Collimation is about to send me insane

[Darth Wader (Wade)]

First I centre the secondary under the focuser so that I can see all three primary mirror clips. Then I use my cheshre/sight tube to adjust the primary so that the black dot is within the annulus in the centre of the primary mirror. The I tighten the locking screws and check it again. All good.

Then I collapse the scope (8" skywatcher).

Then I move it. View for a while, everything looks great, stars are nice pinpoints, plenty of detail in nebulae, moon and planets where possible.

Move scope back inside. I leave the mirror horizontal to let it dry for a few hours before capping and covering. Check collimation, guess what, it's out by as much as 1cm sometimes.

Locking screws and adjusting screws are tight, as are the screws for the spider vanes. Sometimes the mirror moves out of collimation when I tilt it to the zenith.

What is going on? I have been trying to get this right for almost a year now and it will not hold collimation. I read about 8" scopes holding collimation for long periods of time, so I can't understand why i have to collimate every time I use the scope (can sometimes take close to an hour because it's extremely difficult to line up the black dot with the annulus on the primary - the adjusting and locking screws don't seem to be doing their job properly).

I am about to order primary and secondary Bob's Knobs and new springs because I originally thought the movement was due to weak springs (read a few threads here and on CN on this) but now I don't know if the problem goes further. Any advice/comments greatly appreciated before I'm committed. It's driving me nuts.

Cheers
Wade

[bmitchell82 (Brendan)]

well if things are moving around then you have to find the moving part. Process of elimination. you have only a few parts that move

1. Primary mirror is a floating type. is it staying in place
-Take it out leave all the clips on, being careful hold it up so the mirror is vertical. does it move? Yes. use silcone to help secure it. be ware of the mirror clips as they can induce astigmatisim if clipped down.

2. The spidervane/secondary holder.
-Pull it apart look at it does the mirror move? is the double sided tape holding the mirror firmly? is the spider holding the secondary stalk firmly? is the spider attached to the OTA firmly?

3. the primary mirror collimation springs.
-hold the ota mirror and all watch the primary. does it move?
if yes, then your mirror or mirror cell has flop, bigger springs better collimation screws are needed.
if that still doesn't fix it... well you know what to do get a better focuser (you should anyhow makes life easier feather touch or moonlite)
4. Focuser.
-by nature this thing moves around, it is a standard piece of kit that "works" but measureably not so good. with your cheshire in, rack it in and out see if moves the image at all? if it does.? adjust the focuser from underneith where the focus lock is youll see a small panel with four hex heads plus 1 in the middle like the number 5 side of a dice, the middle one is the tension applied by the focusing wheels to the draw tube.

It has to be one of those, there is nothing else. Start at one and move on though till you find the culprit.

[Robh (Rob)]

Wade,

I have a Meade 12 inch truss scope. Each time I disassemble and reassemble the scope, I expect to have to collimate it. Each time I move it as a unit, I expect to have to collimate it. There is enough flex in components to change collimation when moving the scope and tolerances are not perfect, so there will be changes to collimation in final assembly (I should imagine even in a collapsible). Rarely, will I find a perfect alignment from one assembly to the next. Even a full tube scope needs a collimation check after being carted from home to a viewing site.
I had a lot more problems before I put heavier springs in my scope. The standard wimpy springs would not hold the mirror in collimation. So, whenever I undid the locking screws to collimate the scope, the mirror would just flop into a different orientation depending on the angle of the scope. See how you go with the heavier springs and Bob's Knobs. It may take some of the pain out of collimating.

Regards, Rob.

[bird (Anthony Wesley)]

Get good at collimating. With practice it takes only a few minutes, also replace any hard to use bits (screws etc) with easier things like Bobs knobs.

It's worth checking which optical component is moving - primary or secondary. Then you know which bit to re-check.

cheers, Bird

[Waxing_Gibbous (Peter)]

Hey Wade,
Normal thermal expansion and contraction will send a standard 'Newt' out of collimation as well as carting it about. Its inevitable and nothing to worry about.
As Rob suggested, heavier springs make a difference, but you will probably still have to collimate at least every other night. Sometimes you need to re-collimate over the course of an evening - again, not to worry.
I highly recommend a laser collimator, they make the process much faster and don't cost a fortune (80 - 200 dollars).
I was petrified the first time I tried it. Being completely cak-handed, I was prepared to screw it up and spend an hour of two getting it right.
All of 3 minutes later I was ready to go!
These fast mirrors can sense fear. Make them your b****!!!
Peter

[apaulo (Paul)]

Hi Wade
I agree with Robh , I am new to astrophotography and I use a fast 8" f/4 newt I expect to collimate it before I place a ccd or dslr into the eye piece. I was rather scared playing around with my new scope but becoming familar is the best thing, I reckon it took me about 5-10mins to get it right the second time. If all else fails try Brendan's ideas.
Regards apaulo (astrotryo)

[mswhin63 (Malcolm)]

I must admit I alway collimate my skywatcher before imaging seriously. Another thing that you may want to check I noticed on mine, manufacter sets the locking balls (noted by extra black nuts) so that when you raise it, it locks in place. If this is too loose as I found out it can flop a bit. I find this setting extremely fiddly too. Should never have touched mine when I got it.

[mswhin63 (Malcolm)]

Originally Posted by Waxing_Gibbous

I highly recommend a laser collimator, they make the process much faster and don't cost a fortune (80 - 200 dollars).

I agree with laser as well but recently I have noticed an issue with mine. I can easily collimate it but it is not perfect. I have done a V-Block alignment on my laser before and after and it seems perfect.

I am now considering the other method placing a red triangle on the primary, always forget the name.

[Darth Wader (Wade)]

Thanks for the replies all. I have fiddled with the scope and it now seems stable (taking into account Rob's comments about truss dobs). It seems there are no Bob's knobs to fit the primary - adjusting and locking screws are also way too close to each other. Not really an issue, just slightly annoying.

Maybe now I can get some viewing done without worrying.

Cheers
Wade

[Sarge (Rod)]

Wade,
I have a collapsible 12" Dob. I colliminate every time I use it. I use a laser colliminator and although it used to take me up to ten minutes, it is now only a minute or two and I'm ready. With practice it becomes easy and routine. Even though I have to colliminate every time, I wouldn't swap it for any other. It is a fab scope.

Rod