Collimating 250px

[EntropyStar]

Hi,

I'm having real problems collimating my 250px, I star tested it and it was poorly collimated so I thought i'd tackle it...an hour later and I think I've ended up with a rather expensive piece of drainage pipe!

I'm having problems aligning the secondary to the focuser, i'm using a cheshire collimating tool. I can't seem to get all 6 of my mirrors clips in view. When I try and adjust via one screw at a time it seems I can adjust it so I get one clip in view, then when moving onto the other screws to get the other clips in view there's no discernible pattern to when i'm adjusting them bringing them into view. I've zoomed focuser all the way to the bottom so I can bring as much of the clips into view as possible (in theory).

I've got the crosshairs lined up with the cheshire with one clip in shot. If I adjust the other screws I lose that clip as it moves out of shot and the others come into view, but when I try and balance all three screws there's no discernible pattern which'll incrementally allow me to get it centered. It's like i'm trying to control 4 axis of direction with only 3 axis of movement!!

I am chasing water uphill here? I can get the crosshairs lined up perfectly but no clips in view, no matter who much in move the focuser in and out...if the crosshairs are aligned does that mean my secondary is fine and it's time to move onto the primary?

Any thoughts would be greatly appreciated, I've read Astrobaby's guide and others but they don't seem to be helping me at this stage!!

[Moonshane]

with a standard Cheshire the view is not usually wide enough to see all clips. this is where the home made collimation cap comes in. to be honest a colli cap is most useful for checking roundness and position in tube of the secondary. you can align the secondary well enough using the donut and cross hair of the cheshire and effectively ignore the clips if you like. if your secondary looks round, and down the Cheshire, the donut aligns with the cross hairs then you should be fine.

personally, I find that a collimation cap is enough for the secondary and then I use a Cheshire with no cross hairs (a cheap plastic one) to set the primary. this works fine for me.

[EntropyStar]

Hi,

Thank for responding....

I believe the doughnut is aligned properly but I can see a slight oval shape to it, unless that's an optical illusion? It looks like the top righthand side (12-3) bulges slightly further out and as a consequence makes the crosshairs look slightly off centre...although they're aligned with each other. This sound right?

[Moonshane]

When I say the doughnut, I mean the one on the primary. this should be aligned with the cross hairs on the cheshire. don't try to get the cross hairs on the cheshire aligned with the spider vanes as they are unlikely to match.

see the schematics of the slow and fast system and you'll see what I mean. it's a simpler process than many guides would have you believe.

[EntropyStar]

In that case it's way out of line....the doughnut isn't near the crosshairs, just above them.

Back to square 1 then, thanks for your input, I'm gonna leave it tonight and start it again tomorrow.

[RikM]

When I first tried collimating, I got the Cheshire crosshairs mixed up with the reflection of the spider vanes

The Skywatcher Cheshire is just the right size to use as a sight tube and with the focuser racked fully out, the secondary almost fills the view through the hole and this makes it quite easy to see if it is aligned correctly under the focuser. Tweak the secondary screws to align the doghnut with the Cheshire crosshairs, then tweak the primary adjusters to centre the black dot in the doughnut. You may have to repeat this a couple of time until it all matches up. Ignore the clips on the primary.

[Moonshane]

In that case it's way out of line....the doughnut isn't near the crosshairs, just above them.

Back to square 1 then, thanks for your input, I'm gonna leave it tonight and start it again tomorrow.

take a deep breath and you'll get there eventually. you'll find your own way and then will wonder how you didn't get it at first!

a good tip is to cover the shiny angled surface of the cheshire while aligning the secondary. just put a ring of dark paper or whatever around the Cheshire eyepiece. this will leave just the hole at the top and the cross hairs viible. will make aligning the donut easier. then remove the paper and align the black dot into the donut for the primary. simples.

[umadog]

One thing you might want to keep in mind is that if your secondary is slightly under-sized then you mat not be able to see all the mirror clips anyway. The point is that you want to round the secondary and use the cross-hairs of the sight-tube to adjust the secondary so that the cross-hairs hit the donut. You then use the Cheshire portion of the tool to adjust the primary.

Remember that the point of the exercise is to achieve those things. Seeing the mirror clips may be a useful indication that you're on-track, but it's not an indicator that you've aligned things correctly. After all, some Newtonians have no mirror clips at all...