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AT8RC Collimation

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#126 saadabbasi

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Posted 23 March 2013 - 11:00 AM

Howie, I think it is true that the focuser is attached to the primary and since I'm using a 3rd party there's obviously no way that the primary and focuser could have been aligned at the factory.

So what can I do about this?

#127 saadabbasi

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Posted 23 March 2013 - 02:00 PM

Now for the alignment of the scope itself... I'd recommend aligning the primary (and therefore focuser) so the beam hits the center spot on the secondary. Then align the secondary so the return beam hits the laser emitter (or as close as you can easily get).

Repeat these two steps since adjusting the secondary will have moved the laser off the center spot on the secondary. You should converge on a solution.


Jared,

I quite like this method as it's simple but I had a few questions:

When we say that the focuser is attached to the primary do we mean to say that the focuser holder/thread is attached to the primary? In my case, the focuser is a moonlite and naturally Moonlite will probably not be coaxial with the primary as it's a 3rd party focuser.

So, assuming that I get a perfectly collimated laser and insert it into my Moonlite focuser and uppose it doesn't hit the centerspot of my secondary, should I adjust the push-pull screw of the primary to adjust the bring the laser to the centerspot of the secondary?

Now if I move the primary, I will also move the focuser and hence my laser and bring it to the center of the secondary. I then adjust the secondary to bring the beam back on itself - but even though the beam will be back on itself, the beam will not be impacting the secondary centerspot so I adjust the primary again and repeat this process till I don't have to adjust the secondary to bring the beam back on itself?

I have two difficulties with the approach: 1) I am unable to access the push-pull screws of the primary with the Moonlite on. 2) What if the Moonlite is not coaxial with the primary? What steps do I take?

#128 gdd

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Posted 23 March 2013 - 05:25 PM

Because the FeatherTouch focuser has a screw on adapter made specifically for the AT8RC, would the installation be more "plug and play" so you can avoid the need for re-collimation?

I understand the FT has no provision for tip/tilt adjustments, but may they would not be necessary?

Gale

#129 Jared

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Posted 24 March 2013 - 10:09 AM

On the AT Ritcheys, the push/pull screws that adjust the primary also adjust the tilt of the focuser. There is no mechanical adjustment to allow you to separately adjust the primary and focuser.

The Moonlite focuser, I understand, includes collimation adjustments, so you could use those to make certain the focuser was in alignment with the optical axis. Using the laser to check that the focuser is lined up with the secondary spot is a fine way to accomplish this. Even better is to make sure the return reflection from the secondary goes back to the center of the laser (since this would account for differences between the mechanical center of the secondary and the optical center).

If you don't have the Moonlite focuser, then Teleskop Service in Germany sells a push/pull plate that goes between your focuser and scope to accomplish the same task. I use one of these with my Feathertouch.

#130 saadabbasi

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Posted 24 March 2013 - 10:44 AM

Jared, I do have a Moonlite. Just to be sure I understand you, if the laser isn't coming back on itself I should adjust the focuser tilt and not the primary tilt?

Once I tilt the focuser so that it's coaxial with the secondary, how do I ensure the primary is also coaxial with the focuser and secondary?

#131 frebie

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Posted 25 March 2013 - 12:37 PM

On the AT Ritcheys, the push/pull screws that adjust the primary also adjust the tilt of the focuser. There is no mechanical adjustment to allow you to separately adjust the primary and focuser.

The Moonlite focuser, I understand, includes collimation adjustments, so you could use those to make certain the focuser was in alignment with the optical axis. Using the laser to check that the focuser is lined up with the secondary spot is a fine way to accomplish this. Even better is to make sure the return reflection from the secondary goes back to the center of the laser (since this would account for differences between the mechanical center of the secondary and the optical center).

If you don't have the Moonlite focuser, then Teleskop Service in Germany sells a push/pull plate that goes between your focuser and scope to accomplish the same task.


Astro-Tech dealers in the U.S. will have the push-pull focuser collimating rings available shortly in two versions -- one for 6" and 8" R-C's and a second for the 10" and 12" models. Check Astro-Tech dealer websites for pricing and descriptions.

#132 Jared

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Posted 26 March 2013 - 08:25 AM

Jared, I do have a Moonlite. Just to be sure I understand you, if the laser isn't coming back on itself I should adjust the focuser tilt and not the primary tilt?


Assuming the optics themselves are already collimated, yes you should be adjusting the focuser, not the primary.

Using DSI's method, you are going to ensure that the primary and secondary are collimated, but you may still have focuser tilt. One indication would be if stars in the corners had the same orientation of astigmatism, but were different sizes. This is an indication that you are getting different amounts of defocus--likely caused by the focuser not being quite square to the optical axis. Hopefully the difference will be small, but it's definitely there.

