Collimation or more?

[cdlftw]

Hi all,

When I unfocus a star to test collimation, I get a very strange view. It's a perfect circle with a flat top (see attached). Is this the collimation or is it something more serious?

Regards,

Craig.

[Merlin66]

Craig,

Need more info.....

Type and size of the telescope, focuser etc.

I'd say either there's some serious misalignment or something is sitting in the lightbeam....

Ken

[cdlftw]

Hi Ken,

It's a Skywatcher Explorer 130P, with the generic 25MM lens that shipped with it - no Barlow or anything.

Regards,

Craig.

[Doc]

Strange....

I had a similar thing happen to me and it was the bottom of my light shroud cutting across the light path.

But as yours is a solid tube it cannot be that.

[Merlin66]

Hmmm

If it's definately a sharp linear edge then it must be something interfering with the light path...dew shield slipping? the edge of the focuser tube into the main tube?

If it's a hairy wavey edge then it may be tube currents caused by warm air sliding up the inside of the tube........

Have a good look around for possible obstructions - remove the eyepiece and just look through the focuser tube from all angles...

[pvaz]

try pulling the focuser all the way out. when my focuser is in I see a straight edge which is the end of the focuser tube inside the main tube.

[LukeSkywatcher]

That looks alot like the shape of the secondary mirror on the SW 130P. Maybe it is not aligned as well as it should be?

[Astro_Baby]

It could be an obstruction to the light path OR that the secondary is badly misaligned.

Take the eyepiece out and look down the focuser drawtube. Does the secondary mirror appear as a circle to your eye ?

If not thats the problem. If it does then somethng else is amiss. Try racking the focuser out all the way - ie the drawtube away from the scope.

What you are seeing may also be ( unlikley but possible ) a pinched mirror if its a reflector. A pinched mirror usually shows as more of a triangular shape but its possible what you are seeing IS a triangular shape that only appears to be circular. Take a closer look and see.

[LukeSkywatcher]

Just thinking about it further. It COULD also be a crack on the actual 25mm EP lens. I have a 4mm EP and it has a crack on it near the edge that spreads across the lens edge. It happened because i tightened the screws to hold it in place in the focuser of my 90EQ too tightly.

Hold the EP up to a bright light (or a window) and look through it from about an inch away. If there is a dark line on it................it may be a crack or maybe even just a bit of plastic shaving from the interior of the lens casing. It could also be something as simple as an eyelash on the lens of the 25mm EP. I'm still going with my 1st answer though. It looks to me as if the seconday mirror is out of alignment.

[cdlftw]

Thanks everyone for the replies so far, I'm away from the scope at the minute so can't check, but will post findings when I have them. I just hope it's something like the focuser being in, and not something to do with a mirror!