Collimation Question - Laser v. Autocollimator

Started by GregDamiani, 12/20/2005 04:43AM
Posted 12/20/2005 04:43AM Opening Post
Hi All!

I have a collimation question. I am collimating a 14.5" Starmaster (F4.3). I have the Tectron tools (sight tube, Cheshire & Autocollimator) and a Kendrick 2" Laser.

Here's the deal. The mirror is center marked with a red dot and white circle. I use the Tectron tools. I have my wife help with the final adjustments to the primary while I look through the autocollimator. I can get the "white circle" images pretty much overlapped. Yeah!

Not so fast. Then I check with the laser. The beam doesn't fall on center of the primary - slightly off, ditto the return on the secondary and more so for the return on the laser.

I am pretty darn sure the laser is collimated. I have put it in a "V" and rotated the laser, while watching its image about 30 feet away - no circles, it stays put.

I have viewed through the scope after collimating with the laser and with the autocollimator. Planetary images have been good both times.

They cant both be right? What gives? :S

Confused in Florida (a/k/a Gregg)

Thanks!
Posted 12/20/2005 05:05AM | Edited 12/20/2005 05:10AM #1
Hi Gregory,
Well, I don't want to start a bit tiff here about equipment, but here goes.
The Tectron Tools are 1.25" and the Autocollimator is not that well collimated. I owned a set myself as well as the Kendrick Laser.
The problem is mostly the slop in the 1.25" and then partially the autocollimator itself. I was never able to get the Tectron Tools to agree with the laser. I always found the laser to be the better tool and more accurate.
Collimate with the laser as perfectly as possible and then perform a star test and see if the diffraction rings are concentric. If so, then you are collimated and just use the laser.
Oh, I did sell the Tectron Tools and the Kendrick but the Kendrick was quite good. I only sold it because I have the Lasermax as well and I found it to be more useful for more purposes.
I also bought the Jim Fly Autocollimator 2" with the Catseye Tool to replace the Tectron Tools.
Now I know others may not agree with me on this, so I ask you to check it for yourself.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 12/20/2005 06:54PM #2
I am pretty inexperienced at alignment, but I have noticed that several of my focusers introduce error.

When the optic or alignment tool fits loosely into the focuser, it can have a range of positions when you tighten down the locking screw. As the locking screw is near to the outside end of the focuser tube, it tends to cock the optic / alignment tool when you tighten it down.

If you are using a 1.25 inch to 2 inch adaptor, that adds another error component.

Another major problem I have had is when I put something heavy in the focuser, like a heavy optic or a binoviewer, the whole focusing tube cocks in the bearings rather badly. I have given up using my binos until I can come up with a better optic mount.

I think it would be better to have a holder that holds your optics 0 degrees with respect to axial alignment to your scope but displaced laterally half a millimeter, than have the optic centered but 1/2 degree off angle.

Gary Klein

Gary Klein
Posted 01/18/2006 02:52AM #3
Hi All!

To the extent others may be faced with the problem I described in this post back in December - and since I solved the problem - here goes.


First I tried various iterations of fine adjustments to the focuser with slight improvements each time. Views were good, so I dropped it.

I needed to talk to Rick Singmaster about another issue, but we discussed collimation. His advice was that even if the two didn't perfectly agree, if the ghosts were around 1/4 overlapped in the autocollimator after the laser was used, I'd likely never have seeing good enough to notice the difference. Observation bore this out. I coulndn't detect which alignment - laser or autocollimator - was better.

Rick advised that the focuser could contribute to the error, non perfect centering of tools in the focuser could contribute - but in his experience it is the position of the secondary from the primary.

I just cleaned the primary and was collimating after putting the cell back in the scope. I used the laser, then autocollimator. There was a difference. I adjusted the secondary down towards the primary to try and reduce or eliminate the error of the laser missing the primary center dot. I then adjusted as usual to get the return beam dead center on the laser collimator. I put the autocollimator in to fine adjust - and WOW!! Perfect overlap!! No adjustment needed. Rick was 100% right on!

Gregg
Posted 01/30/2006 07:32PM | Edited 01/30/2006 07:33PM #4
As long as the method used shows a perfect star test when completed, I think there is no wrong method. I think there are easy and hard methods though.
I like the Cat's Eye tools for quick and easy and the ability to do it while it is daylight. The Cat's Eye aligns the primary perfectly when used with the supplied triangle and then following up with the Infinity Auto-Collimator you get perfect alignment with the secondary all while it is fully daylight.
If it is getting dark, I will line the secondary to the center of the primary and then place the Cat's Eye in and align the primary and then check the Secondary with the Auto Collimator and it is usually very close, but I go ahead and tweak it anyway.
At night I usually only need a touch up after a couple of hours and the LaserMax is fine for that.
I always follow up with a star test. The star test is the final word on whether the collimation is correct, it never lies. wink
So, whatever method you like, just check afterward with a star test and if it agrees, then your method works great! 8)

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager