6 replies to this topic

[radu.marinescu]

Hello. I am currenty trying to perfect my collimation and fix tilt in my 10" SkyWatcher 250PDS f/5.

 

I am curios to where exactly should the primary mirror be? Like, if you completely unscrew the lock screws for the primary mirror, you can then turn each collimation screw until the mirror basically almost touches the frame. You can move it up or down in the tube (placed vertically). My question is: to achieve a perfect adjustment, what that distance should be? Should I move it down until max and then start the collimation process from there or what?

 

 

[gstrumol]

It shouldn't matter. Think of how an SCT operates (yes, I know you have a Newtonian, but bear with me). Moving the primary forward and backwards adjusts the focus. If there is perfect collimation this movement doesn't affect it at all. The same holds true for your Newtonian. As long as it's properly aligned for collimation the fore-aft position of the mirror is irrelevant (as long as the converging bean doesn't exceed the secondary area of course).

[Neanderthal]

Couple points to keep in mind:

 

1. The shorter/tighter the spring - the better it will hold collimation.

2. Be aware of where the ends of your collimation bolts are in relation to the mirror. If you have a late-model GSO-made scope, you can gouge the back of your mirror if you tighten the bolts all the way.

3. Moving the mirror up or down is known as "walking". It will affect where the focal plane is, so if you adjust it too far in either direction, you might make it hard for a particular EP to come to focus. Trial and error will quickly show if you've gone too far either way.

 

I like to bring the mirror down towards the bottom, then "walk" it back up about 2 full turns with the collimation screw. Then, mark 1 screw and try not to ever use it, you should be able to make your necessary adjustments with the other 2 screws. This will prevent walking the mirror way out of position. If your collimation screws are stiff enough, you don't need to use those locking screws at all, and in fact, they are more of a hassle than they are worth.

Edited by Neanderthal, 22 March 2023 - 07:41 AM.

[ngc7319_20]

I would just emphasize what @Neanderthal said about the springs.  If there is too little pressure on the springs, the mirror's weight will influence the collimation adjustments.  When you finally tighten the "push" screws, the effect of the springs is minimized.  But in the mean time, the collimation will wander all over the place as the springs sag, and you are trying to adjust the "pull" nuts, and view the laser, etc.

[ButterFly]

Keep the primary as far away from the secondary as you can.  Leave some room for adjustment of its tilt.  The closer the primary is to the secondary, the more the secondary vignettes.  It's not much of an issue for visual use only.

[MellonLake]

Like others have said, maximize the spring tension if you can.  Moving the primary up and down changes the position of the focal plane in the drawtube so if you are imaging, you may want to move the plane up and down a bit to get focal plane where you want it for your camera but other than that, tighten the screws up until they are nearly tight then back them off a little bit to allow for collimation.  I keep mine as tight as possible in both Dobs.

 

Rob

[radu.marinescu]

Hello everyone. I adjusted the 

 

Like others have said, maximize the spring tension if you can.  Moving the primary up and down changes the position of the focal plane in the drawtube so if you are imaging, you may want to move the plane up and down a bit to get focal plane where you want it for your camera but other than that, tighten the screws up until they are nearly tight then back them off a little bit to allow for collimation.  I keep mine as tight as possible in both Dobs.

 

Rob

 

 

Keep the primary as far away from the secondary as you can.  Leave some room for adjustment of its tilt.  The closer the primary is to the secondary, the more the secondary vignettes.  It's not much of an issue for visual use only.

 

 

I would just emphasize what @Neanderthal said about the springs.  If there is too little pressure on the springs, the mirror's weight will influence the collimation adjustments.  When you finally tighten the "push" screws, the effect of the springs is minimized.  But in the mean time, the collimation will wander all over the place as the springs sag, and you are trying to adjust the "pull" nuts, and view the laser, etc.

 

 

Couple points to keep in mind:

 

1. The shorter/tighter the spring - the better it will hold collimation.

2. Be aware of where the ends of your collimation bolts are in relation to the mirror. If you have a late-model GSO-made scope, you can gouge the back of your mirror if you tighten the bolts all the way.

3. Moving the mirror up or down is known as "walking". It will affect where the focal plane is, so if you adjust it too far in either direction, you might make it hard for a particular EP to come to focus. Trial and error will quickly show if you've gone too far either way.

 

I like to bring the mirror down towards the bottom, then "walk" it back up about 2 full turns with the collimation screw. Then, mark 1 screw and try not to ever use it, you should be able to make your necessary adjustments with the other 2 screws. This will prevent walking the mirror way out of position. If your collimation screws are stiff enough, you don't need to use those locking screws at all, and in fact, they are more of a hassle than they are worth.

 

 

It shouldn't matter. Think of how an SCT operates (yes, I know you have a Newtonian, but bear with me). Moving the primary forward and backwards adjusts the focus. If there is perfect collimation this movement doesn't affect it at all. The same holds true for your Newtonian. As long as it's properly aligned for collimation the fore-aft position of the mirror is irrelevant (as long as the converging bean doesn't exceed the secondary area of course).

Hello everyone. I adjusted the collimation as you all suggested and my images are almost perfect. I found out that my laser collimator was bad, the beam was displaced and I could not collimate the laser itself. So I completely ditcher the laser for astro, keeping it for the entertainment of my cat and switched to full visual collimation using a cap (one that I bought and can confirmed the gap is perfectly centered). I did push my primary mirror as back as I could, tensioning the swings at max, then turned the screws half a turn to have enough adjustment space. I made sure the lock screws were turned very very gently and one at a time so they didn't ruin my collimation when tighten up. I also moved my secondary mirror a bit forth because I found out that it wasn't centered in my focuser either. In the end I achieved the sharpest images I got with my 10" Newtonian so thatnk you all for your advices. This community is so cool because of people that always help others with very quick advice.

Here is the result. A 4h luminance stack made of 300s subs. The image is downloaded from facebook beacuse I'm not at laptop so it's slightly compressed. There are still some little elongated stars in the bottom right corner but 95% of the issues are gone because of a proper collimation this time.

 

Edited by radu.marinescu, 25 March 2023 - 10:57 AM.