Collimating a Celestron C11 (C8, C9.25) [Archive]

OneOfOne

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

sparky

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

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

24-07-2007, 09:22 PM

Hi Eddy

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

Many thanks

David

sparky

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

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

24-07-2007, 10:11 PM

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

Great help

David

OneOfOne

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 :whistle:

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

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

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

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

26-07-2007, 09:36 AM

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

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....

:help: ?

rumples riot

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 (http://www.bobsknobs.com/pdf/Instructions_English.pdf)shows in more graphically.

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

casstony

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

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...:lol:

casstony

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

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

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

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.

whmacs

27-07-2007, 09:05 PM

Hi Guys,
I've installed Bob's knobs on my 11" SCT (Purchased from Bob). I've got a bit of a write up on my site on how to install them and collimate the scope. This can be viewed at: http://www.stephenmacmillan.com/astro/tipsandtricks.html

I need to update my site, but the new URL for Bob's knobs is:
http://www.bobsknobs.com/
Regards,
Stephen

casstony

28-07-2007, 12:10 AM

Nice site Stephen. I'd be interested to see the procedure for centering the corrector and secondary that you mention in your collimation section.

OneOfOne

29-07-2007, 11:03 AM

We were determined to get somewhere last night (Saturday). Unfortunately, the clouds rolled in late afternoon and things weren't looking good. I went outside a few times after tea to see the sky clouded over :mad2: Finally around 8 the sky was clear YAY! So I started setting up the mount and scope. I just bypassed the mount alignment procedure and swung around to suitable star, bright enough and away from the nearly full Moon.

I put the 24 Pan in to get the widest field possible and did a quick focus. The best I could obtain was a huge comet! So I defocussed the image. Instead of getting a donut, I got something more like a burger ring! The thickest part was about four times wider than the thinest...hence the problems. While I was waiting for my wife to come out to give me a hand, I thought I would give it a bit of a go on my own. So I chose a screw at random and turned it, anti clockwise seemed as good as any. When I looked it appeared the image was slightly less burger ring and more donut...so I turned a bit more and recentered. Eventually I could see it was about as close as I could get in one direction, so chose another screw. After a few more tweaks, it looked pretty good. Time to put in a shorter eyepiece. With the 10mm the image looked like a pretty good imitation of a donut.

Now I tried to focus the image, pretty damn good if I say so myself. By now my wife came out and I said I was nearly done. A couple more tweaks and I was happy :D In total, I guess I had to move one screw nearly two full turns and half a turn or so on another (of course who knows how much of this was caused by the previous attempts).

So I decided to turn it onto something simple and move to Mimosa but as I tried to focus, I couldn't get it right. No matter what I did, I couldn't get it right. :( I went back inside to try another eyepiece and on the way back I noticed the corrector was all frosted up! :doh: You don't get problems like this with a Newt...

So next time I will put the dew shield on before I start to get serious. By now it was about time to come inside and warm up anyway.

So, morals of the story?

If the alignment is REALLY bad and you can only see "comets" use the defocused method to get it into a ballpark. It works really well.
When you are collimated, put the dew shield on (or turn on the heater).Now I just need a clear night so I can reset the finder and laser centering and then I might even try aligning the computer for the first time....:D

Rigel003

29-07-2007, 11:53 AM

Don't forget that the 3 screws adjust the tilt of the secondary support around a central spindle in a kind of rocking arrangement. If you tighten one screw you must first loosen another, or loosen both of the others a smaller amount so there is no slack at any stage. Unless you do this and keep it reasonably tight, your collimation won't hold.

rumples riot

29-07-2007, 12:01 PM

Tony, the white paper technique just gets you close enough to make collimation quick. Not to worry though, it is just a tip. I would be a bit concerned though about not seeing concentric circles. I have not seen this before and have owned a lot of SCT's and helped people sort their collimation out. I have no doubt this does happen, although I would not accept a scope that performed in this manner as the scope is limited by the alignment.

One of one, glad to hear you got your collimation sorted. Increasing the power will give greater collimation and will help each time you setup. Only high level collimation will give you the superb views these scopes can produce. Only on rare nights of excellent seeing (ie 8.5/10 +) can you use the slight tweeks with a focused star. You can then see the airy disk without it being distorted by the seeing. In average to good seeing, it is not advisable to used the focus star technique for collimating an SCT.

Anyway, let us know if we can be of more assistance.

casstony

29-07-2007, 01:23 PM

Rumples, the scope that showed non-concentric circles was a 10" f/6.3. In spite of this the scope showed pretty good images, but other problems developed in the warranty period and I sent it back. For contrast, a 90's era 8" f/6.3 that I currently have shows nice concentric circles looking into the tube but gives softer images due to higher than average spherical aberration. So I certainly agree with you that misalignment of the optical axis is not desirable, but it may be one of the lesser evils that can afflict schmidt cassegrains.

OneOfOne

29-07-2007, 01:59 PM

After I got the collimation, that was the next thing I checked, although I was checking them each time I made an adjustment in the final stages. All the screws seemed to be tight...at least without getting a shifter to give them a final tighten :eyepop:

I would have liked to check it out at a higher magnification, but by the time I was happy, the image was beginning to get "soft" from the condensation.

