Advice on replacement mirror or repair?

[rnobleeddy]

I bought an old TS Optics 150mm Newt in a pretty poor condition (but priced accordingly) as a project to get more acquainted with taking scopes apart and making mods.  

I knew the primary mirror was probably corroded, and that has been confirmed when attempting to clean it. 

The original model is uncertain (TS150, perhaps F/6) but I don't anticipate the quality is/was particularly good - I'd expect it to be on par with the mirrors you'd find in a Sky-Watcher 150PDS, for example.

With that in mind, I just wanted to confirm:

- although the pics are poor, that the mirror is beyond use?

- re-coating is going to be uneconomical?

 

And if so - does anyone know a good source of replacement mirrors? Obviously the aim here isn't to save money, else I wouldn't have started, but if the cost starts to approach that of a new 150mm Newt then I may start to question my choices! I guess the fallback is to put wanted ads here and on ABS and just wait. I have patience!

 

[PeterStudz]

Is it possible to get the thing back together and in reasonable working condition? You might be pleasantly surprised at what you can see even with a mirror in poor condition. 

[Rallemikken]

Just bought an 8" f5 mirror from lens-store on ebay. Seems to be good quality, delivered on time, well packaged, but I have not actually used it yet....   They have very reasonably priced 6" mirrors to,  considering buying a set and build me an 6" f8. One thing is sure, their service was excellent.

Maybe like cursing in the church, but have you tried to polish it? I have repaired small scratches on mirrors a few times, amazing what you can do with the right attitude. Btw, not much to loose if it is spoiled.

[ONIKKINEN]

Tried cleaning with some strong cleaning solutions first? Not just a light spray and touch i mean, proper solvent and all. If youre planning on recoating the mirror it costs nothing to try and nuke the filth off it first. But i reckon that would still show a nice image actually because its not that filthy even. All newtonians have a massive and entirely opaque scratch bang on in the middle of the optical axis that blocks as much as 35% of the light on the way to the mirror. I am of course talking of the secondary mirror 😆. If you cant see the secondary mirror in use, why would you be able to see some small scuffs fractions of a percent the size of the secondary? Chances are you will not notice anything is wrong.

That is a professional telescope with bullet holes through it and marks from banging on the mirror with a hammer. Telescope didn't care about them or really even notice, the resulted scuffs resulted in a 1% drop in light.

[rnobleeddy]

1 hour ago, PeterStudz said:

Is it possible to get the thing back together and in reasonable working condition? You might be pleasantly surprised at what you can see even with a mirror in poor condition. 

 

26 minutes ago, ONIKKINEN said:

Tried cleaning with some strong cleaning solutions first? Not just a light spray and touch i mean, proper solvent and all. If youre planning on recoating the mirror it costs nothing to try and nuke the filth off it first. But i reckon that would still show a nice image actually because its not that filthy even. All newtonians have a massive and entirely opaque scratch bang on in the middle of the optical axis that blocks as much as 35% of the light on the way to the mirror. I am of course talking of the secondary mirror 😆. If you cant see the secondary mirror in use, why would you be able to see some small scuffs fractions of a percent the size of the secondary? Chances are you will not notice anything is wrong.

That is a professional telescope with bullet holes through it and marks from banging on the mirror with a hammer. Telescope didn't care about them or really even notice, the resulted scuffs resulted in a 1% drop in light.

Thanks - both good points!  I know the general suggestion is that mirrors can be pretty bad and still produce good results, but I guess convincing yourself of that is harder. 

It's certainly free and easy to both try a more aggressive cleaning approach (rather than soapy water) and try using it. The secondary is in good condition and cleaned up nicely. I can put up with  the ropey single speed crayford focuser,  so it is entirely usable.

Edited December 5, 2021 by rnobleeddy

[rnobleeddy]

40 minutes ago, Rallemikken said:

Just bought an 8" f5 mirror from lens-store on ebay. Seems to be good quality, delivered on time, well packaged, but I have not actually used it yet....   They have very reasonably priced 6" mirrors to,  considering buying a set and build me an 6" f8. One thing is sure, their service was excellent.

Maybe like cursing in the church, but have you tried to polish it? I have repaired small scratches on mirrors a few times, amazing what you can do with the right attitude. Btw, not much to loose if it is spoiled.

Found them. Agree the prices are good, although it appears a couple of places will recoat the one I have for closer to £80. Of course, I'd actually forgotten how much a new 150mm Newt would cost, so it appears both options are in budget! 

I'm new to worrying about mirrors - do you have any tips for trying to polish it?

[Rallemikken]

57 minutes ago, rnobleeddy said:

 do you have any tips for trying to polish it?

I have used cotton balls and clean water, just as when I clean them. The surface aluminium is soft, and fluid to a certain extend. It is the glass under the aluminium that determines how the mirror performes, as long as you can get the surface coating even. Scrathes and small dots can be polished out, but if the coating is corroded and partly missing, you are in trouble. Work gently from the sides and onto the fault. With that said, I don't think a small defect makes any difference, but it calms my brain to make any imperfection disappear. Many possible variables in this hobby, nice to eliminate some. The claimed reflectivness of theese mirrors lies around 95 percent, so they are far from perfect when new.