The Collimation Saga : Part 2

[F1Bird]

So, 2 weeks on and my scope is still out of action! I keep thinking that I'm getting there but then something doesn't work as the guides say it should! Getting very frustrated but am determined to get it sorted. My next question is about lining the Cheshire up in the focuser. When I look down the empty focuser (having first put the various bits of coloured paper in the tube to block reflections and show the edge of the secondary more easily) I see the secondary seemingly centred in the tube with an equally spaced band of coloured paper around it. Then I put the Cheshire in the focuser and the secondary seems to sit low in the view, ie there is a much bigger area of coloured paper visible above the secondary than below. However, moving the secondary up and down vertically is not a movement that can be made. Also, you can also move the Cheshire about in the tube and it seems to be somewhat arbitrary as to what exact position it will be in each time you tighten the thumb screws to lock it in place in the focuser. How much of an issue is this? Surely this will have a big impact as the position of the mirrors in respect to the end of the tube is reliant on its position in the focuser?

Second question - can someone explain the structure of the secondary mirror holder please? The guides refer to the tilt adjusters but then also state that once you have adjusted the tilt of the secondary you should tighten the tilt screws. Will "tightening" the tilt screws not just move the mirror more?!

Sorry to be so monumentally inept at this but I'm not getting to spend much time fiddling with the scope and so its taking me forever!

[Charic]

The central bolt/screw allows movement of the secondary mirror to or away from the primary mirror. It also allows for rotation. The 3 Alan bolts adjust the tilt of the mirror. Adjust like a 3 legged milking stool, if you adjust one, you'll need to balance the others. Its a pity  someone doesn't live  closer to you to assist. Collimation is pretty easy, but for you its not the case at present, which is spoiling your enjoyment, and  lack of use.  I just used a cap and the astro baby guide to good results. There' s just something small that is  eluding you from getting the result of a collimated telescope. It will work eventually.

In its simplest form, the secondary just needs to look central in the focuser. If thats the case, then next your looking through the focuser to see the three mounts that support the primary mirror. Lastly the Primary is adjusted to see the centre spot reflected to the inside of the collimation cap. A small white paper insert to the cap helps. You`ll need bright Daylight down the tube to see the reflected circle through the pinhole of a collimation cap. 

That works very well on my 200P, and I cant get much better using a laser.

Wishing you success

[itmo]

The secondary mirror adjustment is usually like this:

The 3 small adjustment screws screw in to push the mirror away and they control the tilt. To tilt you need to tighten one and loosen two or vice versa. The center screw is the only one actually screwed to the mirror holder and it controls the "altitude" of the mirror. It has a spring around it to keep the mirror pushed away from the spider and to enable the tilting. What you really want to do is to approximately center the secondary mirror under the focuser. It doesnt need to be that accurate. If you think you have a problem with the cheshire, try to rotate it to see if the "picture" moves and put the scope so that the focuser is straight up so the cheshire floats in the focuser without tightening. 

The secondary only really needs to be adjusted so the oculars can "see" the whole primary mirror. The primary mirror collimation is the really important bit. 

[Tonys]

If the secondary mirror cell doesn't appear centred under the focuser it could be due to it not being centred across the scope tube, as looking down the empty focuser can be misleading. If so, the spider veins can be adjusted via the screws to correct this. When I first did my collimation I first used a pair of dividers to measure the distances between the secondary cell and tube along each vein and adjusted to make them the same, which in my case did the trick. The adjustment must be made in small steps, slackening one screw just a touch before tightening the opposite one, repeating until centred. Regarding the arbitrary position of the cheshire in the focuser, I use a combination cheshire/sight tube and it is a close fit into the focuser, just like an eyepiece would be. I'm not sure what kind yours is, but to be honest there shouldn't be significant play when it in the focuser. One other thing that could cause the secondary not to appear centred is if the focuser itself is not square to the scope tube. My Skyliner 200p was way off on arrival, but happily has adjustment screws. If you have this problem you will see it just by looking at the joint between the focuser and the tube.

