Trouble collimating my new SW 200P Dob

[Gibbo]

Hi all

I am trying to collimate my new SW 200p as it was way out and have hit an issue. The problem lies when adjusting the primary mirror. I loosen the locking screws and then adjust using the adjustment screws. Once I get it to what appears perfect I tighten the locking screws lightly but this then moves the little circle on the mirror away from the centre point again!!!

Does anyone have any advice about how to overcome this?

Many thanks

[sunshine185]

Tighten them very slightly, one at a time, if you tighten one and it moves the mirror too much, enlighten it again and move to the next one.

[RikM]

Don't over tighten the locking screws. I actually don't tighten them at all. I haven't had any problems loosing collimation during a session, but I do check it every time and adjust when necessary.

[Gibbo]

Thanks very much for the advice. I will give it a go

-

[Gloris]

i had the same problem collimating my brothers lil reflector earlier, until i realised that you need to do as sunshine said, just tighten the locking screws very lightly an very slowly. So basicaly turn one screw a quarter step, check its still aligned, turn the next screw quarter step, check its aligned, turn the last screw a quarter step, check its aligned....

and don't do them "tight", i literally just turned them until it could JUST feel them start to bite...basically no tighter than you could turn them by hand. It's quite hard to explain, but you'll probably know what I mean when you are doing it. It'll feel almost like, the screws are free and loose, not "doing anything", when they are unscrewed, but just do them up until you feel that they are "touching" something.

This worked for me, and made it very easy to keep collimation correct, if you do them up any tighter than this it will go WAY out again.

I found this guide to be very helpful. Follow it closely, carefully, and take your time and you'll get some great results...

Astro Babys Guide to Collimation

[RikM]

i had the same problem collimating my brothers lil reflector earlier, until i realised that you need to do as sunshine said, just tighten the locking screws very lightly an very slowly. So basicaly turn one screw a quarter step, check its still aligned, turn the next screw quarter step, check its aligned, turn the last screw a quarter step, check its aligned....

and don't do them "tight", i literally just turned them until it could JUST feel them start to bite...basically no tighter than you could turn them by hand. It's quite hard to explain, but you'll probably know what I mean when you are doing it. It'll feel almost like, the screws are free and loose, not "doing anything", when they are unscrewed, but just do them up until you feel that they are "touching" something.

This worked for me, and made it very easy to keep collimation correct, if you do them up any tighter than this it will go WAY out again.

I found this guide to be very helpful. Follow it closely, carefully, and take your time and you'll get some great results...

Astro Babys Guide to Collimation

Perfect

[Gibbo]

Thanks all for the advice. I think I am there. Thought I would do a very quick star test. I pointed the scope at betelgeuse as a test. When I focused in on it with the standard skywatcher 10mm eyepiece it looked like it was on fire. Is this how it should look?

I moved the scope to Jupiter and it looked more fuzzy than before I collimated it.

Does it sound like its not collimated properly? I am very new to this and have not looked at much as I have only had the scope a couple of days

Thanks

[sunshine185]

You need to do a startest on something that hardly moves, polaris is the best, as the star needs to be in the centre of the ep. You don't want to be doing a test on something that is continually drifting if you can help it.

[RikM]

It could just be that the scope need to cool down a bit or maybe the seeing is poor. I don't put much stock in star testing my own scopes because it too easy to get wrong and my local seeing is rarely good enough to be able to star test properly.

Using a high magnification eyepiece and with the star in the exact centre of the view, just a little out of focus. I see the expected set concentric rings with a dark centre on one side of focus and a light centre the other side of focus. But I don't often see steady clear rings, there are normally shimmering swirly patches.

[Gibbo]

Thanks Rik. Things did improve slightly once I left the scope to cool down for about an hour

Is it normal to have to align the primary mirror after every viewing? When I checked the collimation again this morning it was out again. Does the big temperature change when moving the scope in and out of the house affect the collimation? Does the mirror expand and contract and knock it out of alignment?

Keith

[RikM]

Thanks Rik. Things did improve slightly once I left the scope to cool down for about an hour

Is it normal to have to align the primary mirror after every viewing? When I checked the collimation again this morning it was out again. Does the big temperature change when moving the scope in and out of the house affect the collimation? Does the mirror expand and contract and knock it out of alignment?

Keith

I have to tweak the primary at least every other session. I am not sure, in my case, if it is the temperature change or humping the scope and mount together through an obstacle course (lounge and kitchen) and out to the garden and back again every session.

[Gibbo]

Thanks again Rik

Just noticed that in the horizontal position the primary mirror seems to be aligned. As I move the scope up to almost vertical the little circle on the primary mirror seems to creep away from the black centre and then looks out of alignment!!!!

Does that sound normal? Which one do I believe?

Thanks

Keith

[LukeSkywatcher]

Allow a bit of overshoot. This way when you tighten things up, you should be on target.

[Tiddles]

I've just collimated my new 150mm Dob for the first time and took a while to work out the assembly before I started. It would appear that the mirror sits on 3 squishy pads. With the lock screws loosened the adjustment screws can be screwed in or out to adjust the mirror position. It would appear that the mirror needs to sit against these pads so that they are slightly squished. If all the adjustment screws are moved in the mirror would sit above these pads and not have the same stability. It would be easy to screw the adjusters in and move the mirror up the OTA. Once I understood this collimation became very straightforward. After achieving correct set up, I gently tightened the lock screws to hold it steady.

Of course, I may have got my thinking all wrong.