Once I tilt the focuser so that it's coaxial with the secondary, how do I ensure the primary is also coaxial with the focuser and secondary?


This adjustment shouldn't affect the primary and secondary. Just do a star test on-axis to make sure you don't see any on-axis coma. You shouldn't--focuser tilt doesn't affect mirror collimation.

#133 saadabbasi

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Posted 26 March 2013 - 09:55 AM

Jared, I do have a Moonlite. Just to be sure I understand you, if the laser isn't coming back on itself I should adjust the focuser tilt and not the primary tilt?


Assuming the optics themselves are already collimated, yes you should be adjusting the focuser, not the primary.

Using DSI's method, you are going to ensure that the primary and secondary are collimated, but you may still have focuser tilt. One indication would be if stars in the corners had the same orientation of astigmatism, but were different sizes. This is an indication that you are getting different amounts of defocus--likely caused by the focuser not being quite square to the optical axis. Hopefully the difference will be small, but it's definitely there.

Once I tilt the focuser so that it's coaxial with the secondary, how do I ensure the primary is also coaxial with the focuser and secondary?


This adjustment shouldn't affect the primary and secondary. Just do a star test on-axis to make sure you don't see any on-axis coma. You shouldn't--focuser tilt doesn't affect mirror collimation.


Thanks. I think I'm finally getting a better feel for this. The DSI method seems quite time consuming and I was thinking of giving this method a try: http://deepspaceplac...rccollimate.php

I think it's the same method except more intuitive due to liveview. I have a D700 whose 35mm field might helpful in getting things right but I intend to use my KAF-8300 based camera for actual imaging. I think using the D700 would ensure that I'm getting an even field (not flat, just evenly curved in all 4 corners).

So I am thinking of using this method to ensure optical collimation is OK. Once that's done, I'll adjust the Moonlite based on the laser and not touch the secondary/primary while doing that.

Thanks Jared. This has been very helpful!

#134 saadabbasi

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Posted 26 March 2013 - 02:22 PM

I was having a hard time explaining what I see through the chesire so I decided to take a picture. This is without the chesire, of course. I will try to take one with a chesire tomorrow morning. However, you can still see that an outer circle (in gray) is not concentric with the inner circle. What does this tell us?

Posted Image

#135 rainycityastro

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Posted 26 March 2013 - 07:06 PM

It appears that your baffle is not aligned with the main tube. since the baffle is orthogonal to the main mirror, aligning the main mirror will fix what you see.

BTW, my experience is that the moonlite focuser has too many degrees of freedom and wobbles when rotated. My experience is that the stock focuser is actually aligned with the baffle. So I would first do all collimation with the stock focuser and then put the moonlite on and align the focuser.

#136 Jared

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Posted 27 March 2013 - 01:13 AM

Oh, that brings up a really good point! I found with my Feathertouch that any time I used it to rotate my field of view, it threw off the orthogonality just a touch. Once you get the orthogonality of the focuser addressed--whatever focuser you use--DON'T ROTATE IT. Probably won't matter with a smaller CCD, but once you get up to 35mm size it's enough to matter--even with a good quality focuser like a Moonlite or a Feathertouch.

As to what Saad is showing in his picture... I'm not quite certain what is out of alignment. It looks like the primary baffle as seen in the secondary, but I'm not certain. One thing to keep in mind is that just because the primary baffle is supposed to be aligned with the primary mirror doesn't mean it is aligned with the primary. I would recommend relying on the photographic methods for final collimation, not the mechanical ones.

#137 philming

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Posted 24 July 2013 - 03:11 AM

Hi all !
About Focuser tilt : I was wondering if this method would be OK to verify if any tilt was actually there :

• Find a nice flat surface (make sure with a good spirit level)
• put the RC on that surface, secondary mirror side down, primary facing down
• Use your spirit level on the focuser again and make sure it's OK
This way, one should be able to make sure there's no tilt on the focuser right ?

I have a Takahashi collimation microscope, and I find it a little complicated to collimate the primary mirror with it. The round circle one can see moving about when messing around the primary collimation screws seem to be slightly smaller than the secondary cage (the concentric circles you can see in the taka scope i guess). This round circle is moving behind all the concentric circles one can see through the taka scope. So I find it rather hard to make sure the primary is OK with this device.
Any suggestion about this ?

Regards

#138 Jared

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Posted 24 July 2013 - 07:19 AM

I doubt that would be effective for two reasons. First, because I'm not sure it will be precise enough, and second because even if it is precise enough there is no guarantee that the optical axis is perfectly perpendicular to the mechanical front edge of the scope. Give it a try, though, and tell me if I am wrong.






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