Using the Celestron C90 telescope

  1. Hi, I have a few questions about telescopes;
    (But a quick disclaimer: I know almost nothing about telescopes or astronomy)
    My uncle lent me his Celestron C90 telescope (he said it would be a good thing to start with) and I tried it out just now. I saw the moon with it, and it was incredible; one of the most beautiful things I've ever seen. The weird thing was, when I tried to look at stars, I would see a pretty blurry ball of light (which is what I expected), but in the middle there was a black ball. Is that a problem with the telescope, or the eyepiece, or does that always happen?
    To learn to see more things, I got an app for my phone called Night Sky that I hope will help me know what I'm looking at. Is that enough or should I also get a book?
    Also, when should I move on to a better scope? (And what would that scope be?)
    What resources could I use to learn more about telescopes and astronomy?
    Is the scope I'm using now good to start with?

    PS. I'm planning to take a summer class in astronomy (as well as physics, of course :D) in a university here in Chile, so I hope to learn more there.
  2. jcsd
  3. jedishrfu

    Staff: Mentor

    this is a small scope good for beginners who want to look at the moon and the planets but to see fainter objects you'll need one with a bigger mirror or lens. I have an 8" Orion reflector (can see as faint as 14th magnitude) while bulky is good for viewing:

    A book is good for learning about the telescope selection, observational tricks and terminology. The apps like Distant Suns or Starmap Pro on iPad are great because they use the internal compass and orient to match the sky. They also provide a lot of detailed though sometime arcane info on stars, clusters and other objects...

    A book to consider is the DK Universe book by Martin Rees:

    and for a simpler book:

    and smpler still:

    The Orion constellation is a good one to start with the sword hanging from the belt has several Messier objects of interest.

    For a lighter scope I also use the Orion Starmax which is probably quite similar to your scope: scope

    On a perfect night with no moon or terrestrial lights you should be able to see 13th magnitude stars but the reality where I live in the suburbs maybe 10th magnitude at best.
  4. Chronos

    Chronos 9,771
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    Gold Member

    The 'black ball' in the center of your field of view is probably the central obstruction, which every telescope has - save for refractors. My guess is the scope was slightly out of focus, and tweaking the focus should solve that problem.
  5. davenn

    davenn 3,389
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    yup, that was my thought too.

    guitarphysics ... if you moved the scope straight from the moon to the stars, then you would have had to refocus the scope. If you didnt, then yes, they may well have been blurry balls of light :)

  6. This is bad advice and just simply wrong! The moon for focusing purposes is at infinite distance just like the stars (and anything more than a few hundred/thousand meters away).

    Now if he changed eyepieces without refocusing that could give the donut effect of unfocused stars.
  7. Bandersnatch

    Bandersnatch 1,200
    Gold Member

    Here's my twopence:

    Your C90 is a fine telescope, its compact dimensions shouldn't be mistaken for low performance.
    I wouldn't bother with upgrading until you gain thorough understanding of its nuts and bolts, and come to really feel that it is limiting you in your exploration of the night sky.

    A very useful "tool" that helps in orienting yourself in the sky, is to simply learn as many constellations as you can. I find them easier to memorise if I read the mythology behind their names.
    Later on you might want to learn about the equatorial coordinate system that most star charts use.

    Cornell University's Curious About Astronomy site ( is a great resource for learning about the subject. It contains links to extra resources and a sort of very informative FAQ.
    You should probably start by navigating to "stargazing" and "telescopes" sections.

    And a last bit of advice regarding the use of your telescope - always make sure you focus your eyepiece. You will need to do that every time you change it.
    All the details on the Moon should look crisp and clear, and the stars should always be points - you will never see the disc of a star, and if you do, it only means you're out of focus.
    As other said, if you're badly out of focus, you'll see the shadow of the secondary mirror in the centre of your telescope's aperture.
    Last edited: Apr 28, 2013
  8. Chronos

    Chronos 9,771
    Science Advisor
    Gold Member

    One possible explanation is the 'kidney bean effect' which can occur when using a wide field eyepiece. If the dark spot moves around when you move your head, that is a probable explanation. The solution is to pull your eye further away from the eyepiece. When your eye is too close to the eyepiece, part of the image can miss your pupil. Another possibility is you did not allow your eye to adjust from looking at the very bright moon to a relatively faint star field. If the size of your secondary obstruction image is close to the size of your pupil, it can result in a dark spot in the center of your field of view. You need to allow your eye to adjust to low light conditions for a few minutes before looking at faint objects. Many moon viewers use a filter to dim the image of the moon to a reasonable level for this reason.
  9. Thank you all very much for the help! I'm gonna check out a lot of those resources, they look really good. As for the "donut effect", I don't think it has much to do with what most of you described (except maybe Chronos, you might be right). The weird thing is, when I "unzoom" (sorry, don't know how else to say it), the ball in the middle of the star donut gets smaller; when I zoom, it gets bigger. When I adjust/zoom the scope, the hole will move along with it. I don't think the problem has anything to do with the scope being unfocused (although it might, I don't know). I'll try looking through the eyepiece from farther away next time, hopefully that will help.
    Also, I heard that today saturn would be very visible, so I tried finding it in the sky and looking at it through my scope, but I couldn't find it :(.
    Last edited: Apr 28, 2013
  10. davenn

    davenn 3,389
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    Did you actually find it in the sky without your scope ?
    Dont know where you are from so I cant tell you yet which way to look
    Have you been in astronomy long enough to recognise many of the constellations so that if I told you which constellation saturn is in you would find it ?

    Here's one site you can use for locating Saturn ....

    Last edited: Apr 29, 2013
  11. Bandersnatch

    Bandersnatch 1,200
    Gold Member

    Ah, but that is exactly what's to be expected.
    You see, the knobs by the eyepiece are not for "zooming". They are for adjusting the focus of the eyepiece. There is a point between the two extreme positions of the knobs when the stuff you're looking at is the least blurry - the stars are the most point-like. This is the properly focused position, and you should always keep it at that.
    Having it in any other position is like wearing somebody elses spectacles.

    The "ball in the middle" indeed ought to be moving together with your scope's field of view, as it is a part of the thing. It's the shadow of the round spot you can see in the centre of the telescope's aperture(where the light enters). That's the secondary mirror that reflects light from the main mirror back to the eypiece, and it will be visible if the latter is poorly focused. The more out of focus, the more of it you'll see.
    C90 is a Maksutov-Cassegrain type of telescope. You can find a lot about the workings of their internal optics on the net, e.g.:

    To "zoom" with your telescope, you have to exchange the whole eyepiece for one with a shorter focal length.

    I'm guessing your unle did give you a few extra ones with the telescope? If so, then examine them and look for any markings that say something like "32 mm", or whatever lenght it says. That's the focal length of the eyepiece's optics, and the shorter it is the higher magnification you can achieve witht he same telescope, albeit at the cost of losing image quality.
    If you don't have any extra eyepieces, then you're stuck with the one "zoom" setting you have, and no amount of knob turning can change that.
    However, these are easy to find in shops, and are the cheapest part of a telescope. That's the first thing you'll want to upgrade.
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