Dust on primary

[Conna]

I have noticed that when I have focused my 150p Dob on say a planet (last night Saturn) the planet goes in and out of focus as it moves across the FOV. Is this dust on the primary? If so how do I get rid? I don't want to chance anything DIY in case i do the wrong thing, have looked down the OTA and there is a bit of dust on the primary.

so need some guidance.

Thanks in advance

[Steve Ward]

I would say that it is much more likely to be the atmospheric conditions affecting the 'seeing' making the planet appear to be going in and out of focus , even a filthy mirror will not affect the view to a great extent.

I would leave the mirror well alone .

[RobH]

I agree with Steve. It'll be seeing having that effect.

Dust on the primary will have no effect at all unless it's absolutley covered with a lot of it. Even then, the effect would be even, and have no effect on focus.

You should see the state of the mirror on my 12 inch dob (actually, you shouldn't!).....it really needs recoating, but even so, still works pretty well.

Cheers

Rob

[peter shah]

i agree, dust will not effect the performance of your scope....dont feel tempted to clean it, you will do more damage to the perfomance than a few spec of dust

[Conna]

Well I never, I didn't expect seeing to have such an effect. I am a beginner and only now properly understand about "seeing" I never expected it to make such a differance.

Thanks for the advice

[laser_jock99]

Best not touched unless absolutely necessary. I cleaned my 12" mirror the other day. The dust is now replaced with IPA drying streaks....

[perfrej]

I'd say the in and out of focus over the FOV problem is a consequence of the field not being flat. A newton is not flat and anything not in the dead center will be out of focus forther out in the field of view. A multi-purpose coma corrector cures that, even for visual.

/per