Laser vs Cheshire collimation; my experience

[bobson]

I have Bintel 12’’ Dobsonian telescope, when I bought it they said its all checked, collimated and ready for use. I can’t say they didn’t check it or collimate it but it was totally out of collimation when I got it. Maybe ‘cos of transport…

Since collimation is an issue with bigger Dob scopes I decided after reading many reviews to buy Laser collimator. I got GSO Deluxe laser collimator from Andrews. They said its collimated, and it was pretty much there but still it was making small circles when turned around in two “V” shape holders made from chipboard and distance approx. 5 meters from the wall.

So I decided to collimate it. I had to find very small alan key to be able to adjust the laser light. There are 3 adjusting screws. BTW, the switch is one big screw on the body of the Laser collimator, be careful with it, not to turn it in too much ‘cos it can damage it. I started adjusting it and after a while it was much worse than it was at the beginning.
Just when I thought to give up I managed to make it as straight as possible.

When I put it into focuser and turn it on, the laser dot was about 5-7 cm away from the centre! I should mention it that I used film canister to collimate it before. The return beam was hitting somewhere around the focuser body, not even going into focuser. I noticed two laser dots on the secondary mirror. I was sure that my secondary is OK but just needs tilting a bit to hit primary centre. I loosen the adjusting screws on the secondary mirror and adjusted it so that the beam was exactly on the centre of the primary mirror. Now I could still see two laser dots on secondary but closer to each other. The return beam was on the corner of the 45 degree cut out on the collimator. This is really handy tool; you can be at the back adjusting screws without having to go up and down to see how you are progressing. Once I could see return beam in the centre of the 45 degree cut out I knew it’s collimated. I looked at the secondary mirror and I could see only one laser dot, GOOD! I checked it with film canister and it was in centre. The problem is it was in centre before as well with film canister when I didn’t have laser, but found out that it was out 5-7 cm.

That night I checked it out, and it was very good, star tested it, it was very good too.
I made Barlow laser collimator from my old 2x Barlow. All I did is, I took Barlow cover and made hole on it, since it cannot fit into focuser with cover on, I took it off, than I took 1.25 inch adapter out of focuser, put Barlow through it and than plugged it with Barlow plug with the small hole on it and small white sticker I made before and then put the 1.25 inch adapter with Barlow in it together into focuser. Now, when I put laser in the Barlow, the beam was much wider, the idea is to see centre mark from primary on the white sticker on the Barlow. But since Barlow is too short I needed mirror to be able to see it inside the focuser looking from the top of the tube. I got one from my wife, with the handle, put it in and I could see that I need very small fine adjustments to make.
Now turning focuser in or out all the way, its always in centre.

So, all in all, laser collimation is fun and easy, although I needed to put new batteries in the laser, but they supplied spare ones anyway.
Then I got tempted to buy collimation eyepiece to compare with laser, I got Skywatcher Newtonian Cheshire from Andrews. When I put it into focuser I could see it’s perfectly collimated, but when I moved my eye around a peep hole a bit I noticed it goes out. I found out that peep hole even though looks very small is too big. So I found black sticker, put it over the hole, rubbed it hard around the hole so I could see where the hole is, and then took the needle and made very small prick in the centre. I put it into focuser, now when I looked through it I could not move my eye around, but just look straight. There is 45 degree cut out on it too, you have to turn it that way so light is on it to reflect the centre circle and that way you can see perfectly what you're doing.

Personally I find that laser collimator is much easier to do than with Cheshire, much more fun as well, and I dare to say more accurate too. Of course, laser collimator have to be collimated itself first. I find that it’s easy to find out if laser is not collimated, but Cheshire you do not have any means to check it. If the peep hole is not in line with cross, on the bigger scopes it could be out up to 1 cm if not more. To the naked eye it looks all fine, but like I said how can we be sure it is? I found out that mine is well centered since both laser and Cheshire and star test are showing good collimation.

I should mention that all the time during collimation I made sure that I put both laser and Cheshire the same way into focuser I would put eyepiece. So in my case, the screw that holds 1.25 inch adapter to focuser is always on the bottom and screw that holds eyepiece is on the top, opposite the adopter screw. This is very important, to keep consistency, otherwise you could make collimation for any given position of the either adopter or eyepiece, or Barlow for that matter. Tightening holding screws should be also consistent, as close to each time as possible. With Cheshire you will not notice difference, but with laser you will, a bit tighter or looser and you will notice it.

[erick]

Bob, you are describing my experience almost precisely! Down to the struggle to collimate the GSO laser and how I almost gave up, but suddenly I was on top of it!

