collimation 200p newt

[acey]

Forget about mirror clips they're not relevant. On many scopes it's impossible to get all clips in view.

You say you have a laser and cheshire. In that case start with the laser in the focuser, look down the telescope tube at the laser spot on the primary, and adjust the secondary until the spot is centred. Then take out the laser and use the cheshire to adjust the primary until the spot and ring coincide. If after doing this you find that the cross hairs don't align with their reflection, then start again. Keep doing it until the cross hairs align with their reflection, and the laser spot remains centred when you re-check it.

Remember the whole point of collimation is not so you get a perfect view of the insides of your telescope, it's so you get a decent view of the stars. Don't worry if it's not absolutely perfect. Star test is the best check. As long as you can bring stars to good focus, with the images looking near enough the same just inside or outside focus, you're there.

[661-pete]

I think, the laser may be the best 'way to go' for you. One of the problems with a sight tube or cheshire is, you end up seeing a lot of concentric rings and you have to understand what each 'bit' represents. I have found it confusing. Aligning with a laser, on the other hand, what you need to do is obvious.

In my experience, the cheshire is marginally more accurate than even the perfectly-collimated laser, but I think that, in my case, it's because the laser is heavier and slightly flexes the focuser when you insert it. Anyway, I tend to get it almost right with the laser, then finish off with a cheshire. If it's dark, I get my wife to shine a torch into the diagonal aperture on the cheshire, so I can see what I'm doing!

Tip: do your laser collimation with the scope horizontal and the focusser pointing straight upwards - and don't tighten the laser in the focusser too hard.

Of course the laser itself has to be collimated, before you use it. Hopefully it will be pretty close when you receive it, but you cannot be sure. I think there are threads here explaining how to collimate a laser.

Also the primary has to be centre-spotted with a small paper ring. If not already done, this is something you can quite easily do yourself, if you're prepared to take the primary out! When you do so, put marks on the edge of the mirror cell and the tube so that you re-assemble it in exactly the same orientation.

Good luck.

[Skiddins]

Yes I've read it about 20 times have the printout in front of me and also did it step by step with astroshed video and it looked right but obviously its not.

I have just ordered a laser collimator so I will see how that goes. I know its probably something stupid but I dont know what. Also I need to look at it tomorrow and see if I can get the spider out and its web.

Jen

I bought a Hotech laser collimator to do mine after a couple of months or frustration with collimation (my neice knocked my telescope over onto a stone floor:()

If you have the 'doughnut' on your primary mirror it's easy peasy. Mine had no markings at all so it took the mirror out and added my own centre spot.

I very carefully adjusted the secondary to line the laser up with the centre spot, then set about the primary.

I also have a Cheshire collimator but found it a pain to use, as oce everything looked perfect in it, the laser showed that it wasn't.

I used this video to get an idea of what to do;

I know some of the more experienced users frown at the use of a laser (as they can be out of collimation themselves) but I found it made things instantly much easier.

You can use this the check that the laser is collimated properly before you start;

http://www.youtube.com/user/astronomyshed#p/u/15/bE09_X43UUQ

Skiddins

[umadog]

I didn't measure them, but photos suggest that the secondary mirror is a little too small. You will never be able to see all three clips at the same time. The correct thing to do under the circumstances is to centre the primary mirror using the cross hair and ignore the mirror clips. I think you're getting there. You were just side-tracked into doing something which is not possible. Just centre your primary and proceed to the primary adjustment screws.

You may or may not find a laser to be useful. They can be easier to use but they themselves need to be aligned. If they're not aligned then you'll get headaches as what the laser tells you won't match with what the cheshire tells you. I would suggest you complete the procedure with the Cheshire then insert the laser and see what it's telling you. Remember that you may need to iterate back and forth with the Cheshire, adjusting the secondary and the primary more than once, until alignment is reached. Don't fret, once you've figured this out it will take under 5 minutes from scratch. Please note that until the collimation process is completed, it may be possible for you to think that different tools are telling you different things. In other words the different tools may appear to disagree because the scope isn't collimated, not because one of the tools is wrong.

