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Guys , i am so frustrated .I can't collimate my newt. I m here with my chesire eyepiece trying to adjust the secondary but when i turn the  centre screw, the mirror moves a lot and then shifts back to where it was. And turning the little screws arouns it ddnt so much either. I don't have much time before my session (2 hrs) pls help

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Work out which way the mirror needs to travel. Grab hold of the support (the bit that the center screw goes into. Loosen the screw a couple of turns. Then use the three outter screws to adjust. You’ll need to push the support up (or tighten the center screw) see if you have got it right!

Don’t worry about getting it bob on. You can spend hours messing about rather than observing.

Good luck.

Paul

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I dont really understand , whatever i do i can't get these three adjustment screws to work!

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Okay - so, first things first! The big screw is to set the overall position (front to back of the tube) and allow for rotation of the secondary holder with respect to the focuser. Rotation isn't critical, but try and get the secondary mirror aligned centrally through the cheshire and get it looking "round" - a good way to easily see the edges and get contrast is to put something bright like a sheet of paper opposite the focuser in the tube so the secondary is silhouetted.

The little screws are there to adjust tilt of the secondary once it's roughly in place, and this is one of the only really really important things - you need to get the mirror pointing at the centre of the primary. If those screws are completely loose then they might well have fallen out of their threads, and from the amount of movement you're getting it sounds likely, so the trick is to rotate and position the mirror holder so you can get those little screws engaged with the threads again.  You might have to tighten the big screw to bring the holder back - the small screws won't protrude very far. If you have a small mirror and torch you should be able to look down the side of the mirror holder (in the "gap" between holder and vane centre).

Once you've got 'em re-engaged you should be able to crack on. http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/ has some great tips on how to use a cheshire to do your collimation.

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First slacken off the three screws around the secondary a little, otherwise you won't be able to turn the centre screw at all. When you turn the centre screw the secondary mirror should move fairly quickly up and down the tube. Just adjust it until it looks centred with the focuser, i.e. the gap between the mirror and the edge of the focuser is equal on both sides. Then with your cheshire, make small adjustments to each of the little screws until you can see all of the mirror clips holding the primary mirror (there may be 3 or more of them depending how large your scope is) spaced equally around the edge of your view. The best way to think of these screws is as a three-way balancing act, propping up the mirror, so when you tighten them up don't do each one tight in turn or you'll knock everything out of alignment again. Just turn each one a fraction of a turn at a time until they all become tight, and you should still be able to see the primary mirror clips spaced equally about. Then it's just a case of using your cheshire to get the dot in the middle of the circle.

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Posted (edited)

help/twist it using your hand...

and if you do it for the first time... unfortunately, I doubt you will manage to place the secondary correctly during an hour...

Edited by RolandKol

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You never, never never touch the centre screw

Secures the secondary mirror in place, and as you found, secondary mirror has moved out of alignment

Only adjust the 3 outer screws, loosening one, tightening another until aligned with dot on the primary mirror

I use a laser collimate to do my Dob

Use lowest of brightness on collimator, and shines onto primary mirror

The 3 outer screws are Allen Key, not cross head  screws like centre screw

 

 

 

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4 hours ago, cletrac1922 said:

You never, never never touch the centre screw

Well I would agree that it only really needs doing when you first set the secondary up, but it is needed to get the secondary in the correct position up and down the tube relative to the focuser. Once that is set, it shouldn't change so yes, just adjusting the three outer screws should be all you do on a regular basis.

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Thanks for all the support guys i basically did what you told me by accident and somehow managed to somehow collimate it . Sorry for the lack or responses ,have been kinda busy.

Thanks

Kronos

  • Like 2

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14 hours ago, Stu said:

Well I would agree that it only really needs doing when you first set the secondary up, but it is needed to get the secondary in the correct position up and down the tube relative to the focuser. Once that is set, it shouldn't change so yes, just adjusting the three outer screws should be all you do on a regular basis.

I am out couple of times per month doing presentation in primary schools, scout/guide groups with my club

Very rarely need to touch collimation

If just using Dob at home, and not doing any travelling, the will be no need to re-collimate

John

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I found the 3 screws round the edge would not budge until I took the pressure off them with middle one.  Then I found the secondary always shifted as I did them up right at the last moment, so what I found I needed to do was to allow for this bit of movement when I positioned it to tighten the screws.  HTH

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Posted (edited)
On 25/03/2019 at 03:20, cletrac1922 said:

You never, never never touch the centre screw

Secures the secondary mirror in place, and as you found, secondary mirror has moved out of alignment

 

 

 

 

I disagree with this statement. In fact, to get a proper understand how the secondary mirror mechanism works I removed my secondary mirror completely and then put it back on. There are plenty of good guides on the web for this. Just be careful not to touch the mirror or drop it. And keep the tube horizontal so nothing falls down towards the primary mirror. 

You most likely don't need to adjust this screw (and certainly don't need to remove the mirror), but I found it was a good exercise to make me more familiar with the design.

Edited by Viktiste

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Posted (edited)

Maybe the “never, never, never touch the center screw” was a bit strong. I suspect that John was picturing a loosened secondary crashing expensively into a newly knackered primary.

The secondaries on my scopes rarely needs touching, but when they do, a little loosen of the center screw does make things easier for me, on my scopes.

Paul

Edited by Paul73

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Just to clear up issues respect to centre screw

Have had club members, who have purchased a second hand Dob, and when attempting to collimate, over loosened the centre screw, and the entire secondary mirror twisted out of aliment

Took me ages to get secondary centrally aligned, and then complete collimation

I use a laser collimator 

Sometime might need to slightly slacken off centre screw to adjust secondary mirror, using 3 outside screws, if over tight

Had a new club member a while ago, turn up with a department store 4'' Dob, and was not able to align secondary mirror, just the cheapness of material used

John 

 

 

 

 

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