Cheshire question..

[emadmoussa]

Ah well, while it's not perfectly aligned with the focuser, I gave it a star test last night and the in-focus and out-focus stars seemed 95% circular, I think I need to give my primary a minor tweak. Apart from that, everything was pin sharp.

[Daniel-K]

Ah well, while it's not perfectly aligned with the focuser, I gave it a star test last night and the in-focus and out-focus stars seemed 95% circular, I think I need to give my primary a minor tweak. Apart from that, everything was pin sharp.

i wouldnt get obsessed over it for visual,

[Moonshane]

if your focuser looks pretty much square and the OTA has not been bashed so much that it's dented then there's plenty of adjustment in the optics to account for a small error in the focuser alignment.

you cannot see the primary by looking into the side of the cheshire - you only look down the small hole.

remember that the procedure is done in the order stated for the following reasons (I am assuming your focuser is pretty much there and that your vanes are equally spaced) and also calling your sight tube/cheshire combo a cheshire):

step 1 - align the secondary centrally under the focuser. what this does is ensure that the cheshire central hole is in line with the centre of the secondary. this ensures that all light hitting the secondary (ignoring efficiency of coatings) is reflected through the eyepiece. this adjustment involves the movement up or down the OTA (using the central bolt) and or rotation of the secondary assembly. the effect of this being wrong by a fraction is that you don't get all the light your primary gathers but you may not see this at the eyepiece.

step 2 - align the secondary with the primary. this ensures that the centre of the primary points at the centre of the secondary. this ensures that all light gathered by the primary is sent up the tube and then reflected by the secondary which in turn (as we just centered it) is all sent up through the eyepiece. this adjustment involves the three secondary adjusters and your are centering the doughnut on the primary with the (usually blurry and large) cross hairs of the cheshire. when done for the first time you may have to go back through step 1 and step 2 a few times before they are right. the good news is once done they hardly need adjustment again unless you hit the scope hard and even then.... the effects of this being our by a fraction are similar to step 1 and not critical. do try and get the adjustment as close as you can though.

step 3 - align the primary with the other components of the optical axis which were adjusted in steps 1 & 2. this ensures that the efficiency of the system is at its highest. it involves simply adjusting the three primary adjusters until the dot (which is the hole in the cheshire) is centred in the donut. do this very critically as small adjustments make a real difference here. the effect of even a slightly off primary adjustment is often seen at the eyepiece and affects contrast, sharpness, levels of detail and even appearance of coma. this adjustment is so easy and quick I'd recommend you check it every time you observe. maybe check steps 1 & 2 every now and again.

I hope this helps a bit as collimation is really quite simple once you get the hang of it and especially so with a cheshire.

much of the above is repeated here which I intended as a simpler guide than many available online.

http://stargazerslounge.com/topic/197640-collimation-and-star-hopping/

[emadmoussa]

Everything is perfectly. centered and aligned now. Thanks Shane! I can see it now...

[Pig]

Easy when you know

[Steve Ward]

27:30 onwards for centring focuser .... 

[emadmoussa]

Didnt need to adjust the focuser though. Just moved the secondary and aligned accordingly.

[michaeldurban]

once you know how to 'cheshire' a laser colli becomes totally obsolete..

even in the dark...

I started using a laser colli...way too unpredictable, even when you use a

self centering adapter.

like I said in another post..it's still not 'perfect' but it is perfect for visual use.

(the 'white spot is a light reflection, the centre spot cannot be seen in this pic but

the crosshairs are dead centre over the spot)

If you do need to use a laser use the barlowed laser technique by Nils Olof Carlin.

(and even that technique is only applicable to the primary)

good luck, try and try again, you will fall in love with the accuracy the cheshire has..

and always will have..

[michaeldurban]

and please don't fret with the 'squaring of the focuser'..it has no impact on

your collimation..unless of course it is so skew that you have to do a handstand to look

through it...

[michaeldurban]

one last remark..

Dion (if that is his name) from Astronomyshed makes good and informative

video's on youtube..

but I think he just goes overboard with his squaring, and spotting and collimation

and and and....

** follow collimation guides (like Astrobaby's) but please do not think that Dion's

collimation videos are the standard, they are very involved and in most case

absolutely unnecessary.