Dob arrived - collimation issues

[LaFey]

My dob arrived (see: http://www.iceinspace.com.au/forum/showthread.php?t=9167 )! I am now going crazy about collimation (consulting the excellent article at: http://skyandtelescope.com/printable/howto/scopes/article_790.asp)... I've been all afternoon around my scope doing it and wishing I had plastic arms in order to reach the primary screws while looking through the Cheshire :) (in fact the secondary alignment was much more difficult...). I need to do the star test now...

One of the spider vanes was bent (not by me! out of the box) out of shape but I only noticed it while looking through the collimation cap since one of the lines was "fatter" than the others - I aligned it the best I could with my fingers and I believe it is nearly as good as the others in terms of width. It isn't yet straight as it should be, but I believe fiddling with it too much will do more harm than good.

Ok, so - here's my question: While I am aligning the secondary with the focuser, using the Cheshire crosshairs, I use the three collimation screws in the secondary - fine. Done it. The next phase should be to center the primary mirror's reflection. But since this is done with the same three screws am I not messing what I did earlier on?? :shrug: What's the point of the first step then? I suppose it is important IF you need to use the center screw in the secondary to center it... Am I right?

[ving]

spider bent from shop? send it back! thats how i'd start....

shouldnt you use a sight tube to align the secondary to the focuser?

[LaFey]

Ving: Yes you are totally right! My Cheshire also works as a sight tube. Sorry I forgot to mention that.

On returning the scope to shop - I only saw it bent after I had already assembled the dob base. Sending the telescope back would involve time and money I can't afford right now. I think the re-alignment I did is pretty good but of course, not 100% perfect.

[ving]

when aligning the secondary via sight tube you should be using the middle screw to move the secondary backwards and forwards... you are trying to get the secondary directly under the line of view and centered under the focuser. after this use the 3 collimation screws inthe secondary to center the primary in your fov.

[Starkler]

One thing you should do that isnt often mentioned on these collimation tutorial pages is to accurately centre your spider in the tube. Dont assume that it will come correctly fixed from the factory, mine wasnt :(

You should only need to do this once. If its off to one side you can mess around for ages wondering whats wrong when the crosshairs never line up with their reflections.

After you have done that align the secondary to the focuser tube and then adjust tilt so you can see the reflection of the primary and its mirror clips centred under the focuser. Yes this is the fiddly part for sure.

Third step of course is to adjust the primary centre spot to line up with the cheshire illuminated ring, not the crosshair.

If and only if the secondary is aligned correctly will the crosshair reflections stack on top of each other in the centre of the reflected centre spot as viewed in the cheshire. If its out a bit it means that your optical axis isnt aligned to the focuser axis, giving a slightly tilted focal plane.

[LaFey]

Thanks! That was my doubt. The secondary was well centered from factory (thankfully!).

Since I've done everything correctly I'm keeping my fingers crossed the star test won't let me down...

[cjmarsh81]

Starkler. Your comments lead me to believe that my spider vanes need adjusting. Doh!. I can never get the lines of the crosshairs to line up spot on. Oh well, I knew it wasn't perfect.

[Vermin]

Yes you are, well spotted!

Collimation from badly aligned optics is an iterative process and will require you to go back and forth between steps to get it spot on.

Keep to the recommended order of steps, but don't be afraid to go back a step and repeat if required. You will quickly home in on alignment this way and in future the steps will only need to be performed once in the usual order.

[Starkler]

Can we/should we, pin this thread?

[ving]

dont think its necesary geoff :)

[Starkler]

Ok Dave, you can answer this question next time its asked ;)

[LaFey]

Thanks for your replies!

http://img133.imagevenue.com/loc69/th_03801_views.JPG
I picked up a pencil and did a small drawing of what I saw when looking through the (left side) sight tube/cheshire and (right) collimation cap.

The crosshair reflections stack on top of the reflected centre spot and on top of the spider vanes. This looks good - what doesn't look good is the area around the reflected centre of the Cheshire and of the reflected collimation cap - should it not be perfectly round? The drawings are a bit exaggerated but the area is definitely not circular: it grows elliptical (?) towards the upper right edge of the view.

[matt]

I could be mistaken but I think what you're describing (or have drawn) is actually good news. I think your collimation has been successful enough to include the secondary offset, which appears as that slight skewing around the central obstruction.

I think a collimated reflector shows this slight shift when the offset's about right???

What you need to do now is get it out under clear skies and steady seeing and tweak the collimation using a star test.

Well done!:thumbsup:

[RAJAH235]

LaFey, sounds about right. 2ndary offset is what you are seeing.
Here..> http://home.att.net/~dale.keller/atm/newtonians/notes/diagonaloffset.htm
HTH. :D L.

[Blue Skies]

That looks good to me too. It is, as the others say, just the secondary offset you are seeing. As long as the dot on the primary is in the centre of the cheshire you should have a good sharp image.

[LaFey]

I tried to do the star test today but the sky had much turbulency...

[ving]

let us know how your star test goes after you hae done it... looks good tho :)

[LaFey]

Sure, I will! :)

I want to try and observe Schwassmann-Wachmann 3 comet so I want to do the star test as soon as possible. Unfortunately I have to wait for a good weekened to move away from the city - from what I am able to see (or not!) the turbulence from my house is intolerable.