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SCT Collimation Failure: Can I call out the AA


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Sometimes I just wish it was that easy... excuse me if I rant.. feeling the need to vent.. :BangHead:

I fitted Bobs Knobs to the C9.25 tonight... boy did I not expect to still be trying to collimate the thing some 5 hours later!

Frozen & dejected I've now given up..  on one of the clearest nights for a long time! Arghhh.

Not sure whether it's just my incompetence or what but try as I may I just cant seem to get this thing centred. The best I've got it is with all 3 adjusters loose. Maybe I just need more practice but I got better results with the original screws & a screwdriver. There must be a better way other than waiting for a real star & wasting a good clear night. 

Think I'll take the whole thing apart & start from scratch tomorrow. Any tips on getting a good starting position?

I took a before & after. The focus is way off too.

Before fitting Bobs Knobs

post-11176-0-78144700-1397173938_thumb.j

Best I could manage after.

post-11176-0-12232300-1397173940_thumb.j

Current best focus with a Bahtinov... dire!

post-11176-0-52935800-1397173937_thumb.j

post-11176-0-52935800-1397173937_thumb.j

post-11176-0-78144700-1397173938_thumb.j

post-11176-0-12232300-1397173940_thumb.j

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To get it roughly in place, a laser should work. Take off the diagonal and put the laser in the visual back - then aim to get the laser dot in the middle of the angled face of the collimator.

Although this won't be collimated (you'll need to star test) it is a start.

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Sorry to hear you are having problems, it shouldn't take more than 5 minutes to collimate from scratch. Your second image is only a tweak away from your first image, are you possibly overdoing the adjustments?, these are very sensitive and 1/8 of a turn can make a huge difference. The consolation is that you won't do any harm and that you will get it right.  :smiley: 

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The hardest part is figuring which screw to turn, by how much, and in which direction. The star image needs to be a little more in focus than you show (my experience) and final tweaks are done ideally on the in-focus diffraction rings with the star in the middle of the FOV and using your highest power eyepiece (only possible on nights with excellent seeing and the scope is cooled down completely).

To choose which screw - poke a finger in front of the corrector while looking throught the eyepiece and observe where the distortion appears, move your finger around the periphery until the distortion is nearest the position where the rings are mostly compressed. That will now be pointing to the screw which needs adjusting. If your finger is pointing between two screws then the one to adjust is the one on the other side, but the direction you turn it will be reversed).Turn that screw 1/4 turn or so and observe the effect on the rings - you're looking for changes in the concentricity of them. A 1/4 turn is a large adjustment but starting with that will leave you in doubt which way you need to turn the screw because the changes will be obvious, if you start with delicate tweaks then you might be left guessing which way direction is the way to go. Reduce the amount you turn the screw as you progress. If the rings appear squeezed together even more then you turned the screw the wrong way so reverse the direction. You will have to move the star image back to the center of the FOV after each adjustment although fine tweaks at the end will not move the star very far.

ChrisH

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As ChrisH says , use a finger to find which adjuster to turn and only turn it a couple of degree's, when i changed my adjusters from philips screws to knobs i removed the secondary it has a small keyway so it only goes back in the same position, i then lined the secondary  up so it looked level, placed it back in its holder, used a laser to get the dot centred, then used a out of focus star and a finger over the corrector plate to find which knob to adjust, i haven't tried it yet but think placing a DSLR in the visual back and  using  APT to display on a monitor would make it very easy as the knobs being turned can not be viewed through the EP at the same time but you would see the out of focus star alter on a monitor screen and this is where you would see to much adjustment or to little, the other option is the Hotech Lazer adjuster its a bit expensive and when i used mine it worked, but i have a wooden obby floor so any movement made it vibrate, but on a solid floor or better still a jig to hold the Scope and the ringed plate would make the job a lot easier....

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

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A bit late now, after fitting them (apologies), but there is a way of replacing the screws and maintaining collimation.

My method is as follows (if scope is in good collimation before screw replacement):
 

  • Place laser collimator in visual back and with a felt marker, mark the spot on the angled face that the laser falls on.
  • Do not move laser and keep it switched on.
  • Replace each screw, one at a time - adjusting the screw until the laser dot falls on the same place as marked after each screw is replaced.
  • Star test to confirm

Makes it a 10 minute job.

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Not sure what the problem is.  Difficult to see the problem being Bob's knobs.

If you have enough room, 30m should be enough, you can buy or make an artificial star to get a pretty accurate daytime collimation.  When you get a night of good seeing you can tweak further by doing a visual high power collimation (x1000+ magnification).  Doing a day time collimation is so much more relaxed, you are usually at a more comfortable angle and it may be more convenient to recruit a friend to help out.

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Thanks for all the very helpful comments. The rest of my scopes being fracs I'm obviously a bit green to doing this from scratch as my previous checks only needed a slight tweek.

This is on the Obsy mount so I'm doing this up a stepladder staring down at the laptop screen as I make adjustments. I have a moonlite/CCD hanging off the back. Tom your idea for installing sounds better than following the instructions that came with them. I should have asked for advice first I guess! I do have a Hotec laser somewhere, when I can find it.

What seems to happen is I get to the point as in the piccy above. Then as said I hold my finger toward each adjuster so I check which one to adjust. I then seem to end up with the concentric rings going a bit wavy & not opening up as expected. I end up loosening 2 of them & tightening one to try and force centering but it's as if it wont go quite enough & end up with a figure of 8 looking circle. Concentric rings looking more like a Zebra!

