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Meade 127ED Star Test Question

Started by mforster, 08/20/2008 10:07AM
Posted 08/20/2008 10:07AM Opening Post
Hello,
I purchased a meade 127ed on amart awhile back and have been learning how to collimate and center the lenses. I have done quite a bit of star testing on this scope and I am puzzled with the results. To begin with the scope does have some astigmastim which rotates 90 degrees from inside and outside focus. I think the images I get of jupiter are fairly good but my infocus high power star images are not what I would like. Stars are not a round ball with diffraction rings around it. Sometimes it is round, then elongated, and sometimes little spikes. I have a 76mm scope that has some astigmatims as well, maybe worse than the meade 127ed but still give a nice high power image of a star, round with a couple of diffraction rings around it. So I would think the Meade should be able to do that as well.
Anyway, I do star tests with and without a diagonal. at powers from 266 to 342x. I have done most of the testing on nights of good seeing, and use different stars, mostly altair and vega. What I see outside of focus is some color within the rings but concentric rings, well defined with a bulleyes in the middle. Inside of focus I don't see the color, and the rings are still fairly well defined but not as defined as the outside of focus rings, but the puzzling part is that the bulleyes is not centered inside of focus, it is off to one side. I have collimated and centered the lenses many times I cant seem to improve the off center bulleyes. If it was a collimation or centering issue wouldnt it show up in both the outside and inside star patterns???

Thanks for your help.

Martin
Posted 08/20/2008 10:25AM #1
Hi Martin,
I have the same scope, but it gives an almost perfect star test. Some color of course, but nice rings. I would think that the elements may be not aligned correctly or rotated correctly, but I have never had to touch my lens, so I have no idea about the collimation.
There is some information on the Yahoo group on the lens and collimation though.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 08/20/2008 09:19PM #2
Martin:

I don't have any direct experience with the Meade scope but something in your posting makes me wonder if it is even the lens. In the last part, you say the out of focus star is concentric on one side of focus and off on the other side of focus. First thing that comes to mind is the focuser not being square with the optical axis. If this is the case, as you rack the eyepiece in and out it changes its position with respect to the center of the light cone. Racking farther each way should have the bullseye moving back and forth from one side of focus to the other.

Is there any way you can rotate the lens cell and retest? If its the lens, the axis of astimatism will rotate an equal amount. If it doesn't change, I'd suspect the tail end might be the problem.

Just a thought.

Clear, sunny skies.
Tom Masterson
Coronado-Lunt-Daystar Solar Filters forum Moderator
Posted 08/21/2008 04:59AM #3
Martin Forster said:

Hello,
I purchased a meade 127ed on amart awhile back and have been learning how to collimate and center the lenses. I have done quite a bit of star testing on this scope and I am puzzled with the results. To begin with the scope does have some astigmastim which rotates 90 degrees from inside and outside focus. I think the images I get of jupiter are fairly good but my infocus high power star images are not what I would like. Stars are not a round ball with diffraction rings around it. Sometimes it is round, then elongated, and sometimes little spikes. I have a 76mm scope that has some astigmatims as well, maybe worse than the meade 127ed but still give a nice high power image of a star, round with a couple of diffraction rings around it. So I would think the Meade should be able to do that as well.
Anyway, I do star tests with and without a diagonal. at powers from 266 to 342x. I have done most of the testing on nights of good seeing, and use different stars, mostly altair and vega. What I see outside of focus is some color within the rings but concentric rings, well defined with a bulleyes in the middle. Inside of focus I don't see the color, and the rings are still fairly well defined but not as defined as the outside of focus rings, but the puzzling part is that the bulleyes is not centered inside of focus, it is off to one side. I have collimated and centered the lenses many times I cant seem to improve the off center bulleyes. If it was a collimation or centering issue wouldnt it show up in both the outside and inside star patterns???

Thanks for your help.

Martin

Step one is to TRIPLE CHECK the astigmatism is not in your eyes or eyepiece.


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Posted 08/21/2008 05:09AM #4
The three radial screws that scoot one element with respect to the other--may be pinching that element. These of course on the "new" cell, and not the front push pull 6ea used for cell collimation.

"--Granted, that's a worse case scenario. The destruction might in fact be ... limited to our own galaxy."