Collimation

[Badbart01]

About to drive me crazy... I have a laser alignment tool which I got every lined up with. I decided to check by using the collimation eyepiece cap supplied with the scope it was no where close. So I aligned visually using the cap but can't get it completely aligned. I can't seem to get the secondary centered under the focuser. Couple question collimate focuser all the way in or out? Should I trust the laser or do visual? Might send the laser back.

[turboscrew]

Is your laser collimated? Put it in the focuser and rotate it. Does the spot move in a circle?
Most lasers can be collimated, but not all. There are usually 3 screws to do that, but they might be hidden.
Also, the secondary shadow may not be circular due to the "secondary offset".

[JayTee]

If you are not following this guide, I think you should be
https://www.astro-baby.com/astrobaby/he ... reflector/

Cheers

[Badbart01]

Good read, saved it to my home page. I followed it, using the contrasting paper on the secondary mirror helped a lot. I think I'm sending the laser back if I have to check and calibrate the calibrater it seems useless...lol. I'm close but not perfect think I will use it as is and see how it looks.

[OzEclipse]

I measured your image in Photoshop. The following comments are dependent upon the assumption that camera lens being centred and looking down the optic axis. Also, these things can be tricky to interpret from photos.

I drew the white circle centred on the black dot in the middle of the cap. The middle of the cap is centred on the primary mirror so the secondary is collimated to the optic axis. You should get pretty good views with this. However the image of the secondary is not spatially centred on the primary indicating a tilt of the primary.

The secondary looks like it is centred on the eyepiece barrel. The silhouette of the secondary isn't centred on the primary. The image is off centre by an apparent amount of 10mm if this is a 250mm primary. I don't think the spider vanes need adjustment.

I think you need to adjust the primary to fix this. I have put labels and arrows to indicate the axis of the primary you need to adjust and ticks to indicate the axis that is correct and doesn't need to be adjusted. Of course there are three screws and I don't know where they are so adjusting that axis is going to involve adjustment of all three. After you do that then you may need to adjust the secondary. You should be able to centre the eyepiece cap on both the primary and secondary this time.

[Badbart01]

First, thanks for the feedback. I worked on the alignment a little more. Attached are three pictures one with focuser all the way in, one that is way out of alignment the focuser is full extended and the last one the focuser is half way. I opted to alignment with the focuser half way, still not perfect.

[turboscrew]

BTW, I've heard that you should only use two screws to adjust. If you don't touch the 3rd one, the mirror doesn't crawl up or down, and two screws should be enough for adjustments.

[SkyHiker]

Just in case, don't expect the view to be symmetrical. Here's a good geometrical analysis also discussing different types of secondary (with and without offset): https://www.catseyecollimation.com/pensack.pdf .

[OzEclipse]

Hi Badbart1,
If I understand your last post, the three pictures are all taken only with the focuser at different heights but no changes to the optical element alignment between photos?

If this is correct then it possibly indicates a focuser tilt, the axis of the focuser is not parallel to the optical axis.

You can try to shim it into alignment but this is going o be a slow trial and error process. I am not aware of any simple method to determine and correct focuser tilt.

The other approach is to collimate at the average eyepiece height when the eyepieces are focused on a star. If your eyepieces are all similar, then this is easy but different eyepiece designs can focus at quite different heights. Plossl's focus at the field stop usually up near the end of the barrel while wide angle designs such as Naglers, Baader Hyperions and others have a negative lens up front and focus internally.

Joe

[Badbart01]

You are correct all three pictures were taken with no change accept the focuser. At the very end of travel the focuser kicks upward I probably will never reach that point. After looking at the pictures of the alignment I believe the secondary mirror needs to be moved closer to the mount (left by the pic). The reflection of the focuser tube is not centered in the reflection on the secondary mirror.