Gaps between the tube and mirror holders.

[CumbrianGadgey]

Hi,

As I said on a couple of posts recently, my relatively new Skywatcher Explorer 130 was well collimated when it arrived and has not moved a millimetre since. This is good.

I am dreading the whole process of collimation, which to me sounds like a cross between a collie and a dalmation...I know nothing about dog breeds, either.

Anyway, I digress. When I first inspected the scope, the first thing I noticed about it was that both the primary and secondary holders touch the tube in places but not in others. It is almost as if the holders are too large and have been tightened down too hard because there are either smaller gaps or no gap at each of the four fixing screws.

It looks as if something is not circular, either the tube or the holders, (without stripping it and using callipers, it's hard to tell). If it all has been overtightened to the point it's deformed the tube, what the heck has that done to the important bits?

Could this cause a misalignment problem that as a newbie, I simply haven't noticed? Is it unusual for a Skywatcher 130 to be like this?

Let me know what you think, please.

[Radar2312]

Hi,

I'm no expert but here's my 'take' on it ---

The 'tube' is merely a device for holding the mirrors and focusser in place, relative to each other. Therefore, if collimation is good, then happy days. You can find many pictures of Dobsonians where there is no tube, merely a framework to hold all the bits in place.

I may be wrong, but that's how I see it!

HTH

[sbooder]

If you think something is not circular in the view it is most likely to be the secondary not square on to the focuser.  The reason the secondary is an ellipse is so when square it appears as a circle, always start from that premise when collimating.  The way to do it is twist the secondary by holding the stem not the glass and twist it around its axis until you see it as a circle through the Cheshire/collimating cap and then check it has the same spacing all the way round between edge of mirror and wall of cap/Cheshire.

[Skyline]

This video illustrates it better:

[NGC 1502]

Hi,

I'm no expert but here's my 'take' on it ---

The 'tube' is merely a device for holding the mirrors and focusser in place, relative to each other. Therefore, if collimation is good, then happy days. You can find many pictures of Dobsonians where there is no tube, merely a framework to hold all the bits in place.

I may be wrong, but that's how I see it!

HTH

I think that post # 2 is correct (if I understood the opening post correctly).

The tube can be any shape, square, octagonal, circular, a bit deformed, or not there at all as in a truss tube.

It's the relationship between primary & secondary mirrors and eyepiece that's important ( collimation )  the tube, mirror cells & spider just hold the optical components relative to each other.

Other than the appearance, the shape of the tube is not important.

HTH,  Ed.

[sbooder]

I think that post # 2 is correct (if I understood the opening post correctly).

The tube can be any shape, square, octagonal, circular, a bit deformed, or not there at all as in a truss tube.

It's the relationship between primary & secondary mirrors and eyepiece that's important ( collimation )  the tube, mirror cells & spider just hold the optical components relative to each other.

Other than the appearance, the shape of the tube is not important.

HTH,  Ed.

Quite right.  The tube is nothing more than a way of holding all the elements of the scope, as long as their relationship to each other is square...as said before, "happy days".

[Baffa]

I remember being quite concerned with collimation until I recieved my newtonian. 

The first thing I did, well, almost, was to loosing all the collimation screws, bolts and what not. I thought I HAD to do learn this the hard way or I will avoid it all and enjoy my bad views, haha. After following different guides it wasn't hard at all to collimate, I used an old plössl without lenses instead of a collimation cap, cheshire collimator and a laser collimator, with and without a barlow. I'm quite pleased I did this the hard way, now I just check my collimation, usually it's good but sometimes it requires a small 5 min tweak to be good again.

[Baffa]

Oh, forgot to say, it is really easier to collimate than I thought it would be, so it wasn't really that "hard", just seems a bit overwhelming until you try it

[CumbrianGadgey]

Thanks folks.

The first thing I did when I got the scope set up was to stick a collimating cap into the focuser and have a gander at the mirrors and clips. All looked to be bang on and the scope performed well. I think from what you are saying that the uneven gaps are not therefore a problem.

It's just that never having had a telescope before I got worried by the sight.

[Charic]

My 200P had a gap on arrival? Does yours look like this image.

I loosed the external screws, and rotated the collar till I found a better fit. All good now.

[ChrisLX200]

One thing the tube (of whatever type) must do is hold the focuser in position, so I would be less than impressed at receiving a scope looking like the one above. Add some weight (a camera etc) and it's going to move if not bolted to something firm - a thin floppy (and apparently unsupported) aluminium tube is anything but that... The ring at the front is supposed to supply some reinforcement and rigidity but it needs to be actually connected to the tube to do that

ChrisH

[Charic]

ChrisLX200.......Its not something I noted straight away! I was just looking at taking the spider out for the fun of it, and just messing about with the components. I've nothing to be afraid of with the components, and its a great learning curve. But that's how I noticed it. So I had to take the end off anyway. With some rotation of the end cap ring ring, it allowed me to re-align to the screw holes, and fix it back together. I haven't noticed anything untoward since. Like the OP said, it just seemed to not fit?