I just bought my first telescope - a Celestron CDC 1100. It came with a 40mm lens so with a primary focal length of 2800mm the power should be 70x.

My problem is that I've looked at Sirius and Mars and they look clearer to my naked eye than with my telescope.

I am able to focus from way out so I can see the 'blind spot' of the secondary mirror, go through 'focused' and then continue moving the mirror in the same direction until I again see the blind spot. So I know I'm focusing through when it should be 'good'. But, like I said, what I see is a relatively unfocused spot of light that is clearer and steadier with my naked eye.

Admittedly, my 'seeing conditions' aren't very good since I am looking through my window in a suburban neighborhood where light pollution is pretty bad. That being said, I still see Mars and Sirius quite brightly - it's just that they never come into clear focus - nor are they any larger than when I look using my eyes. And when I move my eye around the eyepiece I see streaks of light in different directions originating from the target.

There are many stars to be seen, though they are just pinpricks as I would expect given the seeing conditions. What I mean to say is that I cannot see all those stars with my naked eye but can see tens or hundreds more looking through the scope so I know it is gathering plenty of light as an 11" scope should.

I tried both with and without the diagonal with no discernable difference.

I have not collimated but supposedly it was collimated before leaving the factory and I would hope that even if it were off a bit from shipping I would still be able to see a sizable though fuzzy image of Mars instead of just a small speck of orangish light.

Any thoughts as to what I may have done wrong or what may be causing this lack of clarity?

  • $\begingroup$ Did you try it outside, without a window in-between? The window might cause light refraction. $\endgroup$
    – Gerald
    Commented Mar 2, 2014 at 13:10
  • $\begingroup$ Are you asking 'Why is Mars fuzzy?' (you are shooting through a window) or is it 'Why isn't Mars bigger in my field of view?' $\endgroup$
    – Jeremy
    Commented Mar 6, 2014 at 1:50
  • $\begingroup$ @Jeremy, both. It was very small and very fuzzy - even when I thought it should have been in focus. I will add another comment to your answer below. $\endgroup$ Commented Mar 6, 2014 at 3:07
  • $\begingroup$ Tracy, Where in So. Cal are you? there are lots of good astronomy clubs throughout the southland. I live in OC, and we have one of the best clubs around (The Orange County Astronomers). With around 800 members and a very active outreach and Astroimagers group, not to mention, one of the best dark sky sites of any club (and I've visted a lot). Our site might not be the darkest, but it's pretty good for light polluted So. Cal. as we have a good set of Amenities including a permanently mounted 22" Cassegrain in the clubs observatory, member observatories and Pads, with Electric and Wi-fi througho $\endgroup$
    – CraigBob
    Commented Mar 7, 2014 at 0:46
  • $\begingroup$ I don't see how to PM you. I am in Irvine. I looked at the OC club web page but didn't go any further than that. It looks like a great club. $\endgroup$ Commented Mar 7, 2014 at 0:55

5 Answers 5


You're experiencing a few issues all of which have one solution. Take the Scope outside. Let it acclimate to the ambient air tempurature, and then try looking.

The 1st issue is the mirror needs to be the same temp as the air you're observing in. Since you are indoors you have a very warm optical path this causes distortion and trouble focusing.

2: Because you're looking through a piece of window glass (or more if it's a multipane window) you're introducing distortions that way. It's no wonder you can't reach focus.

Light pollution won't really affect Mars or the other planets too much, so that's not your worry.

Why are you observing from inside anyway?

