Do I need my mirrors recoated?

[John48231]

Hi All, Im looking for some advice on my dobsonian telescope.  Following this collimation guide - https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

When defocusing on a star I am getting black spots on the white donut like at the bottom of the guide, the above guide suggests the optical surfaces are damaged but my mirrors look ok.  Does this mean that my mirrors need to be replaced?  or can I get them recoated?

Im a beginner so looking for any advice I can get on this one.

Thanks

[Peter Drew]

Mirrors don't need replacing unless broken, new coatings restore them as new.  If the mirror looks ok then it probably is.     🙂 

[Stu]

7 hours ago, John48231 said:

Hi All, Im looking for some advice on my dobsonian telescope.  Following this collimation guide - https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

When defocusing on a star I am getting black spots on the white donut like at the bottom of the guide, the above guide suggests the optical surfaces are damaged but my mirrors look ok.  Does this mean that my mirrors need to be replaced?  or can I get them recoated?

Im a beginner so looking for any advice I can get on this one.

Thanks

If you rotate the eyepiece, do they move? If they move, they are on the eyepiece. I doubt you would see too much from the mirrors as they are a long way from the focal plane

[CraigT82]

As above, Most likely dust specs on the eyepiece lenses. If the dark areas are dynamic (constantly move around in the image) then you’re probably just looking through poor seeing.  

[John48231]

Thanks for the responses guys,  I will rotate the eyepiece to see if the spots move and report back. If the clouds ever go away that is, been 2 weeks now

Edited February 12, 2023 by John48231

[Ricochet]

14 hours ago, John48231 said:

am getting black spots on the white donut

If you can see a white doughnut you have defocused way too far and are seeing dirt, most likely on the eye lens of the eyepiece. You need to use a high power eyepiece with a focal length equal to your telescope's focal ratio or smaller, get perfect focus and then defocus by maybe a millimetre either side of that.