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AT8RC Collimation

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#1 lineman_16735

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Posted 23 June 2009 - 11:31 AM

I have one of these scopes in my hands at this time. I have read the supplied collimation instructions and they are suprisingly detailed. Matter of fact the included instruction and info about the scope are better than any scope I have owned. The instructions state that any adjustment to the primary mirror screws will void the warranty. Wassup with that :question: I started my collimation procedure with a Howie Glatter holgraphic that I picked up at the Cherry Springs Star Party. I attached the grid pattern to the collimator and inserted it in the focuser. It is obvious that the primary is off quite a bit. At this point I have no choice but to try to adjust the primary. First I adjust the primary to center the secondary shadow in the holographic pattern. Easy enough. I then insert laser in single beam mode and adjust the secondary to return the laser beam back to the center of the focuser, in other words fold the laser back onto itself. Replacing the holographic laser back into the focuser, the secondary shadow is no longer centered in the primary. Ugh :confused: I decide to start over. this time I adjust only half the error out of the primary, and then about half the error out of the secondary. I repeat this 5 or 6 times. Things are much much closer now, but this method makes little sense to my pea sized brain. I tried a star test and the secondary shadow appears off to about the same degree as the laser shows. Am I going about this correctly? Any suggestions? FWIW I have confirmed the secondary holder is indeed centered on the mechanical axis.

#2 Gama

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Posted 23 June 2009 - 12:16 PM

The primary mirror was pre aligned with the secondary. Adjusting tose will now make it very hard to align now.
The primary's are laser aligned at the factory using a "Real laser system", so now its alot of trial and error im afraid.
You may check out the RCOS method of calibration for the RC's.

Theo.

#3 lineman_16735

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Posted 23 June 2009 - 01:46 PM

Hi Theo,

It may have been pre aligned at the factory but it wasn't aligned when I received it. I'm sure it was caused by shipping.

The primary mirror "should" be aligned with the mechanical axis, as should the secondary mirror holder. From there the tip, tilt of the secondary should align to the baffle, no? Just a bit ago I checked the collimation using my autocollimator. I centered all of the reflections starting with the primary. I then used my Glatter holographic laser attachment and all agreed :jump: I touched up the secondary return beam with my Glatter barlow attachment. A quick check with the auto collimator agreed again. First light tonight hopefuly and a run through CCDInspector will be the first order of business.

#4 blueman

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Posted 23 June 2009 - 04:45 PM

I too own one of these and did adjust everything. First of all, the 6 screws on the focuser flange, do not adjust the primary, but rather just the baffle tube and focuser, is what I understand. But I did adjust them, but I had no warranty anyway as it was used.
I used my eye, looking through a cheshire with out a cross hair. Then I just adjusted the screws until I had the secondary perfectly centered in the baffle tube. Then I used a Lasermax Laser with hologrid to align the secondary, by using the projection of the hologrid out onto a wall. This is the beam being reflected by the secondary back to the primary and then back to the secondary and casting a shaddow of the secondar on the wall with the grid showing how it is centered in the beam. I then checked it with an Autocollimator and the reflections were very concentric. This got me right on the money and star tests showed perfect concentric diffraction rings.
Fairly simple compared to the VC200l really or even a regular SCT.
Blueman

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#5 lineman_16735

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Posted 23 June 2009 - 05:57 PM

The screws on the rear cell do move the primary. The manual calls them, "Primary Mirror Alignment screws" The baffle tube I suspect is fixed to the back of the primary. I also used a holographic laser. It is my understanding that you should first check the secondary cell to be sure it is centerd. I did just that and it is indeed centered, very well. Next using a holgraphic laser pointed at a wall or some other flat surface you will see the shadow of the secondary as you explained. If the secondary shadow is centered in the laser's grid then the primary is square with the secondary. As a final step you should adjust the secondary with a dot style laser, folding the beam back onto itself. I got this info from here . I'm not saying what you did didn't work but the primary does move with the adjustment screws on the rear cell.

#6 blueman

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Posted 24 June 2009 - 10:41 PM

Hi Chris, I guess that the screws must move the primary, though I thought that it was just to move the focuser and baffle tube.
My method did work well though, the star test showed that. But what ever way that gets you a good star test is fine.

:)
Blueman

#7 lineman_16735

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Posted 25 June 2009 - 12:23 PM

I'm still unable to colliamte this scope. The problem that I "think" I am encountering is that the focuser moves with the primary making a RCOS type collimation impossible? I have spent a lot of time on this and stll can not get it. It has to be error on my part but I can't fiqure out what I'm doing wrong? All I know is when the focuser is centered on the secondary the primary is off. Adjusting the primary affects the secondary? I'm open to any and all suggestions.

