6 replies to this topic

[Vitkus23]

Hello everyone!

 

I have recently bought used Meade lx65 6" ACF OTA.

After delivery I decided to collimate it as it might have had a rough ride during shipping. And I have run into a problem I never seen before.

I added some photos to show what I mean. The defocused star donut is fuzzy on one side. No mater how much I adjusted the screws this fuzziness persists. Please tell me that my telescope is not a lemon.

Attached Thumbnails

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Edited by Vitkus23, 10 February 2025 - 12:17 AM.

[RichA]

The secondary mirror holder may be off-centre of the corrector plate or the plate is off-centre.   It's not a hypercritical issue, unless you see it impacting the actual diffraction pattern at high power.  You can use a caliper to measure the positioning of the secondary but it requires removing the corrector plate.  But if it were me, I wouldn't touch it, the problem could be the alignment of the photographic gear used to image the defocused diffraction pattern.

[vidrazor]

Well, A) what camera are you attaching to the scope and B) how are you attaching it? If you're attaching it via an eyepiece mount, your camera's sensor will be canted and axially misaligned to the optics of the scope.

[Bill Barlow]

The collimation is slightly off in the 6-7 o’clock position where the fresnel rings are a bit wider.  Try and adjust this, then see how focused stars look as well as planetary detail on Mars and Jupiter. 

[carolinaskies]

Well, A) what camera are you attaching to the scope and B) how are you attaching it? If you're attaching it via an eyepiece mount, your camera's sensor will be canted and axially misaligned to the optics of the scope.

For the OP:

This is a very important point when using a camera for collimation.  Use a direct connection to the visual back when possible and be aware of 'tilt' of the camera which can cause the apparent out of focus you see.  Remember in collimation what you are seeking is centralization of the optics via the ring pattern.  

Is the visual back the factory standard single screw pressure point or the more modern tension ring designs?   If the first, then it can be causing tilt if using a camera with a 1.25" nose.  

When using an eyepiece do you see the same fuzzyness?  

Most tilt is from attachment sag, though occassionally it can be sensor tilt inside the camera, depending on the particular camera and accessories. 

[vidrazor]

This is a very important point when using a camera for collimation.  Use a direct connection to the visual back when possible and be aware of 'tilt' of the camera which can cause the apparent out of focus you see.  Remember in collimation what you are seeking is centralization of the optics via the ring pattern. 

I remove the eyepiece mounts altogether and and use an SCT to 48mm (or 42mm) adapter, and screw the camera(s) directly to the scope.
 

[BucketDave]

I collimated my ACF last year with a Hotech Collimator.

After some unsuccessful trials, i figured out that the key to success was to remove the secondary mirror (use the three outer screws). This lets the central laser bounce back from the Hotech mirror at the visual back and that gives you your main optical axis. I found that i had to use a tilt plate ahead of the Hotech mirror to get the laser to reflect back on itself. That told me that my baffle tube is not properly aligned with the optical axis. Once that penny dropped, I was able to collimate it properly. But i still need to use the tilt plate when imaging.

I think i calculated that my baffle was about 1/4 deg off axis. That's worth 100 microns across an APSC sized sensor - certainly enough to affect focus.

Dave

Edited by BucketDave, 11 February 2025 - 05:20 PM.