Collimating and mirror cleaning

[doppler (Rick)]

Originally Posted by julianh72

The post that I responded to:

"who wants a rusty car"

Caustic soda actually PROTECTS exposed steel from rust, so it won't make your car rust, but it is EXTREMELY aggressive to aluminium, and it might damage the paintwork and plastics - and it could well be damaging to at least SOME (but by no means all) optical coatings and mirrors.

Yes, I would definitely avoid using caustic soda products in cleaning any sensitive equipment.

Like I said - "sorry to be pedantic"!

I have recently aquired a 10" f4.5 dob salvaged from the tip. The mirror was in bad condition but the scope was not worth spending much $ to get the mirror proffesionaly recoated. I have had reasonable success in doing a home re-silver on the primary. The hardest part was removing the old coatings. Caustic soda is supposed to dissolve aluminium, and yes it did remove a little bit of the coating (I could feel the chemical reaction heat through the rubber gloves) Nitric acid is recomended to remove mirror coatings, but is not an easy chemical to obtain in Aus. My final resort was hydrochloric acid mixed with a trace of copper sulphate (copper reacts with the hydrochloric acid to make a weak nitric acid). Even then it took a lot of hard rubbing to fully clean the old coatings off.

The point here is, the overcoatings are very durable but the area of caution is to avoid scratching the surface. Soak the mirror first then run water over it, I use a spray bottle to remove large dust particles. Then I use cotton wool (under water) to remove the fine dust. I then stand the mirror nearly vertical on a towel and rinse with distilled water.

Cheers Rick

[Steffen]

Originally Posted by doppler

The point here is, the overcoatings are very durable but the area of caution is to avoid scratching the surface. Soak the mirror first then run water over it, I use a spray bottle to remove large dust particles. Then I use cotton wool (under water) to remove the fine dust. I then stand the mirror nearly vertical on a towel and rinse with distilled water.

That's basically what I do too, only I use kitchen paper instead of cotton wool. I also usually need to use isopropanol to get rid of the eucalyptus oil film that deposits on my mirrors.

Cheers
Steffen.

[julianh72 (Julian)]

Originally Posted by doppler

(copper reacts with the hydrochloric acid to make a weak nitric acid).

Errr .... no! There's no nitrates in copper sulphate, so it can't make nitric acid. (Nitric acid passivates aluminium, so I wouldn't have thought it was ideal for stripping aluminium from glass.) Sulphuric acid perhaps, but the hydrochloric acid should do the job anyway, I would have thought.

[Pinwheel (Doug)]

Originally Posted by julianh72

The post that I responded to:

"who wants a rusty car"

I worked in the technical division at Selley's Chemicals and the point was never wash a car with dish washing detergent because the Caustic soda causes rust, "So who wants a rusty car" statement was supposed to create a mental image that Caustic Soda is a corrosive not to be used on a Primary mirror surface for cleaning.

[doppler (Rick)]

Originally Posted by julianh72

Errr .... no! There's no nitrates in copper sulphate, so it can't make nitric acid. (Nitric acid passivates aluminium, so I wouldn't have thought it was ideal for stripping aluminium from glass.) Sulphuric acid perhaps, but the hydrochloric acid should do the job anyway, I would have thought.

My bad....I loved chemistry at school but was never good at formulas for reactions. Caustic soda can remove the aluminium coating but nitric acid is used to disolve silver from a silver coated mirror. A really stong acid is needed to remove the protective overcoat put on commercially manufactured mirrors. I surprised myself that I was able to make a usable coating for the mirror through improvising chemical sources, due to the unavailability of quality chemicals.

"Chemically, nitric acid is made by bubbling nitrogen dioxide into water. So the objective in this approach is to generate nitrogen dioxide. This can be done by reacting hydrochloric acid, a nitrate salt and copper. Around 80grams of sodium nitrate, over 30 grams of copper and 100mL of hydrochloric acid are the quantities needed. The exact amount is not critical. For useable concentrations, the amount of water being converted should be small, around 20-50mL."

[sn1987a (Barry)]

So strap your mirror to the bonnet and drive through a car wash. Clean mirror, clean car.

[doppler (Rick)]

I found this interesting post about using acetone to clean optics. http://www.rfroyce.com/cleaning_new.htm

[Steffen]

Interesting comment about the Viva towels, they happen to be my medium of choice, too!

Instead of acetone I use Diggers isopropanol, it's easier to work with, doesn't evaporate as quickly and still does the job.

Cheers
Steffen.

[sn1987a (Barry)]

Originally Posted by bobbyf

Whilst we are on the subject of collimation, could i have some opinions as to whether mine is in good shape?
Its a 10" F4.7, and the picture was taken with my phone up against an old film holder with a hole in the centre.
The focuser was all the way out, as per the instructions on one of the above links, but i cant see any mirror clips until i move the focuser inwards. They look Ok when i do, but i wondered whether i need to move my secondary mirror to see the clips when the focuser is all the way out.
Any advice you could give would be appreciated
Cheers

Bob

Hi Bob,

For what it's worth I skipped all that film canister stuff. If you invest in a Glatter laser collimator and the Catseye tool with sight tube all your collimation problems are solved. These tools will last a lifetime so are well worth the investment and make collimation a breeze.

[bobbyf (Bob)]

Thanks for your reply Barry, i'll take a look at your suggestions