Collimation

[Talitha]

I had a feeling this was going to happen. When i look through the open eyepiece hole, the view looks like this photograph of a well-aligned secondary. My view also shows the backside of the focusser's drawtube to be a bit off-centered, but i think it's the 'offset' i've read about. My question is about using my Meade laser collimator. When i place it in the eyepiece opening and turn it on, the laser dot is about an inch (give or take a bit) from the donut on the primary mirror. The solution for this, according to the scope's manual and the laser's directions, are to fiddle with the secondary.

But when i adjust the secondary and place the laser in the donut, the empty eyepiece view is skewed to the side. At that point, i'm not sure if the secondary has been un-aligned or not but according to the linked image, it has. Any advice? Maybe a different kind of collimator will help, i don't know. Also, should i keep working on this indoor collimation procedure, or just wait till the scope dolly gets here so i can move the scope onto the deck for some on-the-job training?

[Ogri]

I haven't got a clue about collimation, not having a Newt (yet), but certainly wish you luck getting it all sorted.

I doubt it will be long before good advice turns up!

[Talitha]

Thanks for your kind thoughts, i'm hoping to figure things out before winter hits. (Apparently i haven't a clue about collimation either, lol.)

[yeti monster]

Carol, have you rotated the laser collimator device in the focuser tube, to check that it is itself collimated?

One happy with that, concentrate on getting the secondary to aim the laser at the donut.

Then adjust the primary to bounce the laser back up to its origin. TopHouse showed me this with his Holtec, and I have made a DIY device to do this job myself (thus saving several earth quids).

[TopHouse]

To be a little clearer, I don't know if your focuser is a compression type or the 2 screws, but put the collimater in, and make it so that it's JUST loose enough to be able to rotate it. Now look down the tube and rotate the collimater in the focuser. Does the red dot on the primary stay still or does it draw a circle?

If it draws a circle the collimater is out and needs collimating itself.

Also, do you have a sight tube or collicap, or even the film cannister with a hole in it?

P.S If you pay my air fare I'd be happy to come over and do it for you.

@Yeti, did you use an empty cornflakes packet, an old fairy liquid bottle and some stickyback plastic?

[yeti monster]

@Yeti, did you use an empty cornflakes packet, an old fairy liquid bottle and some stickyback plastic?

Ooo, better than that TH, a laser mount from astroboot and a couple of sections of ABS waste pipe, it works very well indeed, I can aim the laser down my cheshire and collimate it using the screws on the laser mount. Rotating the assembly proves collimation of the laser. Then I adjusted the secondary to the dot central on the primary, rechecked collimation of the laser and continued to home in. Onece satesfied with that, I brought the returning beam back up to the origin. It may look cheap and nasty, but it certainly did the trick, especially after building up my Yeti- Crayford dual speed focuser. I now have reliable collimation AND decent focusing on the SNT, no doubt you'll remember how terrible it was before.........

[TopHouse]

You mean you've taken those old bike wheels off the focuser?

[TopHouse]

I've ordered one (well two actually) of those new fangled center spots from Jim Fly, I'm now thinking a video might be on the books about collimation and how lasers lie regarding secondary adjustment. Could make for some interesting discussion. Technical videos with broad northern English accents just HAVE to be taken seriously!

[yeti monster]

You mean you've taken those old bike wheels off the focuser?

I thought they were off a wheel barrow!

See the DIY-Yeti-Crayford Dual speed Special:

http://stargazerslounge.com/diy-astronomer/104253-crayford-sct-snt-conversion.html

Technical videos with broad northern English accents just HAVE to be taken seriously!

I can do a rather good "Fred Dibnah"

[yeti monster]

Oh, I do plan a wee photo blog of the "Blue Peter collimatable laser cheshire collimator device", when I get home........

[Talitha]

Thanks!

No, i hadn't tried that, but just went downstairs and did... there's no wobble at all except the expected amount of slop from the necessary looseness of the collimator in the eyepiece holder (compression ring). It took a few tries to get the pressure just right. Earlier in the day though, i'd read something online which suggested cradling the collimator in one set (preferably two sets) of wood which are squared, and in the shape of a 'V' (think manger). Turning the collimator in the 'V' would rotate it and show any mis-collimation of the laser. I didn't have any squared pieces of wood, so i simply rolled the collimator on the kitchen countertop, and the laser dot scooted along the backsplash in a straight line, which was good... no wobble whatsoever.