  • $\begingroup$ Inside? :) Well, honestly, I didn't want to get it dirty just yet. It's my first telescope and I think it's the first one I've ever looked through though I may be forgetting from my childhood. I grew up reading Astronomy magazine more than 30 years ago and always wanted one. Now that I'm middle aged I decided to indulge my earlier interest. So I wanted to keep it pristine for as long as I could. However, I will take it out tonight and see what Jupiter looks like - if it's clear. Continued in the next comment... $\endgroup$ Commented Mar 5, 2014 at 20:37
  • $\begingroup$ Though both of you have indicated the same course of action I do not understand why this would be causing my problem. I'll address this in two ways, my biggest problem is not focusing - it's not being able to see more than a small pinprick of light coming from anything other than the moon. No matter how I focus the pinpoint of light doesn't show me mars. I looked around the web to see if I could find the proper magnification for planetary observation but didn't find much guidance. I assumed the stock eyepiece would allow viewing at least Mars. However, I ordered a lens kit today-continued $\endgroup$ Commented Mar 5, 2014 at 20:42
  • $\begingroup$ The new lens kit will provide much more magnification than the 70x I noted in the first paragraph. The second part of my confusion is the temperature differential (I understand the part about the window potentially causing focusing problems.) Inside, the telescope is the same temperature as the air around it so there is no uneven temperature gradients on the mirrors that could cause focusing issues. Having air outside different from inside seems no different to me than the air outside on the ground being 100 degrees different than the air in the upper atmosphere. Continued... $\endgroup$ Commented Mar 5, 2014 at 20:46
  • $\begingroup$ I know there is an index of refraction between different types of materials such as water and air (and air and windows) which causes the light to bend but I have not heard of such an index between a temperature gradient such as inside and outside my house. Is this temperature gradient really a big problem or is it just one more factor in the 'seeing conditions'? $\endgroup$ Commented Mar 5, 2014 at 20:52

Light pollution doesn't disturb the focussing, just the contrast. But non-uniform refraction of the window can.

So try to open the window, if you can, or take a try outside.

  • 2
    $\begingroup$ just opening the window won't do all that much, as you're causing a mix of warm and cold air in front of the telescope, but atmospheric blooming at its worst. $\endgroup$
    – jwenting
    Commented Mar 3, 2014 at 15:11
  • 1
    $\begingroup$ The windows aren't perfectly plane, therefore cause distortions. That's at least my experience with observing sunspots with a solar projector. If temperature differences cause problems, the only other thing I can recommend is trying the telescope under free sky. $\endgroup$
    – Gerald
    Commented Mar 3, 2014 at 20:19
  • $\begingroup$ I will be taking it outside tonight and look at the moon and Jupiter. However, I still don't have a good feeling about being able to see Jupiter any more clearly than I can inside. I hope I'm wrong but am a bit pessimistic. I also ordered a lens kit to allow more magnification. I'm hoping this will provide better views of the skies. $\endgroup$ Commented Mar 5, 2014 at 20:57

If you are asking 'Why is Mars so small in my Field of View, I thought it would be larger' then you need to consider how big you can expect to see it for various eyepieces.

Right now (2014 03 06) Mars is approx 12.6 arcsec in apparent size. It can be as big as 25 arcsec, and as small as 3 arcsec. (Why the variation? We orbit the sun at a different rate to Mars.)

What is your field of view? Well, for a 2800mm focal length telescope and if your 40mm focal length eyepiece is a Celestron Plossl, then your true field of view will be about 3/4 of a degree (0.74 deg). That is 2664 arcsecs.

Mars will therefore cover 0.0047 of the field of view. That isn't big. 0.5% of the diameter of what you see in the eyepiece. So, it will be barely more than a pin-prick.

If you had a Celestron 7.5mm Plossl, for a magnification of 373x, then the true field of view for that combination with your telescope would be 0.14 degrees, or 504 arcsec. Then Mars will cover 0.025 of the field of view, or 2.5% of the diameter of the view. So if you mentally divide the field into 10 equally sized stripes or rings, Mars will cover about a quarter of one of those imaginary divisions. Still not big, but a lot bigger than what you are currently seeing.

You could try going bigger using a shorter focal length eyepiece, but 400x is about the usable limit. Sorry, you aren't going to get an eye-piece filling view of Mars, not unless you're riding on the Mars Global Surveyor.

To use the equipment to get a picture of Mars that will impress your friends, you might try getting a camera with a small chip with a nice fine pixel size to record video frames. These you then 'stack' using software to remove the noise that comes from various sources including atmospheric refraction variations, and you end up with an image that is often quite impressive, and orders of magnitude better than what you can perceive with the naked eye.

