Primary Mirror Cleaning for a Used Dob - Question

[Asmaroth72 (Scott)]

Hi all,
First time poster, long time trawler.
Disclaimer - I am a newbie when it comes to astronomy hardware and have not owned a scope before.
I recently bought my first scope - a second-hand 10" dobsonian and on initial inspection didn't notice the condition of the primary mirror.
On getting it home I removed the mirror to give it a clean in accordance with the unit's instructions and many tutorials from Youtube.
The first two images show the initial condition. The rest are after the first clean.
My first take is mould as I know that it had been stored in a garage for about 2 years.
My first question is:
A sink with room temperature water, one drop of dishwashing liquid, one capful of isopropyl alcohol - how long can I leave it in this solution? What are your recommendations for duration?
My second question is:
Am I flogging a dead horse here? Will this not come clean?
My third and last questions is:
Should I bite the bullet and have it recoated? (I live in Newcastle so can drive to Palmway Optical), has anyone done this recently and how much am I looking at to have this done? FYI - I bought the scope second-hand due to limited funds as I could not afford a new one.


Cheers all,
Scott T

[N1 (Mirko)]

Gday Scott,

To answer your questions -

1) I leave mine to stew in the solution for about 5 minutes but I don't use a timer, deviations don't really matter all that much. I also vary the amount of detergent depending on gunk levels

2) That does look like damage to the coating, however your clean looks pretty good and will still have made a difference particularly for contrast, and

3) you might find you can install the mirror and enjoy reasonably good views despite the damage, until you're ready to send it off for a recoat. I expect a 10" might be marginal in terms of cost between getting it recoated or moving it on* and buying a new one. I know the latter will apply to my 8" when its time is up. Also have close look at those small black bits. If they are mechanical damage going into the actual glass, well that's an even stronger case for a new mirror.

*to a keen ATMer or similar. I wouldn't just chuck it.

[Asmaroth72 (Scott)]

Originally Posted by N1

Gday Scott,

To answer your questions -

1) I leave mine to stew in the solution for about 5 minutes but I don't use a timer, deviations don't really matter all that much. I also vary the amount of detergent depending on gunk levels

2) That does look like damage to the coating, however your clean looks pretty good and will still have made a difference particularly for contrast, and

3) you might find you can install the mirror and enjoy reasonably good views despite the damage, until you're ready to send it off for a recoat. I expect a 10" might be marginal in terms of cost between getting it recoated or moving it on* and buying a new one. I know the latter will apply to my 8" when its time is up. Also have close look at those small black bits. If they are mechanical damage going into the actual glass, well that's an even stronger case for a new mirror.

*to a keen ATMer or similar. I wouldn't just chuck it.

Thanks for your quick response!


1. It is on it's third clean now and I have used extra detergent to see if that makes any difference.


2. I will have to clarify that the images 'after the first clean' puts it in a good light however there is still a considerable amount of gunk. And,


3. While in the solution I have very lightly run my fingertips over the surface and not felt any scratches/gouges so believe it is all non-mechanical damage. I have asked for a quote to recoat and will start saving if it is considerably > bank account.


What would you say would be the MAXIMUM amount of time I can leave it in the solution? The manual states, "let it soak for several minutes (or hours if it's a very dirty mirror)" however I have seen images of mirrors where water has seeped in under the coating at the edges due to being left in for hours.


Cheers,
Scott T

[TimHarbour (Tim)]

Hi Scott.
I emailed Wayne a couple of months ago and he sent back an email with a pricing list for different size mirrors and coating options.
I too have a used 10 inch but haven't tried cleaning the mirror yet.
Basically a recoat is $395 Aluminium + Quartz R>91% or $455 Aluminum+enhanced R>95% for a 10 inch mirror.

He also suggested saving some money by removing the coating yourself to save some time/labour. You would contact him for advice on how to do this.

Just remember too that the recoat is going to be well above the factory pushed out one as far as finish quality goes. Just my 2c if you're going to keep it.

If you shine a torch behind the mirror you will see the condition of the coating from the top too.

Tim.

[wavelandscott (Scott)]

I might suggest installing and using it for a while before worrying overly much about having it recoated.

I am of the opinion that many folks are too gentle with their gear…no I am not suggesting rubbing it with sandpaper or a grinder but a soft scrub with your fingertips in the solution and then a good pure water rinse and dry will do as much good as possible (careful not to rub off your center spot).

After that I would reinstall the mirror get good collimation and enjoy it. After you gain some more experience then maybe consider if new coating is needed.

A very dirty mirror can still give very good views…and remember the earliest reflecting telescope makers would have been thrilled with the amount of light you will gather with what you have (check out how much light speculum reflects)

Use it and enjoy it for a while before taking additional action…is my advice.

[Adskie180 (Adam)]

Hi Azmaroth72

Any updates on this?

Cheers,

[Asmaroth72 (Scott)]

Hi Adskie,
I joined a local astronomy group and mentioned the condition of the primary to some of the experts there.
Their take on it was that if it is not affecting the image then there would be no need to have it recoated.
I can say that for visual viewing I can see no artifacts on what I am looking at. I don't plan to use the scope for astrophotography, which is where I believe it may affect the images.
I liaised with Palmway Optical who sent me a pricelist should I change my mind.
Cheers,
Scott

[Adskie180 (Adam)]

Azmaroth,

Thanks for the fast response, any chance you could share with me the price list.

Cheers,
Adam

[Asmaroth72 (Scott)]

Hi Adskie,
Sent a PM.

Cheers,
Scott