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C8 with Fastar secondary - help !!!

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#1 dweller25


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Posted 14 November 2012 - 09:17 AM

I have a one year old Chinese made C8 with a fastar secondary that is 1mm too high in the N/S direction on the corrector.

It did not come like this so has moved at some point.

I took the corrector off and removed the secondary mirror to try and loosen the secondary holder but no joy - is it glued or just very tight ??

Anyone any experience of this ??

#2 Eddgie


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Posted 14 November 2012 - 06:43 PM

David... First, 1mm of movement will not make any difference.

The hole for the secondary mirror of course is not so close in tolerance as to make removing and replaceing the secondary difficult because it is designed to be removed.

I am not sure if you did from your discription, but one millimeter of play is nothing to be concerned about.

What exactly happens when you try to remove the secondary mirror assembly from the corrector?

To remove the secondary mirror assembly, you point the telescope slightly upward (or higher if you like) and completely unscrew the ring around the outside of the mirror cell.

Remove the ring and set it on the side.

Now, grasp the little Fastar "Knob" at the center between the three collimation screws. This little knob has a little lip on it that you can grasp with your fingertips. Again, this is the little round thing at the center of the three collimation screws with "Fastar" written on it.

The first time I removed mine, it was a tiny bit snug, but it did not take much pressure to pull it out.

So, not sure at all why you took the corrector off, but the secondary mirror itself should lift out from the front with litte or no effort.

Not saying that yours might not be stuck somehow, but the secondary assembly is basically just sitting in the holder with nothing to retain it but the large ring.

#3 dweller25


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Posted 15 November 2012 - 02:21 PM

Hello Eddgie, thanks for the reply.

I have attached a picture of what I was trying to do...

I removed the Fastar secondary (A)by just unscrewing the outer ring.

That left the internal baffle (B) on one side of the corrector and the Fastar mount © on the other side.

I actually freed the two by using a hair drier to warm up the baffle and it simply unscrewed from the Fastar mount.

Then I moved the whole lot 1mm South, tightened it all back up and measured to make sure it was central on the corrector.

Reassembled the whole scope and did a daylight collimation by looking for concentricity at the front of the scope.

Then did a star test and it was almost perfect - just a slight tweak, took it up to x580 and it was still perfect.

I'm sure the 1mm does not make a difference in the real world but now I have both mechanical and optical alignment which makes me happy :)

The clouds don't though :bawling: :bawling: :)

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