[NGC 1502]

The mention in post # 4 about adjusting the spider vanes to shift the secondary laterally across the tube, should sort something mentioned in post # 1 -

Quote - " moving the secondary up and down vertically is not a movement that can be made "

It can be achieved by adjusting the spider vanes, that's assuming that " up and down " refers to when looking into the focuser with the tube horizontal.

As also mentioned, you can use a divider ( or calipers ) to measure the vane adjustment ( careful you don't drop them ) or a piece of paper.

Hope you sort it, Ed.

[itmo]

You can move the secondary every which way you want, alteast on normal skywatcher tubes. You can rotate it , you can tilt it  and you can translate it on all 3 axis. What is important that you eliminate all rotation and make sure the spider vanes are tight enough to twang. The secondary should be square under the focuser and the focuser should be squared the first time you remove the secondary mirror. 

[F1Bird]

Thanks for the replies. The bad news is that the spider vanes on my scope (astromaster 114eq) are fixed, in fact they are moulded to the tube so no adjustment is possible. The collimation has been fine until recently so given that the vanes are fixed I think my problem must be elsewhere. Once more into the breach......

[Charic]

Thanks for the replies. The bad news is that the spider vanes on my scope (astromaster 114eq) are fixed, in fact they are moulded to the tube so no adjustment is possible. The collimation has been fine until recently so given that the vanes are fixed I think my problem must be elsewhere. Once more into the breach......

Not so bad, as its one thing less to adjust? When you look into the focuser tube without a lens, the secondary mirror should appear to be circular with the same amount of spacing around the secondary mirror in comparison to the circle of the focuser tube itself ( when you look down the focuser tube or for example a toilet roll holder, the tube creates a circular image, the secondary mirrors image should appear to be circular within the tube that your looking through) That's the first priority. If you look at the mirror from any other angle,  its elliptical in shape. If rotated whilst looking down the focuser tube, you should see the change from elliptical to circular. Next stage is to be able to see the 3 primary mirror clips, looking  down through the focuser tube. If you can see the 3 clips, everything is just about aligned. A collimation cap will just  help to keep your eye centred to ensure that everything aligns up. If your primary mirror does not have a marked centre spot (the Doughnut)  then  you wont see a reflection of this  surrounding  your eye if/when looking through a collimation cap. 

[wookie1965]

Using the coloured paper again position the secondary under the focuser, make sure your focuser is not loose or got play. before any of this back the three adjustment screws off slightly to give the centre bolt room for manoeuvre  once it is in position we can use the three adjustment screws to present a full circle remember one has to backed off to turn another when it is right just check all three are tight so that the secondary wont move.

[F1Bird]

Thanks all. I don't think yesterday was the best day to be asking for collimation advice! Think I'll keep a low profile for a bit!

[NGC 1502]

Thanks all. I don't think yesterday was the best day to be asking for collimation advice! Think I'll keep a low profile for a bit!

Don't worry mate, we all have bad days 

In post # 7, you mentioned that " the collimation has been fine until recently so given that the vanes are fixed I think that my problem must be elsewhere "

If it were me, I'd take a break, come back to it fresh, and hopefully will get there.  

Just do your best using the adjustments the scope has, and the one's it doesn't have ( fixed moulded vanes ) are one less bit to fiddle with.

Hope you sort it, Ed.

[bingevader]

This is a Bird-Jones design isn't it? With the spherical mirror and the barlow/correcting lens at the end of the focuser.

I don't think you can use a collimating tool until the barlow/correcting lens is removed.

I had a quick search around t'internet and adjusting the scondary seems to be achieved solely by looking down the focus tube without a collimator, as you have already done!

Cheers

[F1Bird]

When you say without a collimating tool do you mean a laser or any kind of tool? Will the Cheshire still be ok to adjust the primary? It's just I tried it by sight and it looked ok but was well out on a star test so I hoped the Cheshire would help!

[bingevader]

They don't seem to use a tool at all for the secondary, but then in order to use a laser or cheshire, you need to remove the barlow/correcting lens before collimating the Primary, apparently.

Have a search on the web, there are a couple of videos that go through the process, but it does look a little daunting!

