Dirty 8" mirror

[John0z (John)]

Originally Posted by erick

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.

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 (John)]

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 (Ken)]

John, your mirror has come up fantastic. No need to replace that one.

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!!!