Is this collimated?

[Coastliner]

Hi all,

New Skywatcher skyliner 200P, straight out of the box. I thought I would have a go at checking the collimation for the first time. This is a photo taken through the hole in a diy collimation cap. Everything looks ok to me, what do the experts think? The spider vanes look to be crossing smack in the center of the donut on the mirror. Ignore the sparkly thing, it's not a star cluster, 'tis nought but a lamshade :-)

Does everything look aligned?

Neil.

[alan potts]

To be honest I can't tell only from that you have to have the camera fitted to the scope you can't just offer up a phone and snap away.

When you have a clear night take it outside and try focus on a star like say Altair, that still high up, see what that looks like, defocus a little with your eyepiece and look and see if the shadow from the secondry is in the centre, if not it needed a little work. There are tools like lasers you can buy to make the job easier, it is a worry at first but we have all been there.

Alan

[Coastliner]

Alan,

Ok, thanks. I thought that the photo would be enough - it seems not! I'll take it ouside as soon as I can.

[almcl]

I am no expert, but something about the secondary doesn't look quite right.

If the secondary is at 45° it should appear as a circle, whereas yours seems to be elongated (attached image).  Not sure what the cause might be, perhaps some more knowledgeable members will enlighten both of us? 

[bingevader]

In a recent thread, someone suggested not using the primary mirror clips as a guide, but it's the way I've always done it!

I can only see 2 on your picture, there should be another at around 4 o'clock.

Are you using a step by step guide to help you work through the process?

Astrobaby's guide is a good place to start.

[Coastliner]

I think I'd better try taking another photo with a camera fitted to the focuser at 90 degrees, rather than pointing a smartphone down the hole and hoping for the best. I see what you mean by the elliptical nature of the secondary though, although this may be caused by the camera lens being at the wrong angle.  I'll try Alan's suggestion of imaging a bright star first, I'll post the result here. May be a while though, weather not very good at the mo!

[C7tsj]

Also, taking a picture freestyle is likely to produce a 'squint' image. You will never get a square on picture without a camera being attached to the focusser.

Can you post how you get on with this as I need to collimate my scope too and a bit nervous to make a start.

Mark

[Coastliner]

:-) Mark - blind leading the blind then! Have a look at this:

http://www.garyseronik.com/?q=node/169

Another slant on collimation. Also, I have made my own collimation cap out of a 35mm film container - I cut the bottom off with a Stanley knife and used a small nail to punch a hole through the rather convenient indentaion in the middle of the cap. The whole thing fits nicely into the focuser, that's what I took the photo through. I have a Cheshire as well, but I will get around to using that later.

Yes, I will post when I've had a go, hopefully this weekend.

Neil.

[Charic]

To be honest I can't tell only from that you have to have the camera fitted to the scope you can't just offer up a phone and snap away.

When you have a clear night take it outside and try focus on a star like say Altair, that still high up, see what that looks like, defocus a little with your eyepiece and look and see if the shadow from the secondry is in the centre, if not it needed a little work. There are tools like lasers you can buy to make the job easier, it is a worry at first but we have all been there.

Alan

Hi Alan

I dont understand? are you suggesting, that by defocusing  you can see if the secondary is central? I'm aware when defocusing on a Star you will see the 'Airy Disk'. This is the diffraction pattern caused by Star light entering the aperture, causing the circular wave pattern, visible when eitherside of critical focus. I didn't think the secondary had any influence on the airy disk, or am I missing some other technique  here?

[Dave In Vermont]

The secondary must be aligned center to the primary, while also sending the image square into the draw-tube. If this is off, the airy-disk will show this by being off-side with the rings unequal and slanted off somewhere.

A good rough (!) collimation of fast Newts helps before doing the star-test. I'll add two pictures to demonstrate. The first one shows the laser-collimator. The beam will bounce off the secondary to primary back to secondary back up to laser. The bulls-eye on the laser should be spot-on. The second picture shows the laser dead-center on the center-mark of the primary. It's a bit hard to see clearly as it rather overwhelms the camera.

Clear & Dark Skies,

Dave

[Moonshane]

Looks like the secondary needs rotating to me. always difficult to see what's correct though with these sort of photos as you cannot tell what's what

[Charic]

The secondary must be aligned center to the primary, while also sending the image square into the draw-tube. If this is off, the airy-disk will show this by being off-side with the rings unequal and slanted off somewhere.

Dave

.............off course it does Doh! ....had a senior moment!  I'd read  alans text to the fact that the shadow in the centre is caused by the secondary mirror alone? Its not! But if the secondary is off-centre, so too will the wave pattern, indicating that the secondary is truly not aligned.

Only takes a moment to forget!  

[Dave In Vermont]

My 200mm F/4 Newt, above, has it's own set of quirks - as most scopes seem to. Factor in it's funky 3-vane secondary holder, and it looks weird. But once tuned to it's maximum extent - it performs superbly. Just had the old girl re-mirrored, and am busily tuning her now. I just got a MallinCam Jr PRO, so it needs to behave it's very best.

Happy Tweaking,

Dave

[Jason D]

 The spider vanes look to be crossing smack in the center of the donut on the mirror.

