Telescope Reviews: Intrafocal seems collimated, extrafocal no... why?

This is a very fast mirror. Anything that is not quite in alignment will show up.

There could be several factors. If the focuser is changing its orientation at all though its range of travel, this can cause the symptom. It only has to change a TINY bit.

To check for this is simple. Using a VERY high power eyepiece with a narrow field of view and the brightest star available. Center the star. De-focus until the diffraction patter TOTALLY FILLS the field of view. Adjust the telescope's aim slightly if necessary. You want it so the outer ring is JUST JUST JUST shy of the field stop all the way around the edge of the field.

Now, travel through focus so that you acheive the exact same condition on the other side of focus. If when you get to the other side, the defocus pattern touches is no longer perfectly centered, then this is your problem.

If the focuser is not perfectly perpendicular to the optical axis, when you "Collimate" it you are simply adjusting it for the best performace at that point.

Now when you do this, you are in fact setting doing a good "Collimation" but only for the VERY exact center of the field. When you do this, one side of the feild though will be sharper than the other side. At low powers you won't see it, but it is there. At high powers, the coma already in the system might mask the extra coma on one side of the field.

Anyway, this is my guess. Because your mirror is so fast, only the VERY VERY center of the field will be diffraction limited, and if your focuser is off even a tiny bit, as you range though focus, you will see the patern change because you are intercepting the caustic cone in different spots.

Of course if you see that the pattern does move as you range though focus, then you will need to shim the focuser until you have removed even the tinest bit of tilt.

Of course this might NOT be your problem, but my bet is that it is...

Other issues could be the the eyepice or the 2" to 1.25" adapeter is not axially perfectly aligned or square. Rotate them to see.

If the focuser draw tube itself has any slop or play (Side to side in the barrel) this can cause your problem too.

Await your findings.

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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)

The night sky is my mistress. She seduces me away from all other lovers.