn't going to work very well. I can usually just about get away with a 2x barlow on 25 / 20mm eyepieces, but moving to my 8mm makes things a bit fuzzy so that may be worth a try. Hang onto the others though, because on a good night I'm sure the views will be amazing ūüĎć

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11 hours ago, random said:

I have a Celestron NexStar 8SE and have found that using the supplied 25mm eyepiece gives substantially better quality than Celestron X-Cel LX 5mm and 9mm eyepieces. I appreciate that lower magnification is likely to give a clearer view but the X-Cel give very unclear views. Is this to be expected?

I have the same scope and i have to say , that 25mm Celestron EP is really good ... its the first EP i use in EVERY session . I cant speak for the X-Cel range but i have invested in 3 of the Luminos range , the 31mm ( huge!) the 15mm and the 10mm .... these EPs are superb , but the 25mm has a permanent place in my collection . Just my opinion of course. 

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