Yes, i made a cap from a film cannister but TBH i used the Meade collimater instead, because it's supposedly so much more accurate. To use the film cannister properly (IIRC) i'd insert the bottom end of it into the eyepiece holder and then see if my eye is centered in the donut? (Thanks so much for your offer TopHouse, but i just spent your air fare on the scope... please forgive. )

[Llamanaut]

Im not familiar with a meade colliamtor i have a hotech....just to check where your at carol... you have a meade collimator with a compression ring, its callibrated, but the red dot is outside the donut? Youve tried aligning the secondary mirror manually by use of grub screws etc... until it is in the donut? Check?

Next step if you have a primary manual adjustment on your scope which im guessing you do is to align the primary mirror. Is your laser collimator like a hotech? Does it have a cross or centrepoint with which to align the primary?

Once the secondary is donut aligned the primary should be manually adjusted with finger screws until its aligned with the collimator... i usually tighten all grub screws lightly finger tight then slowly unscrew one by one until the primary mirror reflection lines up with the centrepoint of the collimator, then hopefully everything is lined up abd good to go... forgive me what particular brand of light bucket do you have?

[beamish]

Hello Carol,

a few thoughts/suggestions if I may.

The doughnuts are not always perfectly centred

I would have thought for your 16"LB the only laser I would trust to do the job would be a HoTech

In absence of a HoTech I would plum for the investment of any or all of the following,

CatsEye collicap/cheshire/AutoCollimator- these together would be the ultimate tool to collimate the 16" to within a whisker of it's life !

Personally I would now be tempted to ditch the laser and go for alternatives.

"Use the Force Carol, trust your inner instincts"

Honestly the CatsEye stuff will blow your mind !

[TheThing]

Talitha,

Have you tried the Andy's Shot Glass video tutorial to collimating? Includes a very good piece on using a laser. Very clear and easy to understand.

Hope that helps?

[Mark at Beaufort]

Carol. I checked my laser by shining the red dot through a refractor. I have a garage which quite long so I placed the frac one end and a target area the other. I tighten the laser in the focuser so that I could still rotate the laser. I aimed the laser on the centre of the target and rotated - the red dot never moved so I knew that the laser was collimated.

This is at least one way to check the laser without building a 'v' block. Once you have established that the laser is ok then you can proceed with collimating the newt. Hope this helps.

Mark

[Sam]

Carol,

With my LB 16" I collimate it with a collimation cap first as best as I can. I then finish off the fine tuning with a star test. I've found this gives me the best collimation. I had used a laser collimator but I got no where near as good results as doing it the manual way.

Sam

[TopHouse]

If you do the rotate in the focuser with the collimater you don't need V blocks, it's basically the same principle.

Sam is correct though that you should first center secondary up using a collicap/sight tube.

[Astro_Baby]

The problem with andys shot glass is he doesnt cover offset that a fast newt would show. The 16" LB is a I think about F4.5 and as such is a pretty fast scope (only .5) quicker than a lot but that .5 is pretty critical. EPs that work tolerably well at F5 will be rough at F4.5 and completely lose the plot at F4.

Back to collimation.

This is one of the reasons I dislike lasers - they can often lead to these sorts of problems where centring the laser on to the primary ends up with the secondary showing rotation error through the EP.

Carol - can I suggest you take a look at my collimation guide which does cover offset. Offest is hard to explain in text but simple (relatively) in pictures. Take a look at the last few pics of my guide here Astro Babys Guide to Collimation and you will see the two differing patterns between a slow scope (classic collimation pattern) and a fast scope (offset collimation pattern).

To directly address your collimation woes let me suggest you get the sceondary perfectly circular under the drawtube.

Now adjust the tilt of the sceondary and make sure all thre mirror clips on the primary (I think there are atchullay 4 on a LB) are visible at the edge of the secondary refelection.

Try the lasr from that point on BUT if it still produces strange results I'd either (a) chuck it away and get a Hotech or ( Get a cheshire because about 90% of laser collimation woes are caused by duff lasers and the fact that using a laser you cant set up the secondary alignment reliably.

Have a red through my guide and it may help your thinking. Bear in mind my guide is written for a Cheshire based procedure BUT it will work with only a colicap.

[Jason D]

When i place it in the eyepiece opening and turn it on, the laser dot is about an inch (give or take a bit) from the donut on the primary mirror. The solution for this, according to the scope's manual and the laser's directions, are to fiddle with the secondary.

But when i adjust the secondary and place the laser in the donut, the empty eyepiece view is skewed to the side.

