Collimating a Celestron C11 (C8, C9.25)

OneOfOne (Trevor) · #1 24-07-2007, 07:55 PM

Hi guys,

It was clear tonight in old Melbourne town so I left work early enough that I could set up the scope in the backyard and align the finder, laser etc. After a bit of fiddling I finally got the finder aligned fairly close to the centre of the eyepiece field. Then I turned the laser on a tree over the back fence and aligned the laser with the cross hairs. I know there will be some problems with parallax, but at least they should be in a ballpark.

When the Sun finally went down I turned the scope on and pointed it at a bright star. Upon looking into the eyepiece the view was...less than perfect

The collimation is way out. It is so far out the star looks more like a comet at its best focus. I then pulled out the diagonal and put the eyepiece in on its own to check it wasn't the diagonal at fault...same problem.

So I grabbed a screw driver and thought i would adjust the collimation, but this raises a couple of questions for anyone familiar with the current model SCTs from Celestron. At the front there are three small phillips head screws which are over what looks like a plastic cover with C11 on it. Are these the collimation screws? Or are these just screws that hold the cover on? The picture in the Celestron manual looks totally different and talks about turning a cover to reveal the collimation screws. I don't want to undo these to find I did the wrong thing.

Also, anyone have any tips on how to get started where the collimation is so bad? Or got a few minutes on their hands

(and would like to see a CGE). Even looking at trees through the day gives a poor image.

sparky (Eddy) · #2 24-07-2007, 09:17 PM

Hi One of one
on my C11 the 3 screws that are visable from the front of the SCt are the collimation screws there is no cover. I have replaced these (one by one no pun intended) with Bobs Knobs. Once collimated I got fantastic views of the heavens.

Eddy

David Johns · #3 24-07-2007, 09:18 PM

I am not sure whether this will be of any help, but I have a fairly new CPC-11 and have recently adjusted collimation due to annoying coma. On my scope there are 3 philips head adjustment screws under a lift off plastic cover at the centre of the corrector plate (where the secondary is). It was a bit tricky using a screwdriver (a product called "bobs knobs" can be fitted in place of the screws to make adjustments easier), but I did manage to remove the coma fairly easily by slightly turning a couple of the screws. I found the following to be a useful resource: http://legault.club.fr/collim.html

I hope this helps - good luck

Regards

David

David Johns · #4 24-07-2007, 09:22 PM

Hi Eddy

Where did you buy Bobs Knobs ... I presume they are not expensive?

Many thanks

David

sparky (Eddy) · #5 24-07-2007, 09:31 PM

Hi David
got them fRomm Optcorp I think for about AUS$20. Be carful to get the correct type for your C11. I had to swap mine (dohhh) Optcorp were v undestanding. There are different types plastic and metal.

Eddy

norm · #6 24-07-2007, 09:38 PM

Hi Dave,

Bintel stock Bob's knobs these days too. Might me easier to get them locally.

David Johns · #7 24-07-2007, 10:11 PM

Thanks Norm and Eddy .... I'm ordering them tomorrow.

Great help

David

OneOfOne (Trevor) · #8 25-07-2007, 07:48 AM

I was going to order some Knobs but I didn't know which ones to order and thought it was better to wait. I will certainly give Bintel a call, it is only an extra 10 minutes on the way home...problem is I often pick up something else

As for the screws on the front, I thought they were the collimation screws but they looked a little small to do the job, I was expecting something a bit more substantial. If it is clear tonight, I might be able to give it another go...expecially if I can pick up some knobs.

rumples riot · #9 25-07-2007, 01:39 PM

One, get the ones for a C11. There are 3 screws on the secondary assembly and there might be one in the centre. Do not loosen the centre one.

To adjust collimation simply move the screws a little. You don't need to adjust it much. make sure you recentre the star image after each adjustment.

With Bobs knobs, you install them one at a time. Meaning, you unscrew the phillips head screw and then immediately replace it with a bobs knob. Then do the next. It is really easy to do, all you need to do is follow the instructions. Once they are installed collimation can be adjusted really quickly.

Best of luck.
If you need more help let me know via PM. I have owned quite a few SCT's now and each has had bobs knobs installed.

David Johns · #10 25-07-2007, 07:52 PM

Oneofone ... just to let you know that I contacted Bintel today but they didn't have Bobs Knobs to fit my 11" Celstron ... so I contacted Shell-Lap Supplies (tel: 08 8352 3166) and they had them! Good luck.

Regards

David

KG8 · #11 26-07-2007, 08:11 AM

When you do the job "assuming daytime" first align on a small distant object and as you replace each screw re-centre the object in the EP. If you can see stars at the moment you don't have "bad" collimation. Bad collimation is when all you can see in the EP is a blurr, and adjusting the knobs simply adjusts the level of blurr. You don't want to go there believe me.
I also think this a job best done with the scope near horizontal. That way, if something goes wrong, you wont drop the secondary onto the primary.

OneOfOne (Trevor) · #12 26-07-2007, 09:36 AM

Quote:

Originally Posted by David Johns

Oneofone ... just to let you know that I contacted Bintel today but they didn't have Bobs Knobs to fit my 11" Celstron ... so I contacted Shell-Lap Supplies (tel: 08 8352 3166) and they had them! Good luck.

Regards

David

I also rang Bintel and of course got the same answer. I tried a couple of other stores in Melbourne too, without success. In the end I just ordered them directly from Bob!

I was hoping to have another go last night, but the cloud soon put an end to that!

OneOfOne (Trevor) · #13 26-07-2007, 08:48 PM

Wednesday night I put the laser collimator into the eyepiece and adjusted the secondary until the return beam came straight back onto the target. I know this is not going to produce anything like perfect collimation but I figured it may at least get it into a ballpark of correct. Originally the return beam was only just inside the target.

