Dobs newbie collimation (yeah,i know,sorry)

[JohnR]

Had a 10' dobs for about a month now and think i'm ready to have a go at properly collimating it.I've done it to a fashion a few times but as of yet have only had to adjust the primary a little to get the centre dot lined up,i havent touched the secondary.

Using a pinhole lens cap the secondary is lined up nice and concentrically and the photo is what i see through a cheshire

I've read Astrobabys tuturial a lot,and that offset looks about right for my f5 dob to my newbie eyes (the primary needs to go up/left a tiny bit to centre it but i'm not worried about that)

What i'm not sure about how to correct is the crosshairs of the cheshire not realy lining up with the vanes,and it looks likes the crosshairs in the centre of the pic need to move to the right.

Is this correct and how do i go about adjusting it ?

Thamks for your time in advance

[JohnR]

another pic,not as good quality,but i think shows the (mis)allignment better

[Buzz Lightyear]

Hi John,

I had the same problem, my cheshire cross was out from the vanes, not sure what i did was right, but i adjusted the primary mirror with the little allen screws until i re-aligned both cross hairs and then made sure the mirror was dead center, then i had to adjust the secondary mirror to get the dots aligned, now my view looks like yours, but with the cross hairs matching.

would be interested to see if i had done it corectly.

P.

[sixela]

another pic,not as good quality,but i think shows the (mis)allignment better

The cross-hairs, because of diagonal offset, in general do not always have to be superposed with the spider vanes.

But if this photo is actually taken on axis there are two issues:

-the tilt of the secondary isn't set so that the centre spot reflection is under the cross-hairs of the sight tube

-the tilt of the primary isn't set so that the Cheshire ring reflection is concentric with the centre spot reflection.

[sixela]

would be interested to see if i had done it cor[r]ectly.

In general, no (in particular, it's hard to say, but if the correct view has a secondary reflection silhouette that is offset and spider vanes not under the sight tube cross-hairs, "fixing" what should be ignored will "unfix" what is important).

[Jason D]

What i'm not sure about how to correct is the crosshairs of the cheshire not realy lining up with the vanes,and it looks likes the crosshairs in the centre of the pic need to move to the right.

Is this correct and how do i go about adjusting it ?

Ignore the spider vanes. They do not have to line up. See attachments.

[Buzz Lightyear]

sixela,

great, now i have fixed, what should never have been fixed, how do i unfix what i have fixed to get it right?

p.

[sixela]

Simply use secondary tilt to line up the centre spot reflection under the cross-hairs, use primary tilt to make the centre spot reflection concentric with the Cheshire ring reflection, and ignore the spider vanes and the offset between the secondary reflection's silhouette and the rest.

[Buzz Lightyear]

cheers for that, you have 10 karma points.

[JohnR]

Thanks Jason and Sixela,i'll forget about the spider vanes then.

I doubt either pic is on axis,i was just holding a camera to the cheshire as best i could.In the first pic you can see i must have moved the cheshire out of line with the camera -i only just noticed that now .

The biggest hurdle is for me to understand what it is i am actually looking at,when i've got that a bit more sorted i'll feel more confidant about what to adjust.

I do see that the centre point isnt under the cross hairs so that will be my first port of call

Thanks

[LukeSkywatcher]

Yeah forget the spider vanes. They are distracting. If you get the black dot inside the "donut" ring..................you are pretty much bang on.

[JohnR]

Well i made a meal of that in the end.

As i said before i've never touched the secondary.The screw and the 3 allen bolts were so tight that i thought i'd best leave well alone,but the allen nearest the focuser did move a fraction.I checked the other 2 and they would now move too.Unfortunetely,even though i'd only slackened each bolt by a handful of degrees.just enough to take some pressure off I thought - the mirror moved - a lot

I admit to panicking quite a bit at this point

I have got it all back pretty much how it was,and i'll have another go when i'm thinking a bit clearer.

[Taff]

Hi' I have found this site good it is a movie showing you how to collimate a scope. www.andysshotglass.com/collimating.html

[JohnR]

Thanks Taff,pity you cant ffw and rwd the video though. astrobabys howto is great,nice and straightforward and easy to follow.

I've had another go,taking my time ,and now have the secondary lined up well i think.

[sixela]

The one thing about the Andy Shotglass video is that it seems to imply that everything (including the silhouette of the secondary's reflection) can be made concentric, and for fast scopes that just isn't right.

It's driven many people to the brink of insanity.

[Buzz Lightyear]

Hi JohnR,

I am in the same boat, the video helped alot, i will not be adjusting the secondary mirror again in a hurry, but at least i know what to do now.

Good luck.

P.

[Jason D]

I made a mod few years ago along with my Bob's Knobs upgrade that made adjusting the secondary mirror even easier. I made two washers from a Milk container and inserted them between the set screws and the secondary mirror holder. Not only the washers protect the secondary mirror holder but more importantly they added elasticity which allows me to fine tune the secondary mirror by adjusting one set screw without touching the other two. Think of it as adding micro springs to each set screw. The mod has been used by many and the feedback has been positive.

If you do not want to remove the secondary mirror to insert the washers, you can make temporary washers as shown in the second diagram.

Jason

[James]

I made a mod few years ago along with my Bob's Knobs upgrade that made adjusting the secondary mirror even easier. I made two washers from a Milk container and inserted them between the set screws and the secondary mirror holder. Not only the washers protect the secondary mirror holder but more importantly they added elasticity which allows me to fine tune the secondary mirror by adjusting one set screw without touching the other two. Think of it as adding micro springs to each set screw. The mod has been used by many and the feedback has been positive.

That's a pretty good idea and will help me no end. Unfortunately over time I've gouged small holes in my secondary holder and adjusting the secondary can be a real pain....

James

[JohnR]

That does seem a great idea Jason.I have to admit i was surprised (scared lol) how tight the bolts and screw were as delivered

[irishbloke]

Unfortunately over time I've gouged small holes in my secondary holder

James

James if u take the secondar holder off and glue a steel washer to back of it, it flattens the back and the steel wont pit as easy.

[Jason D]

James if u take the secondar holder off and glue a steel washer to back of it, it flattens the back and the steel wont pit as easy.

The fundamental advantage of using two plastic washers made from a milk container over a steel washer is elasticity. Plastic elasticity will act like micro springs enabling you to make fine adjustments with ease.

Jason

[irishbloke]

I actually used both the steel washer to fix my pitted holder and the washers to give the elasticity and avoid the holder turning as I adjusted the secondary thumb screws.

[Jason D]

I actually used both the steel washer to fix my pitted holder and the washers to give the elasticity and avoid the holder turning as I adjusted the secondary thumb screws.

You do not need both. In fact inserting both type of washers might be a problem for some scopes as there might not be enough clearance or gap to accommodate all these washers. The plastic washers made from a milk container will suffice. I found the main advantage of the plastic elasticity is not to prevent the holder from turning but rather to allow me to twist one set screw without the need to unscrew the other two. This is a huge advantage when fine adjusting the secondary mirror. And of course the plastic washers will prevent the set screws from gouging the secondary mirror holder.

Jason

[Astro_Baby]

I couldnt cope with another collimation thread I thought my head would explode if I tried to answer another one but its good to see the Gurus have helped you out

[JohnR]

I at least understand the principles of the job now,or more accurately,i know where i'm going wrong.

The problem i have with my 10in skywatcher dob is it difficult to line up the secondary properly.You can't get a view of the edge of the primary to centre it (without moving your eye offline and guestimating that each of the 6 clips are roughly the same distance out of shot)

I'm thinking i prob need a laser just to line up the secondary,unless anyone has any suggestions? Ta