SCT Collimating & reticule eyepiece

[Astrobod]

I've just had to replace the secondary mirror screws, and have replaced them with some "Bob's Knobs".  I've read guides on collimating and they seem straight forward,  but some say you need to use a reticule eyepiece.  Some advise would be very gratefully received,

Ron

[John]

When I've owned SCT's I've found the best tool to help with collimation is Polaris, ie: a star test, and a reasonably high power eyepiece.

[Astrobod]

Thanks,  I'll get it roughly collimated with a friend then get out on the next clear night and fine tune it.

[Tinker1947]

A star is likely the best way to collimate a SCT, the Hotech Collimator is expensive and really needs to be bench mounted, so when the scope is moved back the mount a simple knock could throw it out again......

http://www.firstlightoptics.com/hotech-collimation-tools/hotech-advanced-ct-laser-collimator.html

[Klingon]

Ron, I have an 11 inch Celestron SCT and replaced the mirror screws with Bobs Knobs. Let your scope cool down to eliminate circulating heat in the tube which is seen during collimation as disturbance. Have the scope as vertical as possible on a star. I use a 14mm EP to get the diffraction rings set. The central star should vanish centrally into the diffraction rings when making the adjustments in and out with the EP, the diffraction rings should be perfect. It took me a few hours to master the adjustment procedure, I check it out regularly as I carry the scope to the observation decking in the garden. Hope this helps.

[Astrobod]

Thanks Klingon, I knew changing the secondary screws was the easy part :))

[YKSE]

Changing the screws may be easy, you still need to be very careful though. Change only ONE screw at a time, DON'T uncrew all three of them, because there's nothing else there to hold the corrector plate in place, getting the corrector plate back in place is much more work than collimating a SCT.

[Astrobod]

Thanks YKSE,

[Mr Spock]

Changing the screws may be easy, you still need to be very careful though. Change only ONE screw at a time, DON'T uncrew all three of them, because there's nothing else there to hold the corrector plate in place, getting the corrector plate back in place is much more work than collimating a SCT. 

That's a bit confusing. The three collimation screws (to be replaced by Bob's Knobs) are on the secondary assembly and have nothing to do with the corrector plate.

As you have to collimate anyway after fitting Bob's Knobs, it's better to take the secondary assembly out to fit them. The secondary assembly has a threaded collar on the outside which unscrews, allowing the assembly to be pulled out - this is how you use Fastar. When refitting just remember which way up the word Fastar was and you should have no trouble at all. The whole job takes a few minutes

I always collimate my SCT with a star test. It's the only way to get it spot on.

[Astrobod]

That's the way I did it, removed the secondary mirror from the corrector plate by undoing the knurled retaining ring.   Removed the secondary assembly, changed all three screws for Bob's Knobs, then popped it back into the locating ring, aligning the tiny pin and tightened the locking nut.  Fastar word is the same way up.  I've done a rough alignment visually, now just need a nice clear steady night or two for the star alignment.  This part I've never done, so patience and practice :))

[Ruud]

I too have Bob's knobs and was planning to use them as soon as my Celestron 6SE would need collimation. Somehow it has not yet lost its collimation and I never found the courage to install the new knobs.

Yet, from the videos I've seen it should not be difficult: an out of focus star, once centred in the eyepiece,  should show a concentric doughnut.

Still, I have cold feet about the process.

[YKSE]

I'm glad that you got things sorted. Someone else (such as me) may not be as handy as you

http://www.cloudynights.com/topic/520033-sct-collimation-hell/

[Astrobod]

I had no choice really. I bought my 11" Celestron secondhand for a really good price. Hand a few software issues, hand controller got a bit sluggish and fading LCD display. Bought a new one, then had to upgrade the motor control board to get all working together. Then one of the secondary mirror screws  came out whilst trying to make a small correction. So went for the Bobs Knobs upgrade.

[Don Pensack]

I always seemed to run into bad seeing when I chose to collimate my SCT.

So I started using a Christmas tree ornament hung in a tree a couple hundred feet away to the north.

The sun reflects off the bulb as a point, which creates a star image in the scope, complete with diffraction rings.  The seeing conditions in a couple hundred feet of air are always good, and you can take your time since you aren't wasting observing time to do it.  Plus, you can look straight through (how you should when collimating) and the eyepiece isn't pointed toward the ground.

