Collimation shift on SW newt

[kirkster501]

Hey all,

I've adjusted my Newt using the AstroBaby method using collimation cap and cheshire. Its absolutely crack on when no higher than 70 degrees to the horizon. However, when the scope is more vertical it drifts out a bit. Drifts such that the small circle moves half way off the centre spot. So not massively out but definitely moves off.

My question is why should this happen? I am nipping the collimation lock screws with a screwdriver as well to prevent it moving. If I don't do that it will drift out of collimation significantly.

Any thoughts please?

Steve

[Lorne]

Make sure the clips holding the mirror onto the cell aren't too loose, whilst making sure they're not too tight either of course

[LeeB]

Also make sure your secondary spider vanes arnt too loose.

Replace springs on mirror with stronger ones is an option , but shouldnt be necessary.

[Moonshane]

I suspect it's the primary springs or clips. to address the clips, just check that they are 'just right'. with the springs, I normally tighten the colts all the way in as finger tight as you can. then back off only about 3/4 turn (which gives plenty of room to collimate) and this will hold the mirror more firmly.

[kirkster501]

Not sure I follow regarding the springs guys? What about them? They seem to be OK to me and mirror does not appear to physically move at all.... ?

[laser_jock99]

The Skywatcher standard issue springs aren't all that strong and the 12" is a heavy mirror. Better off fitting stronger primary cell springs like Bob's Knobs (or you can buy cheaper ones in the UK). This will help mirror shift issues.

Standard vs uprated springs

Also take a read of this thread regarding the same issues:

[scarp15]

I replaced the primary springs on my 12" flextube recently and it does make a difference at holding collimation, as the pictures in the above post show, they are much stronger than the factory skywatcher offerings.

[kirkster501]

Thanks. What do these springs actually do? Is the weight of the mirror compressing them all the time and the standard ones are not up to it? Its only a tenner to get upgraded ones from Bob's Knobs.

That thread you link to, LaserJock, mentions a guy who does not tighten the lock screws on the mirror - the ones I have been saying I tighten with a screwdriver!

[Scott]

I believe that guy might be moonshane who is somewhat of a dob guru. I'd definately NOT use a screwdriver on the primary locking screws.

[laser_jock99]

Thanks. What do these springs actually do? Is the weight of the mirror compressing them all the time and the standard ones are not up to it? Its only a tenner to get upgraded ones from Bob's Knobs.

That thread you link to, LaserJock, mentions a guy who does not tighten the lock screws on the mirror - the ones I have been saying I tighten with a screwdriver!

I don't use the 'mirror locking' screws all they do is mess up collimation I have found. In fact I have recently replaced mine with more springs.....

[Moonshane]

not sure about dob guru but I take locking bolts out of any scope that has then - complete waste of time. beefier springs is a good thing for 12" and larger. incidentally, anyone know the diameter of the larger springs? 2mm?

[kirkster501]

Definitely. 100%. Been looking at my scope mirror cell this evening and been playing around with the collimation now that I can reliably get it back into collimation again. The "locking screws" completely screw up the Collimation when the scope moves, on my scope at any rate!!!! I move the scope around the sky with these locking screws loosened and the scope keeps collimation just fine. I lock the collimation with these screws and the collimation moves!!! They are useless!!!!! Easily rectified, just don't tighten them! As you say LaserJock, I will look into removing the locking screws completely and putting some extra strong springs in their place.

[Moonshane]

you'll possibly find the original ones are OK but see how it goes. if you tighten the collimation bolts as mentioned, it may be enough. if not then get some beefier ones.

[kirkster501]

you'll possibly find the original ones are OK but see how it goes. if you tighten the collimation bolts as mentioned, it may be enough. if not then get some beefier ones.

Think I am going to get the beefier ones anyway. There is still a very small amount of shift, albeit very much less than with the "locking bolts" tightened. As folks have said, I think its down to the springs being compressed as the scope moves into a more vertical orientation.

Other than Bob's Knobs anyone know a good place to get these from? Will get them from Bob if need be but always happy for alternatives!

[laser_jock99]

not sure about dob guru but I take locking bolts out of any scope that has then - complete waste of time. beefier springs is a good thing for 12" and larger. incidentally, anyone know the diameter of the larger springs? 2mm?

The wire diameter is 1.6mm, outside diameter 13.72mm, free length 12.7mm, no. of coils 4.2

I upgraded the stock Skywatcher (well GSO!) springs on my 200mm F4 Newt too. That doesn't shift at all now. For 'Belts & Braces' stability I will also do the six spring mod on this scope as well.

Think I am going to get the beefier ones anyway. There is still a very small amount of shift, albeit very much less than with the "locking bolts" tightened. As folks have said, I think its down to the springs being compressed as the scope moves into a more vertical orientation.

Other than Bob's Knobs anyone know a good place to get these from? Will get them from Bob if need be but always happy for alternatives!

I purchased my springs from SPRINGMASTERS LTD in Reditch. The part number for them is C5514570 £1.04 each.

If you are thinking of doing the six spring mod you'll need six of them!

http://www.springmasters.com/compression-springs/147-165.html#one

If you are thinking of shoving the mirror 10-15mm further up the tube for AP reasons then you'll need the longer version C5514610 which is 25mm long.

[Moonshane]

cheers, that's great info. even better to know they work.

[LeeB]

when building scopes I use high load radio control car springs for the primaries, they are loads cheaper than bobs knobs, they are plastic coated and you can select what strength you need. The ones i tend to use are dayglow yellow, easy to spot in the dark too!! , in the last dob they support a 8.5 inch mirror ( 60mm thick!!) no probs.

[kirkster501]

Fantastic info LaserJock - brilliant. Six will be ordered tomorrow!

EDIT Ah.... £15 minimum order + VAT +P/P