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Paracorr2 impacting collimation of a fast Newtonian

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As you may know, I purchased a 20" f3.6 dobsonian earlier this month and I have now had a few short sessions with the scope.

My initial findings were that I was getting better stars with the Baader MarkV binoviewer using a Baader x1.7 Newtonian glass path corrector than I was getting with the Ethos and Paracorr2 combination.
I was getting on okay with the Ethos21 but the stars were really hard to focus with the Ethos 13 and Ethos10. The brighter stars did not come to a focus with four nice diffraction spikes.

I did some reading up on the internet after my last session and found this useful document: https://www.cloudynights.com/topic/373440-collimating-a-fast-newt-with-paracorr/
It talks about performing collimation with the paracorr in place.
I also found several contradictory articles saying that the paracorr should not be in while collimating.

Anyway, as I own a Howie Glatter laser (with 1mm stop adjuster) which can be used with a paracorr, I decided to give it a go…

I did some testing with the laser and a nicely centred laser without paracorr moves about 1cm when the paracorr is added.

WARNING: If you decide to follow me then make sure your paracorr is not in position “A” when inserting the laser as the 1mm stop adjuster with hit the lens of the paracorr. I decided to use the mid-position “E” for my collimation as E21 uses “A” and E13 uses “H”

I had the focuser racked fully in for stability then added the paracorr which stays in place for the whole collimation process of secondary AND primary adjustment.
The laser dot is made larger by its journey through the paracorr but still shows up nicely on the primary. I adjusted the secondary to centre the spot then added the Glatter Tublug and adjusted the primary.
I went around the loop again and made some more minor tweaks until all was stable with a good result coming from the secondary and primary.

RESULT: I am happy to report a positive outcome, there is a marked improvement in stars and I am able to get good focus with the E13 & E10. M13 globular was much better. Much sharper image of Jupiter than I was getting before.

Thoughts & Questions:
1. I do not know how many Dobs are out there of similar or faster speeds than my f3.6 and how many of those scopes are using a Paracorr2 with Ethos 100 degree eyepieces, but I would be interested to hear from anyone who has had similar issues with collimation (which is critical at these fast speeds) being affected.
2. This seems another reason to go for Glatter collimation tools as they are capable of working through a paracorr2 coma corrector.
3. It’s interesting that the Baader x1.7 Newtonian GPC did not impact on collimation the way the paracorr2 seems to !


Comments welcome,


Edited by alanjgreen
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hi Alan

interesting observations - ive got an OO 12" F4 Newt and also use a paracorr2, typically i use 12mm,17mm Naglers and 24mm Meade UWA (all 82deg). ive found the 12mm at times will not produce sharp stars - however when go to 17mm or 24mm sharpness improves - initially i thought this was down to seeing conditions as some nights been really sharp with 12mm. I did another collimation recently and it appeared to improve the view, although this was on a different night so there were 2 factors at play. I collimate without the paracorr 2 currently, using cheshire EP and laser collimator. 

Note i may be wrong on this - My guess is collimation is about alignment and light rays being centered to a point, hence it doesn't feel like having a paracorr in the light train should affect collimation, as assuming the rays are 'parallel' then its just the focus distance and paracorr setting that should correct the view.

One observation ive noticed with scope, the laser dot seems to move slightly when elevate scope thorough about 60 degrees in altitude - interested to see if others get this as could be another factor at play (note i have got locking screws tightened for this)



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I use a 20" F3.5 with a Paracorr so quite similar. I set the secondary up with the Paracorr IN with a laser collimator which has itself been collimated. I then collimate the primary by eye with a collicap as close as I can get it and then refine it on a star image. For me this gives excellent results for edge to edge images in 82 degree FOV eyepieces and lunar and planetary images at high power.  :icon_biggrin:

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I have a f4.6 so not as critical. I vary how I collimate just to learn from different experiences. Sometimes just with a collimation cap, sometimes also with a laser, and more recently with a laser through the paracorr 2. 

The laser through the paracorr seems to be better than a laser alone for collimating the primary as it doesn't matter if the laser is collimated. As long as I've got an illuminated image of the centre spot coming back I am fine.

I would say collimation cap is best for me (but slightly more hassle) followed by a laser through a paracorr but I haven't tested this scientifically.

I don't get alignment shift as my ota is a solid tube and on top of that is reinforced around the secondary, plus I use naglers and Delos and don't use power mates which all keeps the weight and leverage on the focuser down a bit.

What I do notice is collimation shift if the focuser or paracorr are moved in/out so I collimate with the focuser at 15mm out (where it is when I use the paracorr) and with the paracorr at whichever setting I'm expecting to use the most or at an average setting.

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26 minutes ago, Paz said:

What I do notice is collimation shift if the focuser or paracorr are moved in/out so I collimate with the focuser at 15mm out (where it is when I use the paracorr) and with the paracorr at whichever setting I'm expecting to use the most or at an average setting.


I too see the collimatiion shift if I rack out the focuser with the laser & tublug still in (after I have completed collimation with it racked in).

I have a 2.5 inch travel focuser which is about 2 inch racked out under normal cyclops (one eye) viewing. Maybe I will try collimating with the focuser racked out at this position to see if there is further improvement to be had. I was worried about sag due to the weight with the focuser racked out plus paracorr2 plus tublug plus glatter 2 inch laser. I have a feathertouch focuser so maybe it will take it?

Only way is to test it out...



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I always collimate my 16" truss with my Howie glatter and the paracorr in place. The paracorr acts as a slight Barlow and makes it easy to see the reflection of the doughnut on the secondary, so I don't bother using the Tublug.

Edited by Scooot
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