17 replies to this topic

[bokemon]

Hello folks, 

I usually collimate with just a cheshire and laser.  But with all the drama I have been facing lately regarding thumb screws and focuser motion, I'd like to be able to collimate the scope with the closest to actual use configuration.  That means coma corrector and high power eyepiece.  For the primary mirror, I assume it is the same as an SCT?  Usually the seeing is too bad to view a star's airy disk, so I assume it is more like checking the concentricity of the poisson spot and secondary shadow rings?  But what about the secondary?  Is there a way to star-collimate that?

 

[mayhem13]

Visual star collimation relies on precise axial collimation where the star is centered in the FOV. Where things get misidentified or misdiagnosed are typically issues with poor seeing or poorly figured optics.

 

Major secondary axis tilt and rotation should be mostly set and forget once….especially for imagers who need an evenly illuminated FOV. This can be difficult in F4 scopes whose secondary is offset for those that don’t recognize what to look for. Setting this should not invoke any optical elements at all and should rarely need adjusting. 
 

Axial is another story……that can be done visually with a CC in place……but I’ve always found the barlowed laser method easier…..typically takes me under 2 minutes to perform with a stable secondary mirror and focuser.

[Asbytec]

But what about the secondary?  Is there a way to star-collimate that?

The reference for star collimation is tilting the primary optical axis and best focus location to the field center, so it's limited to primary axial alignment. There is no visual reference I am aware of that can reliably tilt the diagonal flat. Set it and leave it.

 

The star image is formed by the primary, so tilt the primary to center best focus for coma. Slightly defocus a star in the center of the field of view. The star should expand evenly to 1 concentric ring (as seeing allows) with the Poisson spot centered in it.

 

If the slightly defocused star expands asymmetrically then coma is present, and the best focus location is somewhere else. Tilt the primary so the star moves in the direction of the coma tail or asymmetric bulge. See Lockwood's collimation technique. 

Edited by Asbytec, 21 April 2025 - 07:33 AM.

[briansalomon1]

I align my primary mirror using both the diffraction pattern of a star and always leave paracorr in.

 

While I have the eyepiece/paracorr slightly defocused, and have decided where the rings are slightly nonconcentric, I'll stick my hand into the light path in different places until I have it directly over the nonconcentric portion, then I adjust the primary alignment screw or screws to "squeeze" all the rings into a perfectly concentric pattern. I find that's the easiest way to decide which screws need to be adjusted.

 

Lockwood says to "be critical" about how concentric the diffraction pattern is.

Edited by briansalomon1, 21 April 2025 - 09:58 AM.

[dave brock]

While I have the eyepiece/paracorr slightly defocused, and have decided where the rings are slightly nonconcentric, I'll stick my hand into the light path in different places until I have it directly over the nonconcentric portion, then I adjust the primary alignment screw or screws to "squeeze" all the rings into a perfectly concentric pattern. I find that's the easiest way to decide which screws need to be adjusted.

I move the telescope to move the star from the field centre to the position in the field that gives the best collimation then adjust the primary to bring the star back to the field centre. If I had to move the scope, say, down and a little right, I know the mirror needs to be tilted up and a little left so it's obvious which bolts to adjust.

[bokemon]

There is no visual reference I am aware of that can reliably tilt the diagonal flat. Set it and leave it.

ok, then

[Starman1]

Hello folks, 

I usually collimate with just a cheshire and laser.  But with all the drama I have been facing lately regarding thumb screws and focuser motion, I'd like to be able to collimate the scope with the closest to actual use configuration.  That means coma corrector and high power eyepiece. 

 

High power eyepiece--yes, for the primary mirror only.

Coma corrector--no.

 

For the primary mirror, I assume it is the same as an SCT?  Usually the seeing is too bad to view a star's airy disk, so I assume it is more like checking the concentricity of the poisson spot and secondary shadow rings? 

 

No, a bit closer to focus, with perhaps only 2 rings showing outside the spot.