You asked what the "cap" is called. The collimation cap IS a Cheshire. I know nobody calls it that, but it is. Here are some precision machined Cheshires. As you can see, they're simply expensive collimation "caps": ASTRO PHOTOS

A telescope does not have to be perfectly collimated to generate views. As your scope is right now it will work. The views may be slightly blurry, but it will work. It has to. You should see cloud bands on Jupiter with it if the planet is sufficiently high in the sky. The image will be clearer if the scope is collimated, that's all. It helps if you understand what the collimation process is doing. Once you get that, then you'll see that your scope has to work even when it's out of collimation. For example, you're not changing the distance between the optical elements (much) so you're not changing the location of the focal plane and you will always be able to reach focus. The process of centring the primary ensures that later steps don't produce a tilted focal plane and it ensures that light from the primary mirror isn't being lost.

If you can't see all of the primary mirror at the focal point then the secondary mirror is too small and you're loosing a little light. I thought mass-produced scopes tended to have secondaries that are on the large side. But it seems this isn't always the case as yours is on the small side. This isn't a big deal and you probably wouldn't notice the difference between your secondary and a correctly sized one. If you like, I tell you the process for figuring out much light is being lost. It's not hard.

[661-pete]

I didn't measure them, but photos suggest that the secondary mirror is a little too small.

Secondary size is a big subject for debate amongst Newtonian owners! For most people, I reckon, you're stuck with the size of the secondary that came with the telescope!

For visual work an undersized secondary won't matter too much, and will actually be of benefit when observing planets, since you will get better contrast. But for deep sky work and especially astroimaging, you will lose some of your effective aperture and get vignetting. On the other hand, an over-sized secondary is also bad, because it will cause diffraction effects.

I don't know if there's a simple answer.

Concentrate on the collimation, at any rate. It should be achievable. The ultimate test, of course, is the star test. Observe a bright star at high magnification, very slightly out of focus, and see if the blurring is symmetrical.

[jmfdiver]

I have now done it with the laser collimator. (still can't see the clips ) Only taking it outside will be proof of the pudding but tonight is Friday and it's my night out, so it will have to wait for tomorrow.

Jen

Thanks for all the comments especially Umadog Cheers

[umadog]

"For visual work an undersized secondary won't matter too much, and will actually be of benefit when observing planets, since you will get better contrast."

I know this goes way beyond what the OP asked, but since we're on the topic, I thought I'd go into this. I recently had long discussions with an optician and structure makers on secondary size and this is what I've learned:

The effect of secondary size on contrast is over-stated. Newtonians with secondary sizes under 20% by aperture produce views which are indistinguishable from an unobstructed scope (barring diffraction spikes, of course). There is lots of theory on the topic in Suiter's book, should you wish to verify this. The effect of the secondary begins to become noticeable as we approach 25% obstruction by diameter. By 30% is very noticeable but good views are still possible. In other words, so long as the secondary size is below 20% you'll see no contrast reduction by its presence. Going to longer focal lengths to reduce secondary size to values much smaller than 20% isn't going to make a noticeable difference to contrast. You're optimising something you can't see. I suspect the reason why the idea caught on is because mirrors with larger focal ratios (and so smaller secondaries) are easier to make and so tend to produce better views. In addition, these scopes are substantially more tolerant of collimation errors. Miscollimation will, of course, have a major effect of contrast.

There isn't really any point at which the secondary becomes "over-sized." This is a "how long is a piece of string" issue. It's true that there is a size at which the very centre of the field becomes illuminated by the whole of the primary but outside of this point the illumination drops. It's a personal choice, but the fully illuminate field that is often recommended is 0.1" to 0.2". Futhermore, secondary defects are more likely to occur closer to the edges of the mirror. When an object is placed in the field centre it will be unaffected by such defects (should they exist) if the secondary is "over-sized." Note that this latter point I have heard by word of mouth and not seen supporting data on the distribution of secondary mirror fabrication errors.

Finally, there is a reason for under-sizing the secondary. Most mass-produced primary mirrors tend to have defects at the edges (turned edges). These defects can reduce contrast across the FOV. Defects in the primary are more likely and more serious than at the secondary. An under-sized secondary effectively masks the mirror edge and solves the problem. You're better off with a masked primary if the edge is bad.

