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Adjusting the Secondary

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#1 StarTrooper

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Posted 03 July 2020 - 02:29 AM

When the secondary is in the correct position should the secondary's screw be fully tightened or not? If fully tightened it's very difficult to collimate with the secondary's three collimation knobs


Edited by StarTrooper, 03 July 2020 - 05:55 AM.


#2 TOMDEY

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Posted 03 July 2020 - 03:37 AM

Most mass-produced Newtonians have terrible adjustment finesse on the folding flats. Yours sounds like one of them.    Tom



#3 Asbytec

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Posted 03 July 2020 - 05:53 AM

Once one has the secondary in the correct position in the sight tube should the secondary's screw be fully tightened or not? If fully tightened it's very difficult to collimate with the secondary's three collimation knobs

Mine are snug. Yes, it can be difficult to adjust the primary if the screws are tight. I might suggest another method of using the secondary screws so they work together rather than against each other. We tend to tilt the secondary like we do the primary mirror by adjusting the three screws independently of each other. You can adjust tilt by tightening one screw, then loosening the other two by turning them slightly in the opposite direction by the same amount.

 

So, once your secondary is nicely centered under the focuser, you should complete the secondary position step by refining your secondary rotation so the primary mirror is centered on the secondary major axis. This has the benefit of also bringing the primary center mark in the same plane as the focuser axis. Then, use the one secondary screw also in line with the focuser axis to tilt the secondary so the reflection of the primary center moves under the cross hair in a single direction along the plane of the focuser axis. Tighten that screw, loosen the other two to allow the tilt from the first screw. Ideally, you will finish secondary tilt by snugging the single screw so the secondary will have some tension on it. 

 

If you are using the three adjustment screws independently, I'd suggest you're doing so because the primary center is seen anywhere in the secondary mirror. It's much easier to rotate the secondary so the primary mirror and the center mark are centered on the secondary major axis. When you do, everything is in the same plane as the focuser axis, including at least one of the three secondary adjustment screws. The secondary only need tilt in that plane to achieve focuser axial collimation. The beauty is, your secondary offset will also point at the primary instead of somewhere else because the secondary is properly rotated to start with. Part of the problem is, using the two screws orthogonal to the focuser axis independently will tilt the secondary away from focuser center and induce unwanted secondary rotation. Tilt in the plane of the focuser axis avoids those tilt and rotation "errors" and helps maintain your collimation signatures. 

 

Below are a couple of illustrations. It makes collimation so much easier when you can approach the primary center (mostly) from a single direction instead of three different directions. You can do this with a laser, too. The principle is the same as with a cross hair. If needed, small amounts of orthogonal tilt are okay. If you miss the primary center mark by tilting along the focuser/secondary major axis, back that screw off to slightly adjust rotation, then tighten that screw again until you nail the cross hair in the center. Or use a tiny bit of orthogonal tilt to nail it.

 

Focuser Collimation and Secondary Rotation.jpg

 

Focuser Collimation Final Approach.jpg


Edited by Asbytec, 03 July 2020 - 06:25 AM.

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#4 StarTrooper

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Posted 03 July 2020 - 05:43 PM

Before one collimates should the secondary mirror’s bolt be locked down so the secondary does not move? 


Edited by StarTrooper, 03 July 2020 - 05:43 PM.


#5 makeitso

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Posted 03 July 2020 - 06:00 PM

Moving the secondary is part of the collimation process so I have to say no. You need to get the secondary collimation right during the process, then the primary can be collimated.

 

Remember, I’m not one of the experts in this area though.

 

Jack



#6 StarTrooper

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Posted 03 July 2020 - 06:07 PM

Moving the secondary is part of the collimation process so I have to say no. You need to get the secondary collimation right during the process, then the primary can be collimated.

 

Remember, I’m not one of the experts in this area though.

 

Jack

 

Sounds reasonable Jack. I’ll go with that. Thanks for the input.


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#7 Asbytec

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Posted 03 July 2020 - 06:54 PM

Jack is right, you have to play with the center bolt a little to center the secondary under the focuser toward or away from the primary. After that, it can be snug to hold the secondary in place. The three adjustment screws will add the tension you might need or prefer to hold the tilt and rotation. The secondary will tilt around the center bolt pivot. 

 

If you are constantly tightening the adjustment screws trying to tilt the secondary, of course they can become excessively tight and we lose some ability to tilt the secondary. You'll need to loosen one or two of them a tiny bit to allow the tilt from the third screw. You kind of walk the tilt into place that way. If you're lucky, the final turn of the last screw will ensure the secondary is snug. 


Edited by Asbytec, 03 July 2020 - 06:58 PM.

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#8 Kunama

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Posted 03 July 2020 - 07:42 PM

Before one collimates should the secondary mirror’s bolt be locked down so the secondary does not move? 

The centre bolt is used to set the vertical position along the OTA axis as well as the rotation of the secondary in relation to the focuser axis. Once you have done that you can lock it in place and then use just the 3 or 4 collimating screws to adjust the tilt of the secondary to align the reflected focuser axis with the centre mark of the primary mirror.

 

Then align the primary using two of the three collimation screws under the primary...

 

I don't use Bob's Knobs, to me it much easier and more precise to use an Allen key for those adjustments.  Bobs Knobs might be fine for small secondaries but mine weighs about 1200 grams so  the collimation screws need to be tight.....


Edited by Kunama, 03 July 2020 - 07:45 PM.

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#9 StarTrooper

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Posted 04 July 2020 - 07:48 AM

Thanks to ALL who replied. Every reply is beneficial to me.



#10 airbleeder

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Posted 04 July 2020 - 08:38 AM

   I tried Bob's knobs, but went back to an allen wrench on my small secondaries. I could never get the knobs tight enough to suit me.


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#11 bleep

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Posted 05 July 2020 - 10:42 AM

My secondary bobs knobs are pretty snug.  Not too tight where its almost impossible to turn but snug so the mirror stays collimated but I can still adjust when necessary.  If that makes sense?!




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