  • $\begingroup$ Thank you Jeremy for the math. I appreciate the information and will be sure to use it in the future when needed. $\endgroup$ Commented Mar 6, 2014 at 3:15

Regarding the fact of using the scope indoors: it's understandable, but silly. Unless you live inside a clean room (like those ones in labs), your scope will get dirty anyway, because an average person's room is not free of dust and particles. In fact, it might be even worse, because you won't move the scope frequently and the dust is definitely going to build up. Besides, cleaning a scope is not that difficult, as long as you have the patience to put everything back together later, and then doing the re-collimation. Also, the dust grains on the mirrors are not big enough to depreciate the image, so you might as well just not bother with them. I can guarantee to you that the only dust that cause Astronomers trouble is the dust up there, in space, and rarely this one down here.

Other things to check: collimation and quality of the optics. Things can get bumpy during transport, so it is possible that the mirrors got misaligned or maybe you got damaged optics (or just poor quality ones). A collimation problem can be easily identified: you'll get comet-like distortions (tails) in stars and planets. Poor quality optics is problematic, because it is generally the item that buyers do not check, so the manufacturer may shove the cheapest optics elements in the package to get a lower price tag. So, if that's the problem, you might want a replacement, or new (better) elements.


Much like Jeremy has answered, Mars will appear just larger than a point, you will be able to discern a disk, however with minimal detail. For an overview of the disk you will see, I two versions of a virtual telescope in Wolfram Mathemtica; one with an orange disk and another using models of the planets. Anyway, using the orange disk variant, I input the CPC1100 details and the resulting view will match my output.

The field of view using a 40mm eyepiece without a barlow results in a 0.74degree apparent field of view, this equates to 2664 arcseconds. For reference Mars right now has an apparent diameter of 4.5 arcseconds, in your field of view that is .0017% of the apparent field of view (as represented by the orange disk).

eg. 40mm eyepiece @70x magnification w/no barlowenter image description here

You telescope has a maximum useful magnification of 560x (2x telescope aperture) however it is airy disk limited to 481.2x. This means that a star will only appear 'point like' up to 481.2x magnification, above that the disk becomes increasingly blurrier to a point where you can no longer discern a point it appears as a cloud.

As a point of reference to the size of Mars using different equipment;

40mm eyepiece @210x magnification w/3x barlowenter image description here

10mm eyepiece @280x magnification w/no barlowenter image description here

17mm eyepiece @494x magnification w/3x barlowenter image description here

On another note apart from what you 'should see', is what you can see. Aperture plays the ONLY role in telescope design which defines your clarity. The primary mirror diameter is a light bucket, a larger mirror is a higher image resolution (more discernible features/clearer image). Magnification also plays a role in clarity, each time you increase the magnification by 2x, you reduce the brightness by 4x; increase by 3x, you reduce brightness by 9x. So brightness is directly proportional to magnification. The highest possible brightness you can achieve, is your lowest possible magnification. How aperture works by collecting more light; allows a brighter image for a larger telescope than a smaller one at the same magnification, which is why so many people get 'aperture fever' and think big is best (for clarity yes). Your focal length is the determining factor for image size, meaning a 20inch telescope won't see an object larger than a 5inch at the same focal length.

You are correct in saying that both Mars and Sirius appear brighter with the naked eye than your lowest magnification; referring above, if your pupil diameter is 7mm, then the object appears at a factor of hundreds of times dimmer through your telescope.

I can provide material to calculate your own virtual telescope, it's parameters and viewing expectations if you are an avid user of Mathematica. Hope this has cleared up some information for you.

  • $\begingroup$ I haven't used Mathematica since my college days nearly 20 years ago and am impressed by the images you produced with it. Well done. The issue was resolved by simply taking it outside. My comments to that point were deleted for some reason. I was able to see one of the polar caps, the rings of Saturn and the bands of Jupiter. I am hopeful that I will be able to see the red spot at some point soon. $\endgroup$ Commented Jan 26, 2015 at 17:38

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