Focus Issue

[Freddy]

Guys, this may be my 2nd dumb question this week - but here goes!

I just bought 2 new eyepieces, an ED 12mm and 18mm. I have a Celestron Nexstar 6 which came supplied with a 25mm eyepiece.

The problem is this - when I tested the eyepieces today by focusing on a tree about 400 metres away the image was crisp and clear with the 25mm Celestron. When I put in the 18mm and the 12mm ED eyepieces in the image was not very clear and slightly blurred and no amount of adjusting the focus could get me a really clear image. Any suggestions??

[themos]

Eyepieces are usually not "parfocal". This means that you generally need to rack the focuser in or out when changing eyepieces. Nearby targets are focused further "out" from the scope. It could be that you don't have enough "out" travel in your focuser for the other 2 eyepieces. Is the image the clearest when you rack the focuser all out with them? If so, that could be your problem and you'd need a focuser extender (looks like a barlow without any glass).

[Freddy]

Thanks Thermos - I am a complete Newbie so bear with me! I haven't racked the focuser out yet but just plonked them in as far as they go and tightened the screw! I will try to experiment with with them as you suggest and see if that makes a difference.

[themos]

Confused! You said "no amount of adjusting the focus" and "I haven't racked the focuser out yet". What scope are you using?

[themos]

Yes, I see, a NexStar. That focuser moves the main mirror instead of the eyepiece. Maybe others familiar with that scope can help.

[vds]

I wonder if the tree is too close for that combination of scope and eyepiece?

vds

[Victor_MO]

That could be an option, I use to focus on objects very far, a couple of miles may be (the scope manual says "over 1.5 mile"). so may be your 400m tree is too close for your eyepieces!... Keep trying!

[AndyH]

400Mtr is plenty far, to reach focus with a moving mirror focus system IMO.

When you put the eyepiece in the diagonal and tighten up the thumscrew, rack the focus knob both in and out and you should eventually find the focus for both those eyepieces.

The stuff below was taken from page 26 of the users manual, here

http://www.celestron.com/c3/images/files/downloads/1152120348_1106811069manua.pdf

Hopefully this will help you out ?

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The NexStar's focusing mechanism controls the primary mirror which is mounted on a ring that slides back and forth on the primary baffle tube. The focusing knob, which moves the primary mirror, is on the rear cell of the telescope just below the star diagonal and eyepiece. Turn the focusing knob until the image is sharp. If the knob will not turn, it has reached the end of its travel on the focusing mechanism. Turn the knob in the opposite direction until the image is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer object and counterclockwise for a more distant object. A single turn of the focusing knob moves the primary mirror only slightly. Therefore, it will take many turns (about 30) to go from close focus (approximately 25 feet) to infinity. Figure 3-3 The emblem on the end of the focus knob shows the correct rotational direction for focusing the NexStar.

For astronomical viewing, out of focus star images are very diffuse, making them difficult to see. If you turn the focus knob too quickly, you can go right through focus without seeing the image. To avoid this problem, your first astronomical target should be a bright object (like the Moon or a planet) so that the image is visible even when out of focus. Critical focusing is best accomplished when the focusing knob is turned in such a manner that the mirror moves against the pull of gravity. In doing so, any mirror shift is minimized. For astronomical observing, both visually and photographically, this is done by turning the focus knob counterclockwise.

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Cheers,

Andy.