Since I got my 16" Astro Sky dob, I've encountered what appears to be a new dimension in the collimation procedure - UTA position. I marked my poles to insure I put them on the same each time. But I still get what appears as a misalignment of focuser to secondary, even though these parts have not moved. My laser is pointed at center of primary and the primary is collimated with catseye cheshire. But I see that the reflection of my laser off the primary does not converge on the secondary surface with the spot where the laser hits the secondary on first pass. Any procedure or other suggestions for avoiding having to tweak the focuser each time?
Truss Collimation Procedure
Started by pwp818, 06/21/2008 07:27PM
Posted 06/21/2008 07:27PM
Opening Post
Posted 06/21/2008 07:30PM
#1
Hi Paul,
Are you using a 1.25" or 2" laser? Have you checked the collimation of the laser?
Are you using a 1.25" or 2" laser? Have you checked the collimation of the laser?
[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue
Amateur Imager
Posted 06/22/2008 07:48PM
#2
Dear Paul:
It is possible that the catseye is causing an issue.
In larger scopes, the secondary wants to be visually centered on the focuser, but displaced slightly off axis to the primary, away from the focuser. This allows the secondary to grab the full cone of light, without being oversize.
As a result, if your scope is perfectly collimated, the secondary will appear to be off center in the reflection from the primary. This is because it is off center, intentionally.
The laser should nail the center of the primary and bounce back to the center of the laser. It will also appear slightly off center where it reflects on the secondary.
The secondary in this case is also actually displaced off axis slightly toward the primary, but appears centered in a cheshire.
In small scopes the displacement would be very small and not worth doing, so your secondary is probably on-axis in that one.
Gary Klein
Does this make sense?
It is possible that the catseye is causing an issue.
In larger scopes, the secondary wants to be visually centered on the focuser, but displaced slightly off axis to the primary, away from the focuser. This allows the secondary to grab the full cone of light, without being oversize.
As a result, if your scope is perfectly collimated, the secondary will appear to be off center in the reflection from the primary. This is because it is off center, intentionally.
The laser should nail the center of the primary and bounce back to the center of the laser. It will also appear slightly off center where it reflects on the secondary.
The secondary in this case is also actually displaced off axis slightly toward the primary, but appears centered in a cheshire.
In small scopes the displacement would be very small and not worth doing, so your secondary is probably on-axis in that one.
Gary Klein
Does this make sense?
Posted 06/24/2008 05:33PM
#3
Just to update, I followed Floyd's tip about installing a 2nd pin to keep the UTA from swinging around the locking pins. Actually, James Grigar at Astro Sky recommended that I not drill, but rather use flat pieces of wood glued to the truss pole connector pieces just below the locking pins, as shown in the attached photo. Net result is that I can now get the scope collimated quickly after removing and replacing the UTA, since this preserves angular placement of the UTA to the poles from one observing session to the next.
Attached Image:
Funding Member