The only additional step I took with the unbarlowed-GSO laser was to put one turn of contact (yes, the exercise book stuff) around the 1.25" barrel so the laser was a snug fit into the 2"-->1.25" adapter, without having to tighten the holding screw much at all. I could then wiggle the laser and the spot hardly moved on the primary mirror.

Two further steps are 1) wind the focusser all the way in and out and check that the spot on the primary mirror stays fairly unchanged. I think this is telling you how square the focusser is and how the draw tube is moving, and 2) tip the OTA from horizontal to vertical and watch if the return spot back at the laser moves much. I think this is telling you about the mechanical performance of the tube and also whether the primary mirror is "sagging" on the springs.

[bobson]

Yes Erick,

I did that test, wind the focuser all the way in and out, and the beam stayed in the same spot. Also tilted tube up and down and again all the same. I actually got the idea for "V" shape laser testing tool from you, very simple and it works fine. Thanks mate!

The only thing I noticed are the locking screws on the primary. You know when I finished all the adjusting and as they always say try to tighten instead loosing it 'cos of weak springs. I find that I have to tighten locking screws unevenly, otherwise the laser beam goes out of centre a bit, just a bit but that shouldn't happen? I don't use screwdriver for this at all, only for adjusting screws 'cos I can't turn them with fingers.
The collimation stays good for a long time, I take care taking it in and out. Don't know hows it going to be once I transport it out to dark sky, if this rain ever stops.

The other day I had to take primary mirror out 'cos the bolts and nuts that hold the round plastic bit on the sides of the tube that sit on the mount became very loose! I couln't reach in to hold the bloody nut, so I had to take primary mirror out the tube and tighten it. Someone told me I should use naylock nuts for this, but I hope I tighten them enough.

cheers

BTW, I noticed you have Televue 3x Barlow, I have the same one. You could make Barlowed Laser with that one very easy, and its long maybe enough to see it when wind focuser in. The bottom cover where its written TeleVue (black cap) you can drill very small hole, make sticker from white paper and punch hole on it. The best one would be the one that you can peel it off if you want to without leaving glue on it. Stick it on the cap, take 1.25 inch adopter out from the focuser, put Barlow in and from other side plug it with the cap with that white sticker on it, and together like that put it back into focuser. This way you don't have to worry about damaging your TeleVue Barlow. Now you can put laser into Barlow and fine tune it :)

[erick]

Good to hear Bob.

Yes, I hear people saying that screwing up the locking screws just pushes the primary mirror back out of collimation.

In my view, if the collimation stays pretty close as the tube is moved from horizontal to vertical, all is fine and leave the screws off the primary miror cell. If the collimation moves, then solve it another way, rather than trying to use the locking screws. Seems there are two approaches with the GSO. First, replace the springs with stronger ones. Second, put the existing springs under greater compression by using the three collimating screws to pull the mirror further down the tube - a good centimetre or more will help. The only possible downside (as I discovered on a Bintel BT-402 I did this to last week) is that the small GSO plossls might no longer come into focus - the focusser bottoms out in its travel prior to focus.

I've been fortunate not to have any problem with the trunnions on the 12". They are firmly in place. I did tighten them on the 8", but I could reach down from the spider end to hold the nuts.

On my 12" GSO (and my 8" GSO prior to that), I have only tightened the locking screws for transport.

Yes, I have used both my GSO 2x Barlow and my TV 3x Barlow for barlowing a laser collimator. My approach is to use a piece of paper or card with a hole punched in it (just your regular hole punch), hold it inside the tube at the bottom of the focusser, allow the barlowed laser beam to pass through the hole, and the reflection of the centre "spot" appears back on the paper. I have to admit that I've only ever put the barlow in place after I have set the collimation with the un-barlowed laser and never found I needed to adjust it further.

[batema]

We have bought a 12 " Dob GSO and are pretty excited about the views we were getting. We knew it needed collimation but were still quite happy. Of course the weatheer has set in so we have not been able to do any viewing after we manually did collimated without any tools. I have attached what our mirror set up looked like through the eyepiece and we were quite excited with the views. Do you think we will get even more so after we fix the collimation. The red circle is the outline of the base of the tube.

Mark

[batema]

Sorry reduced image

Mark

[bobson]

Hi Mark,

Put it this way, I thought everything was OK with my collimation with film canister and then I found out that it was good 5-7cm away from the centre with the laser collimator.

What does this mean is that even though star test was OK, and viewing was good I was not using full potential of the mirror. This is mostly apparent with higher magnification.

The hole on the film canister has to be very small, so when you move your eye around it doesn't shift the picture out of centre. That's why I even had to block hole that came with my new Skywatcher Cheshire collimator eyepiece and make smaller one instead.

