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Collimating 8"SCT Help!

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#1 lenrabinowitz

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Posted 04 December 2016 - 06:30 AM

Help!

 

Put Bob's Knob's on my 8" a while back.  For the most part not a problem, but now it seems that I have tweaked once too often and cannot get the thing into alignment.

 

Any tips or ideas?  I am more than happy to put the original screws back in.  I spent a lot of time on this in the last few days.  It seems like it needs to move in a certain direction and it just won't go there.

 

I'm thinking I should remove them and redo the whole thing from scratch, maybe with the original screws?

 

HELP!

 

Len 



#2 James Cunningham

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Posted 04 December 2016 - 07:19 AM

Do not take the Bob's knobs off.  They make it much easier to collimation.  Stand back about 8 feet back from the scope during the day with the business end level with your eyesight.  You should see concentric circles.  Adjust the knobs to get it as close as you can to having all of the circles exactly round.  With that, you will be close..  At night, enlarge the star to get the central dot as close to the center as possible.  Then enlarge the star even more until you can see concentric circles.  Try each knob to see what it does.  Your turns on the knobs she knobs should be very slight.  Look at what each adjustment does to see if it makes it better or worse.  If worse, turn the knob back and go to another knob and do the same thing.  Make sure you re-center the star after each adjustment.  I use Polaris.  Keep adjusting each knob to get those rings equally spaced and round.  If the knob you are turning will no longer turn, loosen the other knobs (remember, all turns should be slight).  It may take a while but keep going.  At the end next, you want each knob to be fairly tight so that the collimation stays set.

jim


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#3 Tel

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Posted 04 December 2016 - 07:43 AM

Hi Len,

 

I take you've already looked at this set of instructions ? It is popularly regarded by SCT owners and easy to follow. .

 

http://www.starrynig...Collimation.htm

 

Note that the diagram calls for adjustment when you are standing in front of the corrector plate.

 

Let us know how you get on ?

 

Best regards,

Tel


Edited by Tel, 04 December 2016 - 07:50 AM.

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#4 lenrabinowitz

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Posted 04 December 2016 - 08:06 AM

I will take another look at all of this.  What I meant by removing them (I know you don't take them all off) was to kind of get it back to square one.  I've had those knobs on for months without problem.  They make it easy to collimate, but they can come loose and they make it too easy to tweak all of the time.

 

I'm upgrading to the 8" EdgeHD anyway.  I just don't want to sell this one out of collimation that's all.

 

Thanks!  I will post updates.



#5 leveye

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Posted 04 December 2016 - 09:48 AM

Sometimes I use a camera with a swivel screen attached to the scope. I turn the screen around and then zoom in on the de-focused star so I can see what the adjustments are doing while I'm doing it.


Edited by leveye, 04 December 2016 - 09:49 AM.

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#6 mclewis1

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Posted 04 December 2016 - 11:56 AM

If you are already reasonably close in collimation then you don't need to bother with any visual standing in front of the scope stuff. You also don't need to work very far out of focus. Tweaking collimation is best done at high magnifications and in focus over just very slightly out of focus. 

 

Using a camera can make things a lot easier. Using software like MetaGuide can also make things easier (this does analysis and will also keep the star centered if you enable the mount control connection).

 

Len, If you are just selling the scope then just tweak it visually as well as you can and make sure the knobs are not loose at all ... this will help keep things close during shipping. But remember that shipping will usually mess up fine collimation so the buyer should always be prepared to touch things up when they receive the scope (so don't worry about it too much).


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#7 lenrabinowitz

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Posted 04 December 2016 - 12:20 PM

Sometimes I use a camera with a swivel screen attached to the scope. I turn the screen around and then zoom in on the de-focused star so I can see what the adjustments are doing while I'm doing it.

 

That my friend is a great idea that I will try tonight.  Thanks!


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#8 lenrabinowitz

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Posted 04 December 2016 - 12:22 PM

That is one of the problems with Bob's Knobs:  Shipping or not, how do I get them tight enough so that they don't wiggle out of alignment without screwing up said collimation?



#9 leveye

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Posted 04 December 2016 - 12:42 PM

That is one of the problems with Bob's Knobs:  Shipping or not, how do I get them tight enough so that they don't wiggle out of alignment without screwing up said collimation?

