GSO GS-530 and tube gap

[SuperG]

This is the 150mm (6") "imaging optimised" Newtonian version I'm talking about here. The OTA came seriously out of collimation.

(1)I've had serious trouble positioning the secondary for good collimation. After rough rotational positioning by eye I've tighten the central screw and then adjust the outside three. What's curious is that when looking at the primary mirror, the laser is super bright on the ring marking but every time I get to centre it appears dim. The laser dot in the collimator looks ok though.

(2)During the day time when looking down the tube towards the primary mirror I can see daylight around the edges of the mirror. Is this normal. It seems that GSO must have put a spacer in the tube to move it back a bit. If normal I guess I'll need to put a sock on the mirror end just in case light leaks in.

thanks

[Shiraz (Ray)]

when you get the laser centered, it reflects directly off the mirror, back towards the laser and you cannot see it well. When it is off centre (on the donut), it scatters everywhere and you can see it. Adjust the secondary to put it in the centre (where it is nearly invisible) and, provided your primary mirror is not too far out, you should get a bright reflected dot on the angled plate in your laser collimator. Make that dot disappear down the hole by adjusting the primary mirror and you should be collimated.

It is normal to see light around the edges - GSO left room for air to circulate around the mirror. If you find that you get too much scattered light and have to put a sock on it, put a fan in the sock to move air around on the mirror.

[glend (Glen)]

Is this your first newt? It's fairly normal for the laser to go dimmer when it hits the centre donut as Ray said. Hard to say how seriously out of collimation it was without looking but my experience buying a number of GSO dobs and imaging newts is that secondaries don't rotate but may need a tweak as transport can bump them out. I would not suggest you mess around with the central lock screw as that controls offset of the mirror. If your just adjusting the angle the three screws are all you need, loosen one tighten one etc o make your adjustmeny. Have your downloaded Astrobaby's Collimation Guide - worth reading.

[SuperG]

Thanks Shiraz and Glen. I guess I was after reassurance. When the OTA arrived I could not adjust the three outer screws enough to bring the laser to the center of the circle. Unfortunately, I had to rotate a little. Good to know about the ventilation part.