#8 blueman

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Posted 25 June 2009 - 03:30 PM

Hi,
What you say is correct, but please do this.
Using just your eye or a 35mm film canister with a hole in the cap or a cheshire with out a cross hair, look through the focuser and adjust the 6 screws at the focuser base until you see that the seconday holder is exactly centered. You will see that there is no gap anwhere around the perimeter of the holder. It will take movements of all the screws to accomplish this and each will effect the other. When you have this completed, make sure the srews are tight without upsetting the alignment.
Then you can use the laser to do the rest of the job, watching the projection on the wall until the secondary shaddow is centered correctly. You do not want to touch the primary again as this will indeed change everything.
This method produces perfectly aligned star diffraction rings.
Blueman

#9 lineman_16735

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Posted 26 June 2009 - 01:01 AM

Thanks Floyd. This is exactly what I have been doing though. Centering the secondary with the focuser/baffle tube is no problem. The problem happens when I adjust the secondary, either centering it in a holograph or folding a single laser point back onto itself. When I adjust the secondary and then check the the focsuer/secondary alignment with a sight tube, autocollimator or holographic laser the baffle tube is no longer centered on the secondary. It is a small amount as I can just see a crescent moon shaped annulus on one side of the secondary. I have spent WAY too many hours trying to dial this scope in and still haven't gotten it. I'm lost as to what to try next? I appreciate your help.

#10 blueman

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Posted 26 June 2009 - 01:56 AM

Hi,
So you center the secondary in the focuser tube by looking through the focuser while adjusting the 6 screws?
I found that once I had that done, then the adjusting of the secondary had no effect on the centering of the secondary holder in the baffle tube.
Are you using the secondary holder for the centering or are you looking at the reflection in the secondary?
I use the metal holder for centering the secondary, not the reflection.
Blueman

#11 lineman_16735

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Posted 26 June 2009 - 12:13 PM

Yes I center the secondary by looking through the focuser and adjusting the primary. The secondary holder is smaller than the secondary, I can not see that through the focuser :question:

#12 blueman

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Posted 26 June 2009 - 04:07 PM

Hi,
I am referring to the metal case that holds the secondary and not the reflection of the secondary seen from the focuser.

There is a method that is used for the VC200L that is for centering the secondary in the focuser baffle tube, that is basically the same thing I did. However the AT8RC has a larger holder and it just fits the circle of the baffle tube. But if you put your eye to the edge of the openeing in the focuser tube, you can see the gap if it is not exactly centered, it shows as a small crescent of light, very thin, with the other side dark. I found that when it is properly centered you can not see any crescent of light at all.
Then once I had done this and moved to the secondary, adjusting it with the secondary shaddow projection, did not change the look of the secondary in the baffle tube.

This is where I am not sure you are having the problem. Why is should change for you , I am not certain. I would wonder if you were not so far out of alignment that the tilt on the secondary holder is causing this.
Blueman

#13 lineman_16735

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Posted 26 June 2009 - 05:24 PM

I have a VC200L which I have no problem collimation so I am familiar with that procedure. I now have the primary/baffle aligned with the secondary holder. I am having some issues with proper secondary alignment though. Floyd please check your PM's.

Chris

#14 blueman

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Posted 26 June 2009 - 07:06 PM

Hi Chris,
PM received and replied. :)
Blueman

#15 waassaabee

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Posted 26 June 2009 - 11:21 PM

UNFAIR!! Taking the conversation offline is like tearing the last pages out of a novel. :mad:

#16 blueman

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Posted 27 June 2009 - 12:02 AM

Nothing like that, he just wanted to speak to me personally, not take it off line. :)
Blueman

#17 Gama

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Posted 27 June 2009 - 01:53 AM

Just to add fuel to the fire, are you maintaining the right mirror seperation distance ?, as this is also critical for collimation. Moving the mirror about is going to change the distances of the mirrors. Even a 1mm starts to affect the collimation.

Theo

#18 drprovi57

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Posted 27 June 2009 - 07:25 AM

I am also curious - I have on order the AT10RC (later this summer) and i currently own a 14.5 RCOS - I use the Tak alignment scope on my RCOS and I adjust all three elements (primary, secondary holder center, secondary tilt) - why does AT not want the primary mirror be adjusted? It may move in shipment? Also - with my RCOS, I end up using star-alignment for final adjustment and only adjust the primary - NOT the secondary. But it appears the ATRC one makes final adjustment using the seconday - can someone explain.

