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Concentre collimation


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I recently purchased one of these off member. 

I have never used one before so just tried and I think I have it right would someone mind checking for me and if I have to adjust anything let me know what it is please. 20230515_113706.thumb.jpg.c46c9c761e3f85beaf02c7b4d0d090c3.jpg

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Never used one, but I'm wanting to see the feedback.  Might have to get one from flo (and one of the magnetic handles) for the dob.

Always filled with doubt about the alignment of a my secondary mirror.

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2 hours ago, wookie1965 said:

I recently purchased one of these off member. 

I have never used one before so just tried and I think I have it right would someone mind checking for me and if I have to adjust anything let me know what it is please. 20230515_113706.thumb.jpg.c46c9c761e3f85beaf02c7b4d0d090c3.jpg

20230515_113901.jpg

20230515_113810.jpg

20230515_113734.jpg

Looks to be slightly out and not concentric (excuse the pun). The outermost ring visible on the concenter is nearer the edge on the right hand side of your photos which should be equal on both sides. I used one on my 12” f5 to set my secondary after doing some modifications to the secondary mirror holder. I also can’t really see the edge of the focuser draw tube. HTH.

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I use a concentre too. Make sure the secondary is adjusted for up/down the tube, and tilt/rotation, first - undo all the screws if necessary to get it directly under the concentre with the outer ring aligned. You can then adjust the three screws to align the secondary with the primary.

The secondary is one of those fiddly things to adjust. But once it's right you'll likely never have to touch it again.

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1 hour ago, bosun21 said:

Your secondary 

Thank you I guessed as much now sorted dead centre just need a star test.

1 hour ago, Mr Spock said:

I use a concentre too. Make sure the secondary is adjusted for up/down the tube, and tilt/rotation, first - undo all the screws if necessary to get it directly under the concentre with the outer ring aligned. You can then adjust the three screws to align the secondary with the primary.

The secondary is one of those fiddly things to adjust. But once it's right you'll likely never have to touch it again.

This is not my first reflector I have had a 6" a 8" before I got the starsense but its first time using a concentre I have a laser but it needs collimating I have been trying for months but I am getting nowhere it draws a circle about 15mm from 24ft away just cannot get it any tighter, thank you for your reply.

Edited by wookie1965
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I might be wrong, but if you are using the laser only to align the primary only you can put it in a Barlow.  Then it doesn't matter if it is aligned.

You just need to centre the reflection of the primary centre circle on the centre of the laser collimator

Edited by Ratlet
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If you place a sheet of paper behind the focuser and take picture through a cap it will be easy to judge but if it’s correctly registered then the circles should be concentric in relation to the outside edge of the secondary (green) and the reflection of the primary edge (red).

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49 minutes ago, Spile said:

The secondary looks great but your primary appears to need a slight tweak. That is assuming it is still uncentred when looking through the cap?

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Yes I thought as much I need to change the springs which I am trying to source every time I move the dob it goes out a bit, only takes a tweak get it in but with better springs it will hold collimation better and I dont need to touch the secondary at all now.

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2 hours ago, wookie1965 said:

I need to change the springs

Hi

If you want the collimation to hold on any budget Dobsonian at all tube angles (not just the angle at which you collimated), you'll need to seal the primary mirror to prevent lateral movement and stiffen the tube, as well as fit better springs. 
It's easy to do. We outline the procedure at the url below.. On a Dobsonian, you may need a pair of tube rings with aluminium profile top and bottom. This is for a gso, but Chinese Newtonians are similar:

https://linuxcb.blogspot.com/2022/06/gso-tube-support_22.html

Note, on a 10" steel tube, you may also have to reinforce the tube where the focuser is mounted. @laser_jock99 has articles on this.

Cheers and HTH

Edited by alacant
orto spell
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Gosh I can see how - if not properly understood and/or used correctly, this device could cause a lot of confusion.

It’s a myth that *everything* should be concentric; in fact it’s physically impossible to do so because:

  1. If the secondary has been mounted according to secondary offset rules then the shadow of the secondary will appear be off-centre (as shown in Spiles diagram above) since the secondary is (intentionally) no longer exactly physically centred with respect to the spider vanes. This is normal and correct
  2. If the secondary hasn’t been mounted with secondary offset, then the spider vanes will appear be slightly off-centre, because the optical axis in a non-offset scope is slightly skewed relative to the tube. This is also normal and correct.

In a properly-collimated Newtonian, the following three conditions need to be met:

  • The outline of the secondary mirror should be centred relative to the sight-tube/collimator
  • The outline of the primary should be centred within that secondary outline
  • The primary’s reflection of the sight-tube/collimator should be centred relative to the primary’s centre-spot

The position of the secondary shadow and the position of the spider vanes can be ignored, and in practice, will tend to lie somewhere between 1 and 2 above.

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9 minutes ago, bosun21 said:

Upgrading the springs alone sorted out the problem of holding collimation for my 12”. I have only needed to tweak it once in 5 months which I did with the locking screws.

Guess it wasn’t fully locked-down then 😉

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I had a 150p and a 200p I was always tweaking them, I sold the 150 and got springs for the 200 a friend on here (Shane) who flocked my 150p for me said screw the collimation screws in all the way then back them off 1 and a half turns I did this with my 200p with the upgraded springs and it held collimation for the next 18months before a stupidly sold it. 

I had been to a few meetings as well so it was rocked and rolled in the car but was spot on every time oh I do miss that scope.

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1 hour ago, great_bear said:

the springs are out of the equation

Hi

The idea is that with adequate springs, the locking of the cell becomes an unnecessary source of further misalignment. Remember that locking a cell does nothing to prevent lateral movement of -especially in the OP's case- heavy mirrors within the same.

Recommendation: six springs. Use the -loose- locking screws simply as retainers for the three extra springs.

Cheers

Edited by alacant
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