Screwed up my Dobs? - sorted now (+ mirror fixings advice!)

[rikyuu]

Help! After getting close to a good shot of Saturn over the past few days, I finally cleaned my mirror (very carefully following plenty of advice) and used a new laser collimator to collimate the scope. But now I'm getting stange viewing, please help!

Stars are looking like 2 dots next to each other, as I go away from focus, they open out into a triangle instead of a circle. Going out of focus further creates the usual donut, but with 3 points around the edge, what have I done?!! I recollimated using the hole in the lens cap technique as before, but it's still the same. I just can't get good viewing. Why am I seeing these double stars and 3 points when out of focus? Images added

Please help, panic is setting in, as I'm not sure what is wrong.

Thanks!

(click to enlarge)

(click to enlarge)

[Ant]

First thing it don't panic.

I am no expert (there are others that will be able to advise).

But providing that all you've done is carefully clean the mirror then I'm sure whatever the problem is will be fixable.

I could be something as simple as the the little clips that hold the mirror in place are too tight - there are three of them. Could be something else.

I'm sure that someone will have seen something similar!

Ant

[rikyuu]

First thing it don't panic.

I am no expert (there are others that will be able to advise).

But providing that all you've done is carefully clean the mirror then I'm sure whatever the problem is will be fixable.

I could be something as simple as the the little clips that hold the mirror in place are too tight - there are three of them. Could be something else.

I'm sure that someone will have seen something similar!

Ant

Thank you Ant, I think that might have been it :hello1:. I loosened the (actually 6) clips on the mirror, recollimated with the laser and a quick check outside, everything seems ok. Not had chance to fully test it yet as it's getting late, but I think that's sorted it. Thank you so much, I was really panicing that I had screwed up the mirror by cleaning it somehow, although I coudn't think how.

I'm amazed that the mirror can be distorted as the glass is over 1 inch thick, like a submarine window! (Although I know glass is a slow moving liquid!). I was trying to think why it was 3 points I was seeing in particular but couldn't have imagined it was the rubber clips.

I'll have to wait for another night to try it out fully, but hopefully it's ok now.

I would be really interested to hear if anyone else has seen this issue before, it's quite a shocker , hope I don't have too many like that!

PS, looking at the second image, it does look like there are 6 sides forming to the 'donut' and I remeber going further out of focus did create a kind of hexagon shape

[Ant]

Hopefully you're sorted - glad you took pictures because I have never seen that before (but I've never removed a mirror from the cell for cleaning before).

So do you think you're sorted now or have you a gone from a 3 sided doughnut to a six sided one?

Ant

[rikyuu]

Sorry, the two pictures were both from when it wasn't sorted, just one more out of focus than the other. Now I'm seeing proper donuts and Saturn was pretty clear so hopefully ok now.

Cheers

[Ant]

Result!

That's excellent news. I bet that's a relief?

Ant

[russ]

Excellent news. And well done Ant on the advice. :hello1:

Let us know how that view of Saturn is! Fingers crossed for a stunning view.

Russ

[rikyuu]

Yes thank you again Ant. I managed to find a gap in the clouds for an hour tonight and gave it another check. On my max maginification (375x, 6.4mm lens with 2x barlow) I could still just see 6 faint forming dots around the diffraction rings when going out of focus. I loosened the 6 mirror clips a little further until the mirror was just held in by them and the 6 points vanished. Now getting perfectly round diffration rings and a lovely view of Saturn (which I never tire of).

I'm guessing not many people remove the primary mirror (especially as beginners, but I'm an engineer you see, I have to dismantle and investigate everything!!), but the tightness of the retaining clips is certainly something to watch out for when reassembling it.

I spoke to my colleague about it, who went on an optics course, he said it would only need to bend the mirror by a matter of wavelengths of light to create this type of distortion and 1 inch is really quite thin for a 10" mirror.