The knobs should not be tight. Remember it is a pivot system you must sometimes loosen (sometimes 2 knobs) one side and then adjust the one knob to move the secondary. Then just very gently snug them all so they are not loose and things will stay in place.


Edited by leveye, 04 December 2016 - 12:43 PM.

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#10 lenrabinowitz

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Posted 04 December 2016 - 02:45 PM

I did what James suggested from about ten feet away and got it looking pretty good that way.  Hard to tell unless you stand perfectly still!  Going to try the camera trick tonight and just get it "close enough for rock and roll.". Keep you fingers crossed!


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#11 lenrabinowitz

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Posted 05 December 2016 - 12:11 PM

The camera trick actually worked.  I think I have it pretty good now.  In the pictures I took last night the stars look pretty good.  Thanks!  Now if I just win the 8" Edge HD I am bidding on... wish me luck!



#12 lenrabinowitz

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Posted 06 December 2016 - 03:52 PM

Back out of collimation again.  Crap!  The Bob's Knobs just seem too easy to knock out of alignment.  Any suggestions on putting the originals back in?



#13 Tel

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Posted 07 December 2016 - 03:35 AM

Hi Len,

 

I'm afraid that while many 'scope operators swear by "Bob's Knobs", many swear at them for the very reason you mention : i.e. they don't hold adjustment. (On a personal note I can't judge. I had a set made up for me in the UK some 10 years ago but, working on the principle that "if it ain't broke, don't fix it", I never fitted them to my old Nexstar 8i OTA).

 

But as to replacing them once again with the original screws, it goes without saying that each "Bob's Knob" must be replaced "one at a time".

 

I'd propose therefore that you set up the 'scope pointing, (if possible) at Polaris and remove one of the "Knobs". Replace it then with the original screw and follow Rod Mollise' "Rough" collimation instruction.(See below). Carry out the same operation with the replacement of the other two "Knobs" and once all have been "roughly" brought into optical alignment, you can then begin with the refinements.

 

Here's Rod's link:

 

http://skywatch.brai...collimation.pdf

 

Hoping this helps,

 

Best regards,

Tel   

 

PS. If you wish to attempt the operation in daylight, you can of course use a pukka artificial star or failing owning one, a Christmas bauble, strategically placed beyond the minimum focusing distance of your 'scope, (25-30yds.?) to catch the rays of the Sun. This should give a similar effect to a star. If you use this technique though, do make your final fine adjustments using a real star of about the second magnitude. (e.g. Polaris).


Edited by Tel, 07 December 2016 - 03:41 AM.

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#14 lenrabinowitz

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Posted 07 December 2016 - 06:29 AM

Thanks Tel!  I am actually going to sell this tube as I got a good deal on a used Edge HD.  Rough collimation will be good enough, but I don't want to put it up for sale completely off like this.  I will give it a shot.



#15 Tel

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Posted 07 December 2016 - 05:07 PM

Best of luck, Len. I hope your sale is successful !   :fingerscrossed:

 

Best regards,

Tel



#16 lenrabinowitz

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Posted 07 December 2016 - 05:40 PM

It will sell.  It's perfectly good it just needs some collimation.  It has some cosmetic scratches on it, but the optics are perfectly good.  I think they are 899.00 new?  I am thinking may 500 for this one; it's only about 1 1/2 years old.



#17 Tel

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Posted 07 December 2016 - 06:25 PM

Sounds OK, Len, but only you would know  :bow: ..

 

I, of course, I work in UK ┬ús !   :waytogo:  

 

Best regards,

Tel



#18 diaspora04

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Posted 23 February 2019 - 10:16 AM

The tip about daytime collimation from the business end of the scope was extremely helpful.  I just experienced my first attempt at collimating my 2 month old Celestron 6SE using this method.  The scope was a little out of collimation, and I seem to have corrected it with very small adjustments -- after going the wrong way initially.  Daylight, indoors conditions made the process easy and reasonably stress free for a newb.  I am looking forward to fine-tuning using a star as soon as clear skies permit.

 

Conclusion -- the collimation process is now less mysterious.


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#19 De Lorme

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Posted 23 February 2019 - 07:55 PM

I've used the same method that Tel suggested and it does work great.   Another collimation tool that works{in conjunction}great is using a Duncan Mask.  Helps to really know if your going the right way.

 

http://alpha-lyrae.c...-a-duncan-mask/


Edited by De Lorme, 23 February 2019 - 07:57 PM.



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