Dust Doughnuts Problem

[michaelmorris]

I've been imaging the Moon and planets now for quite a while. Whenever I use a barlow, I am plagued with dust doughnuts all over the image. A thorough clean of all components seems to solve it for a while, and then they slowly accumulate again.

The doughnuts stay still when I rotate the camera and filter, but leave the barlow clamped to the eyepeice holder, so I am assuming that the dust is on the barlow, is this correct?

[Shibby]

No, the opposite I would say! If they're staying still, they must be on the camera or the filter...

[Catanonia]

I would agree, get a CCD cleaner.

[michaelmorris]

No, the opposite I would say! If they're staying still, they must be on the camera or the filter...

Doh! Now you know why I asked.

My next question then is why do these dust doughtnuts only show up when I use a barlow?

[themos]

Higher f-number, the more closed the light cone is. At low f-numbers the shadow spreads too much and is not as discernible.

[michaelmorris]

Higher f-number, the more closed the light cone is. At low f-numbers the shadow spreads too much and is not as discernible.

Thanks

[Catanonia]

Higher f-number, the more closed the light cone is. At low f-numbers the shadow spreads too much and is not as discernible.

arhhhh, nice explaination, alwa ys bugged me too