Marks on Meade 12" Dob Mirror

[vk5cp (Chris)]

Hi all,

I have a 12" Mead Lightbridge. There are a few marks which appear to be on the underside of the mirror. Do they make any difference to the scopes capacity to resolve objects? If so how to a get a new mirror (or the old one re-silvered)?

Also I have a darkish disk in the centre of the viewing area, I have adjusted the mirror with my laser setup divide and bobs knobs, but its still there. What do I need to adjust to remove it? Or is it related to the marks on the mirror.?

Thanks in advance for answering my newbie questions.

Chris

[BlackWidow (Mardy)]

Hi Chris.. If I understand what your asking correctly you have nothing to worry about. Marks on the back of the mirror are normal. Often marks are made a registration points or other info when the mirror went through it's grinding.

If you have a black dot in the center of your mirror when looking inside of your DOB from the top, this is also normal. Some have a dot and others a small circle. This is used to collimate your scope. The very center of the mirror tends not to reflect light into your secondary mirror and is not seen when viewing objects.

Hope this helps with your questions


Mardy

[vk5cp (Chris)]

Thanks Mardy. Whist the marks look like they are on the back of the mirroI can see them from the front of it. I will try and take a photo on the weekend and post it.

The black image is blurry, I understand what you mean about the secondary mirror obscuring the field of view. I thought I might not have got it adjusted correctly., When I see the moon the edges are very sharp, its just the middle thats a bit blurry.

[BlackWidow (Mardy)]

Pictures would be good. Not sure if your scope is new or used, but if your focus is out in the center of the image you may have a collimating issue. If you focus on a star the start winding the focus out you should see the star turn into a donut with a dark center. The dark center should be round and perfectly centered in the middle of the donut. If it's off center or out of round you are not correctly collimated. You can look up collimating on a DOB on Google and you will see some examples

[White Rabbit]

I have the same scope and I also have black marks on the mirror. Mine look like black fungus or a black water mark but aren't. They appear on silver part of the mirror. Not sure what caused it but it still works just fine