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Please Look At My Collimation

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#1 Lucky 777

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Posted 23 October 2020 - 12:18 PM

This is a Celestron 150mm F5 newt.  The secondary and spider were removed for cleaning.  I felt I had it sufficiently collimated before and had what my novice eye thought were acceptable visual results.  Planning to use this for some DSO photography and started doubting my work.  What I struggled with most was the rotation of the secondary on the central axis and the position of the secondary in relation to being centered under the focuser.  This view is down the 1.25" focuser, all the way out.  A Hotech laser collimation tool says I'm good but the dark shadow of the secondary in the primary isn't dead center.

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#2 Lucky 777

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Posted 23 October 2020 - 02:28 PM

Did some more reading and answered my own question.  offset Capture.JPG



#3 SteveG

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Posted 23 October 2020 - 03:29 PM

We don't have a point of reference. If your camaera was exactly centered, then there are many errors. Do you have a collimating cap, or combo "cheshire" tool?



#4 Lucky 777

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Posted 23 October 2020 - 06:04 PM

I have a home made cap and the laser.  Not sure if the camera was perfectly centered or not.  I have a Celestron neximage camera I can put in the focuser and use to get a better image.  Suppose I need to invest in a cheshire?



#5 Asbytec

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Posted 23 October 2020 - 06:08 PM

A white paper background is handy for seeing the edge of the secondary, and your laser might not be collimated or seated well in the focuser. From what I can tell, the white donut on the primary center is nowhere near the center of the focuser axis. When the focuser is collimated to the primary center, the focuser axis and the primary center, both, need to coincide at the center of the focuser draw tube. The edge of the secondary will also be concentric with the draw tube. It's not. 

 

Initially, it looks as though when the primary reflection becomes centered under the focuser, it will no longer be centered in the secondary. It will overhand the secondary by quite a bit. So, the secondary needs to move closer to the primary at around 2 O'clock, and to the upper left a little about 10 O'clock. Moving the secondary will mess up your current configuration, but that's okay. Once the secondary is moved into position, realign the focuser axis (the laser) back onto the primary center and see where you are. 

 

If your laser is telling you this is collimated, please check the laser beam is well collimated by rotating it and watching the dot move around on a distant wall. The laser dot needs to be accurate to within a small diameter orbit transcribed on a distant wall. The laser may not be seated (or register) well in the focuser, too, causing it to tilt a little. 

 

Untitled.jpg


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#6 Asbytec

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Posted 23 October 2020 - 06:20 PM

I have a home made cap and the laser.  Not sure if the camera was perfectly centered or not.  I have a Celestron neximage camera I can put in the focuser and use to get a better image.  Suppose I need to invest in a cheshire?

A Cheshire is for collimating the primary mirror axis, just like your collimation cap. They are functionally the same tool. A site tube (with crosshair) can be used to better place your secondary and the crosshair functions to align the focuser axis just like the laser. A Combo tool is both a Cheshire and a site tube, so it can do both focuser and primary alignments. Before you invest in another tool, check to see if your laser is properly aligned and that is is properly seated in the focuser. The Hotech is supposed to square itself with the focuser, but it may not be. 

 

There may be some parallax error in the image, but I can tell your secondary is tilted away from center by looking at the angle indicated by the gap between the secondary holder and the spider hub (at lower left in your image). That in itself means nothing, really, except the secondary does appear to be tilted opposite of where it needs to be. Even with some parallax error, from what I see collimation is pretty far off. That includes any parallax error. Go ahead and try your Nextimage camera, just for kicks. Cell phones can be finicky. 


Edited by Asbytec, 23 October 2020 - 06:23 PM.


#7 Lucky 777

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Posted 23 October 2020 - 07:57 PM

Ok, I have a white piece of paper behind the secondary.  Made some adjustments and got to here.  Still using a phone...as soon as I mentioned the other camera I knew that was a stupid idea because it has no lens.851 Capture.JPG



#8 Asbytec

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Posted 23 October 2020 - 08:43 PM

No lens? Thought so but wasn't sure. :lol:

It looks better. I'm away from my computer, so can't look hard. But, it's looking better.

#9 Lucky 777

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Posted 24 October 2020 - 07:08 AM

By the way, the Hotech laser is collimated.  It had been serviced by them earlier this year and I double checked it myself.  At first I was extremely disappointed with the thing, it would not stay square in the focuser tube as I tightened the collar. The rubber rings seemed to push out unevenly and shift the position a little.  The fix for that was to remove the rubber rings and aluminum spacer between them and apply a very light coating of silicone grease to the inside of the rings.  Now it works smooth and always stays square with the focuser tube.  My other self induced error was that the home made collimating cap I was using has the sight hole slightly off center. 

 

Thank you for the suggestions that got me back on track.  It was too late to do a star collimation check last night and it's expected to be cloudy here the rest of the weekend.