Cheers

[Charic]

If there appears to be a lens in the base of the focuser tube, its therefore deemed as a Bird-Jones type. Its a corrector lens to simulate a longer focal length in a shorter tube. Don't be tempted to remove that lens either. If this is a Bird-Jones, just centre the secondary mirror to the focuser, and ensure you can then see the primary mirror clips from looking through the focuser.  That's about it for set-up on this type of telescope. I have something similar with the 127EQ in my shed. I managed to centre spot it just for fun, but makes no difference to the set-up.

Pages 26/27   here http://www.celestron.com/c3/images/files/downloads/1218147775_astromasterinst.pdf

The focuser dust cap can be turned into a collimation cap, just needs a 0.5 - 1mm hole in its centre. Insert a white piece of paper inside the cap also with a small hole) this helps with the final observation of alignment, makes it easier to see.

You will get there soon.

[bingevader]

There you go, what Charic said! It was the removal of the lens that looked the daunting bit to me. Maybe best to leave it alone. Celestron do offer a collimating service, but I'm sure it'd be at a cost.

Cheers

[Charic]

I'm sure the bog standard spherical mirrors in the Astro and my powerseeker, are not as critical as Parabolic mirrors, so the alignment is not critical. You just need to be able to see the primary mirror clips from the view through the focuser. The companies insists you send the telescope  back for 'critical collimation' ? 

[bambuko]

If there appears to be a lens in the base of the focuser tube, its therefore deemed as a Bird-Jones type....

Correct me please if I am wrong, but I thought that Bird-Jones design had corrector lens in front of secondary (ie between secondary and primary mirror)?

I have TAL-120 which has corrector lens at the base of focuser tube, but as far as I knew it was never referred to as Bird-Jones?

Or is it simply that Bird-Jones refers to the type of corrector (...a simple doublet corrector of Jones-Bird type...) regardless of where it is fitted (i.e. in front of secondary or between secondary and eyepiece?)

In TAL-120 primary mirror is not adjustable at all and the only collimation possible is that of secondary.

It makes for a bit more complicated collimation because you have to adjust to suit both primary mirror and focuser in one go (instead of more "normal" design, where you can adjust in two steps i.e. secondary ve focuser, followed by primary).

[Charic]

I think .a simple doublet corrector of Jones-Bird type hits the nail.

In the Celestron 127EQ its fitted to the base of the focuser

As you say its a TYPE of corrector (Barlow) It corrects for the shortness of a  compact Newtonian, cost cutting maybe?

[bambuko]

It permits shorted telescope tube as well as:

...Corrective tele-extender lens - A simple doublet of negative power placed at the bottom of a focuser in fast Newtonians would, by extending converging cone, make possible reduction in the minimum size of the diagonal mirror. It can also be designed to reduce coma, while inducing low spherical aberration and acceptable astigmatism...

the quote is from here: sub-aperture corrector

[Charic]

If you compare the length of the OTA to the quoted Focal length, you may find that the focal length shouldn't  fit, unless corrected( I'm talking Newts, Not Cats) A Bird-Jones type corrector is then incorporated.

Ive just looked at another thread by your good self, showing the secondary mirror and corrector lens. Quite a tidy solution, typically Russian, built like a tank? 

I believe that the corrector on my 127EQ allows for a shorter tube construction. The corrector/Barlow increases the focal length, and the corrector also  helps to refine any aberration caused by the spherical mirror

[Charic]

F1Bird.......Have you fixed it yet?

[bambuko]

...I've just looked at another thread by your good self, showing the secondary mirror and corrector lens. Quite a tidy solution, typically Russian, built like a tank?...

Indeed it is built like a brick s***house.

Having said so, still PITA - I need to adjust/shim the focuser but don't know how to do it without touching/removing corrector lens (and that is definitely the last thing I want to do  ) .

Anyway... enough all this OT stuff , as you say it is about F1Bird and his collimation (I will now stop drifting)

[Charic]

Now you've done it!..............F1Bird is the clue?

[Peter Drew]

One of the problems with collimating this design is that the Barlow/corrector gives a very reduced image of the optical components making it difficult to judge by eye in what manner components need adjusting. To be honest, Ive really only had success doing this on a star image whilst adjusting the secondary.