Lens query

[crumblepenny]

Can you help me please? I am very new to star gazing and am using a Celestron Astromaster 130. I was looking at stars tonight (the moon wasn't visible) and I could see a spoke effect over the star through my lens. Is there any way I can eliminate this spoke effect? I don't get it when I'm looking at the moon. Thank you.

[Dom1961]

You don't normally look at stars,theyre just... Points of light. What you are explaining is having your star out of focus. Using the focus wheels on the side of the focuser or where the eyepiece goes in, turn it until it appears the smallest and the sharpest. You are seeing the shadow of the secondary mirror and the vanes. A book like turn left at orion will help you find objects suitable for viewing with your scope. M13 a globular cluster in hercules should be visible. Along with planets,but I'll let more experienced members advise targets.

[LukeSkywatcher]

You don't normally look at stars,theyre just... Points of light. What you are explaining is having your star out of focus. Using the focus wheels on the side of the focuser or where the eyepiece goes in, turn it until it appears the smallest and the sharpest. You are seeing the shadow of the secondary mirror and the vanes. A book like turn left at orion will help you find objects suitable for viewing with your scope. M13 a globular cluster in hercules should be visible. Along with planets,but I'll let more experienced members advise targets.

I am not so sure. If it was a star out of focus it would look like a donut. The spoke effect Dom is talking about is known as diffraction spikes. They are caused by the spider vanes on certain types of scopes (IIRC). Not sure how to overcome them. I'm sure someone else can advise better.

[brantuk]

Yup - once properly focused the spike effect will disappear. Focusing on the moon which is a lot closer, will be different to focusing on a deep sky object which is further away. So you will have to refocus for each different object you observe - a bit like using binoculars. Slowly scan through the full focus range until you get a good sharp image each time - eventually you will know roughly where the focus point is and will get there very quickly.  Hth

[crumblepenny]

Thanks for your help and advice, very much appreciated.

[LukeSkywatcher]

I stand corrected. 

[John]

If the "spokes" were black against the bright disk of the star it means that the telescope is not in focus.

When it is in focus a bright star will appear as a point of light with 4 faint bright spikes, like this:

The spikes of light are caused by the support vanes for the smaller, secondary mirror so can't be eliminated. They will be much less apparent or invisible on dimmer stars.

[ronin]

As the Celestron Astromaster 130 is a reflector it sounds like you simply have a diffraction pattern caused by the spiders holding the secondary in place.

This is normal on a reflector.

[crumblepenny]

This all makes sense to me now - thank you all for your help.

[LukeSkywatcher]

I stand corrected. 

Or do i ?.

[Stu]

Or do i ?.

No Paul, you were correct, they were diffraction spikes.

As mentioned they only really show up on brighter stars, in focus you will see the bright pin point star in the centre of a fine cross of light caused by the secondary support vanes.

You do get it on planets but the spikes are more diffuse so may not be as obvious.