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/astr...andtricks.html
I need to update my site, but the new URL for Bob's knobs is: http://www.bobsknobs.com/
Regards,
Stephen
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 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 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! 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....
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.
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.
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.
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.
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
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.
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?
Graeme .... the Bobs knobs arrived today and the information sheet (different to what I had) directed me to 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.
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.
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.
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.
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.
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.
That sketch looks like the scope is not properly cooled. If the weather holds up and you check your scope around midnight you might find the flare has gone. Changing temperature has a large effect on sct's.
If you figure out that the scope is ok, there are after market coolers that can be used to speed up cooling of sct's.
Also a clear sign that your scope is way out of collimation. That sketch shows that your collimation needs a lot of work. Not a mechanical error. So don't worry. Once you get collimation sorted you will wonder what all the fuss was about.
The collimation on the scope seems to be fixed now, but that is another thread!
Nearly half a turn of one screw and a bee's dick on another one and it was done. I think one of the screws may have been done a little tight that may have caused extra distortion of the collimation image.
It is really hard to over tighten with Bob's knobs. The little plastic shims prevent overtightening and the screws need to be firm enough that they will not move and not so tight that you cannot move them at all.
Glad to here that you got the scope collimated and just remember to continue to collimate your scope each time you move it. Doing this will increase you skills and keep your scope in fine collimation.
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. 
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).
I went back inside to try another eyepiece and on the way back I noticed the corrector was all frosted up! 
You don't get problems like this with a Newt...
