Ways to check if mirror donut is not centeral?

[Luke]

I suspect the donut on my GSO dob's mirror may be off centre. Any ideas how I could confirm this without having to take the mirror out and measure it? Or should I just stop messing about and take the mirror out?

Thanks for any advice!

Luke

[rory]

I think I read somewhere where they took a photo and used that to work out measurements.

Can't be sure how Exactly it was done .

The pic was uploaded on to their PC and some software used to work out centre .

If your good with all thing tech / computers then that might be an avenue, if not I suspect dropping out the mirror cradle and carefully physically measuring it is probably the only other way.

[Moonshane]

taking it out is easy but a photo of the reflected primary (i.e. on the secondary) down the focuser would give a way to measure it also. what makes you think it's not central?

[Luke]

Thanks, Rory and Shane

Shane, what makes me wonder is that a Paracorr 2 made a huge difference to the view with my F5 GSO, more than I would expect given what folks say about the benefit of a Paracorr on an F5 dob being marginal. It isn't marginal on mine, it's a clear as daylight difference! The scope has gone from being a bit disappointing visually (not as good as my SW dobs) to being a sweet performer.

So I wonder if the slight Barlow effect of the Paracorr 2 allows more tolerance for the collimation. Also, high power views with my GSO are a bit iffy.

I have also seen a few reports of donuts not being central on GSO mirrors.

[Steve Ward]

Maybe a photo and Al's Collimation aid would help determine it .. ?

http://www.iceinspace.com.au/forum/showthread.php?t=62696

[Luke]

Thanks, Steve, I'll check it out! It would be great to know now if it is out and then I could wait until Summer if it is to sort it out, when the dob is on holiday

[sbooder]

Take out the mirror, put it on a piece of big enough paper, draw round the mirror using nice sharp hard pencil. cut out the circle, fold in half and in half again, snip the point, open out the paper, give it an iron and then lay over the mirror.  If the donut is in the cut out you are good to go.

[Moonshane]

Thanks, Rory and Shane

Shane, what makes me wonder is that a Paracorr 2 made a huge difference to the view with my F5 GSO, more than I would expect given what folks say about the benefit of a Paracorr on an F5 dob being marginal. It isn't marginal on mine, it's a clear as daylight difference! The scope has gone from being a bit disappointing visually (not as good as my SW dobs) to being a sweet performer.

So I wonder if the slight Barlow effect of the Paracorr 2 allows more tolerance for the collimation. Also, high power views with my GSO are a bit iffy.

I have also seen a few reports of donuts not being central on GSO mirrors.

I'd expect a noticable difference even with an f5 scope - this was considered quite fast a few years ago I believe.

I'd personally just take the mirror out and then measure (using a piece of paper marked with two marks for where the centre would be and where the edge should be - measure the diameter first). do this from several points around the circumference and they should all be in the centre and edge. if not, calculate what distance and direction you need the spot to be in and mark it. I'd personally fit another circle over the top of the old one if off, but they are easy enough to remove.

I suspect it will be OK though.