Newtonian Secondary Central Collimation Screw

[Jiggy 67]

I think my SW 200PDS is pretty well collimated and I have never touched the position of the secondary mirror under the focuser. However, I have recently took delivery of a Concentric Collimating tool and decided to just check on the position of the secondary under the focuser.........pretty good I thought, but as human nature dictates, I decided to "tweak" it a little, make use of the concentric and make it even better........big mistake, i should have left well alone.

Now, although the concentric tool is an outstanding idea which should make collimation very easy and accurate I find it difficult to use, hard to focus the concentric lines, made worse by the fact that it's usually quite dark in the tube......I know I'm going to get a lot of replies about using a Cheshire, I know all about Cheshire's and that isn't the point of this post. My confusion lies in the operation of the central screw in the secondary holder.

What I've read suggests that the central screw is an adjustment screw used to move the holder up and down the tube. It is also said that once adjusted, there shouldn't be a need to adjust it again, unless the scope takes a knock or similar. The three screws next to the central screw are tilt adjusters. My confusion is this:

1. What does the central screw tighten up against?....if indeed, it supposed to tighten up against anything. What is to stop it loosening/moving over time and therefore, shifting the position of the secondary. I have read instructions directing me to tighten the screw at the end of the process, but surely any tightening/loosening of the screw will move the secondary as it's an adjustment screw.
2. The only way I can think of to tighten the screw is against the other three tilt screws......is this correct? This also worry's me as these screws need to be movable for daily collimation

Any help to end this confusion would be appreciated. I have collimated the scope but don't really know if I've done it correctly, I just ended up tightening the central screw against the 3 adjusters

[Mand1]

I couldn’t resist tweaking my secondary as well. Ive come to the same conclusion as you.
Back off the 3 tilt adjustment screws then turn the central screw to get the secondary where you want it. At this point the secondary holder is wobbly so try to hold it so it’s not tilted, then screw in the 3 tilt adjustment screws a small amount so they just touch the secondary holder.

The secondary holder is now not as wobbly. This is where I made tiny adjustments to the 3 tilt screws to get best alignment and tighten the central screw so the secondary holder is held tight against the 3 tilt screws.

Needs patience, some back and forth and more than 2 hands would have been useful.

Hope this makes sense and is of some use.

Mand

Edited November 18, 2020 by Mand1

[spike95609]

You are right, the big central screw moves the secondary up and down the tube, and the 3 small ones control the tilt. In order for the central one to move at all though you'll need to slacken off the other three a little bit. Once you've done this you should find that the big screw turns easily, and all you need to do is get the secondary spaced as equally as you can get it on either side of the focuser, which will mean that they are aligned. Next you can move on to the three smaller screws to align the secondary with the primary. With this I'd just use a collimation cap, and the only thing you're trying to do here is make sure that you can see the three primary mirror clips (I think there's three in the 200) spaced equally around the edge in your view, as this will mean that your secondary is looking straight down the tube at the primary. The three screws work as a sort of balancing act, so you'll need to play around with them a fair bit to get an even view of all of the clips. Once this is done, you'll need to tighten the screws back up, but as it will trash your alignment to do them one at a time, tighten them in rotation, about an eighth of a turn each time. Then check your primary as all of this messing about will probably have thrown that out of aligntment unless everything was perfect before you started!

[Peter Drew]

This problem underlines the benefit of the secondary holder "mod" where a disc is introduced between the three adjusting screws and the top of the holder.  This simple mod allows the holder to be turned without being wobbly loose and the tightening of the three adjusting screws doesn't readjust the collimated setting.  Plenty of detail how to do this elsewhere on this forum.      🙂

[Jiggy 67]

Thanks for the replies, at least I know I’m basically doing it right