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XT8 collimation cap off-center?

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#1 mirpis

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Posted 20 October 2021 - 10:32 PM

So, the reflective white circle in the collimation cap that came with my XT8 is off-center, and I suspect the hole may also be, but I'm not sure. I emailed Orion and they sent me a replacement that came today, and it seems to be off-center in the exact same way. Are they supposed to be like this for some reason, or am I just unlucky?



#2 Shinzawai

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Posted 21 October 2021 - 02:54 AM

Do you think the peephole in your collimation cap is not centered? Easily measured with calipers. Or the donut center maker that is on your primary mirror? If the second, and it's definitely possible, the (only) way to check is to just remove the primary mirror, or leave in its holder and measure with a measuring tape. 

 

If the latter and you need to go through the trouble, although not particularly hard, to remove and get a new center marker, it could be handy to just go for a Catseye triangle marker. Arguably the 'gold standard' when it comes to collimation.


Edited by Shinzawai, 21 October 2021 - 02:54 AM.


#3 belliott4488

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Posted 21 October 2021 - 06:45 AM

One way to check the centering of the peep hole in the collimation cap is just to rotate it in the eyepiece holder while you watch the refection. If center mark on the primary appears to move around, then your peep hole is likely not centered.


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#4 Noto

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Posted 21 October 2021 - 11:01 AM

So, the reflective white circle in the collimation cap that came with my XT8 is off-center, and I suspect the hole may also be, but I'm not sure. I emailed Orion and they sent me a replacement that came today, and it seems to be off-center in the exact same way. Are they supposed to be like this for some reason, or am I just unlucky?

I purchased an XT8+ earlier in the year, and I noticed the same thing. The actual hole is in the middle, but whomever placed the reflective circle on the inside was off by quite a bit. Because of that, I found the reflective part kind of useless. I only used the cap once during collimation, but have since moved on to other tools (cap still "works", but the off-center reflective part makes it kind of useless).

 

You could easily fix this yourself, as Orion isn't going to inspect the part before sending you a replacement (not this kind of part anyway).



#5 belliott4488

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Posted 21 October 2021 - 11:10 AM

I got one of these on the recommendation of several CN folk: https://www.rigelsys.com/telecoll.html, and I've been happy with it.



#6 SteveG

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Posted 21 October 2021 - 11:44 AM

It is the hole that matters, not the reflective surface.



#7 MellonLake

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Posted 21 October 2021 - 03:40 PM

It is the hole that matters, not the reflective surface.

Just to clarify Steve's point.  It is the positioning of the hole in the reflective surface and the hole in the cap that are important.  Both must be centred in the drawtube.   The outer diameter of the reflective surface can be off centre but the hole in the reflective surface needs to be centred. 

 

Rob



#8 mac57

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Posted 21 October 2021 - 05:31 PM

These caps are obviously molded, and the hole position shouldn't vary.  



#9 SteveG

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Posted 21 October 2021 - 08:09 PM

Just to clarify Steve's point.  It is the positioning of the hole in the reflective surface and the hole in the cap that are important.  Both must be centred in the drawtube.   The outer diameter of the reflective surface can be off centre but the hole in the reflective surface needs to be centred. 

 

Rob

The reflective surface can be off, but yes, the hole must be centered in the cap, and go through the reflective surface. Only the hole needs to be centered in the drawtube.



#10 MellonLake

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Posted 22 October 2021 - 06:52 AM

The reflective surfaces in some collimation caps are stickers that can, and sometimes are, positioned with the hole slightly off centre.  

 

Rob  



#11 Vic Menard

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Posted 22 October 2021 - 06:54 AM

So, the reflective white circle in the collimation cap that came with my XT8 is off-center, and I suspect the hole may also be, but I'm not sure. I emailed Orion and they sent me a replacement that came today, and it seems to be off-center in the exact same way. Are they supposed to be like this for some reason, or am I just unlucky?

A picture might be helpful (perhaps with a ruler showing the offset "reflective white circle).

 

Generally speaking, the primary mirror tilt alignment is corrected by centering the collimation cap pupil (hole) relative to the dark silhouette of the primary mirror center marker/donut. With most collimation caps I've seen, the area surrounding the pupil is painted white or covered with reflective white tape (to enhance the visibility of the pupil). If the tape perforation isn't well aligned with the pupil, that could affect the view of the pupil. 

 

That said, you could simply take a few reads at different collimation cap rotations and reduce the offset error, or, at f/6 you have some "leeway" in the required precision, which allows for a read error of 2mm (with an accurately centered pupil). At f/6, I would reject any tool with an error over 1mm (which would then require a "perfect" alignment read).


Edited by Vic Menard, 22 October 2021 - 12:44 PM.


#12 Ulmer Spatz

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Posted 22 October 2021 - 07:36 AM

As others have said, if the hole in the reflective circle is concentric with the peep hole drilled into the plastic, you're good to go. I suspect you're looking at the circumference of the reflective circle and seeing some variation in the distance of that circle to the flexible hold-down tangs. That's applying impossible standards of accuracy to a cheap plastic part with (intentionally) flexible features. 

 

For that matter, the draw tube of your xt8 may not be exactly concentric with the focuser at all times, and the focuser may not be at precisely 90 degrees to the optical axis of the scope. But it'll all work together anyway to give you impressive views. Look at the stars and try to ignore manufacturing tolerances.  :-) 


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