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Confusd by collimation problem


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Hi ... I've been away from astronomy (and Stargazers Lounge) for a couple of years for various reasons. I'm back now and have a problem with a Sky-Watcher 200p reflector on an EQ5 that I have acquired 2nd hand. And the problem is ... yes, collimation.

The issue is, I think, with the secondary mirror. It seems to be misaligned because I can't seem to get anything on the scope collimated (I never was an expert collimator). Please see the attached image: following Astro Baby's advice I put (pink) coloured card in front of the primary mirror to highlight the position of the secondary. Looking down the focusing tube the secondary (pink circle) seems to be way off centre and appears to be nearly touching the outline of the focusing tube on the right-handed side. The shape of the secondary in the image seems to be pretty circular ... but it seems to be in the wrong position.

I have tried adjusting the secondary's locking screw but that didn't change its off-centre position. I have also fiddled with the tiny Allen bolt adjustors ... but I don't want to fiddle anymore!

So, any advice on how I can centralise the secondary as viewed through the focusing tube?

Many thanks, Julian

20160520_164105.jpg

Edited by JulianFR
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It looks like you are not taking the image straight down the focuser tube; try a pinhole to ensure that you are looking straight down it.

If you are, the next thing to check is that the focuser is properly aligned. Remove the secondary and make a mark on the tube exactly opposite the focuser centre, then check that the focuser is centred on this.

If it is, then replace the secondary and tweak the spider vane lengths to get it in the right place.

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Not an expert on collimation, but I believe short f number Newtonian telescopes have the secondary mirror slightly offset from centre. I have an f5 newt which had this offset, so with your scope been an f5.9 it may also have this offset applied to the secondary too. If so the offset is normal. 

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Thanks for the replies. I did use a pinhole collimation cap, as well as a Cheshire - and the offset was the same. I  removed the collimation cap for the photo to make the image clearer and easier to take.secondary mirror centred.jpg

Re the f number, the scope is f5 and I know from Astro Baby's collimation guide that this means there will be an asymmetrical view. But my image seems quite extreme ... and Astro Baby used an image of her own scope, which is the same as mine, that appears to be well centred (attached).

Because I'm appallingly cack-handed I would prefer not to remove the whole secondary assembly unless I have to ?

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Judging by the picture (the angle of which may be deceptive), I would suggest that the secondary is too far towards the primary and too far "down" (towards the bottom-right of your photo). To move the secondary away from the primary loosen all three allen screws and tighten the central screw. Once that is in a reasonable position to raise the secondary you need to tighten the bottom allen screws(s) while loosening the top allen screw(s). Given how far it appears to be out I would be inclined to loosen all three screws and, while looking through a cheshire, take hold of the secondary stalk, hold the secondary in the correct position and then tighten all three allen screws up again so that the mirror is in a reasonable position to start collimation.

The offset is given by Secondary minor axis / (4 * f ratio) which for your 200p I make 52/(4*5) = 2.6mm which is small enough that it probably wouldn't really be noticeable, or matter given that secondaries tend to be slightly oversized.

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OK ... that sounds good: loosen the three Allen adjusters and then take up the resulting slack by tightening the central locking screw to raise the secondary away from the primary. I'll try that tomorrow morning and let you know how it goes.

In this meantime, any other suggestions very gratefully received (I'm desperate!)

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Also, you might measure the distance from the secondary holder to the inside of the ota on each spider vane.  They should be the same on each and if not that may put the secondary where it is in the photo.

 

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Look at the side of your secondary holder with a mirror.  are the adjusters all about the same length?

The problem in the photo with the pink circle (which should be central in the drawttube) is corrected with the vane,  the grubscrews or central bolt.

 

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?  I'm just about to look at he secondary now actually. My big fear has been that this might be a terminal problem for the scope ... but everyone's answers have been very positive!

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Just an update ... I followed advice here and adjusted the secondary outwards/ away from the primary ( a lot!) and also checked and adjusted the vanes ... and at !last I got recent collimation ? I tried it last night and tonight and the results were good, with sharp views of Jupiter n particular.

The the trickiest thing was managing the EQ4: it's the first equatorial mount I have owned and I (and the scope) were tying each other in knots. There must be a technique to this 

!

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On 25/05/2016 at 02:08, JulianFR said:

The the trickiest thing was managing the EQ4: it's the first equatorial mount I have owned and I (and the scope) were tying each other in knots. There must be a technique to this 

!

Tube rings are your friend! They allow you to rotate the OTA without it slipping through the mounting rings. They tend to be a tad pricey for what they are, but you may find an embroidery hoop outer that fits your OTA - very much cheaper.

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      Hi guys, 

      I have been trying really hard to collimate my new Skywatcher 200/1200 Dobsonian and after trying for several days (:)), this is the best I have managed to achieve so far. I know that it's not perfect but could you please let me know if this is adequate or do you think I need to make some further adjustments (and, if so, please let me know which ones)?
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