 

But what about the secondary?  Is there a way to star-collimate that?

 

Photographically, yes.  Visually, no.

Why do you feel you aren't getting good collimation with the tools you have?  Could it be your laser is not collimated?

[152ED]

I used an old Lumicon coma corrector in my 10" f/5 dob for years, and was able to easily collimate the scope on a star with a high powered eyepiece, just like my SCT, with the coma corrector in place.  In a pinch (if I was lazy), it was possible to tilt the coma corrector slightly in its slip-fit holder to null out any on-axis coma, instead of tweaking the primary screws.

 

Now I just ditch the correctors and deal with fuzzy edges.  If I want a perfect wide field view I'll use a refractor. 

[Asbytec]

I move the telescope to move the star from the field centre to the position in the field that gives the best collimation then adjust the primary to bring the star back to the field centre. If I had to move the scope, say, down and a little right, I know the mirror needs to be tilted up and a little left so it's obvious which bolts to adjust.

I've done that, too. Move a star around the field looking for the best focus location. If you can find it at high power, it should show you where the primary axis is relative to your field of view. 

[bokemon]

 

 

Why do you feel you aren't getting good collimation with the tools you have?  Could it be your laser is not collimated?

My laser is fine.  But when I put it in the focuser that uses only one thumb screw, I can make it flop back and forth in the drawtube (rotates along the thumb screw axis).  How am I to align the secondary when this happens?

[Neanderthal]

My laser is fine. But when I put it in the focuser that uses only one thumb screw, I can make it flop back and forth in the drawtube (rotates along the thumb screw axis). How am I to align the secondary when this happens?

Sounds like using a Howie Glatter Paralyzer would serve you well - it's designed to be secured into the drawtube with just one screw.

[Asbytec]

Sounds like using a Howie Glatter Paralyzer would serve you well - it's designed to be secured into the drawtube with just one screw.

Absolutely. I use a Paralyzer for reasons related to its design, but it "pulls" my FarPoint laser in nice and firm. No room for any slop. The laser is nice and parallel to the focuser. 

[bokemon]

lol at "paralyzer" vs parallizer vs parallelizer.

This seems like a good device but in my case I can't use it because I am also using a 2" coma corrector.

Ideally, I would have a focuser drawtube with a built-in APM twist lock thing, with an also twist-locking 2" -> 1.25" adapter.  This is what came with my APM refractor.

Edited by bokemon, 29 April 2025 - 04:23 PM.

[Starman1]

lol at "paralyzer" vs parallizer vs parallelizer.

This seems like a good device but in my case I can't use it because I am also using a 2" coma corrector.

Ideally, I would have a focuser drawtube with a built-in APM twist lock thing, with an also twist-locking 2" -> 1.25" adapter.  This is what came with my APM refractor.

The Howie Glatter Parallizer adapter would fit your coma corrector just fine.

It's a standard 2" to 1.25" adapter, but its design automatically centers the adapter in the scope and automatically centers the eyepiece in the adapter.

I attach the manual I wrote for its use, which has an illustration that explains it quite well.

Attached Files

•  The Howie Glatter Parallizer.docx   86.87KB   4 downloads

[bokemon]

No, I mean the coma corrector will still get held in by the 1 thumb screw and flop around.

[Asbytec]

No, I mean the coma corrector will still get held in by the 1 thumb screw and flop around.

So, what I do with my 2" adapter (in you case the coma corrector), is tighten all three screws if it has them. I also have a twist lock 2" adapter that holds my 1.25" Paralyzer. I use the single screw to set the Paralyzer then tighten the twist lock. I think that works. The Paralyzer is set, then tightened. Try that. 

[Starman1]

No, I mean the coma corrector will still get held in by the 1 thumb screw and flop around.

Time to add a second thumb screw 120° away from the single one.

Two screws will fix anything in the focuser and prevent any wiggle.

• Drill hole
• Tap hole
• Install thumbscrew (find anything at McMaster-Carr online)

[CHASLX200]

I never use my P when i collimate.