I do this now with refractors and Maksutovs as well because it is simply the easiest way to collimate.  Not that using Polaris is bad, but the ornament is a lot easier.

[Astrobod]

Thanks Don I'll give it a try. Also you don't have to worry about tracking to keep the point of light centred. At least then you only have to fine tune collimating on a real star

[Highburymark]

Just out of interest, last year I did a very simple collimating test at home to see if it worked. After doing a normal star test, which showed my SCT needed slight adjustment, I got some aluminium foil, and stretched it over a hard surface, then made the smallest pinprick possible. I then used this to make an artificial star, and from a distance of about 30ft, it showed exactly the same result as the star test - only brighter. Collimating the scope inside with a non moving target was easy, and when I subsequently did another star test outside, the scope was spot on. The key is to make the smallest hole possible in the foil - which must be flush against the hard surface. I am sure that this process will not work for everyone, and may not be as precise as other techniques, but it is certainly an easy and cheap way to check basic collimating.

[Astrobod]

Thanks Mark,  I'ii give it a go.

[Riemann]

How are you getting on with this Astrobod?   I would be interested to know how you have found the different methods.        

Certainly I found the my C11 difficult to collimate and I am not absolutely certain I have finished yet.   Like you I bought mine second hand and the collimation was miles out so I had not choice but to tackle it.    I got nowhere with the screws and bought an equivalent to Bob's knobs from a generic hardware supplier (as per some thread elsewhere on the forum).   

I tried using Thierry Legault's three stage collimation process, ( http://www.astrophoto.fr/collim.html  ) which seems to be a write up of the classic collimation method with some physics thrown in.   I repeated the first stage over and over and thought I had everything symmetric but still could not get a diffraction pattern on the second stage.   Others may be able to do this but it was beyond my skill level - obviously the symmetry needed in the first stage is just so precise.

So I switched to the Duncan method, made my cardboard cut out, waited for a clear night and got the three lines centred in just a few minutes.   The lines are very small but that does not really affect the method adversely.   I took the cardboard cut out off and went back to the Thierry Legault method to check.   I had a nice symmetric diffraction pattern in stage 2, so that was huge step forward and possibly job done.

I have tried to the third stage of the Thierry Legault process and cannot see an Airy disc at all.    Seeing probably has not been good enough to get one but I also note that he says "except in big telescopes for whom the Airy pattern is rarely or even never visible".    Anyone know if you can get an Airy disc with a C11?

BTW there is another thread devoted to collimating 11 inch SCTs here

which I found very interesting.  

Peter

[YKSE]

Peter,

If you've not been able to collimate a SCT yet, I would suggest that you follow the method suggested here

http://www.astromart.com/articles/article.asp?article_id=548

This is simply the best SCT collimation guide for first timer, IMHO.

Uusing a ball(metal or glass) or Christmas ornament, lited from side as Don suggested should get the work done too, it gives you chance to use the scope when weather is this bad

[Ken82]

I found collimating my c11 very easy with bobs knobs. Put the star in the middle of eyepiece at high magnification and de focus so you can see the deffraction lines or airy disk . Then use a pencil to point at one of the three screws whilst looking through the scope . You need to point towards the screw opposite where the deffraction is happening. When you point to the correct screw , tighten it just a touch . The. Re centre the star and check again. 

[Riemann]

Ykse and Ken, thanks for the replies.   I think I probably have got it fully collimated, certainly I have symmetric concentric ring pattern when it is out of focus.

Ken, can you see the Airy disk round a star when your C11 is in focus?

Peter

[YKSE]

You should see Airy disk with an eyepiece of 10mm or higher mag. I check my collimation with 6mm +2x barlow on mag 0 (Vega or Capella) in focus when seeing is good, with concentric ring and dead centered dot. Indoor test can be done with the same hig mag too.

[Klingon]

Taking Dons advice I put a polished spherical ball "garden ornament" into the field next to me on this sunny afternoon, about 100 meters away. The suns reflection allowed me to make a few minor adjustment to my Bobs Knobs on the 11 SCT CPC giving me concentric diffraction rings. On this freezing night I have just confirmed the settings on the star above, a quick easy collimation procedure, great stuff.

[Astrobod]

Hi Peter, sorry about the delay in replying to you, been working away.  Yes collimated all OK, lot easier than I thought, maybe I was just lucky.