Have I just ruined my primary mirror?!

[Vega]

Please help, very worried astronomer!

After 12 years of use, I cleaned my primary mirror the other day taking great care not to ruin any coatings. I researched and followed several videos and guides online.

I submerged the mirror in a bowl of luke-warm water with one drop of fairy liquid diluted. Let it soak then rinsed. Then used an individual cotton wool piece under it's own weight for one, smooth brush across the mirror in one direction. Repeated this with new cotton wool until the whole mirror was cleaned. I then did the final rinse with de-ionised water and left almost upright to dry (most water drained off right away anyway).

I put the mirror back in the back plate and all back in it's place in the original orientation (used markers for this). Next I re-collimated all seemed fine. Got it bang on. 

Anyhow, after trying first light tonight, it seemed like I could not achieve focus (see video below). This was my DSLR attached to the scope on 3rd magnitude star Almaaz. When getting close to focus, it appears to develop streaks then goes through the focus point (star does not appear to reach true focus), followed by the streaks again in the other axis. The out-of-focus circle appears to look more like a hexagon rather than a circle.

I've also taken a photo of the primary mirror before treatment, after and now in the scope tube. If you look closely on the third image, I 'think' I can see faint streaks across the mirror from each cotton wool clean. Obviously being a flash photo you can see every speck of dust on there but this is to be expected.

My worrying questions are:

-Is my mirror ruined?
-Should I attempt to somehow remove the streaks?
-If so how?
-Am I missing something and something else could be causing the focusing issue?

 

[Ouroboros]

@Vega I wonder whether you have distorted the mirror slightly by over tightening the screws that hold the mirror.  Loosening off a little might help.  More expert opinion will be along in a minute. 

Edited April 16, 2020 by Ouroboros

[mikeDnight]

You may have overtightened the mirror restraints. They should barely touch the mirror otherwise they deform the figure by twisting the glass.

[jonathan]

When I cleaned my primary I just placed it on a face flannel in the bath and used a warm shower to rinse, then very lightly moved a blob of soapy (washing up liquid) suds around the surface with my fingers, rinsed, then used de-ionised water for the final rinse.  I didn't have any streaks after that.

I would suggest that you have another go with the soapy suds, don't bother with a cotton bud, just use a very light touch with your fingers (no pressure at all).  De-ionised water again, but leave it at a 45 degree angle somewhere warm on a towel.

Examine the mirror before fitting, flash photo it if you like to check for invisible streaks.

 

EDIT: Kind of surprosed you haven't flocked the inside of your tube, I did this on my Skywatcher 150P and I think it did make a slight noticeable difference in contrast, must surely be worth while for an imager?  I concentrated on the upper area near the secondary mirror (also flocked the edges of the secondary) so probably almost half of my tube is flocked.  Trick is to vacuum it (gently) using the brush attachment to pull out any loose flock bits before use.

Edited April 16, 2020 by jonathan

[johninderby]

When I clean a really dirty mirror I soak it in soapy water then rinse off with plain water and dab dry with a cotton balls then clean with Baader Optical Wonder fluid using a brand new cloth. Leaves a spotless mirror.

Only time I would use deionised water is if cleaning a plain aluminised mirror. Mirrors from Skywatcher etc. do have protective coatings.

Also when tightening the mirror clips I tighten them until they just contact the mirror then back off about an 1/8 of a turn.

Edited April 16, 2020 by johninderby

[Vega]

Ouroboros and MIkednight you both may be right. I tightened them quite tight from memory. It’s been screwed in for a few days now. I presume the twisting I f the glass will not be permanent? 

[steppenwolf]

The issue here is almost certainly pinched optics during the re-assembly process - the mirror clips should touch but not 'grip' the mirror. The sharp 'points' on the defocused star pretty much point to this.

[Vega]

Thanks for confirming Steppenwolf. Will loosen them first thing tomorrow! Shame I missed a gorgeous night for imaging tonight. 

Jonathan, will certainly consider flocking

 

[Jkulin]

Can I ask, has anyone cleaned their mirror whilst in situ, on a warm breezy day with my truss it would be very easy to do and then let the air dry it out, as an RC it would make enormous sense to leave it in, the only thing electrical would be the fans and if done with the OTA on its side or even slightly pointing down, it would make good sense?

[Gina]

I think pinched optics too.  A dirty mirror wouldn't do that and I can't see that you could have done it any damage in cleaning that would give that effect..

