Collimation Problem

[ZOG]

I got a laser collimator this week so I set about collimating the flextube. I first used the great idea I picked up on here of cutting a piece of card to the same size as the secondary, punching a hole in the centre, and fixing it to the secondary to find the centre of the mirror. The secondary had to be moved back a lot, the laser dot was a full half inch higher up the mirror than the hole in the card. I then adjusted the secondary so the dot was bang on the centre spot of the primary.

A few tweaks of the primary and the return beam was dead centre of the target on the collimator. I felt dead proud of myself, this collimation stuff is dead easy isn't it ?

Last night, at last I had a clear sky so out went the scope, colimator in, a slight tweak to the primary needed. I let the scope cool down for an hour and started observing, as usual with the 32mm and 26mm plossls first. The moon, and Venus looked crisp and resolved well. Orion was above the rooftops so I swung round to M42. Four trapezium stars shining brightly in the 32mm, which usually means fairly good seeing to me. I decided to up the power to see If I could resolve the E and F components and this is where the problems started.

The celestron 17 and 12mm plossls wouldn't quite give me the pin sharp stars that the 32mm did, and the Meade 3000 5mm was unusable, in the past this has been a good performer for me.

I don't understand what I've done wrong, the laser is still showing good collimation but the EP's are telling a different story. I've just ordered a Televue 40mm plossl so I'd like to get this sorted as soon as I can.

Any suggestions gratefully received.

[Astronut]

Forget about finding the centre of the secondary. I assume you're talking about the geometric centre and this isn't important. The main thing to do with the secondary is to position it under the focuser tube so it looks circular and has an even gap around it ie. it looks concentric with the bottom of the focuser tube. For this, the best tool is a sight tube or failing that, a collimation cap. You can make a DIY cap with an old film canister lid (but who uses film these days ).

The thing to remember about the secondary is the 45 degree angle means the optical centre is not the geometric centre because the edge closest to the primary is further away from your eye than the other.

[Doc]

I've had the same problems in my 16" LB for ages. Collimation looks spot on, widefield eyepieces work a treat and then you try splitting doubles with a 8mm and the stars are not pin point spots of light.

I've tried everything. Positioning the secondry so it looks concentric inside a collicap then laser collimating. Tried moving the primary up and down a tad.

I'm lost as well. I now only use the LB for deep space stuff which it excells at. Splitting doubles I use the Tal refractor.

[ZOG]

It seems you're right. I have a collimation cap but didn't use it because of the hole in the card idea. Just stuck the cap in, and I had an eliptical secondary with just 2 primary mirror clips showing. Done the neccesary adjustments and stuck the laser back in. The dot was way off centre. The scope is now collimated so next time I get a clear sky (2012?), I'll try it again.

Just goes to show though, the scope looked collimated, but it wasn't.

[ZOG]

I've had the same problems in my 16" LB for ages. Collimation looks spot on, widefield eyepieces work a treat and then you try splitting doubles with a 8mm and the stars are not pin point spots of light.

I've tried everything. Positioning the secondry so it looks concentric inside a collicap then laser collimating. Tried moving the primary up and down a tad.

I'm lost as well. I now only use the LB for deep space stuff which it excells at. Splitting doubles I use the Tal refractor.

To be honest Doc, I had no problem splitting doubles before I fiddled with the collimation.

It appears that the best tool for this job is going to be a cheshire to get good collimation at home, and the laser just to tweak the scope after moving it outside.

[adamsp123]

I am not entirely sure but if the scope is a "fast" then shouldn't the secondary be offset ie not exactly in line with the EP, I know my F/4 SNT has quite an offset.

Matbe I teling Grandma to suck eggs here if so...sorry

Pete

[adamsp123]

Now I can't speel!

[Astronut]

I'm assuming the secondary on your truss Skywatcher is the same as on my solid tube Skywatcher so if you look at the back of the secondary from the open end of the tube you'll notice (use a tape measure to confirm) the secondary is deliberately mounted on it's holder non-centrally so that the distance from the edge furthest from the focuser to the central mount edge is more than the distance from the central mount edge to the secondary edge nearest the focuser. This introduces a deliberate mechanical transverse offset negating the need to adjust the spider vane lengths. The longitudinal offset is introduced during collimation ie. moving the secondary towards or away from the primary with the secondary holder's central bolt. The transverse and longitudinal offsets are only a small amount (more as the 'scope's aperture increases and/or focal ratio decreases).

When collimation is complete, the reflection of the secondary in the primary looks offset to the right ie. towards the primary (it looks non-central to the spider vanes) and this is perfectly normal on an f/5 'scope such as the Skywatcher. On a longer focal ratio 'scope eg. f/8, the offset is hardly noticable and the secondary reflection within the primary reflection looks concentric with the primary reflection and more or less centred within the spider vane reflections.

The point of the offset is to achieve full field illumination and the whole primary including the 3 clips can be seen at the focuser.

Here's a good read:

http://web.telia.com/~u41105032/kolli/kolli.html

You don't need to know or calculate how much offset is needed - just use a colli-cap or sight-tube (which is better because it reduces the gap seen around the secondary edge) and centre the secondary so it looks circular and concentric to the bottom edge of the focuser tube (or sight tube if you're using one). Lasers are useless for this fiddly step in collimation.