So, yes, the effect of a slightly under-sized secondary may be insignificant or may even be beneficial. But it's worth knowing how secondary size can effect contrast because it's not as simple as "smaller secondary means better views." If you operate under that principle then you may end up with design mistakes.

[theodoros1978]

Forget about mirror clips they're not relevant. On many scopes it's impossible to get all clips in view.

You say you have a laser and cheshire. In that case start with the laser in the focuser, look down the telescope tube at the laser spot on the primary, and adjust the secondary until the spot is centred. Then take out the laser and use the cheshire to adjust the primary until the spot and ring coincide. If after doing this you find that the cross hairs don't align with their reflection, then start again. Keep doing it until the cross hairs align with their reflection, and the laser spot remains centred when you re-check it.

Remember the whole point of collimation is not so you get a perfect view of the insides of your telescope, it's so you get a decent view of the stars. Don't worry if it's not absolutely perfect. Star test is the best check. As long as you can bring stars to good focus, with the images looking near enough the same just inside or outside focus, you're there.

Hey acey,

Read your entry and solves kind of my problem.

Few days ago I decided to buy and collimate my Newtonian with a laser, baader colli III. Before I used a cap.

I did with cap all OK (evev the mirror clips). I put the laser and was surprised that the red dot was out of cyrcle on primary. Odd cos the cap was ok.

Did the collimation with laser, put everything in center and just out of curiosity put the cap and recheck. Everything was at center apart the mirror clips! I could see the 3 of then, instead if 5.

I am confused as I am thinking that laser is not the proper way, maybe loosing viewing ability of scope??? Isn't it better when all clips are shown? Never managed that with laser, only with cap.

Is that a problem!?

[Moonshane]

the reason people say to see all three clips is that this is one way to ensure your optics are centered as the clips act a s a guide. the point is you need to line up your optics and this is what counts.

[theodoros1978]

But how to do it with laser, tried so many times that looked impossible.the laser beam is center but clips always not.

Viewing though is ok and get concentric donut when star testing.

Is there a chance that am loosing viewing ability (sharpness, better images) if clips are not shown?

Wouldn't you use more space on the mirrors if were centered than lines up only?

[umadog]

Yasou Theodoro,

There are two things I'd look into. Firstly, make sure that the laser itself is collimated. Place it in the focuser barrel and rotate it. Does the beam move significantly? If it does, then your laser isn't collimated so you can't trust it and will always have a discrepency between the collimation methods.

Secondly, and I think more likely, I wonder if you have conducted the steps in the right order and if you have done things iteratively. The order is described here: John Reed Home Page Collimation - Cheshire Go through all the steps and then go back to the beginning and repeat until everything is right. IIRC, the secondary being off to one side may create the situation you describe. Hopefully someone who knows for sure will chip in.

The fact that you're getting concentric rings in the star test is good. That's what counts, after all. However you may be loosing a little light if the primary isn't centered in the secondary. You won't loose sharpness.

[jmfdiver]

Now I can actually see stars but they are all doughnut shaped. That was with the focuser (which I squared) Aligned , then collimated with laser (which I built a v rack to test collimation) to the primary and checked with the cheshire.

Jen

[jmfdiver]

Eureka iv'e done it no more donuts for me. I had inadvertently left something to do with attaching my camera in the focuser. Now I see stars lots of em.

Yippee

:D:hello2:

[umadog]

ROLF! Good to hear you're sorted. Enjoy the views!

[jmfdiver]

In a way Umadog it was a good learning experience as I have squared the focuser using a tutorial, put an extra tube ring on after counter sinking the screw heads, to make rotating the tube for better viewing easy. Put in a washer to stop the grub screws digging into the secondary mirror holder. Built the v rack to collimate the laser collimator. So apart from going mental it's all good.

