Cheshire Collimation

[Glyn80]

Hello,

I'm at my wit's end. I've been trying to get my secondary mirror adjusted on my Skywatcher 200p Dob. But i can't seem to get it central in the Cheshire once it's been aligned with the doughnut on the Primary mirror. I can get the secondary mirror nice and central in the Cheshire before I try and line it up with the primary doughnut. But once i start adjusting the three allen screws on the secondary, to get the doughnut in the centre, the secondary mirror always seems to move upwards and is no longer central in the Cheshire. I can't keep the secondary in the centre of the Cheshire AND get the doughnut in the centre as well. As far as I am aware everything should be central at the end of the collimation.

In the end I just used the collimation cap last night to do the secondary alignment and the result is in the photos below.

I took the telescope out last night and tried the Star Test and it all looked really good, all the circles were concentric. The moon also all looked really clear and in focus and I even found Orion's nebula.

I have attached photos of the view through a Collimation cap and the Cheshire. 

My question is, do I keep on trying to adjust it to get it spot on in the Cheshire? Or does it all look and sound good enough as it is? As this collimation is starting to take all the joy out of owning a telescope 😞

Many Thanks for any suggestions.

 

[Jasonb]

That looks a lot closer than I've managed! I have read that some 'faster' scopes can have the secondary offset and that's the way it's meant to be, so that might be the case with yours (I don't know the focal length of your scope)? Either way, I'm no expert at all, but that looks pretty good and if you're happy with the star test I reckon you're good to go?

[John]

That top pic looks OK to me. Stop worrying about it and get on and use the scope

 

[Glyn80]

Many thanks @Jasonb and @John. I'm glad you both said it looked ok. Yes I was surprised at how good the star test was (I was expecting a lot worse!) I'm going to stop messing around with the collimation now in the endless search for perfection and i'm going to start enjoying the stars! 🙂 

[Jasonb]

12 minutes ago, Glyn80 said:

Many thanks @Jasonb and @John. I'm glad you both said it looked ok. Yes I was surprised at how good the star test was (I was expecting a lot worse!) I'm going to stop messing around with the collimation now in the endless search for perfection and i'm going to start enjoying the stars! 🙂 

Advice I need to start following myself! 😀

[Dark Vader]

1 hour ago, Glyn80 said:

Many thanks @Jasonb and @John. I'm glad you both said it looked ok. Yes I was surprised at how good the star test was (I was expecting a lot worse!) I'm going to stop messing around with the collimation now in the endless search for perfection and i'm going to start enjoying the stars! 🙂 

It's easy to get tied up in the search for perfection. Just to help put your mind at ease a bit, here's my 200p through a collimation cap. Yours doesn't look much different at all to me. If your happy with the views and star test then just enjoy 👍

 

[Ricochet]

1 hour ago, Glyn80 said:

Where in the above photo is the doughnut? I can't actually tell if I can see it or just the reflection of the hole in the cheshire. Is it where I have marked the red dot in the photo below? If so you need to align it with the crosshairs of the cheshire, that I have marked in green. It looks like you might have been trying to align it with the reflection of the spider vanes instead.

[Glyn80]

Hi @Ricochet yes the doughnut is on the red dot. That picture through the cheshire was after I had collimated the telescope by only using the collimating cap. When trying to collimate using only the Cheshire i just can't get it all central. I was definitely using the Cheshire's crosshairs. 

[Pixies]

I agree with the others - just get on and get some use out of your scope. The star test is the final arbiter of collimation.

But when you want to try again, here are a couple of things:

1. Make sure the secondary holder is parallel with the vane-assembly (I'm not sure what the best names for all these parts are, though). Basically, wind the mirror back up the tube (tightening the centre bolt and loosening the adjusters) until the mirror holder is flat up against the top part (with the adjusters just touching). Then, bit-by-bit, move the mirror back down the tube by loosening the centre bolt (anti-clockwise) by half a turn and then taking up the slack by tightening the adjusters by exactly the same amount each. Keep doing this until the secondary is under the focuser. Then you can do the usual adjustments but know that you are starting with the secondary holder exactly parallel with the vanes.

2. I had a similar problem which was down to the fact that the adjusters had dug 'holes/grooves' into the secondary holder. However well I centered the secondary, once I tried to make further adjustments, the holder slipped back into the usual position. This was because of my cack-handedness originally by over-tightening the adjusters. Easily fixed by adding a steel washer between the adjusters and the secondary holder.

[Ricochet]

1 hour ago, Glyn80 said:

Hi @Ricochet yes the doughnut is on the red dot. That picture through the cheshire was after I had collimated the telescope by only using the collimating cap. When trying to collimate using only the Cheshire i just can't get it all central. I was definitely using the Cheshire's crosshairs. 

Get the doughnut under the cross hairs and then align the primary, those are the only things that matter. 

[AstroNebulee]

On 28/12/2020 at 14:14, Pixies said:

I agree with the others - just get on and get some use out of your scope. The star test is the final arbiter of collimation.

But when you want to try again, here are a couple of things:

1. Make sure the secondary holder is parallel with the vane-assembly (I'm not sure what the best names for all these parts are, though). Basically, wind the mirror back up the tube (tightening the centre bolt and loosening the adjusters) until the mirror holder is flat up against the top part (with the adjusters just touching). Then, bit-by-bit, move the mirror back down the tube by loosening the centre bolt (anti-clockwise) by half a turn and then taking up the slack by tightening the adjusters by exactly the same amount each. Keep doing this until the secondary is under the focuser. Then you can do the usual adjustments but know that you are starting with the secondary holder exactly parallel with the vanes.

2. I had a similar problem which was down to the fact that the adjusters had dug 'holes/grooves' into the secondary holder. However well I centered the secondary, once I tried to make further adjustments, the holder slipped back into the usual position. This was because of my cack-handedness originally by over-tightening the adjusters. Easily fixed by adding a steel washer between the adjusters and the secondary holder.

I followed this excellent advice from pixies when collimating my skywatcher explorer 150p and worked a treat for me and very simple to follow. 

[Spile]

On 28/12/2020 at 14:14, Pixies said:

The star test is the final arbiter of collimation.

That was the mantra I followed when I decided to use the homemade cap method rather than using a Cheshire or a laser.

The advice I follow is https://garyseronik.com/no-tools-telescope-collimation and https://www.youtube.com/watch?v=KADpZ_XZkS8&t=1s

I did have problems using the star method to test collimation using the original EPs supplied with my telescope but when I "upgraded" to a Hyperion Mark IV zoom it all worked as in the tutorials.