Newt Primary mirror centre spot

[jam1e1]

Hi

Whilst flocking 10" scope thought i'd check if centre spot on the primary was accurate, checked with digital calipers first - seemed a bit out then tried finding it using string crossed across 6 points, its slightly out as displayed so but a blue dot on it(the ring not glass!) so could tell when colliating which 'edge' to lean towards. Out of interest where do you get new centre spots from if i want to change?

Also i hope the small blue dot wont affect viewing......

Pics attached

cheers Jamie

[ponytale]

jam1e1, try cuting a circle out of paper the same diameter as your mirror ,fold in half very precisely and then fold in half again , snip off the corner point only and you have centre of your circle, use as a mask to mark your mirror, ps the spot lies in the shadow of your secondary so will have no effect on image .

another way is to use drawing tool in word , select donut from shapes menu and scale outer circle to your mirror diameter using rule function and scale centre down to spot size, print and cut out. tony

[Kai]

People have used Ring binder reinforcers to replace the center spot.

[jam1e1]

cheers guys

[theodore]

Hi,

if you are replacing it - don't use a reinforcer ring or similar. My new Orion Optics UK 12" mirror came with one of those, and the first time any dew formed on it and ran down the mirror (which ok I shouldn't have let happen, but it often can) it left a trail of glue residue across the surface. If I were marking a mirror I would use a good quality permanent marker (e.g. Sharpie) and mark a 2-3mm spot at the exact centre (which you can cut in the paper template as above). If you are laser collimating, it will still reflect fine off the marker spot.

BTW, marking the primary anywhere from the centre out to the diameter of the secondary will have no effect because it is in the secondary's shadow. It does not reflect any light.

Theo

[Stephen]

BTW, marking the primary anywhere from the centre out to the diameter of the secondary will have no effect because it is in the secondary's shadow. It does not reflect any light.

Theo

Which can be a god send when removing old centre spots that are araldited on and require a little firm persuasion to remove ....

Slightly scuffed centre on one of my mirrors, but doesn't matter

[assasincz]

I recently added a new mirror centre spot to my 4.5" newtonian, which had non as default. I decided to use a small blackened bolt washer and was thinking about how to get it dead centred on the primary.

I have made target circles on a sheet of A4 paper, with the innermost circle of 114 mm (exactly the mirror's diameter). Having blackened the washer, I have placed it dead centre in the middle of the target and supplied it with a few dabs of quick glue on the top.

Then I took my primary mirror (out of the mirror cell), flipped it over and lowered it carefully to the target, making sure that I have it as perfectly alligned to the innermost circle as possible. When it was resting on the paper, the washer stuck to the mirror surface and before the glue hardened, I remeasured it to make sure it is dead centred - and it was, so no adjustments were not needed.

I am well pleased with the result, although it might not be practical for a 10" mirror. And why have I decided to use a small bolt washer? Well, it was the only thing at hand at the time. Come to think of it though, I am not sure why I have blackened the washer in the first place

By the way, a good way to measure the accuracy of the centre mark might be to take a digital picture of the mirror from above and then check pixel count from the centre to the edges of the mirror.

[acey]

Interesting thread: I wonder how many scopes are sold with centre-rings that aren't really centred (leading to endless headaches when trying to use laser collimators with them).

My old Orion 8" had an unspotted primary and I used a self-adhesive paper disc (didn't need a ring as I didn't have a laser). Never had a problem with glue running.

My Skywatcher 12" came with a marked primary and I've never yet had it out of the tube, but it's due for a clean, and I'll be sure to check that the ring is centred.

[Rustmonkey]

You can buy " hot spots" which is what Dion uses. You would have to ask him where he gets them from though.

[assasincz]

Problem for me though is that a lot of these astro gadgets are unavailable from czech suppliers...I was lucky enough to get hold of the Celestron red light torch...so bolt washers for me it is then