Collimation question (with picture)

[barees63]

Hi All,

I have a 10" GSO dob and I'm still struggling a bit to understand the whole collimation thing. I purchased a catseye cheshire with the center spotting template and today I removed the old center spot, put on the catseye triangle and attempted to collimate my scope. I have also ordered a Techron site tube to configure the secondary correctly but haven't received that yet. The procedure I followed was to use my laser collimator to tilt the secondary so the beam hit the center hole of the catseye spot then used the catseye cheshire to align per instructions by tilting the primary. What I ended up with was something like this:
http://www.zitafarm.com/astro/collimate.jpg

So, my question is, why is the cheshire reflection (gray ring) so far off center? I know my laser collimator is dodgy (although it is correctly collimated), it was so poorly machined that the centre spot moved 2 cm when tightening the focuser set-screw, in the end I wrapped it with tape to make the fit tight like the cheshire (which is a perfect fit). I'm assuming this all has something to do with secondary centering and that I'll be able to sort it with the sight-tube when I get it.. am I on the right track? Thanks! Bruce.

[ausastronomer]

Bruce,

Re-read my reply to your very similar thread on 25/12/05. Basically you need to use a sight tube to get the lateral positioning of the secondary correct.

http://www.iceinspace.com.au/forum/showthread.php?t=6429

CS-John B

[barees63]

Thanks John sorry to repeat myself.. I know I'm being thick :o I'm just not sure what the secondary adjustment will involve..

When you say the distance of the secondary from the Primary, is that what the center screw on the spider is for? I have the 3 tilt screws and a screw in the center.. does that screw raise and lower the secondary mirror?

Thanks for your patience! Bruce.

[ausastronomer]

Bruce,

Yep thats it. Prior to trying to loosen the center screw you need to loosen each of the 3 "collimation screws" a little bit which will then also loosen the center screw. you then adjust the center screw as necessary either loosening or tightening it. If you need to move the secondary further from the primary keep undoing the 3 collimation screws a bit then do the center one up a bit and undo the 3 collimation screws again a little bit and do the center screw up a little again. To move the secondary up or down you sorta work the 3 collimation screws and the center screw together as a pair. The secondary is held in the correct position by the center screw putting pressure in the opposite direction to the 3 collimation screws.

Once you get the secondary in the correct position in relation to the primary you then use the 3 collimation screws only to adjust the secondary tilt. Prior to this step you need the center screw done up with medium pressure so that when the process is finished the 3 collimation screws and the center screw are held fairly firm against each other. Be aware however that the top of some secondary holders is only plastic where the 3 collimation screws rest against so don't go too firm if it is. I actually put a piece of 1.2mm Stainless Steel on the top of my secondary holder for the screws to ride on.

CS-John B

[barees63]

Thankyou! That is exactly what I needed to know.. much appreciated. Btw, do you think it's neccessary to have the OTA horizontal when doing this? I saw somewhere that it's possible for the secondary to drop (onto the primary) when making adjustments.. seems it would be a bit painful to have to tweak it while it's horizontal (more painful to break both mirrors I guess).

Bruce.

[ausastronomer]

Bruce,

I would lay the scope over to almost horizontal and see how much thread you have on the center screw, to establish your "comfort zone". Then you can comfortably work with the scope at about a 45% angle in case you drop a tool down the scope.

CS-John B

[barees63]

Here's another question..:confuse3:

What would be the likely visual effect of the type of collimation error I have? If I'm understanding correctly (and I'm almost certainly not :P ) the light travelling from primary is converging on the secondary toward one end rather than the center, however I do have the focuser axis lined up with the center of the primary (center mark centered in cheshire ring).. so.. does all this simply mean a little less light reaching the eyepiece since it's not hitting the largest part of the secondary? so I can expect dimmer images? or is there more to it..?

Thanks again! (just tell me if I'm being a nuisance!)

PS. another question would be, why could I not simply adjust the secondary so the cheshire ring is centered, using the cheshire eyepiece.. what does a sight-tube add to the equation?

[ausastronomer]

Bruce,

If all the light angles are not at 90 deg you get flaring and significantly increased aberrations. Conduct a little experiment to see for yourself:-

Take a magnifying glass and focus the sun on some dull concrete, hold the magnifying glass at 90 deg to the angle of incidence of the sun then move the glass closer or further from the concrete until the suns light is focused as the smallest possible little round spot. That's how it should be. Now angle the magnifying glass (keeping the distance the same from the concrete) and you will notice the spot fan out on 1 side with a triangular shaped tail. This aint how it should be and this is exactly what happens to your round star images when things are not perfectly at 90 deg. The experiment exaggerates things a little but you get the idea. When the magnification is increased, collimation errors are critical with fast F-ratio scopes.

