Skywatcher-put the centre spot in the centre!

[mike-h]

I found out today that the little white ring that is meant to be at the centre of the primary mirror was actually about 5mm away form the centre.

Ha!

I bought the scope about 12 weeks ago and I spent ages learning how to collimate my skywatcher 250px. I thought I had it spot on (and checked with a barlowed laser).

Given the tiny tolerances for collimating an f4.8 scope I guess that 5mm may make a difference.

I had to take the primary mirror out of my Skywatcher Skyliner 250px today because I had noticed that the mirror was moving slightly in its cell (could hear a slight knocking noise last time I put it on the back seat of the car).

I pulled the mirror cell out and sure enough a few of the 6 clips were loose. I delicately screwed them tighter (not too tight I hope). I then got out the hurricane blower and blew off some of the bits and fuzz off the mirror (very satisfying!).

I then thought I'd check whether the little white ring was in the centre. I was pretty surpised at how far off it was. I decided to replace it and so I peeled off the ring sticker.

Decided to use a tiny blob of 'white tak' to mark the centre rather than the usual paper ring. It was easier to adjust and I thought it may give me a more useful round shadow for when I use a barlowed laser to collimate (the shadow of the paper ring appreared very faintly on the silver window of the laser collimator). It worked well and it saves having to use the 'disk with hole in + mirror pointing up focuser' to check the barlowed lasers return shadow.

The cell was a little tricky to get back in the tube (tight fit and the tube deformed slightly when you pull ot the cell).

Stil its all done and the collimating was a doddle.

If i was doing it again I'd use a slightly bigger blob of white tak to create a better defined shadow.

Next time I buy a newtonian i think i'll check the centre spot before I collimate!

Mike H

[beamish]

Crikey I feel for you mate, that just ain't funny. Bet you got your collimation routine down to a par tho

[Doc]

Maybe he had a few beers while sticking it on in the factory

Glad you sorted it out.

[Milamber]

Odd. Thought I had posted a message here! Anyway, point was - how are you going to use a single beam laser to collimate now if you have no access to the centre of the mirror?

Arthur

[mike-h]

I use the little dot in the centre for alighning the secondary (when the beam is right on top of the dot). I then put a barlow on the laser collimator and use the faint shadow of the dot (which falls onto the silver reflecting window of the laser collimator) to align the secondary. The idea is based on the barlowed laser idea for collimation that I read about in an internet article.

Mike-h

[proflight2000]

I suppose though in essence that you could still allow for a tolerence on 5mm, go on, zap that mirror out and realign the marking. When I had myskywatcher 200 I much preferred the laser to collimate, along with of course, the ever so unexpensive film cannister lid with a small hole drilled in the center.

[Milamber]

The dot will spread the laser somewhat you realise?

[acey]

Alarming to hear how far out the ring marker was. I'll certainly check mine whenever I get round to cleaning my primary - I prefer not to take it out until then.

Can anyone think of a way of checking the centre ring without removing the primary??

[Milamber]

Take a photo of it and measure it on the computer.

[mike-h]

Milamber

I only use the dot to collimate the secondary, so spreading of the beam is not a problem. I put a barlow on the laser for collimating the primary- this means i'm using the spread area of red light to return a shadow of the dot to the readout on the laser collimator. It's supposed to be more accurate than just using the return laser beam.

Mike-h