(journeyman)

07/03/09 06:57 AM

Collimation problem on a LB16

Hi !

First of all, sorry for my english !

I'm using the catseye collimation tools on my lightbridge 16".
I follow all the steps (center the secondary under the focuser, center the primary in the secondary, then center the primary). My focuser is, as well, well centered in the tube
The problem comes when i use the auto-collimator. When i look into it, i see the dark zone i'm suppose to, but when i do the adjustement to get all the triangles together, my secondary goes in that position :
http://img510.imageshack.us/i/secondarymovelj1.jpg/

Everything is well centered but it seems that i loose some light here

Any ideas ?

Thx a lot !

Renaud

(sage)

07/03/09 08:18 AM

Re: Collimation problem on a LB16

It's centered until you use the auto collimator, you say. From the image, it looks as though your diagonal might have to be rotated a bit or moved "up" a bit. But, if you rotate it and use the auto collimator, then you'll likely rotate it back to that location. Maybe a bit of movement "up" might actually "center" it a little better.

I don't use an auto collimator, so hopefully someone who has experience with them will reply. But, from an "eyeball" point of view..."up" might seem the way to go.

(Pooh-Bah)

07/03/09 08:24 AM

Re: Collimation problem on a LB16

Looks like your focuser might not be square with the tube or your spider is not centred, forcing you to tilt the secondary too much??

(journeyman)

07/03/09 08:29 AM

Re: Collimation problem on a LB16

To correct with the auto collimator, i have to rotate the secondary(clockwise if you're in the front of the tube) that's why i've got this black gap on the seconday. I don't understand why the collimation is ok on the autocolimator with this misalignement of the secondary :/

(Mongo)

07/03/09 09:26 AM

Re: Collimation problem on a LB16

Welcome Renaud your English is just fine.

(Post Laureate)

07/03/09 10:27 AM

Re: Collimation problem on a LB16

Quote:

I don't understand why the collimation is ok on the autocolimator with this misalignement of the secondary :/

Jimmy, centering/rounding the secondary mirror under the focuser is accomplished by the sight-tube -- not the autocollimator. Stacking reflections with the autocollimator is independent of the secondary mirror position under the focuser. Check the following animation. Each frame shows good axial alignment but for a different secondary mirror position. Also refer to the following link

Questions:
1- Do you stack the autocollimator reflections by only adjusting the secondary mirror?
2- Do you iterate between the autocollimator and the cheshire until both show good alignment?
3- Do you use CDP (explained in Catseye documentation and website)?
Jason

(journeyman)

07/03/09 01:00 PM

Re: Collimation problem on a LB16

Hi jason !
thx for your replies
I do center the secondary with the ajustable sight tube from catseye. In fact I do all the steps until the autocolimator step. Then if I stack all the triangle, my secondary goes wrong (as the picture show) and it's not well centered under the focuser anymore :/

I just move the secondary in this step. If i use the cheshire in this part, i just go in circles because the autocolimator makes me move the secondary (as shown in the picture) and in the cheshire I have nothing to do except put it in the first place lol

(Post Laureate)

07/03/09 01:17 PM

Re: Collimation problem on a LB16

Jimmy,

If you use the sight-tube to achieve good collimation and good secondary alignment then using the autocollimator and the cheshire should only "fine" tune collimation with small adjustment. It is not clear to mewhy you are dramatically adjusting your secondary mirror after collimating your scope with the sight-tube – this should not be the case.

Your photo suggests a tilt/rotate error. To fix it, you will have to rotate your mirror in such a way to move the top secondary mirror edge as shown in your photo closer to you and the bottom secondary mirror edge away from you. The amount of rotate has to be SMALL -- just a touch. Then only use the secondary mirror adjust setscrews to stack the autocollimator reflections. Do not forget to check your final setup with a cheshire. If it is off, then iterate between the cheshire (by only adjusting the primary mirror) and the autocollimator (by only adjusting the secondary mirror) until both tools show good collimation.
Do you have the XL or XLK version of the autocollimator?

Jason

(professor emeritus)

07/03/09 01:19 PM

Re: Collimation problem on a LB16

Does the secondary allow tilt adjustments? I have the cat's eye set as well. I rotate the secondary around its axis until the cross hairs of my sight tube are centered in the hole in the cat's eye triangular center mark. Then I use the cheshire and center the triangle as described in the instructions. At this point the autocolimator usually has the triangles all in there, but spread out. My secondary has four screws I can use to tilt it without rotating it to get the trinagles to stack up. I usually don't need more than two iterations of cheshire, autocollimator, cheshire... to get everything all lined up.

