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Old 01-05-2019, 06:14 PM
bluesilver (Peter)
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Aligning the mirrors on Dobsonian from scratch

Hi, I have a bit of an issue i think with my 10 inch Skywatcher Dobsonian
It all started last weekend when i went out to have a look at Jupiter.
All i got was a blured orange image.
First of thought it was bad seeing conditions and the like.
But the next night i left the scope inside and out of curiosity, tried looking at Jupiter through the window and a bit later at night when it was higher in the sky, same result, blured orange image.

So i came back to collimation, I collimate it most times before i take it out using a laser, ( also checked that laser was collimated )
Adjusted secondary until red dot was in the middle of the circle on the primary mirror, then adjusted primary mirror until red dot was centre of laser tool.

Results were the same, maybe slightly better, but definitely couldn't tell it was Jupiter i was looking at.

So many days of research and failing attempts to sort this out, i have decided that something has gone wrong with my adjustments.

I desperation i have lessened the centre bolt and secondary mirror has swung around, so figured might as well keep going now and removed secondary to see how the mounting system works.

It is simple enough, but now not sure about reinstalling,
How far out should those 3 screw be from flush, as flush will give you no adjustment as i soon found out.
And also squaring up the secondary.

I am holding back from doing anything with the primary yet, but is there a factory sort of setting to wind the crews back to?
I am just thinking that the primary could also be way out of alignment, or do i not worry about the primary, just align the secondary up with the centre of the primary first?

I hope all that makes sense, I realise i might of gone a tad far by removing the secondary, but in one good way at least if i can get it working again it will teach me a lot in collimation.

Any advice is greatly appreciated.
Thanks.
Peter.
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Old 01-05-2019, 06:32 PM
Saturnine (Jeff)
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Hi Peter
For starters, please read the Newtonian Collimation article in the Projects and Articles Beginners Guide section. The information in there should give you a very good idea of how to align the mirrors of your scope
http://www.iceinspace.com.au/63-390-0-0-1-0.html
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Old 01-05-2019, 06:45 PM
bluesilver (Peter)
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I appreciate the link, yes i have read and printed it out and been going over it the last few days.
I was thinking there must be an ideal way to get the mirror square to the focuser with a laser or something similar and i have read that there is a way to figure out how far the secondary should be from the primary, but unable to find it again.
I was thinking a paper cut out to go over the secondary with a line down the centre, that will get the secondary square with the focuser, then another line marking dead centre on the secondary.
From there if i could work out the distance it is suppose to be from the primary, i could start from there.

It might sound a tad confusing, but just trying to get a good starting point as the secondary and the 3 screws are out, so starting from scratch so to speak.
Thanks.
Peter.
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Old 01-05-2019, 07:02 PM
raymo
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Firstly, collimation has to be a huge amount out before it seriously degrades an image.
Secondly, I have never seen Jupiter described as orange, so I think you may have been looking at Antares which is nearby.
Thirdly, You shouldn't have to tweak the collimation every time you use
the scope. SW dobs[especially collapsibles] hold collimation very well.
For visual use I check mine every few months. For AP every few weeks if it
hasn't been shaken up a lot in the interim.
raymo
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Old 01-05-2019, 07:27 PM
Startrek (Martin)
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The orange tinge may have been a bit of prismatic atmospheric dispersion. If you looked closely there probably was a bit of a blue tinge on the opposite of the planet as well. We’re you viewing Jupiter at lower altitudes say 20 to 40 degrees ? If so, this is a common phenomena
Collimation advice
You will need a Cheshire with cross hair to do the secondary first and the primary last
The laser is only used to do a final alignment of the primary and a check or tweak of both
Cheers
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Old 01-05-2019, 07:43 PM
Startrek (Martin)
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Following my previous post I have used the following guide to collimate my 3 Newtonian scopes from this website -
www.astro-baby.com

Cheers
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Old 01-05-2019, 09:09 PM
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mental4astro (Alexander)
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Hello Peter,

The Astro Baby method of collimation is and excellent one that Martin has posted Follow Astro Baby's method and the secondary mirror will be properly aligned.

Please note a couple of things:
1, You need to deal with the secondary mirror first! Ignore this, and nothing you do will give you properly collimated optics. A Cheshire eyepiece is the tool for sorting out the secondary mirror, including making sure it is square with the focuser.

2, A laser will not do anything for the secondary. A laser is used ONLY to do minute tweak to the secondary to centre the beam in the centre spot of the primary, after you've properly dealt with the secondary. Only then do you adjust the primary mirror.

I too thought I could collimate my Newt/dob exclusively with a laser. I started out collimating just with a Cheshire. A Cheshire will collimate both the secondary and primary mirrors. But a Cheshire is not the easiest tool to use with the primary. So I ditched it for a laser... which I thought would do it all. I bought the laser from an astro retailer too. Problem is that even though astro retailers are well meaning and want to sell you the right tool for the job, they cannot know how competent you are at collimating Newts. So if you ask for a laser, they will sell you a laser, expecting that you know what you are doing. It is not their place to necessarily ask if you know or not... That's the way it is.

So, I started using my laser, and over the next few months instead of the image in my scope improving, it was getting worse and worse! Not only very frustrating, but very flaming annoying as I thought I was doing everything right.

Then, one night, out of sheer exasperation after trying everything I could think of, I looked down the draw tube of the focuser, and to my utmost surprise I saw the secondary mirror was so far out of position it was staggering! The secondary was not square with the, tilted out of position (due to the collimation screws), and not centred with the draw tube by a long margin!!! And this was all so obvious just by eyeballing down the draw tube.

It was only then that I realized that the laser will not deal with the secondary mirror as I thought it would. ONLY a Cheshire eyepiece will deal with the secondary mirror, and only after you've dealt with the secondary do you give the primary mirror any consideration.

And since that most eventful night, I have never had collimation issues. None that could not be traced back to some cock-up of mine...

You need at least a Cheshire to properly collimate your Newt - it will deal with both the secondary mirror and primary. A laser will help, but ONLY for the primary mirror, AND ONLY after you've used the Cheshire to sort out the secondary mirror first.

Alex.
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Old 02-05-2019, 08:34 AM
bluesilver (Peter)
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Thanks for the replies and advice, appreciated.
Yes i was viewing Jupiter from those low altitudes, So that was probably all that was wrong in the first place as i thought my collimation was spot on.
But not to worry, a good lesson learned.
Thanks again for the links, i can see where i might of been going wrong now right at the start.
I have a Cheshire eyepiece and an film canister with a small hole in the cap, so all good there.
Will start again and hopefully all will spot on again.
Appreciated.
Peter.
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