Edited April 16, 2020 by Gina

[Vega]

Will un-tighten today and report back tonight... oh wait clouds rolled in of course! 🙄. 

[Shimrod]

On 16/04/2020 at 22:52, Jkulin said:

Can I ask, has anyone cleaned their mirror whilst in situ, on a warm breezy day with my truss it would be very easy to do and then let the air dry it out, as an RC it would make enormous sense to leave it in, the only thing electrical would be the fans and if done with the OTA on its side or even slightly pointing down, it would make good sense?

I cleaned the mirror on my LX90 without removing it by following the guide:  Cleaning a Meade LX Mirror

[Peter Drew]

Does the mirror really, really need cleaning?      🙂

[Gina]

I cleaned the mirror in my MN190 Mak-Newt in situ.  Didn't want to disturb anything.  Took the focuser off to get at it.  And yes, it certainly did need cleaning!

[wimvb]

15 minutes ago, Gina said:

I cleaned the mirror in my MN190 Mak-Newt in situ.  Didn't want to disturb anything.  Took the focuser off to get at it.  And yes, it certainly did need cleaning!

How did you ever manage that? The mirror is in a baffled, closed tube. I would imagine that taking the cell out would be much easier.

[wimvb]

On 16/04/2020 at 23:33, steppenwolf said:

The issue here is almost certainly pinched optics during the re-assembly process - the mirror clips should touch but not 'grip' the mirror. The sharp 'points' on the defocused star pretty much point to this.

And so does the severe astigmatism. Btw, @Vega, have you had opportunity to test the scope yet. I read a long time ago that the prudent way to fasten the clips is to tighten them only so much that you can just slip a thin sheet of paper between the mirrors and the clips.

And as far as water is concerned, this will always leave stains. When I was still active in the semiconductor industry, we used to apply an iso-propanol rinse (from a squeeze bottle) after a deionised water rinse, and dry with filtered compressed air. Anything to avoid water stains.

[johninderby]

Once restored a Meade SNT10 that had been in a fire and the owner had sold it to me very cheaply as scrap that I might be able to salvage a few parts from.Tube was blackened in places and the miiror was plain black with soot. Had to wash the primary mirror three times in warm soapy water  before I could even see the mirror’s surface. and surprisingly once I’d finished cleaning it the mirror was good as new. Corrector also took a lot of cleaning. The Baader Steeltrack focuser was full of soot and scorched and had to be completely dissasembled and cleaned but also came up like new and used on another scope. SW focuser replacement  Covered the tube in silver vinyl and it was a working though very heavy OTA.

Edited April 18, 2020 by johninderby

[Gina]

43 minutes ago, wimvb said:

How did you ever manage that? The mirror is in a baffled, closed tube. I would imagine that taking the cell out would be much easier.

The hole for the focuser is big enough for me to get my hand in.  As for taking the end off, the screws were rusted in!!

[wimvb]

1 hour ago, Gina said:

The hole for the focuser is big enough for me to get my hand in.  As for taking the end off, the screws were rusted in!!

Ok, that makes sense.

[Dragon_Astro]

Others might confirm this, but a tip I’ve been given in the past is that the mirror grips should be just tight enough to just about slip a piece of paper under? 😊

[HunterHarling]

On 18/04/2020 at 04:05, Gina said:

The hole for the focuser is big enough for me to get my hand in.  As for taking the end off, the screws were rusted in!!

I'm curious why the primary actually needed cleaning, considering that it has a glass corrector in front. My C8's primary is still virtually spotless after a few years of having it.

[Gina]

It was left out in a damp observatory for years.

[Vega]

Hi all sorry for the late report back... blame the weather. We'll what can I say. Stargazerslounge never disappoints! Thank-you all for your sound advice. I would probably have not thought of the tightness issue on the mirror. 

I can now achieve perfect focus. Confessions time, it appears I had tightened the screws as far as they could go. Having loosened them all round and had another go I could see an immediate improvement. Zooming in to 10x on the DSLR live view however still showed the spiking affect. I took it all apart again and loosened each holder until I could see a clear gap, then slowly tightened until barely touching the mirror. I was worried this wouldn't secure the mirror enough but it certainly does keep it solid regardless. Mounted it back up and tried it on a mag 3 star. Perfect focus!

Thanks again

[Filroden]

4 minutes ago, Vega said:

Perfect focus!

Glad you got this resolved. Isn't SGl a wonderful place? 

[johninderby]

Glad it was simply a bit of easily sorted overtightened clips after all. 👍🏻