GeoffW1
11-07-2007, 10:50 PM
Hi,
I've recently taken the primary and secondary mirrors out of my 300mm Bintel Dob, and replaced them. In so doing, I found some suitable stronger replacement springs for the primary collimation screws at Bunnings. These are a bit long, and can become coil-bound before the screws reach the end of their travel, but they work well.
In looking at the secondary and its holder, I noticed the 3 collimation adjustment screws there bear on a metal ring, presumably so they won't gouge the plastic mount. I also noticed that the secondary central screw seems not to be done up absolutely tight, but is about mid-travel when the mirror is centred lengthways under the focuser tube. So, it appears that when the 3 collimation screws bear down on the metal ring, with the central screw pulling the other way, all is rigidly held.
I use a collimated laser with a masked Barlow to do final telescope collimation, although I do get out a sight tube to position the secondary initially. As often described here, the 3 collimation screws are usually then used to centre the laser spot on the primary.
In thinking about what happens when I do this adjustment, I took another look through the sight tube, and found of course that minute changes take place in the apparent appearance and position of the secondary, hardly enough to notice though. The 3 screws are arranged as you Dob owners know at 120 degrees apart, at 3 o'clock, 'then 7 and 11.
The 3 o'clock screw moves my laser spot in a right left direction on the primary as you would expect, and the other 2, at a shallow angle to that. I found that if I tightened the 3 collimation screws moderately, I could reach past the spider, grasp the secondary mirror mount (watch fingers) and rotate it slightly against the pressure of the 4 screws. The movement produced in the laser spot is an arc as you would expect, and is similar in magnitude to that produced by adjusting the 7 and 11 o'clock collimation screws normally.
So, for night to night collimation of the secondary, I believe I can, with the screw pressure adjusted as above, rotate the secondary holder slightly to get the laser spot level along a horizontal axis on the primary, then use just the 3 o'clock screw to bring it into the centre ring on the primary. We are only talking very small movements here.
Also as many of you have mentioned, the overall collimation of the whole unit does not vary much from night to night, even with transport I have found, and small adjustments of the locking screws on the primary end seem to be enough most of the time, say 1/8 turn.
To this end I made a diagram, showing which locking screw goes which way, as viewed in a small hand-held mirror which I use to view the image of the centre-spot ring cast on the end of the laser Barlow unit, up the focuser tube. With this diagram it is now very quick to get the image centred, instead of the frustrating time I used to have trying to guess which primary collimation screw I had to adjust, and which way, and how much.
Comments would be welcomed.
I've recently taken the primary and secondary mirrors out of my 300mm Bintel Dob, and replaced them. In so doing, I found some suitable stronger replacement springs for the primary collimation screws at Bunnings. These are a bit long, and can become coil-bound before the screws reach the end of their travel, but they work well.
In looking at the secondary and its holder, I noticed the 3 collimation adjustment screws there bear on a metal ring, presumably so they won't gouge the plastic mount. I also noticed that the secondary central screw seems not to be done up absolutely tight, but is about mid-travel when the mirror is centred lengthways under the focuser tube. So, it appears that when the 3 collimation screws bear down on the metal ring, with the central screw pulling the other way, all is rigidly held.
I use a collimated laser with a masked Barlow to do final telescope collimation, although I do get out a sight tube to position the secondary initially. As often described here, the 3 collimation screws are usually then used to centre the laser spot on the primary.
In thinking about what happens when I do this adjustment, I took another look through the sight tube, and found of course that minute changes take place in the apparent appearance and position of the secondary, hardly enough to notice though. The 3 screws are arranged as you Dob owners know at 120 degrees apart, at 3 o'clock, 'then 7 and 11.
The 3 o'clock screw moves my laser spot in a right left direction on the primary as you would expect, and the other 2, at a shallow angle to that. I found that if I tightened the 3 collimation screws moderately, I could reach past the spider, grasp the secondary mirror mount (watch fingers) and rotate it slightly against the pressure of the 4 screws. The movement produced in the laser spot is an arc as you would expect, and is similar in magnitude to that produced by adjusting the 7 and 11 o'clock collimation screws normally.
So, for night to night collimation of the secondary, I believe I can, with the screw pressure adjusted as above, rotate the secondary holder slightly to get the laser spot level along a horizontal axis on the primary, then use just the 3 o'clock screw to bring it into the centre ring on the primary. We are only talking very small movements here.
Also as many of you have mentioned, the overall collimation of the whole unit does not vary much from night to night, even with transport I have found, and small adjustments of the locking screws on the primary end seem to be enough most of the time, say 1/8 turn.
To this end I made a diagram, showing which locking screw goes which way, as viewed in a small hand-held mirror which I use to view the image of the centre-spot ring cast on the end of the laser Barlow unit, up the focuser tube. With this diagram it is now very quick to get the image centred, instead of the frustrating time I used to have trying to guess which primary collimation screw I had to adjust, and which way, and how much.
Comments would be welcomed.