Telescope primary mirror.?

Question

Hello,
Having some trouble getting sharp images and have been messing with the collimation for a while with no real change. It's a Meade DS-2130At long tube.

While looking down the focuser tube, the reflection of objects in the primary are always blurry. If I put my hand or any object within about 2' of the tube opening, the view is much sharper. Anything beyond about 2' is really blurry. The reflection of my eye in the secondary looks crisp always. I'm seeing this while adjusting in doors.

My question is: Is this normal? If it is normal, could you explain why?

To summarize: The doughnut is blurry but the hole is sharp. Is this normal?

Thanks for any information

Answer 1

Yes, that is normal, because at twice the focal length you are at the center of curvature, and an image of something at the center of curvature will disappear when seen from the center of curvature. If you get well beyond that point, the image will become clear again, only upside down.

As to why your telescope images are not sharp, it could be because you are using too much magnification. A lot of beginners go straight to the highest magnification and that is a mistake. Start with the lowest power eyepiece and work up to higher magnifications. Usually any magnification above 2 per millimeter (or 50 per inch) of objective diameter will never be clear, and usually the limit is half that due to natural blurring caused by the atmosphere.

Answer 2

A telescope is an instrument designed to be used at infinity focus. The focuser allows adjustment only within a certain small range of infinity to compensate for differnt eyepieces and for near or farsightedness of the observer.
The only true test of a scope's performance is a star Test. All the other fancy lab tests can tell you what is causing a problem. But it is the star test that tells you how good the optics are.
Point the Telescope at any Star. A medium bright one would be fine. Put in a medium power eyepiece and focus as best you can.
When focused the image of the star should be only a small pinpoint of light. Now turn the focuser past the point of focus and the star's image will get bigger until it looks just like a doughnut. (the hole in the middle comes from your secondary mirror). This doughnut should be nice and round and symmetrical. Now turn the focuser the other way past focus and you should again see a nice doughnut. It should look identical to the first doughnut.
If that's the case, your optics are fine. Blurrines during this procedure most likely indicates a need for better collimation. But if you line things up during your collimation procedures, chances are that your images will be just fine.
The problem with checking optics at distances closer than infinity focus is that the distances at which you test things are critical and this can easily get confusing as your question indicates.
Give the star test a try.

Adolph

Answer 3

Collimating a telescope can be a tricky task.

In reflectors there are two points of collimation. The secondary mirror and the primary mirror.

Since you've fiddled with the collimation already, you're going to have to go back and redo it.

I highly recommend you read this website and follow it to collimate your telescope properly.
http://www.atmsite.org/contrib/Carlin/collimation/

You can skip the part on squaring the focuser though.

The website will show you how to collimate the secondary mirror and then the primary mirror.

To collimate the primary mirror properly, fine ajustments to it need to be made after the telescope has "cooled". This isn't neccissary to do every time you use the telescope but it helps to do it once.

After you collimare your telescope you should star test it. To do this, you point the telescope at a bright star, such as Vega, and starting with the lower powered eyepiece, take the telescope out of focus in both directions. The blur will show you the diffraction rings, or airy disk, created by the light interacting with the optics, and will give you information about the quality of the optics and how well your telescope is collimated. It will show you if you need to make adjustments. See this website for the star test.

http://www.company7.com/books/products/testing.html

After your telescope is collimated, you still might not get a good view. This could be due to bad optics but it's important to keep in mind that much "blurring" that amateur astronomers experience is due to environmental factors, such as atmospheric turbulence in the upper layers, or changing temperature causing the telescope to no longer be in equilibrium, or maybe they just didn't let the telescope cool properly.

You could also be trying to use too high of a magnification. Most telescope viewing needs to be done with low power.

Answer 4

Looking into the primary from a distance will not produce a useful image. It will magnify things sufficiently far from it so much that there isn't much to see. A telescope without an eyepiece is not very useful to us. The mirror creates a virtual image at the focal plane that our eye cannot see. The eyepiece views this image and converts it into light our eye can process.

Answer 5

The doughnut will certainly be blurry because you are, and must be in order to collimate, out of focus. Are you checking to see that the hole is in the exact center of the doughnut? If so then your scope is collimated. Good luck.