Jen

[JimStan]

I am leary of laser collimators. We have a couple in the club, but not one of them agrees with the others, and none of them give collimation that makes star images look "right". By "right", I mean that a slightly out of focus star image looks like a set of concentric circles. That of course, is the extreme test. On a "fast" scope, those "target" images can be centered in the center of the view, and off-center as the stars get closer to the edge of the FOV. Very sensitive indicator of proper collimation!

Jim S.

[todd8137]

First some definitions so we're all on the same page:

A collimation cap is a Cheshire. A "sight tube" is a tube with two wire cross hairs at the end. Many places sell a combination tool which does both jobs. It may look different because it has that funny 45 degree cut-out but it's a cheshire/sight-tube combo tool. Here is one such tool: Orion Telescopes: Orion Collimating Telescope Eyepiece

It's very hard to diagnose this stuff on-line but I'll try. Please answer these questions.

A) I want to get this straight: Do you have a sight-tube? Were you able to use the sight-tube to centre the primary? In other words, can you get the mirror centre spot under the cross hairs? I ask because I want to know if you're having trouble with the mechanics of the secondary alignment process.

You indicate that the image of the primary appears physically larger in size than the secondary itself. Is that correct? If so, is this still the case even when the focuser is racked in? If so, please take a photo through the focuser and post it here. I ask because if this really is the case then this may be the answer to your problem. Imagine trying to pass a 2p coin through a 1 cm hole: it ain't happening. Same thing with trying to see all the mirror clips if the secondary mirror is too small. It sounds like yours might be, but that is really unusual for a mass-produced scope. A photo through a racked-in focuser would help to confirm this. If it's the case then I can advise on what to do next.

whooooooooo matron

[RikM]

I am leary of laser collimators. We have a couple in the club, but not one of them agrees with the others, and none of them give collimation that makes star images look "right". By "right", I mean that a slightly out of focus star image looks like a set of concentric circles. That of course, is the extreme test. On a "fast" scope, those "target" images can be centered in the center of the view, and off-center as the stars get closer to the edge of the FOV. Very sensitive indicator of proper collimation!

Jim S.

You can use the coma inherent in a fast scope to improve the collimation further.

If a defocused test star gives concentric rings in the centre of the view, swing the scope so the star is near the edge of the view and note how far over the dark centre shifts within the pattern of rings. Then swing the scope to take the star image to the opposite side of the view and check if the dark centre is offset by a similar amount.

If there is a difference side-to-side then collimation is still a little bit out and can be 'fine tuned' until the images look the same all around the edge of the field.

[Dr_Who_46]

Yasou Theodoro,

There are two things I'd look into. Firstly, make sure that the laser itself is collimated. Place it in the focuser barrel and rotate it. Does the beam move significantly? If it does, then your laser isn't collimated so you can't trust it and will always have a discrepency between the collimation methods.

Secondly, and I think more likely, I wonder if you have conducted the steps in the right order and if you have done things iteratively. The order is described here: John Reed Home Page Collimation - Cheshire Go through all the steps and then go back to the beginning and repeat until everything is right. IIRC, the secondary being off to one side may create the situation you describe. Hopefully someone who knows for sure will chip in.

The fact that you're getting concentric rings in the star test is good. That's what counts, after all. However you may be loosing a little light if the primary isn't centered in the secondary. You won't loose sharpness.

Hmmm cheers for that John Reed bit, as a newbie i find this very easy to understand and do properly now. Valuble me thinks

[RikM]

Yasou Theodoro,

There are two things I'd look into. Firstly, make sure that the laser itself is collimated. Place it in the focuser barrel and rotate it. Does the beam move significantly? If it does, then your laser isn't collimated so you can't trust it and will always have a discrepency between the collimation methods.

Secondly, and I think more likely, I wonder if you have conducted the steps in the right order and if you have done things iteratively. The order is described here: John Reed Home Page Collimation - Cheshire Go through all the steps and then go back to the beginning and repeat until everything is right. IIRC, the secondary being off to one side may create the situation you describe. Hopefully someone who knows for sure will chip in.

The fact that you're getting concentric rings in the star test is good. That's what counts, after all. However you may be loosing a little light if the primary isn't centered in the secondary. You won't loose sharpness.

That is a great link. Nice and clear and step by step in the right order. Thanks for posting.