Skyliner 200P - Collimation Problem

[AdeKing]

Hi All,

This is a quick question to find out whether anyone else out there with a standard Skyliner 200P has experienced this behaviour when collimating the primary mirror.

I have recently upgraded the standard screws with Bobs Knobs to make collimation somewhat simpler and it appears that unlike the Explorer 150P the primary mirror cell adjustment has rubber rings between the primary cell and the base of the tube rather than springs.

When adjusting the collimation I find that pretty much all of the adjustment relies on a single adjustment knob with minor tweaks of the other two.  The result is that the "main" adjustment knob is now incredibly tight, so much that I have given myself blisters on my forefinger and thumb through trying to adjust it further and get the perfect collimation pattern.

I have loosened everything off until only the first thread is engaged and repeated, but the same happens time and again.

Disclaimer - Prior to replacing the adjustment screws with Bob's Knobs I was satisfied with the star tests I had carried out, and so haven't yet re-adjusted the secondary mirror.

I haven't yet removed the primary mirror to double check that the central circle on the primary is truly central, thats the next port of call. 

I follow the same procedure to collimate both the 200P and the 150P (i.e Astrobaby's Collimation Guide, the full pdf version which has been posted by @Dave In Vermont on numerous occasions), and I don't get the same problem on the 150P so it seems specific to my Skyliner 200P.

I have borrowed a friends Howie Glatter laser collimator so I am intending to use that along with the Cheshire to give an indication of whats going on. 

I am going to go all the way back to basics with this and start from the beginning with the secondary mirror, but has anyone else experienced this problem where virtually all of the adjustment is made using a single adjustment screw that then becomes so tight that you can't adjust it any more without wrecking it?

Thanks in advance,

Ade

 

[Peter Drew]

Assuming the 3 adjusting screws are the same length, there must be something not quite square with the mirror cell. The rubber ring compression system doesn't give as much adjustment potential as springs.  

[AdeKing]

@Peter DrewThe three replacement Bobs Knobs are indeed the same length (I measured them before I installed them) so thanks for confirming my own suspicion that there might be something slightly amiss somewhere with the mirror cell.  

I do need to remove the primary mirror to apply a Catseye reflective hotspot which will make collimating in the field much easier, but I've been putting the job off for a while, this now gives me more of a reason to dismantle it and have a good look at everything when it is in pieces.

I will also check to see whether the original central donut is actually in the centre as I've read reports of these frequently not being quite central.

I really wish that the mirror cell had springs supporting it rather than rubber rings, as these really don't allow for much adjustment at all.  I've always wondered why this is the case and can only assume that its because its assumed that a Dob might be handled more roughly than a smaller and faster Newt, though I am restoring a 12" Dob which uses springs beneath the mirror cell, so its a bit of a puzzle.

Ade

[Ricochet]

Did you remove the cell to change the screws? If so check that you put it back on squarely as there's probably a fair bit more movement there than you'd think. If everything is ok mechanically then I would be inclined to check the secondary collimation. 

[AdeKing]

@Ricochet I haven't yet removed the primary cell from the tube, the original collimating screws were simply unscrewed and the replacement Bobs Knobs screwed into the same holes.

However, before I remove the primary cell, I will loosen off the screws holding the cell to the bottom of the tube and see whether I can get the primary cell more squarely aligned.  I hadn't thought of this , D'Oh.

Ade