Here is a photo of an 8" mirror - I have taken the mirror cell off the bottom of a dobsonian. erick sent me some info on cleaning the mirror - thanks Eric.
The V mark looks bad - I could only magnify to 10x and it won't fit under my USB microscope, but I might be able to hand hold it. I will try it out.
I held the camera above it to give an idea of how bad the reflection is.
-John
Last edited by John0z; 14-03-2012 at 02:49 PM.
Reason: Reflection
John, Unfortunately the beginning of the end...the coating is failing.
Your images should still be ok for the moment, but this will get worse over time.
I would recommend that you clean the mirror, before further use.
John, Unfortunately the beginning of the end...the coating is failing.
Your images should still be ok for the moment, but this will get worse over time.
I would recommend that you clean the mirror, before further use.
John.
Thanks John, yes - definitely it needs a clean. This is from a Bintel dob that came with an Argo Navis that I bought from another IIS member on Saturday. Also this will be the first big mirror I will have cleaned.
Yes John, a GSO. I reckon that JohnOz needs to give it a clean and then we'll see how it looks under a Ronchi eyepiece. I reckon JohnOz will still get nice views for his first foray into dobsonian territory, once all aligned and collimated.
John, I suggest that take another shot but focus on your camera lens so we can see the reflectivity. I think you have lots of nice reflective aluminium surface there, under the dirt. So if the dirt blows/washes off nicely, you'll be fine.
How do the support springs look on the mirror? Perhaps stronger ones might be a good move while you have the cell out.
Just after blowing some of the dust and hairs, lint off the mirror. Definitely the coating is failing - here is another shot of it with a torch behind it.
Having seen the state of the 12" mirror at the Snake Valley clubrooms the other night that apparently gave pretty good views, this isn't so bad. Once it is cleaned it will still give service for a while I would hope.
I'm sure I've told the story of how I used an 8" primary at the polished stage with no coating at all for a few months, in IIS. Is still worked fine for really bright objects...but it was amazing at how much brighter things were once I got it coated!
Don't worry John, clean it - use it for the time being
Good advise , I used my 5inch f/13 newt for a while the same with out a coating , awsome on the moon and brighter planets .
but yes a good coating makes a huge differance .I bet its not a filthy as my 10 inch Dob in NZ , is , but that still gives great views , bar a little light scatter .
Brian.
John, I suggest that take another shot but focus on your camera lens so we can see the reflectivity. I think you have lots of nice reflective aluminium surface there, under the dirt. So if the dirt blows/washes off nicely, you'll be fine.
How do the support springs look on the mirror? Perhaps stronger ones might be a good move while you have the cell out.
Eric,
Here is a shot focused on the camera lens - looks better.
Yep that coating looks like it's beginning to fade. I can tell you mine is even worse and still gives very pleasant views actually - it's rather surprising. But give it a clean and you should be ok for quite a while still.
Thanks everyone for your great responses and advice. Here is a shot of the mirror after cleaning - leaning in the corner drying. Focused onto the camera as best as I could. It looks fantastic, much better than originally.
Yes John, a GSO. I reckon that JohnOz needs to give it a clean and then we'll see how it looks under a Ronchi eyepiece. I reckon JohnOz will still get nice views for his first foray into dobsonian territory, once all aligned and collimated.
Good point about a ronchi test Eric. If it's a very nicely figured mirror, it might be worth getting it recoated rather than replaced later on down the track. You know what it's like - hit & miss 'freak mirrors' when it comes to mass produced stuff..I'm sitting on a freak right now & it's called a 'mass produced C11 SCT'
Good point about a ronchi test Eric. If it's a very nicely figured mirror, it might be worth getting it recoated rather than replaced later on down the track. You know what it's like - hit & miss 'freak mirrors' when it comes to mass produced stuff..I'm sitting on a freak right now & it's called a 'mass produced C11 SCT'
Like my 12" GSO mirror. Good enough to build a quality scope around it.
Good luck with the scope John. Should be plain sailing. Hopefully the Argo Navis is already correctly set up for the encoders (steps, direction etc) - you'll find out.
Looks like a superb cleaning result John! Remember to not pull those three mirror clamps up tight. Stop at the thickness of a business card above the mirror surface.
