Cap on Mirror Accident

[deadsimple (Ash)]

I was setting up my week-old Dobsonian tonight and I somehow managed to launch the Finder scope's cap into the air, and it homed in on and fell into the OTA which was near vertical! I shone a torch down there and the mirror seems to have a mark or scratch (don't know which) on there about 5-10mm long

Now the question is .. from people's experiences, is a falling plastic cap enough to scratch a mirror like that or is it more of a removable mark? Would it degrade the image much? Any ideas on how to fix something like that?

I'm about to pull the primary out for the first time (still a newbie) and have a look. Was just hoping for some opinions.

[deadsimple (Ash)]

Update: seems like the mark wasn't permanent, no scratch .. all gone now (very light pressure with moist cotton ball). Now I'm having trouble drying the water that's on the surface of the mirror. The water leaves behind a residue ... reading the mirror cleaning article, I realise I don't have any distilled water (would have been good for me to read that before proceeding but I panic when something is wrong and try to fix it straight away )

Guess I'll head to the servo right now and grab some distilled water, unless someone has a better idea?

[Hagar (Doug)]

Originally Posted by deadsimple

Update: seems like the mark wasn't permanent, no scratch .. all gone now (very light pressure with moist cotton ball). Now I'm having trouble drying the water that's on the surface of the mirror. The water leaves behind a residue ... reading the mirror cleaning article, I realise I don't have any distilled water (would have been good for me to read that before proceeding but I panic when something is wrong and try to fix it straight away )

Guess I'll head to the servo right now and grab some distilled water, unless someone has a better idea?

Make sure it is distilled water and not demineralised water there is a big difference. Small marks on the mirror are of no real concern as you focus way past any marks.

[Rick Petrie]

It is probably best not to try and clean an isolated section of the mirror until you can clean the entire mirror with detergent and then a rinse over the whole mirror with distilled water and left to dry as per recommendations posted here on IIS in the projects and articles section.
A small mark on the mirror will not effect overall performance and, as many have said before, optics should be cleaned only occasionally and when really necessary as mirrors are very fragile and should be handled with extreme care. Some people leave their dusty mirrors for months sometime years without cleaning and views are still good.
If your scope is fairly new you should not need to give your optics a clean for some time.
Cheers Rick

[deadsimple (Ash)]

Originally Posted by Hagar

Make sure it is distilled water and not demineralised water there is a big difference

Oh dear. I asked for distilled water and the guy at the servo gave me demineralised .. I thought it was the same thing . EDIT: I'm using the exact same bottle shown in the tutorial.

I just applied some of it to the part of the mirror that needed the residue removed, didn't help .. still got residue. No permanent harm done with the demineralised water though, right? I hope it's just a case of getting some real distilled water (I assume from the supermarket?) tomorrow and cleaning up the mess?

Thanks, sorry for all the silly questions ... probably would have been best to not touch anything.

EDIT: added extra reply:

Originally Posted by Rick Petrie

It is probably best not to try and clean an isolated section of the mirror until you can clean the entire mirror with detergent and then a rinse over the whole mirror with distilled water and left to dry as per recommendations posted here on IIS in the projects and articles section.

Seeing as I have already attempted to clean an isolate section, should I just continue getting the residue off or just give the whole thing a clean?

Why oh why didn't I wait for replies before cracking it open ... impatience

[Rick Petrie]

Hi again
If you have taken the mirror out it is probably OK to give it a clean whilst apart but be very careful in how you do it and as stated follow those instructions in IIS projects and articles.Only wipe with gentle strokes and don't rub at all. Only apply as much pressure as the weight of the water on the cotton balls.
Only use a couple of drops of dishwashing detergent and make sure you don't let let your mirror dry before rinsing and flushing with distilled water as any traces of detergent will leave a smear on your mirror. Let dry naturally without trying to touch the surface of the mirror. If there are any smears left, it is probably best left alone.
Hope this helps and after taking your mirror apart and reinstalling you will now have to collimate your scope but that's another project that you have to do or you may already know how to do.
Good luck and be very careful!!!!
Cheers Rick

[deadsimple (Ash)]

Thanks for the replies.

I ended up getting most, but not all of the residue off that little section without touching/cleaning the whole mirror - it was incredibly stubborn. It's not that visible when I look at my reflection in the mirror under decent lighting, but I can see some minor bluey-white residue when looking at the mirror at an angle under a strong white light. I guess the main thing that stands out is that most of the mirror is dusty and this 1/5th of the mirror is dustless but with some residue.

Would that be good enough or can it really affect observing and astrophotography?

(I've read in a couple of places that it will always look dirty after a clean if you do the torch test, and just to do it by eye near good light instead - if it looks fine then that's good enough)

[astro_nutt]

Here's another test to try..have the optical tube horizontal remove the dustcap and look at the primary mirror...if it looks clean..then leave it...and if it's looks dusty or has a few tiny smudges...leave it for 6-12 months and then look again...if it hasn't changed much..then leave it for another 6-12 months...your images won't be greatly affected by the amount of dust that would cover a tenth of the area of the centre spotting ring on the primary mirror spread over the surface of the mirror itself!
Relax and enjoy the views!!
Cheers!