brilo pad to clean the mirror?

[astroimpulse]

It's surprising how often I read to leave dirty mirrors as the dirt doesn't detract from the viewing.

That pic of the mirror before it's cleaned, looks filthy. How would that not spoil performance?

[The Warthog]

I actually used a Scotchbrite once. No, really, my daughter worked for 3M, and it was a new product designed to leave no scratches at all on anything. And it was my 4.5" mirror, and I only used it to wipe away my tears when..., er actually to wipe away the last drops of water. It did leave no scratches.

You've done a nice job on that mirror. Now you can go back to your normal blood pressure dosage.

[Ian_A]

Well to be honest, its been washed again ! ! Someone here asked if i checked if the dot was in the middle, when i checked, it wasn't.

I collimated the mirror, bang on i thought, but early yesterday morning i checked on the only bright star left and the image shifted left and right as i went through the focus point....I'm still learning, so understand that its still out if that happens...SO

out came the mirror and out came a plastic rule. The centre dot was around 5mm off centre on one axis, i'm not sure how important that is, but i thought, nothing else to do today, so lets have a go.

fetched the white dot off, (sand paper...haha) then rewashed the mirror. Then using a compass draw an 8" circle, made the hole a little bigger in the middle and cut the circle out. Lay it over the mirror and with a black per-marker, put a dot in the real middle !

the re collimated the mirror, so last night the star test (if i understand how to do it correctly) when in/out of focus the doughnut stayed in the same position...

And i had a nice clear view of jupiter.....Have to say that with the 3x barlow and the 10mm ep, i find the focus is poor, maybe the focuser or just poor quality ep and barlow !

cheers

Ian

[Moonshane]

Mirror cleaning should be considered at intervals comparable with car engine rebuilds.

Olly

this is a superb statement!

(aimed generally obviously not at Olly) don't forget that the worst way to inspect a mirror for cleanliness is to shine a torch down the tube. I suspect even a brand new mirror would look dusty under such an inspection.

as a test, try shining a torch down the tube (don't drop the torch!!) and change the angle of the tube whilst keeping the torch in the same spot. the mirror will look spotless from some angles and filthy from others. then don't ever do it again and enjoy the view instead.

[Moonshane]

I don't know the exact figures but suspect that 5mm is a serious error in terms of enabling accurate collimation in an f5 scope. you have done the right thing here and it sounds like this is confirmed in the star test from the sounds of things.

whether or not to clean is one thing but you have been lucky the centre circle came off OK as I have heard of problems when taking these off that people have actually pulled off coatings! not sure how rare this is but sounds like you did an excellent job all round.

[perrin6]

I found that methylated spirits works better than Isopropyl alcohol (using BP cotton wool ) but I have a particular problem with some sort of oily spots on the mirror,

Alan

[Ian_A]

eased the dot off with a finger nail, then soaked the mirror for a few mins in warm soapy water to remove the glue backing. Washed off with lots of warm water...drained, then wiped (gently) with a new clear cotton clothe. No spirits of anykind used just incase it affected the coating.

[stev74]

And i had a nice clear view of jupiter.....Have to say that with the 3x barlow and the 10mm ep, i find the focus is poor, maybe the focuser or just poor quality ep and barlow !

It's more likely due to the magnification you are at, you would have to have a night with exceptional seeing conditions to get much detail with that combination - with the 10mm EP and 3x barlow the mag is more than 300x.

I find with Jupiter on most nights my 6mm Ortho gives the best, sharpest views at 166x but even then it depends on the seeing conditions. Your 3x barlow and 20mm should give you a much better and sharper view at just under 150x.

[brianb]

I don't know the exact figures but suspect that 5mm is a serious error in terms of enabling accurate collimation in an f5 scope.

Yes. The diameter of the coma free field at f/5 - irrespective of aperture - is 3mm - based on diffraction limiting. You clearly get more than this at low power because the diffraction pattern will not be resolved at low power.

[brianb]

you have been lucky the centre circle came off OK as I have heard of problems when taking these off that people have actually pulled off coatings!

Damaging coatings when trying to clean mirrors is quite common. That's why it's an emergency measure, not routine maintainence.

Actually it doesn't matter if you smudge, damage, or even remove the central bit of the coating as the central part is shadowed by the secondary.