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Not another GSO RC Collimation thread.

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79 replies to this topic

#76 Epox75

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Posted 24 March 2021 - 04:18 AM

I've seen this tool from TS-Optics. It looks interesting, at least seems some sort of affordable bench test and I like the fact that it barely fits the 2inch focuser because the play of the cheshire is something I truly hate. It may be good for a daylight collimation without frustrations.  

 

The only "headscratch" is when the guy loosen the central screw of the secondary... I thought it was off limits. 

 

https://youtu.be/aLwTkyJZM1Y



#77 plane48

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Posted 24 March 2021 - 07:50 AM

In the thread I quoted look specifically at post #50.  There Cedric made a PDF of exactly how to adjust the secondary mirror.  It's a totally logical way of getting the optics truly aligned.  I can't wait to try it out once the weather permits it.



#78 Rasfahan

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Posted 25 March 2021 - 02:58 AM

I've seen this tool from TS-Optics. It looks interesting, at least seems some sort of affordable bench test and I like the fact that it barely fits the 2inch focuser because the play of the cheshire is something I truly hate. It may be good for a daylight collimation without frustrations.  

 

The only "headscratch" is when the guy loosen the central screw of the secondary... I thought it was off limits. 

 

https://youtu.be/aLwTkyJZM1Y

I just used that tool and the result was as good as with the Hotech - which means my optical and mechanical axes are pretty much aligned. Finished off under stars and it took me all of 15 minutes to get collimation good enough for the bad seeing I currently have (2.5“ FWHM over APS-C, some curvature on my 8“ remains, central part at 2“). But then, only the secondary was off. To adjust primary with these kind of tools I have to remove the baffle because it blocks the outer light circle.

 

Loosening that central secondary screw (1/8th of a revolution) allowed me to resolve my problems with a pinched secondary without losing the correct FL, I could only tighten again by 1/16th at the end. FL is now still at 1602mm (From what I gather the correct spec is 1600mm for the 8“, not 1625).

 

Edited to say that mine is the REEGO - which seems the same optical principle in a bit different casing (circular instead of square).


Edited by Rasfahan, 25 March 2021 - 03:05 AM.


#79 David07

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Posted 02 April 2021 - 10:54 AM

I've watched the video on the TS Optics 2" LED collimation tool. Interesting. the device is, essentially, a card with a hole in the centre illuminated with a red LED.

 

Using the TS Optics device, you collimate the secondary mirror by putting the device in the focuser after first removing the extension rings, look at the secondary mirror central doughnut through the hole in the card and adjust the mirror so that the reflection of the hole in the card is centred in the secondary mirror central doughnut. 

 

You collimate the primary by centring the outline of the shade tube with the inner edge of the device. 

 

The method of collimating the secondary is virtually identical with the method I outlined in posts 35, 37 and 38 of this thread. The method of collimating the primary is different to the one I proposed.

 

The drawback of the TS Optics device, to my mind, is that it makes the focuser and shade tube a reference pointing direction and relies on the primary mirror being orthogonal to the pointing direction of the focuser. In my experience, this in not necessarily the case. Nonetheless, the device is cheaper than a laser and probably produces a similar level of collimation.

 

For those who might be interested: I've produced a complete description of collimating an RC using a card with a hole. The description covers collimating an RC8 and RC6, and I'm sure can applied to larger scopes. Es Reid (First Light Optics, UK) had the original idea for this technique and he uses it in his commercial work. The key advantage of this method is that the hole in the card provides a positional reference in the centre of the primary mirror, not a pointing reference - the focuser is not involved at all.  The method aligns the primary and secondary mirrors to a reference line defined by the centre of the primary mirror and the centre of the fixing hole in the secondary spider.  

 

You'll find my write up here:

 

 https://www.dropbox....isc V2.pdf?dl=0

 

There are many photographs to help you and additional notes and photos of the final tests using an artificial or real star. 

 

If you ever wonder if your RC is working at its 'correct' focal length, i.e. where testing with a Ronchi grating gives parallel lines, then this short paper might help you:

 

https://www.dropbox....epiece.pdf?dl=0

 

David


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#80 Kzoech

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Posted 07 April 2021 - 08:12 AM

https://www.dropbox....on 1.4.pdf?dl=0

Ive been using this method for sometime and have pretty much done every one in this thread. By far this is the best, 5-10 mins tops in done.
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