David Johns

29-07-2007, 05:26 PM

Hi Graeme ... I was very interested in your comment regarding loosening and then tightening the screws. I have attempted collimation (CPC-11 once) using a Philips screwdriver and recall none of the three screws were 'tight' when I started - or finished. What you are saying makes sense ... but could you give some more information. I have ordered a set of Bobs Knobs and would like to understand this a little more before I attempt to fit and use them. Incidentally, I have read the installation instructions for Bobs Knobs and I don't recall any mention anything about tightening the screws. How tight is tight?

Many thanks

David

David Johns

30-07-2007, 04:21 PM

Graeme .... the Bobs knobs arrived today and the information sheet (different to what I had) directed me to www.bobsknobs.com (http://www.bobsknobs.com) - this site had more information regarding the secondary plate design and tightening the collimation screws ... very helpful. I am installing mine now (!). Many thanks again for mentioning the importance of tightening the screws.

Regards

David

OneOfOne

20-08-2007, 08:18 AM

Hmm....Last night (Sunday Aug 19) was the first relatively clear night I have had in a couple of weeks so I thought I would give a final few tweaks to the collimation. In the afternoon I took the scope out the front to setup the finder and telrad with the view in the scope. I focused on a power pole several houses away, maybe 500m? (The furthest thing I can see in the back yard is the neighbours tree) I could clearly see the insulator at the top and the wires around it, but just couldn't get it "sharp".

Night time fell so I set up about 7pm. I swung it around to Beta Cen as this was relatively high and put the collimation adjustments at a convenient height. With the 24 Pan in I could focus down close to a point, but there was still a little flaring off to one side. I changed to the 14 Pentax to get a closer look and focussed either side. I could not achieve a nice tight "dot" in the centre. Looking at the diffraction I could see a number of circles, but there was a definite flare to one side, the rings appeared close to concentric but just offset slightly to one side but I couldn't clearly decide where the rings were bunched up. I swung around to Antares, which was directly above but got much the same view. Giving Jupiter a go resulted in four moons that could not be focussed into dots and the belts were barely discernable, obviously collimation was still out.

I am concerned about the flaring, I should have moved my position at the eyepiece to see if the flaring was me but forgot. However, the last time I used the Newt I don't recall seeing any flaring and when checking the collimation I just see rings, no flare, and so it does not seem likely that the flare is ME!

So, is the flaring likely to be from the collimation and so I will eventually get rid of it, or have I bought a Hubble copy that has a problem with something in the optics? I would imagine I would need to return it to Celestron, aka Extravision, for warranty. At the moment, it looks like I will be taking my 8 inch to any public viewings... Hopefully I can get someone who has more experience with SCTs to have a look and give their opinion.

rumples riot

20-08-2007, 09:56 AM

Ok,

Two thiings can be at play here.

One is that the seeing was not good enough to collimate. I know the seeing in Tassie was 5/10 last night, so it is very likely that seeing was not great.

Another is that you scope may have been exhibiting tube currents from the cool down. You scope has to be completely cooled down to collimate. That may take 3-6 hours. Closed systems take a long time to cool.

Also both of the above may combine to cause havoc and make you doubt. If you defocus until you see 2 defraction rings that is enough for collimating. Wait until the seeing is a little better before sending it back.

Yes an experienced operator of SCT will help enormously. They can tell you what the problem is. I suspect it is only the two things above and nothing else.

casstony

20-08-2007, 11:31 AM

These star test images might help. The first is of a reasonable quality sct: http://aberrator.astronomy.net/scopetest/html/cassegrain250_1.html

The second is a poor quality sct: http://aberrator.astronomy.net/scopetest/html/cassegrain_200_4.html

As has been mentioned previously, your star test won't be at its best until the scope has been outdoors for a few hours and temperatures have stopped falling.

OneOfOne

20-08-2007, 01:06 PM

Hmmm...I wish the images looked something like this. Even with the 24mm the smallest I can get a star is a blur with a big flare sticking out on one side. The "rings" are not what I would call circular at all and very jagged. At the best focus, a star completely fills the square inside the reticule eyepiece (10mm). I thought the corrector was fogged up, but I looked with a torch and it appeared completely clear.

I had a look at the Moon with the bino viewer and although the view was impressive, it just lacked a feeling of being in focus. But if I adjusted the focus, it just got worse without ever getting better.

I would have expected that at fairly minimal magnification (with the 24) stars would at least appear as dots rather than a big out of focus smear. My 8 inch Newt will give me "dots" in the reticule when I am aligning the GoTo, just minutes after I take it outside from a heated room.

Tonight is looking good...so I will take it outside and setup before dinner and try to align after.

OneOfOne

20-08-2007, 01:59 PM

I worked out how to use the scanner/fax/copier and send me an email of a sketch of what it looks like in the eyepiece. I hope it makes it a little clearer what I am seeing. One drawing is of the out of focus diffraction, note that the flare is not in the same spot as where the diffraction rings are compressed. The focussed star is just a blur with a bit of a tail, sort of like a comet. This represents about as good as it gets.

From the location of the compression of the diffraction rings, I know the collimation is still out, but I am more concerned with the "flare" at the moment. This would be the appearance through a 14mm. You get something similar with a 24 but you can't really see the rings but you can see the flare.