Cheshire crosshairs

[Planetesimal]

I've a question about using the Cheshire collimator... When I look through, I can line up the shadow of the secondary inside the centring circle on the primary, and the reflection of the spider vanes lines up in the middle of the central reflection. Plus I can see all 3 primary mirror clamps reflected in the secondary.

So far so good, I think I'm pretty well collimated... The question I've got relates to the cross hairs of the collimator itself - when I look through the pinhole in the Cheshire, I can see the crosshairs don't line up with the spider vanes etc. reflected in the secondary.

So, should they?

[Spec-Chum]

Nope, in a well collimated scope they actually shouldn't line up.

[Planetesimal]

Ah, mine is definitely not like this ( although my 'scope is F8, which might make a difference?)...

In the view through the Cheshire, the centre spot is perfectly aligned with the reflection of the spider vanes, but the crosshairs are offset to the right of the centre spot. Looks like I'll have to check the alignment of the secondary.

[Spec-Chum]

Yeah, that picture represents an f6 or faster scope. The offset becomes less as the scope gets slower.

Always align the cross hairs to the centre spot, ignore the spider vanes

[Planetesimal]

Thanks Spec-Chum...

When I star tested with a bright star, I got nice concentric rings on the Airey disk, but focusing in on the star I notice the flaring comes out to the right of the star image just before focus is reached. I guess that would make sense if the cross hairs are to the right of the centre spot...

I'm guessing that it will be the secondary that needs alignment? I'm slightly nervous of messing with the secondary, as I've only ever adjusted the primary until now, given that my "slower" 'scope should be relatively tolerant of mid-alignment compared with F6 and lower...

[Spec-Chum]

No worries.

Only needs a slight tweak by the looks of it.

The secondary being slightly off is not a critical as the primary being off.

[Planetesimal]

Realisation!

I just had another look down the Cheshire, and realised if I roll the draw tube back and forth, the cross-hairs move back and forth across the line of site, and the centre dot apparently travels in and out of the centre circle. Obviously this is not impacting the primary mirror, but effectively this means collimation of the light path is not possible, even if the primary and secondary were spot on, due to excessive slop in the focusser...

[Planetesimal]

Panic over!

I just spent an hour taking the rack & pinion assembly apart and putting it back together, cleaning off the synta gunk (which was literally gluing my fingers together!), and regreasing with lithium grease. I tightened up the focusser wheel assembly and adjusted the grub screws next to the focus-lock thumb screw, and hey presto! Everything now turns smoothly and controllably, there's no more slop, except right at the extremes of travel, and most importantly the collimation holds while adjusting focus (with cross hairs sitting where they're supposed to).

Learning something new every day at the moment!

[Spec-Chum]

Nice one.

I must have been lucky with my stock focuser. It's as smooth and true as you like. I did have an issue with a slight grinding noise some weeks ago but this seems to have resolved itself. I'm guessing some grit had got it but got out again.

[Planetesimal]

The 150PL comes with the old rack & pinion style focusser, I'm guessing the crayford usually fitted to the 200P would be a little better!

Anyway I'm loooking forward to my next shot at Jupiter to see if it makes any difference. Thanks for helping out.

[yesteryeargames]

I am yet to collamate it perfectly , i end up making it much worse every time i have followed all the beginner guides

but the best i can get is this,

the Big black solid circle, 3rd from the centre of the middle of the crosshair i cant seem to centre? is that the reflection of the callmation cap ?

i can get the tiny black dot right in the centre ,

any thoughts please its driving me crazy i have followed the astro baby guide watched dozens of youtube vidoes.

[yesteryeargames]

I have tested it i can see the 4 bands of jupiter and get a good view of stars, altho they twinkle a little ( is this a common issue for dob 200p skyliners )