Hello Neil,

That does not mean much. You should ignore the spider vanes reflection. In some scope, they will cross at the center spot in others they will not just like mine.

This has to do with whether the secondary mirror was mounted with an offset or not. Mine was not but your secondary mirror seems to have been mounted with an offset.

Jason

[Jason D]

When it gets to the secondary mirror alignment, many miss the fact that there are two different alignments. One has to do with centering/rounding the secondary mirror under the focuser for optimal field illumination and the other has to do with eliminating focal plane tilt between the eyepiece and the primary mirror. Typical laser collimators will only take care of the latter alignment -- not the first.

Let me demonstrate. In the attached animations (gif file), each frame will pass the laser collimator test. That is, for each frame the laser beam will hit the primary center then retraces its path to the source -- yet as you can see, the secondary mirror placement is all over the place. Only one frame includes the optimal placement of the secondary mirror under the focuser.

As far as the star test, you need to defocus by a small amount at high magnification. See below. You should not see the reflection of the spider vanes or pirmary clips. This test will tell you about your "axial alignment" , i.e. how well is the eyepiece axis aligned with the primary mirror axis

If you defoucs too much to see the spider vanes reflections as well as the primary mirror clips then you are no longer evaluating the axial alignment but rather how well is your secondary mirror placed under the focuser.

Proper steps for collimation:

1- Round/center the secondary under the focuser for optimal field illumination using a sight-tube or a laser collimator with holographic attachment

2- Fine tune the secondary mirror to eliminate focal planar tilt by directing the laser beam to the primary center -- or by aligning the cross-hairs of the sight-tube with the primary center.

3- Adjust the primary mirror to eliminate focal planar shift (aka coma) by having the laser bean retrace its path  -- of by aligning the sight-tube pupil reflection with the primary center.

(You might need to re-iterate between the above 3 steps)

The third step is the most important collimation alignment followed by the second then the first.

Jason

[Coastliner]

Ah - thanks For your comprehensive reply Jason! It looks as though I've picked up some wrong ideas re collimation along the way. Looking at my initial photo It seems the first thing I have to do is get all four clips visible and go from there. Bull by the horns today, I promise!

Incidentally, how did you take the pictures of your collimation? Did you mount the camera on the drawtube without an eyepiece?

Neil.

[Coastliner]

Jason - and looking at your first pic I would have been concerned that not only do the vanes not cross in the center but also the circular "bits" are not concentric. Reading this and other articles I now realise they don't have to be.

Neil.

[Jason D]

I mounted the camera on a tripod and pointed it appropriately without touching the focuser.

The secondary mirror reflection (shadow dark area) will be shifted towards the primary mirror. This is expected.

[Charic]

Hi all,

New Skywatcher skyliner 200P, straight out of the box. I thought I would have a go at checking the collimation for the first time. This is a photo taken through the hole in a diy collimation cap. Everything looks ok to me, what do the experts think? The spider vanes look to be crossing smack in the center of the donut on the mirror. Ignore the sparkly thing, it's not a star cluster, 'tis nought but a lamshade :-)

Does everything look aligned?

Neil.

Don't rely on Spider vanes being accurately aligned  On my Skyliner, each vane is butted to the secondary hub with two screws. These would also be symmetrical if  the 'L' shaped end of each vane was facing in the same direction? 

You need to look carefully for this issue, unless they messed up in fitting my spider. The 'L'  I mention is the fold of metal on each arm that is folded to allow  the screw to pass through the arm into the hub. 

[Charic]

.......not sure why my edit has not appeared?

I added that I don't 'see' or 'look' at the Spider vanes when collimating my Skyliner, as their not important. What is important is getting the mirror itself correctly aligned at the base of the focuser tube.

[Coastliner]

Ok, I think I've got it - my scope, I think, is now collimated. It took a while to understand what I was seeing throught the Cheshire, but once I understood it was sort of plain sailing.

One thing I'm not sure of, one article (Astrobaby's) says that once you are happy with the alighnment of the secondary one should tighten down the adjustment screws - this alters the alignment though? I must be reading it wrong, I know!

Another point is that I don't find the crosshairs on the Cheshire to be of much use, with my eyes they are so out of focus it is difficult to see them.

Thanks for all your help with this, I've learnt a lot - especially "don't use the sider vanes as crosshairs"! I can see why now.

[Coastliner]

*spider vanes

[Dave In Vermont]

My 200mm F/4 PRIMARY-mirror has 6 screws on the back of the cell. 3 are left loose during adjustment with the other (larger) 3 screws/bolts. After the primary is properly adjusted, then the other 3 (smaller) screws are tightened to hold the primary in the adjusted position.

Perhaps this is what was being referred to.

Clear Skies,

Dave

[Charic]

........some things  just take a little longer to comprehend, but once its there, you wonder what all the fuss is about.? It does get easier and quicker with time, and sooner than later, you'll be expert in your own way, and will be  offering the same help and advice to others new to collimation.

[jetstream]

The mirror should be left "floating" on the springs while using and the jamb screws snubbed up when transporting the telescope.

Jason, greats posts and animations, thanks