9mm vs 25mm eyepiece

[mborger]

Hi everyone. Admittedly a newbie questions here, but I'm still learning my way around my Celestron 127SLT and the planets lately have been more than I could have dreamed of. The other night not only did I see Venus in phase, Jupiter and the four Galilean moons, but I also saw Saturn for the first time and its rings -- absolutely amazing.

One question I have is that the scope came with both 9mm and 25mm eyepieces. The salesman said that I would have no real use for one of them (though I can't remember which). When would each be appropriate? I don't want to waste viewing time with the wrong eyepiece.

Thank you.

[KathyNS]

The 9mm is a bit over-powered for that scope. On a night of particularly good seeing, it might get you some good views, but it's not something you can count on every night. Your normal minimum focal length will be 12mm.

[gcisko]

Hi everyone. Admittedly a newbie questions here, but I'm still learning my way around my Celestron 127SLT and the planets lately have been more than I could have dreamed of. The other night not only did I see Venus in phase, Jupiter and the four Galilean moons, but I also saw Saturn for the first time and its rings -- absolutely amazing.

One question I have is that the scope came with both 9mm and 25mm eyepieces. The salesman said that I would have no real use for one of them (though I can't remember which). When would each be appropriate? I don't want to waste viewing time with the wrong eyepiece.

Thank you.

The 25mm is a definite keeper for wide field views. The 9mm may be a bit too much magnification as Keith says. The reality though it there are times where both are appropriate. Experience will teach you which.

[mborger]

Thanks - so basically, the mm rating is the opposite of a camera (higher mm number is a more magnified view).

To that end, wouldn't a higher-powered eyepiece, like the 9mm, make objects (ex. Saturn and its rings) appear closer than they do in my 25mm?

Perhaps I'm missing a key idea here.

[arsene37]

Hi !

To that end, wouldn't a higher-powered eyepiece, like the 9mm, make objects (ex. Saturn and its rings) appear closer than they do in my 25mm?

You are right : short eyepiece focal length gives greater magnification and smaller true field of view (with usual limits), and long one gives smaller magnification and greater true field of view.
Limitation are : bad definition if your EP has too short focal length, and if your EP has too long focal length, you will not use the extern part of your mirror (light rays will not go inside your eye).

I use a 12 inches dobson, and only in my yard, with no real steps, so I move it from house with homemade wheelbarrow and inflatable wheels ; I put it on pieces of wood to avoid waddles.
Have a good day (or night for you ? )
Arsène

[jerryTheC]

Magnification is scope focal length divided by eyepiece focal length, so a shorter focal length eyepiece gives you higher magnification but less brightness (you're spreading the same amount of light out more).

Planets are nice and bright, so high magnifications work well, but there's also a limit to how much magnification you can usefully use. As a ballpark guide, the everyday max magnification is around your scope's aperture in mm - and you get that with an eyepiece with focal length equal to your scopes f-ratio - so 12mm for an f12 scope.

Given perfect conditions, you can go to around twice that magnification (half the eyepiece focal length) which is what the manufacturers usually quote as the max magnification - but most of the time atmospheric turbulence ("seeing") tends to mush things up, hence the lower everyday guideline.

[TCampbell]

Your scope has a focal ratio of f/12. As a general guideline, you typically will not want to use any eyepiece whose focal length is shorter than the focal ratio of the telescope. In other words, since you have an f/12 scope, you won't want to use any eyepiece with a focal length shorter than 12mm (so the 9mm is out).

This rule isn't absolute... it's just a good guideline.

You can put any eyepiece into the telescope (as long as the barrel size fits in the eyepiece tube.) But the more you magnify, the less sharp the image will be (you are magnifying all of the flaws). The larger the diameter of the telescope, the more it can resolve finer detail. A scope's theoretical useful magnification is 2.5x the diameter of the scope when measured in millimeters or 50x the diameter if measured in inches. Your 127mm scope would have a limit of 254x. It turns out that the useful limit is really based on IDEAL viewing circumstances -- so ideal that they aren't very realistic. You need very steady skies usually only possible if you're on a small island in the middle of a large flat body of water with no warm fronts, cold fronts, or jet streams within 200 miles of you (I basically never get these "ideal" viewing conditions. If you do, it may just be a few nights per year. So using about half of that maximum useful limit tends to be the more practical guideline.

When you do the math... anytime the focal length of the eyepiece is EQUAL to the focal length of the telescope, then the magnification will be equal to 1x the diameter of the scope when measured in millimeters. And that lower end limit tends to be more useful. Every so often I get a night of below-average (even for my location) seeing conditions and I have to back off on the magnification or I won't be happy with the quality of what I can see in the scope.

If you get a night with PARTICULARLY good seeing conditions (really steady skies) then the detail can really pop for you and you'll see features that were never visible to you before. Don't pitch the 9mm in the waste bin... if you get one of those rare better-than-average nights then the 9mm will be useful to you. The lower theoretical limit for your scope is 6mm.

If you wanted an eyepiece with more magnification than your 25mm... but likely to be useful on most nights (unlike the 9mm which requires above-average seeing conditions) then I might suggest looking for something around the 15mm focal length. That's not right down to the 12mm limit so it gives you a little bit of a buffer.

The atmosphere itself works a bit like a lens. Objects near the horizon will be more distorted than objects up high near the zenith. But overall stability will distort views too. The analogy I like to use is to consider a coin in the bottom of a pool. If nobody is making any waves and the water is perfectly still, then you can probably get a clear view of the coin in the bottom of the pool. But now imaging that someone is making some waves in your pool. You'll have difficulty seeing the coin. You may be able to tell that the coin is there, but you'll probably never really get a good view of it. Atmospheric "seeing" conditions are a bit like this.

The scope can only do so much... but then you're at the mercy of the sky quality.

[mwillis321]

I'd also add that it's never really wasting time using any of your EPs on different objects. Although there are some limits and rough guidelines, EPs are highly personal to the specific person using them and the scope they're using them in. I try as many possible magnifications as I reasonably can (obviously if I can't find or bring an object into good focus at a lower magnification I won't try anything smaller) on every object I view from planets to DSOs. So in other words, you won't know what works best for you or what you like most about different EPs and magnifications if you don't spend/waste time using different combinations of them on objects you're viewing.

Although the 9mm is slightly over your normal maximum magnification, the manufacturers usually provide EPs that will work with the scope they've come with, and since you already own the 9mm EP I'd use it, especially on the planets, but a good bit of DSOs will take some magnification well too. Typically you'll want the 25mm EP for locating objects and viewing DSOs. Again try both on everything you view, along with any other combination of EPs you currently have.

Best of luck, and clear, dark skies.

[smeyer8015]

You might want to get a 3rd EP around 18mm. This would give you about 83x.
I highly recommend the Celestron X-Cel LX series. Just under $60. Very good EP for the price.
Then down the road a 30mm and 2x barlow would give you a nice range of magnification.

Scott

[mborger]

This is fantastic. I appreciate everyone of you for providing your opinions and insight. I've read every response and will bookmark this page because I think I'll need to refer to it repeatedly. For now, I'm going to experiment with both EPs and develop some comfort with both and then likely pick up a medium length as recommended a couple times. I'm in San Diego, so we normally have decent skies.

On a related note, is there a page on here with diagrams that explains the math of scopes in general? I'm a visual learner and I think that will dramatically shorten my learning curve.