Hopefully this helps. Sometimes just hearing the procedure in someone else's words can help.

-Brian

(Post Laureate)

07/03/09 01:29 PM

Re: Collimation problem on a LB16

Quote:

Looks like your focuser might not be square with the tube or your spider is not centred, forcing you to tilt the secondary too much??

Hi Keith,
Actually, it is possible to determine whether the issue is due to tilt/rotate or misaligned focuser/spider_vanes. Check attachment.
Jason

(journeyman)

07/03/09 02:13 PM

Re: Collimation problem on a LB16

I found your last attachement very usefull !

So my problem is a rotate problem. But i find the fact that the triangle only stack when the secondary is in that position

(sage)

07/03/09 07:42 PM

Re: Collimation problem on a LB16

Jimmy, as Calibos mentioned, it appears your focuser might not be truly square. Start from scratch to square your focuser once more.

(Post Laureate)

07/04/09 12:07 AM

Re: Collimation problem on a LB16

Quote:

But i find the fact that the triangle only stack when the secondary is in that position

Jimmy, as I mentioned in an earlier post, the autocollimator has nothing to do with the secondary mirror alignment under the focuser. That is, you will be able to stack all reflections of the autocollimator perfectly for different secondary mirror alignments. I pulled out the following animations from the thread I referred to in an earlier post. Every frame represents a good axial alignment yet each frame includes a different secondary mirror alignment.
Jason

(journeyman)

07/04/09 12:01 PM

Re: Collimation problem on a LB16

I think I might have found where the problem comes from...
It looks like the center triangle...is not centered !

here's what I did (tell me if something is not logical there)

i very precisely centered the secondary under the focuser, and the primary reflexion into it with the sight-tube. then i took...a laser !
ok so theoretically the laser spot might be right in the middle of the triangle now (am I wrong ? don't think so). and guess what. The spot is landing in one of the corner of the triangle (my laser is well centered, if i rotate it in the focuser, it's position doesn't move on the primary.)
ok, let's continue anyway...
Still with the laser, i'm centering the primary (this step is normally done with the blackcat cheshire.)
Now last step, if i look in the auto-colimator, i can see all the ghost images really close, and a minor secondary ajustement allow to stake the triangle and only one of it's ghost (of course not the 4 of them, cause the spot is not well centered, am i wrong again ?)

after that, i can check with the sight tube and the laser (and auto-colimator) everything's in place...

what do you think ? Did a miss something there or am i right ?

Renaud

(Vendor - Scope City)

07/04/09 12:28 PM

Re: Collimation problem on a LB16

Quote:

I think I might have found where the problem comes from...
It looks like the center triangle...is not centered !

here's what I did (tell me if something is not logical there)

i very precisely centered the secondary under the focuser, and the primary reflexion into it with the sight-tube. then i took...a laser !
ok so theoretically the laser spot might be right in the middle of the triangle now (am I wrong ? don't think so). and guess what. The spot is landing in one of the corner of the triangle (my laser is well centered, if i rotate it in the focuser, it's position doesn't move on the primary.)
ok, let's continue anyway...
Still with the laser, i'm centering the primary (this step is normally done with the blackcat cheshire.)
Now last step, if i look in the auto-colimator, i can see all the ghost images really close, and a minor secondary ajustement allow to stake the triangle and only one of it's ghost (of course not the 4 of them, cause the spot is not well centered, am i wrong again ?)

after that, i can check with the sight tube and the laser (and auto-colimator) everything's in place...

what do you think ? Did a miss something there or am i right ?

Renaud

This presumes the laser is perfectly collimated and that you have perfectly centered everything else, which is highly unlikely by eye. So I don't think this is a test of the centering of the triangle on the primary.
Try measuring the distance from the center of the triangle to the edge of the mirror in several directions. If it's as close to center as you can read with a tape measure, then the centermark IS centered and the problem is simply miscollimation.

(Post Laureate)

07/04/09 01:43 PM

Re: Collimation problem on a LB16

Jimmy,

I concur with Don. I do not believe your approach in assessing the center mark location is adequate.

There is a much simpler approach. You do have the Catseye tools; therefore, it is fair to assume you still have the transparency template that comes with the tools. Use that template to assess the placement of your center mark.

By the way, all ghost reflections can still be stacked precisely even if the center mark is slightly off.

I still strongly believe your problem is simply a rotate/tilt issue. Based on your drawing, the discrepancy between the primary and secondary mirror edges it too large to be contributed to a “slightly” off-center mark. The center mark will have to be an inch or so off to create the scenario you have described which I doubt to be the case.

Jason

Cloudy Nights Sponsor: Astronomics