Primary Mirror Damage??

[Mariner (Frank)]

Last weekend I pulled out my 12" Dob (lightbridge) which I have not used for a few months. Although it has been covered and in a storage bag I noticed that the primary was quite dusty - certainly not bad enough to warrant the hassle of cleaning it properly, but dusty none the less. Being very short on time and patience I lightly wiped the mirror (yes I know, I am a foolish, foolish man and well deserve all the condemnation that may come my way) with a very soft cloth from a CD/DVD cleaning kit. I don't know what chemicals, if any, this cloth was impregnated with from cleaning CDs but nothing else was added to it for this wiping and the cloth was dry.

I have very sharp eyesight and even with a very close inspection I could assertain no scratching (probably a miracle I know) on the mirror and the reflective surface looks visually unscathed - very bright, silver and reflective. however, when I tried to view saturn under a higher magnification (15mm Plossl or higher) the image was quite poor (not fuzzy as with poor atmospheric conditions). My 3.5 refractor had a decent image with the same EP. It looked as if the image through the Dob had been smeered/smudged slightly. As I said, I can see no damage at all on the primary but, does this necessarily mean the coating isn't damaged?
I don't know what the next step should be to check whether my mirror needs to be recoated or not? I have not cleaned it properly yet, should this be my next step? I am at a loss.....

PS. The Scope was correctly collimated, the EP and Secondary were both clean as well. There was a lot of moisture in the air and I did not have the mirror fan running (although the evening cooled rapidly I don't think that the lack of forced cooling/airflow would have made any difference).

Thanks very much for any assistance.

[renormalised (Carl)]

Hi Frank

Though you should've been more careful with cleaning your scope's mirror, the fact that there was a lot of moisture in the air the night you looked through the scope, meant it was probably warm air just over your mirror causing the poor seeing. The reason why the moisture was there was because the evening cooled rapidly from a relatively hot day, plus the humidity would've been fairly high. Your mirror won't cool as fast as the air, so the boundary layer just above the mirror would actually be warmer than the surrounding air. The fact you never had your mirror fan running would most certainly make a difference. No airflow over the mirror would've prevented it from cooling rapidly enough to equilibrate with the surrounding air, plus it would prevent that boundary layer from dispersing.

Next time you go out observing, turn the mirror fan on for awhile and then check it through an ep on an object. I'll be pretty certain you'll notice the difference

[Rod]

Hi Frank,

If you can't see scratches or sleeks in the coating, it would seem unlikely to be seriously damaged. The smaller diameter refractor will be much less sensitive to atmospheric disturbance and the sealed tube will give a steadier image. It would be interesting to compare the performance with a sub aperture mask on the 12 inch. That might be a better way to test the difference between the two.

Rod

[bojan]

This is psychology in action
I wouldn't worry too much at this stage.
Yes, mirrors should be cleaned with slowly running water only and then washed with distilled water to remove any traces of minerals that are normally dissolved in tap water...
But even if you can see the damage on the mirror itself, the effect on image is minimal - unless you have many paralles streaks - this will result in quite ugly difraction artefacts on bright objects.
Couple of years ago my 10" dobson OTA slipped from my hands (morning dew) and ended on a concrete floor. Secondary was broken into pieces and fell on a primary... I was devastated then but after washing the mirror, I can see only a thin circular track where secondarry impacted the primary, but no effect on image whatsoever. The telescope (with new secondary) is perfectly usable.
So don't worry... just give it a proper wash.
If this doesn't help (which I think it will) then contact Mr Suching - for recoating job

[Mariner (Frank)]

Thank you all for the information - it has made me feel a little more at ease about the condition of the primary mirror. I will give it [the primary] a proper clean and use the fan as well when I next take it out. I am at sea for the next few weeks but will report back with my observations as soon as I return to land. Once again thanks very much for the kind assistance all.
Regards, Frank.

[mark3d]

ahhhhhh really, re-coating is an option? Could you give even the wildest idea of how much that would cost for a 10" primary mirror?

When my son was about 1 he 'drew' with his finger on the mirror. Not knowing the damage skin oils/acids can do, I did not wash it immediately, and it permanently affected the mirror - it looks like the fingerprints are still there even after I washed it.

Like you say psychology in action - I know it's not affecting the image much (or even discernably at all) - but it bugs me.

Of course replacing the primary is a very expensive option.

Thanks for the info!

[astronut (John)]

Chi Qin at Campsie will recoat the mirror for $285.

[renormalised (Carl)]

Originally Posted by astronut

Chi Qin at Campsie will recoat the mirror for $285.

Sounds like a Jedi Knight....will he use "The Force" to coat the mirror??!!

[bojan]

It's a standard process - removal of old Aluminium coating chemically (I believe acid bath is used in a process, it doesn't affect the glass) and then new Al depositing in vacuum chamber.
See here fore more info:
http://en.wikipedia.org/wiki/Amateur_telescope_making
http://stellafane.org/tm/atm/coatings/atm_coatings.html

[mark3d]

Thanks a lot for the info.

I note from this thread that the outcome will be better than a new mirror - with quartz coating etc.

When I have a lazy $285 lying around I'll get it done

[bojan]

Well, my old Coulter Optics 10" (company doesn't exist any more) is still in excellent shape (apart from that bruise)... I bought it back in 1981 (30 year ago) and the original coating is still there. I washed it maybe 4-5 times since.
I was thinking about re-aluminizing and re-coating after the accident, but decided not to do it - no real benefit there (apart from that warm feeling..).. Reason versus Psychology in action