Tonight (Thursday) was relatively clear and the moon getting quite bright now so I figured this would be a perfect time to try again as I could easily see what I was doing. So I set it up in the backyard and pointed it to Mimosa as it was away from the Moon and was not hidden. The image was certainly MUCH better than earlier, it looked like a very bright comet but at least I could see something.

I adjusted a couple of screws and was able to reduce the coma significantly but eventually it became frustrating trying to remember what it looked like before I had made an adjustment, so it was time to call in my trainee collimator (wife). She looked in the eyepiece as I adjusted the screws letting me know if the image got smaller or the tail started to disapear. After a frustrating 30 minutes we had to give up. As I adjusted a screw, the coma no longer appeared to get any better. The image moved across the field and the coma appeared to change direction, but it didn't appear to get much better. I was using a pentax XW-7 so I don't think the coma is in the eyepiece...certainly not to this degree. Adjusting the focus made only a small improvement, but the coma was always there.

So...am I doing something wrong? I there a trick to getting the alignment better. I was expecting that as I turned one screw the coma would get shorter and then get bigger again. Adjusting another screw would bring it in further until eventually I would only need to iterate gradually between two screws....

?

rumples riot · #14 26-07-2007, 10:04 PM

Wow,

Ok where to start.

First, laser collimators don't work on SCT's you need a fixed point source. Either a real star or an artificial star. Then you need to defocus the image until you get 1-2 rings. Then make adjustments to get all the rings concentric. You cannot use a focused star to collimate unless you have superb seeing 9/10 so you can see the airy disk and only then you would be making very minute adjustments.

By the sound of it, you have collimation so far out that you will need to do the white paper trick to get it close. So here is how that goes.

Get some white paper and punch a small hole through it. Set your scope up and lay it horizontal so that you can see all the reflections. Use the white paper by moving back far enough to see 3 distinct images. You will now see how far out your collimation is. Move one of the knobs and then go back to the same place and check to see which way it moved. Keep doing this until you get each reflection concentric

This article shows in more graphically.

Let us know if there is anymore we can help with.

casstony · #15 27-07-2007, 12:11 AM

This is a bit confusing. Working at 400X in focus sounds like you're trying to perfect collimation rather than the collimation being a long way out. Just to clarify, can you clearly see the secondary shadow in a defocused star at lower magnification?

OneOfOne (Trevor) · #16 27-07-2007, 07:51 AM

Sorry, it wasn't the 7, it was the 14 I was using. I had got the eyepieces out of order in their case, so mag was about 200x. Also if I use the 24, the image still looks like a comet, only smaller!

The other night when I pointed to a star it was so badly out that I could see nothing more than a bunch of very feint smudges. That's when I thought I would check which screws to adjust.

After putting the laser in the next day, I was at least able to see something last night. I knew it would not be perfect, and wasn't, but at least it enabled me to get it close enough to be able to make out stars on a night of bright Moon.

At the moment I am not so much worried about seeing rings as at least being able to see focused "dots" rather than comets.

If I defocus the image I can see the shadow of the secondary, but it appears significantly off to one side. If I attempt, at least initially, to adjust the shadow to appear to be in the middle would this at least get it in a better position? Would the star have to be in the centre of the field for this to work, ie. as the image moves off to one side, does the secondary shadow "move" inside the defocussed image? (Haven't actually looked at this before, but I would expect the shadow would stay in the same relative position) I won't get a chance tonight to do anything as we have to go out.

I will have a look at the white paper trick at the weekend, I might even get a chance after work tonight (Friday). Also next time I might try it with the 24 until I get it better as this will allow more movement until the image goes out of the field.

I wish my arm was long enough that I could look in the eyepiece at the same time as adjusting as I think this would give a more immediate feedback...fortunately though, when I stand up, my knuckles don't drag on the ground...

casstony · #17 27-07-2007, 09:19 AM

You can work with the defocused star and secondary shadow - no need to look down the front of the tube for concentric circles. You need to center the secondary shadow by adjusting the collimating screws, but each time you make an adjustment the star should be re-centered. Once the shadow is centered you should have reasonable looking images. From here you can move on to high power in-focus collimation of which I have no experience due to my unstable atmosphere. On the odd occasion I've been able to go to high power but I'm too excited about observing to bother collimating.

If you happened to have a laptop and digital imager you might be able to observe the secondary shadow adjustments on the screen while adjusting the screws - I haven't tried this though as I can reach the front of my 8" sct while looking through the eyepiece.

OneOfOne (Trevor) · #18 27-07-2007, 12:59 PM

I was thinking of centering the shadow last night, but after half an hour in the cold and not appearing to get anywhere, we called it a night. Let you know how I go...now if this cloud would just go away!

rumples riot · #19 27-07-2007, 06:25 PM

Yep you are definitely out of collimation, and yes you can use the defocused star. I suggested the paper trick to get you close during daylight hours and then use the defocused star to get it spot on. Thsi saves time. Don't forget you need to do final collimation when the scope is perfectly cooled.

With the initial defocused star use low power and defocus until you can see 2-3 rings. Then move the collimation knobs or screws to get it closer. As stated don't forget to recenter the star after each movement. One you get it looking reasonably close, use more magnification and check the collimation again.

With the white paper trick I got it close enough to collimate with my C14 and a 9mm EP. That is 434x which is about where you want to end up.

casstony · #20 27-07-2007, 07:50 PM

I didn't mean to pour cold water on your idea Rumples - I just thought it might be simpler to go straight for the defocused star. One potential problem you can run into with looking for concentric circles down the front of the tube is if the optical elements are not all centered along the same axis. In this case when collimation is good on a star you will not see concentric circles down the front of the tube. I don't know how common this is but I have seen one sct in this condition - it still performed quite well.