I definately would recommend one of those tools, either laser or Cheshire. Andrews is selling them $59 for laser, and $49 for Cheshire.

[batema]

Hi Bob and others,

I just came from my mates place where our DOB is stored to the delight of his wife and I would now recommend that every person must have a 12" DOB in there astronomy arsinel. How unbelievable are the views. We have only manually collimated without even a film canister and I am blown away. To spin this thing around and in the space of 1 hour view up to 15 galaxies (huge in the field of view), Helix, Tarantula, ORIONNNNNNNN and others with probably still bad collimation and GSO eyepieces I'm amazed. Will definately go a laser and hopefully better viewing. What a machine.

Mark :D

[bobson]

Hi Mark

Yes, 12" is great scope. Imagine Newton had only 5 cm wide mirror when he invented reflectors!

cheers

[Roger Davis]

Funny, why didn't you stop at para three of your attempts and do what most people would do, a STAR COLLIMATION. Any mechanical device will only get you so close. Your final collimation should always be done on a star with the instrument in question.
Collimation implies mechanical and optical co-alignment. Usually focusers are fine, but sometimes they may not be square to the optical or mechanical axes of your scope. So you need to square it on. This requires removing the secondary mirror and cell.
There is a lot more to collimation than just putting a laser in and hoping it does the job. A laser will not tell you if your secondary is rotated, a Cheshire will.
A good book would be "Perspectives on Collimation" by Vic Menard.

[bobson]

Yeah, read on:

http://w1.411.telia.com/~u41105032/myths/myths.htm

"myth: You have to square the focuser very accurately

I'm not quite sure of even what "square" is supposed to mean - likely it means set perpendicular to the tube, or possibly to the optical axis - or both, always assuming you have made them coincide. There is nothing wrong with doing it, of course, but the secondary is optically flat, and the angle of reflection is not critical. Most secondaries are made to look circular when tilted 45 degrees (to reflect 90 degrees), but if the angle deviates from this by a few degrees, the only consequence is that the secondary will appear slightly elliptic - it won't affect the image.

The important thing is that the focuser and the secondary are lined up, as seen from the focus. If not, you should adjust either, as appropriate - if the secondary seems to be off in a direction from or towards the primary, you can usually move the bolt that holds it in the spider. If the error is "sideways", and the secondary is indeed well centered in the optical tube, it must be the focuser that is off to one side and needs shimming. If this error is left alone, the penalty is that the fully illuminated field at focus won't be centered - a small error here is no disaster, but the center of the field should always cover the center of the field of view, and best with a small margin to avoid the effects of a possible narrow turned-down edge on the secondary. Check with a sight tube in daylight when the telescope is collimated - when the peephole of the sight tube is close to the focal plane (top of the drawtube at a "normal" position), the whole edge of the primary should be visible in the secondary. If the focuser is tilted a little "down" or "up" the tube, this will be compensated for when adjusting the tilt of the secondary - the only consequence is that the fully illuminated field will be slightly elliptic - a thing you will never notice."

"myth: You should always finish collimation by star collimation

At the focal plane of a paraboloid mirror, there is one point where coma, the one-sided aberration, has a minimum. This point is, by definition, the focal point. Locating it can really only be done by star collimation - using a real star, or a sufficiently distant artificial one, collimating the primary mirror for a symmetric star image (high magnification, and image close to focus for best sensitivity) at the center of the field of view. If you put a small peephole at the true focal point and look, you will see a distant reflection of the peephole itself (if its inside is illuminated), as if far behind the mirror surface. The spot on the mirror that the peephole seems to lie behind is - again by definition - the optical center of the main mirror.

The common thing to do is to place a spot at the geometric center of the primary mirror and use it with a Cheshire eyepiece, centering the reflection of the Cheshire behind the spot. Normally this is close enough (I am still waiting to hear about anyone with a mirror where the optical center is significantly offset from a well centered spot!). Using the Cheshire is a lot easier, quicker and better reproducible than star collimation is - particularly with less than excellent seeing. However, if you find the one-sided asymmetry of coma on a star at the center of the field, it can only mean one thing - the center spot isn´t at the true optical center. The best thing is to move it or put a new one, centered in the reflection of the Cheshire (the next best is to make a note of the direction and distance of the offset), for later collimation. Now you know you have the true optical center marked.

Thus, you should take the trouble and do a careful star collimation under good seeing to ensure that the primary is accurately marked - but when this is done once, collimating with a Cheshire or Barlowed laser is easier and more reproducible. There is no point in star collimating every time."