Thanks
Jason

#19 blueman

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Posted 27 June 2009 - 01:45 PM

Hi Jason,
No idea of why they do not wish you to collimate the primary. Maybe the fear that you will loosen the screws and something will fall apart?
But, it would seem that the fixed primary, once adjusted, should say in position with the 6 push-pull screws tightened.
As for why you would adjust the secondary as a final collimation, I would have to guess that is it because they do not wish you to touch the primary adjustments.
However, I did get my scope collimated well with just a laser collimation and Chesire. Star tests are concentric and star images show no signs of mis collimation.
Blueman

#20 lineman_16735

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Posted 27 June 2009 - 02:07 PM

I'm simply tipping/tilting the mirrors so the spacing should remain the close to the same on axis. Other RC's use a moving secondary to focus so I don't think it affects collimation . It may in fact affect the appearence of 5th order coma or astigmatism if the spacing is not maintained within 3 or so mm though? FWIW I am sure the primary is very, well aligned now. It was user error on my part. All of the reflections had my pea sized brain confused :grin: The system is well optically collimated as of now. We spent some time startesting last night and using the scope visually just to confirm optical alignment. I may write a detailed review of this scope once we can spend a few nights imaging with it. So far it looks promising.

#21 waassaabee

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Posted 27 June 2009 - 04:38 PM

Write on ... :rockon:

It sounds like I may need a box of collimation tools though... :money:

#22 lineman_16735

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Posted 27 June 2009 - 06:38 PM

Well, the problem is solved now. It was the focuser all along. Centering the primary is actually very easy as the baffle tube is only slightly bigger than the secondary holder when viewed through the back. Then using a holographic laser adjusting the secondary is easy as well. The problem I was encountering was that when I would collimate the scope with the laser system a check with a sight tube/ chesire or autocollimator it was obvious something was off. When the holographic laser said things were aligned, the autocollimator disagreed, a single laser dot showed that the laser was not hitting the center of the secondary mirror and the laser was not returning to the center of the focuser. So after some brainstorming with blueman we both agreed that the primary was in good alignment as evident by the baffle tube being aligned to the secondary housing. So it could only be one of two things, the focuser or mechanical misalignment of the secondary spider. I first checked the secondary, and I'm happy to report that it is very, very well centered. Matter of fact I have taken a lot of measurements of this scope and everything is nice and round, square and suprisingly well thought out and built. The spider is very strong yet reasonably thin. The secondary adjustments are very easy to get to and the screws although not stainless are not the typical soft metal that I would have expected for a 8" scope for this price. Anyways, I placed a sheet of paper over the baffle tube and turned on the holographic laser, it was obvious the focuser was misaligned severly. Adjusting the focuser tension, or positioning made no difference. Since this focuser is being replaced anyways I decided to do some surgery on it to center it in the baffle tube. Think drill bit, and some set screws ;) The reason I did this is because I wanted to be sure that the scope was mechanically and optically aligned before I installed the Moonlite 2.5" focuser. The Moonlite is a prototype and the flange on this first version, covers the primary collimation screws. Once the GSO focuser was aligned to the baffle I checked the collimation again with the Glatter Holographic laser. I adjusted the secondary then check with the Chesire/sighttube, and auto collimator. All agreed! :jump: I then but on the Glatter balow laser attachment and it was as we say around here "dead nuts"! Now that I know the mechanical/optical alignment are good, I can install the Moonlite and do some test with a camera and CCDInspector.

#23 blueman

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Posted 27 June 2009 - 06:58 PM

Hi Chris,
Very good news! I thought that this could be the culprit and it is great that it was because it was the easiest to fix.
:D
I am very happpy that it has all worked out find.
Blueman

#24 lineman_16735

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Posted 27 June 2009 - 07:21 PM

Write on ... :rockon:

It sounds like I may need a box of collimation tools though... :money:


Nah, if you have a good laser such as Howie's offerings you would be good to go. A sight cap or $20, autocollimator is enough for very good alignment of the primary, IF you have to adjust it. This is a very easy scope to collimate as long as you don't have same issue I had with the stock focuser. It is well known that Astronomics opted not to offer the Feathertouch with the ATRC8 to keep the cost as low as possible. I assume that most who purchase this scope will have plans to purchase an aftermarket focuser.

#25 drprovi57

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Posted 27 June 2009 - 10:36 PM

Good information - but I am still puzzled that the final adjustments are done with the secondary while on RCOS it is done with the primary...

thanks
Jason






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