I'm sure its just me so tonight I'll check everything is seated properly & find my laser so I know where I'm starting from & hope I can find a star. Think I'll have to invest in an artificial star for the future, any recommendations?

Thanks..

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I don't think I would like to look directly at a laser with a telescope for collimation purposes.  :eek:

Me neither, but shine it at a distant ball bearing at an oblique angle and the resultant divergent light will be safe enough.

ChrisH

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Have you tried Al's collimation aid, just google it. It even indicates which screw to turn! I use it with a SPC900 for collimating my C8

Ian

PS People seem to think that there are an awfgul lot of topics on collimation of Newtonians, i think that there are almost as many on the collimation of SCTs :evil:

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Hehe :grin:  10 Mins!!.. Ok just rub it in, that's what I expected last night.. I was taken completely by surprise how obviously incompetent at this I'd be.  :grin:

Tonight's not looking very good at all.. solid high white cloud. I must have a look around for for a decent artificial star.

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I've just been looking at this http://www.firstlightoptics.com/hotech-collimation-tools/hotech-advanced-ct-laser-collimator.html

Not cheap but I wonder if there's anyone on here that's used one?

Some interesting points watching video 2.  I just fitted a Moonlite with the new auto rotator that has collimation adjustment on it. Something I hadn't considered. Also, I hadn't realised about adjusting the actual corrector plate either. They make it look very easy & precise. :smiley:

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I've just been looking at this http://www.firstlightoptics.com/hotech-collimation-tools/hotech-advanced-ct-laser-collimator.html

Not cheap but I wonder if there's anyone on here that's used one?

Some interesting points watching video 2.  I just fitted a Moonlite with the new auto rotator that has collimation adjustment on it. Something I hadn't considered. Also, I hadn't realised about adjusting the actual corrector plate either. They make it look very easy & precise. :smiley:

I have one, it works, but needs a stable base for the scope and the tripod with the target circles, my Obby has a wooden floor so it vibrates, used it the once, its in its padded case so it doesn't collect any dust....:)

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I have one, it works, but needs a stable base for the scope and the tripod with the target circles, my Obby has a wooden floor so it vibrates, used it the once, its in its padded case so it doesn't collect any dust....:)

Hmm my floor's isolated & my piers into over a cubic metre of reinforced concrete so I recon the entire "Riverdance" wouldn't be a problem ;)

Is it as good as they make it look in the video? The more I think about it if it doesn't mean using up valuable clear skies as I did last night I may start seriously considering one

Sent from my iPhone using Tapatalk

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I was being serious earlier - collimation of an SCT isn't a big mystery!

The first thing to do is to get it close as possible indoors. Laser collimators don't help ;)

Place your SCT on a table or similar. Go at least 2-3M away and look down the tube - you will see a whole series of concentric circles, from the primary, secondary, corrector, tube, uncle tom cobbley and all.

Tweak your Bob's Knobs until the concentric circles are approximately as close to perfect as you will get. OK, it's not easy - your head will move around and looking straight down the tube suddenly doesn't sound so easy - but at least this part is indoors and in the light and warmth.

Once that is done you can start your star test. By now all your Bob's Knobs should have even tightness - undo them all by no more than 1/16th of a turn. Yes, it needs to be that accurate.

Use at least x200 and centre the star. Defocus until you see a central dot. You will have an image similar to the first image you show above - note your image before fitting is not correctly collimated ;)

Once you have a star centred, adjust the knobs by a tiny amount, centering the star after each adjustment. You'll soon see if you are going the wrong way. The key is the smallest adjustments possible - you have plenty of time ;)

When you have a few tweaks under your belt, Bob's (knobs) your uncle, so to speak :)

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A bit late now, after fitting them (apologies), but there is a way of replacing the screws and maintaining collimation.

My method is as follows (if scope is in good collimation before screw replacement):

  • Place laser collimator in visual back and with a felt marker, mark the spot on the angled face that the laser falls on.
  • Do not move laser and keep it switched on.
  • Replace each screw, one at a time - adjusting the screw until the laser dot falls on the same place as marked after each screw is replaced.
  • Star test to confirm
Makes it a 10 minute job.

Thats sounds like a good system, thanks for the tip.

Sent from my iPad using Tapatalk

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Sorry to take things a little off topic (kind of), but I've also just replaced the original screws with a set of Bobs Knobs on my LX90. My question is whether the spacers they come with are absolutely necessary, as I can't get the screws to reach thread with them on. I replaced one at a time. The first two were fine and reached with spacers but then when removing the third screw, there was a slight clunk with the release of tension, which was obviously the mirror shifting slightly backward which then prevented the final screw from reaching with its spacer. Reaches fine without, so I simply removed the other two spacers and now have the knobs installed without them. I've performed a course indoor collimation and they seem to be working ok. Am I likely to have issues if I don't bother with the spacers?

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I believe the spacers are meant to be there to prevent damage to the secondary.  Did you slightly loosen all the screws, and not fully tighten the replacement knobs in?  If not you may have caused the mirror housing to tilt when removing the 3rd screw - could explain the clunk you heard

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Yup.. I had exactly the same issue. Its because it tilts & the thread is then off alignment. Make sure the tube is tilted down so the secondary doesn't drop down a bit. What I did was get hold off one off the ones already fitted & pull forward so any slack is taken up.

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