Carol, the secondary mirror adjustment consists of two alignments. The first alignment is centering/rounding the secondary mirror under the focuser. The objective of this alignment is to optimize the secondary mirror position to intercept the most light cones for all stars in the FOV. This alignment is somewhat coarse. Once you complete it the first time, you do not have to deal with it again for a long time. Besides, it is not the most critical collimation alignment. If you are somewhat off, it will NOT impact the resolution and clarity of objects in the middle of the FOV. The only impact is the loss of some brightness for those stars located towards the edge of the FOV.

The second alignment has to do with redirecting the laser beam to strike the middle of the primary mirror center spot. This alignment requires fine adjustments. The purpose of this alignment is different from the first one. It eliminates the tilt between the eyepiece and primary mirror focal planes. In other words, it ensures all stars in the FOV come to focus.

To summarize:

- Centering/rounding the secondary mirror under the focuser is meant to optimize the star brightness across the FOV.

- Redirecting the laser beam to strike the center of the primary mirror spot is meant to bring all stars in the FOV to focus simultaneously.

Even though both alignments involve the secondary mirror and use the same adjustment screws, both are somewhat independent. That is, you can center/round the secondary under the focuser yet have the laser beam strikes the primary mirror 1” off-center. Or you can center the laser beam and lose the centering/rounding alignment of the secondary mirror under the focuser.

Refer to the attached animation. Each frame will redirect the laser beam to the primary center yet only one frame includes a centered/rounded secondary.

The laser collimator deals only with the second alignment -- unless it comes with a holographic attachment which is not the case here. Therefore, what you have experienced has nothing to do with the quality of your laser collimator but rather with not being able to correctly interpret what you have seen.

Refer to the first few pages of this thread

Telescope Reviews: Useful info about secondary mirror alignment

Also refer to this post

Telescope Reviews: Re: Concise thread about autocollimators+improvements

Jason

[Talitha]

Sorry i'm always getting in here so late, but Wisconsin is 6 hours behind BST... might as well be a whole day when you want to reply to posts in a timely fashion. I'd like to say a big thank you to each and every one of you for all of your help and advice. I really appreciate it, and honestly wouldn't have been able to make full sense of the collimation procedure without you.

<<<BIG BIG BIG HUG>>>

Mel and Jason, your material is absolute gold. I'd read (and had also bookmarked) the pages before the scope was delivered, and should have read them a few more times but was suffering from impatience and a bit of information overload. However, after slowing down and taking a few deep breaths, the entire process makes sense now (still need to apply it, but at least it makes sense ). I think my main frustration stemmed from never having set up a dob before, let alone prepare one for it's launch date. Speaking of launch dates, the FedEx tracking site indicates that my scope dolly will be delivered on 20 July.. i'll finally be able to get Lurch out of the living room and out under the stars. Or out under the clouds, for that matter... i really don't give a rip, i just want to see something besides the ceiling.

[Astro_Baby]

Carol - tut tut - as an artist you should know patience is the key

I'm sure when you sit down, take a deep breath, (In the UK we would stop for tea dont you know ) and generally chill you will work it all out easy enough.

This is one of the great things about reflectors you can play even when its cloudy because colimation can ALWAYS be slightly improved

I look forward to hearing your first light report.

[Talitha]

Yeah, i should practice what i preach more often, lol.

Well, methinks i might have done it successfully.. looks good on paper, anyway.

But the final proof is in the pudding as the saying goes, so all i can do now is wait (patiently;)) for the scope dolly to arrive.

Then, Lurch goes to work.

[WWPierre]

Dredging this thread up from the past, because it came up when I googled "collimating a 90mm mak". It does look like the posters know a lot about collimation. I hope they are still hanging around here.

As a moderator at Astronomy Forums | Telescope Forums & Reviews | Astronomy Community, I posted my query there, so if I can ask you to visit our forum, and read this post, my question will be revealed.

[John]

Dredging this thread up from the past....

it only dates from 9 days ago ......

Perhaps you would like to post your question here as well ?

There is lots of collimation knowledge within the membership of Stargazers Lounge

[Talitha]

John, i don't think they want to ask anything, it's quite obvious what was done here.

WWPierre 1) joined SGL 4 months ago, 2) never posted anything till today, and then 3) suddenly decided to use my thread as a springboard to promote another group. Here's what came up when i Googled "collimating a 90mm mak". TBH i think the only reason my thread showed up is because it's recent, and my 90 Mak is listed in my signature line. Makes me wonder if they did the same thing to CN... they're #1 on the list.

I don't know where Admin stands on something like this, but as far as i'm concerned it's a pretty big slice of spam and i hope no one clicks on the links.