[ianmwatson]

All good points regarding the offset. One other thing worth checking if you have problems in future is the laser itself. I made a vee block for mine, if you position the laser collimator in the block and mark the fall of the laser on an opposite wall, then turn the laser through 180 degrees and mark the fall again you may see that the laser isn't true to the holder. I had to adjust mine from the box, it was out by 5 inches! It was a fairly familiar adjustment though, 3 screws, and didn't take long at all.

For a super accurate collimation you can't beat collimating the diffraction rings.

[steppenwolf]

I'm with Ian, it is vital to check the collimation of the collimator itself - you can't assume that it is correct on delivery unfortunately although in any event, I don't think a laser will help with secondary offset adjustment. You might find the attached image of my jig of some use:-

[Merlin66]

I like the centering mask idea for the secondary! I use it!

I agree with big, fast dobbies you need to "correct" for the secondary off-set, but even on a 16" f4 this is only 4.6mm. An easy check is to prepare such a "modified" mask and look at it with the Cheshire.... you'll see the secondary is centred.

The final collimating/testing must be done on a star. A ronchi screen will show if there are any major optical issues and the star diffraction pattern will confirm best collimation.

The 5mm eyepiece I figure will give about X350 magnification; a good test for any optics and seeing.

I also agree 100% that the laser collimator MUST be check for accuracy; the two I've tested so far were well out of alignment.

[Astronut]

Collimation adjustment using a star is, at best, difficult especially with an unguided dob coupled with the usual seeing conditions under UK skies. The only available star is Polaris because you can keep it centered in the FOV. I've used Polaris to check collimation but there's no way I'd attempt any adjustment looking at the diffraction rings. They're usually 'boiling' anyway but in rare steady moments they seem reasonably concentric I don't have a high power eyepiece either so they're a bit on the small side.

I'm not a fan of laser collimators. I used to have one but it's long gone after I got the Catseye kit.

[Merlin66]

ZOG/ AstroNut and others..

Looking at the the issues raised, I've rechecked my "secondary mask" notes and I must admit that the one that I made and used ( on a 17 1/2" f4.5) a couple of years ago WAS NOT set -up as I explained.

I actually had made the hole in the mask 1.7 x offset towards the end of the tube thereby "pushing" the secondary down the tube to compensate for the off-set. This was also in a telescope where the centre of the secondary had also been off-set in the tube AWAY from the focusser.

Unfortunately I don't have a fast newtonian available to re-check and verify. I do believe that if the system is a fast f4 etc then an off-set of:

Offset = T/4F*F

where T is the distance from the centre of the tube to the focus and F is the focal ratio, should be applied.

This formulae, for a 400mm f4 mirror, based on T= 320mm, gives an offset of 5mm, and an off-set to the mask alignment hole of 8.5mm.

When the hole is off-set by this amount (ie 8.5mm) it will appear central to a Cheshire eyepiece etc.

I apologise for the confusion; I should have checked my original notes earlier.. :oops: :oops:

[John]

I've given up with laser collimators having tried a couple without much success. My 8" F/6 dob could resolve the E & F componants in the Trapezium and other tough doubles - I maintained the collimation in that with a simple Skywatcher plastic cheshire eyepiece. I'm now using the plastic cheshire with my Lightbridge 12" and, hey presto, the E & F componants are now seen !.

John

[adamsp123]

Just for information

Here is a link to the Meade LXD manual that shows a picture and notes for the secondary mirror offset on a F/4 SNT.

see pages 40 and 41 link - http://www.meade.com/manuals/TelescopeManuals/LXDseries/LXD-75C.pdf

[johninderby]

Re: Secondary offset

On a dob the secondary offset is irrelevant so don't worry about it. If the light path is not perfectly in line with the tube it doesn't matter. Think of it this way, with a simple dob mount you're aiming the "light path" at whatever you're looking at so if the tube isn't quite in line so what.

However the offset does become important on an equatorially mounted Newtonian as the light path has to be alligned with the tube and therefore the mount, otherwise it won't track properly.

I now only use my laser collimator to set up the secondary and then use a collimating cap to set the primary.It just works better for me that way.

John

[ianmwatson]

Especially if you have a slower scope, like F6 or F8.

[Astronut]

The bottom line is the offset is taken care of as you align the secondary mirror concentric with the focuser. You don't need to calculate the offset amount or take it into account when aligning the secondary unless you're using a mask for a laser as referenced in merlin's post above.

Just be aware that the secondary's offset will be visible in a correctly collimated 'fast' 'scope (f/5, f/4) as an off-centre (towards the primary) reflection of the secondary within the primary's reflection and it is perfectly normal for a fast 'scope.

[ianmwatson]

The radial offset isn't, it must be calculated and engineered into the tube assembly. As you say though, the axial offset is automatically set due to the collimation process. As discussed though, radial offset isn't really worth worrying about in slower scopes.

[Astronut]

Yes you're correct, the transverse or radial offset is built into the 'scope by the manufacturer which is not something that concerns most users except those who have the time and resources to build their own newtonian.

[ZOG]

Thanks for all the advice folks, it's much appreciated.

steppenwolf , I had been looking at your jig, can you tell me, are those plumbing parts or trunking. I can't identify them.

[steppenwolf]

steppenwolf , I had been looking at your jig, can you tell me, are those plumbing parts or trunking. I can't identify them.

Overflow waste - you can get these bits from places like B & Q etc.