Its worth taking the time and learning to collimate your scope properly. With the tools you have and have on order you have all the necessary equipment to do the job properly, now its just time learning and patience.

CS-John B

[33South]

Hey Bruce, heres something I found usefull, not quite as good as having an expert standing next to you, but not bad.

http://www.andysshotglass.com/Collimating.html

About 3.7mb with sound.

[barees63]

well I don't know if I'm ever going to get this.. I just tried to adjust the secondary using the idea of putting paper on the tube opposite the focuser as suggested by raja235 and also described in the catseye instructions, rotated the secondary so it facing square onto the focuser then adjusted the center screw until it appeared to be centered (judging by the outline against the white paper) but now I can't get the laser spot anywhere near the center mark by tilting the secondary, so at this point my scope is basically out of commission I guess - wish I'd bought a refractor. Anyway, thanks for all the attempts to help me, I guess I'll have to wait and see if the sight-tube makes things any clearer for me.

[lost_in_space]

There is an excellent article "No Tears Collimation" in December 2005 Australian Sky and Telescope.

[barees63]

well, petulant whining aside :sad: , I have now got it back to as close as I can get it without some kind cross hair arrangement to really know that it is centered (the sight-tube I guess).. I don't know quite what happened (since I'm not that keen to remove the secondary completely to study how it works) but the secondary holder seemed to have have come adrift in some way, I managed to get it back into place and now have the whole thing back to close to the diagram I posted.



I'll see I can find that issue next time I'm in town, or maybe I can order a back issue direct.. I'll check it out. I have now read literally dozens of articles/threads on collimation and each one made me a little bit more confused :confuse2:.. I'm sure I'll be able to look back and laugh one day and I guess I am learning, having now center-spotted my primary twice (once with a ring and now with the more accurate catseye spot) and spent rather a lot of time twiddling with collimation screws it's slowly starting to make sense :P

[ving]

ahhh! collimation, the bain of every new telescope owner.....

glad you have it sorted. once you know how to do everything its less of a hassle (still a hassle just less) :)

[astrogeek]

I'll never forget the 4 hours I spent collimating my 8" dob for the first time. I almost gave up many times but eventually I got it and now I can collimate perfectly in less than 5 mins. Just keep at it is my advice, and if you still can't get it call your local astronomical society (even if you are not a member), I'm sure you could find someone there who would be willing to come and help you.

Good luck :),
Leon

[RAJAH235]

barees63, did you use both of the collimation articles/links that were posted? The Sky & Telescope makes it simple to do. Andy's Shot glass is also very easy to follow, once you know what to look for....
Just in case you missed the link >
http://skyandtelescope.com/printable/howto/scopes/article_790.asp
Print it out.
HTH a bit. :D L.
ps. Take your time with it.
pps. If you haven't got a 'Chehire' yet, use a film cannister. Cut the bottom off, drill a 1/16 hole in the centre of the cap & use this to collimate.
Oh, 1 more thing, do the adjustments in the day time & place the tube on the bed/lounge to adj. pref facing a window/open door.

[iceman]

Collimation is a must-know skill for every newtonian owner. If you avoid learning it, you're potentially robbing yourself of detail! The different when looking at Saturn between a well collimated scope, and one that's out of collimation, is outstanding.

You're on the right track -one day soon it will just click and you'll be kicking yourself how you missed it :)

[barees63]

Thanks for all the encouragement, I think I'm basically there now - I'm just a bit out on secondary centering and I'm assuming once I have the sight-tube this will become a simple matter to fix..

Cheers, Bruce.

[RAJAH235]

That's why I use a 'Cheshire', then star test, & not a laser. :D L.

[barees63]

I think I need another eyepiece before I can star-test as I only have 25mm and 9mm (and a nasty cheap barlow) at the moment so I can't get the very high magnification, for now I just want to get the best possible "tool-based" collimation I can and then concentrate on enjoying the scope for a while! :-)

(sorry rajah I pulled the comment out from under you.. he was responding to a comment I made then deleted about Andysshotglass maybe being a bit simplistic and especially a bit too optimistic about laser collimation)

[RAJAH235]

np. :D L.