PDA

View Full Version here: : Dirty 8" mirror


John0z
14-03-2012, 01:48 PM
Hi,

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

asimov
14-03-2012, 01:55 PM
Gso?

astronut
14-03-2012, 01:58 PM
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.:)

John0z
14-03-2012, 03:28 PM
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.

erick
14-03-2012, 05:10 PM
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.

erick
14-03-2012, 05:14 PM
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.

John0z
14-03-2012, 05:22 PM
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.

erick
14-03-2012, 05:40 PM
A brand new 8" GSO primary mirror is only $350 from Andrews, John. So you have an easy and not too expensive fix available.

barx1963
14-03-2012, 06:00 PM
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.

Malcolm

asimov
14-03-2012, 06:08 PM
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!:D

Don't worry John, clean it - use it for the time being:thumbsup:

brian nordstrom
14-03-2012, 07:51 PM
:thumbsup: 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.

John0z
15-03-2012, 07:39 AM
Eric,

Here is a shot focused on the camera lens - looks better.

-John

SkyViking
15-03-2012, 08:20 AM
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.

John0z
15-03-2012, 10:46 AM
Hi,

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.

-John

traveller
15-03-2012, 11:07 AM
looking good now john, let us know if you notice any difference in viewing.
Bo

asimov
15-03-2012, 11:28 AM
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' ;):lol:

John0z
15-03-2012, 12:55 PM
Eric, I just checked the Bintel website while looking for other things - they have a 8" f/6 mirror for $219.

For now, I think I will put it back together as it is, then collimate again, and do some star testing - and play around with the Argo Navis .

-John

barx1963
15-03-2012, 04:07 PM
Looking good John, cannot see any flaws after the cleaning!

Malcolm

erick
15-03-2012, 11:56 PM
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.

erick
15-03-2012, 11:58 PM
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.

John0z
16-03-2012, 06:31 AM
Eric,

The mirror clamps are rubber with a small piece of metal for the screws to press on. I did them up sort of loosely - until I saw a little movement in the rubber, so there should not be a lot of pressure on the mirror. Anyway, I will see how it goes. The mirror cell is back in.

I decided to do a rough alignment of the primary mirror. The secondary holder has four vanes, so I adjusted the collimation screws until the central dot on the mirror was reflecting back centrally to the secondary. I.e. I first adjusted the collimation screw that moves the mirror in the direction towards or away from the focuser until the reflection I could see by sighting along the other right angle vanes was centered. Then I adjusted in the other direction by sighting along the vanes pointing to and from the focuser. Does this make sense? So the end result is that by sighting along any vane, I can see the central dot lining up with the reflection of the vane. This should mean that the mirror is now reflecting back to the secondary - not perfectly but should be very close.

After I did this, I went to readjust the secondary. More about this later.

-John

John0z
01-05-2012, 09:18 PM
Hi,

It has been a while since I did the cleaning of the mirror. A lot of things happened as well as the weather. I went to the Star Gaze at Parramatta Park on Thursday, and this prompted me to get my telescopes out.

Earlier this evening, I put the dob in the garage so that it can start to cool down. Then an hour ago, I took it outside - I had a go with the Argo Navis, but found that I needed to align my red-dot finder which took me a little time. By then I decided to just do some viewing. I looked at Alpha Crucis with a 40mm eyepiece - and aligned my finder. Then had a look at the jewel box cluster that is nearby - still good. I think my rough collimation seems to be working since I get nice points for the stars. I had a try at some objects nearer the horizon, trying to find something I can use to do the Argo Navis alignment, but found that the dob kept moving to the vertical position, so obviously I need to have a good look at balancing it.

Also the azimuth rotation is very stiff - takes some effort to turn. Last object I looked at is my favourite - Omega Centauri which was just visible to the naked averted vision. Through the 40mm it was like some cotton wool. I changed to a Meade zoom at 24mm and I can start to see some stars. Going to 10 or 8mm gives lots of stars, so I guess the mirror is working well - the dust and grime is more or less gone, and I get bright stars.

Anyway, the wife called me back inside so that was that. Now I need to get the energy to get my GM8 outside and put my Meade 10" f/10 in use - haven't used that for a few years so hope it still works.

-John

ballaratdragons
01-05-2012, 09:54 PM
John, your mirror has come up fantastic. No need to replace that one. :thumbsup:

Yes, my poor old 12" GSO dob is sitting in the clubroom at SVAA and it has crazing (aluminium corrosion) over most of the surface. It has reduced the actual reflective area by probably 50% and yet it still gives fantastic views. Attested to by several members.

Often we underestimate just what a mirror is capable of.

I have seen fantastic views through mirrors where 90% of the coating has turn black from Crazing!!!