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#10 Asbytec

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Posted 24 October 2020 - 07:38 AM

Okay, yes, much better. The focuser axial tolerance is 3% of the primary diameter, in this case abo0ut 4.5mm. I am guessing the perforation in the donut is about 1/4" (6mm in diameter and 3mm in radius). The red cross hair is right at one radius from the center, so you're within the 4.5mm tolerance as shown. Notice how much more concentric everything looks. Your secondary position looks real good, too. The primary reflection is well seen in the secondary with a fairly consistent dark annulus around it (just a tad away from the primary but fine as is). Your secondary rotation looks pretty good, too, pointing upward toward the primary. I cannot see your primary center mark in relation to the collimation cap peephole, but I trust you can. 

 

Untitled.jpg


Edited by Asbytec, 24 October 2020 - 07:55 AM.


#11 Lucky 777

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Posted 24 October 2020 - 08:29 AM

Thank you so much!



#12 Vic Menard

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Posted 24 October 2020 - 08:48 AM

...I cannot see your primary center mark in relation to the collimation cap peephole, but I trust you can. 

 

From what I can see, the camera centering is pretty good (reference blue circle and cross hairs--as you noted, well within tolerance). The secondary mirror placement is also pretty good (green circle is optimal). But the primary mirror alignment still needs correction (reference the primary mirror center marker/donut (small red circle) to the inside of the focuser drawtube (yellow circle shows correct alignment).

 

I'm a little puzzled that the optimal secondary mirror placement shows that it's not fully offset (needs to be a little closer to the primary mirror end of the OTA), but the silhouette reflection of the secondary mirror surrounding the reflection of the underside of the focuser seems to be showing too much offset? 

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Edited by Vic Menard, 24 October 2020 - 08:54 AM.

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#13 Asbytec

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Posted 24 October 2020 - 08:57 AM

"But the primary mirror alignment still needs correction (reference the primary mirror center marker/donut (small red circle) to the inside of the focuser drawtube (yellow circle shows correct alignment)."

Great catch. I noticed the same as I was positioning a circle on the reflected bottom edge of the focuser. I could not trace a circle concentric to the primary center donut. Now it makes sense. Thanks.

Your cross hair is, also, much closer to the primary center than the program I use. I trust it, but not completely.

Edited by Asbytec, 24 October 2020 - 09:00 AM.


#14 Lucky 777

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Posted 24 October 2020 - 08:57 AM

That is correct, I could only get a photo holding the phone over the empty focuser, you can see the corner of my iPhone in the reflection coming back from the primary.  So at this point I should leave the secondary alone and try to tweak the primary?


Edited by Lucky 777, 24 October 2020 - 08:59 AM.

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#15 Vic Menard

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Posted 24 October 2020 - 09:36 AM

Your cross hair is, also, much closer to the primary center than the program I use. I trust it, but not completely.

I referenced all of the circles to the donut/small red circle. If you look closely, the outermost blue circle is not aligned to the bottom edge of the focuser drawtube, but shifted toward the right, so your annotations are pretty close to what I'm showing (compare the cross hairs to the spider vanes in both annotated images--then shift mine to the left).


Edited by Vic Menard, 24 October 2020 - 09:53 AM.

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#16 Asbytec

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Posted 24 October 2020 - 09:49 AM

"So at this point I should leave the secondary alone and try to tweak the primary?"

Yep. :)

Edit: for the reason Vic explaines below, I'd recommend trying primary collimation with your cap. Any deviation you may "read" with the center spot is 2x the actual error on the focal plane. (It's a similar triangles thing). So, the closer you center the donut in the peep hole means 2x accuracy on the focal plane (actual error where it counts). Imagine 2x perfect. :lol:

Edited by Asbytec, 24 October 2020 - 10:02 AM.


#17 Vic Menard

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Posted 24 October 2020 - 09:52 AM

...So at this point I should leave the secondary alone and try to tweak the primary?

Yes. It should be "near" perfect. The allowable read error (using a Cheshire, Barlowed laser, or a collimation cap--tools that magnify the actual primary mirror error 2X) is a little less than 1.5mm--about 0.05-inch, and you're currently showing about twice the allowable read error. If you're using the Hotech return beam to align the primary mirror, the outgoing beam alignment (laser dot/cross hair relative to the primary mirror center marker/donut) will also need to be "perfect" (if it's not, you'll need to add 1/2 of any residual error to any residual primary mirror alignment error (the actual error, not the read error).



#18 Lucky 777

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Posted 24 October 2020 - 10:05 AM

I have a 2.5x Bader Barlow that will screw to the bottom of the Hotech laser and will see what that gives me since we’re socked in up here in Pennsylvania, but at this point I’m thinking my best results will come from a star collimation once I can get the scope outside.

#19 Vic Menard

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Posted 24 October 2020 - 11:47 AM

...at this point I’m thinking my best results will come from a star collimation once I can get the scope outside.

This is the star collimation method I prefer:  http://www.loptics.c.../starshape.html



#20 Lucky 777

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Posted 24 October 2020 - 03:13 PM

Interesting read...thanks! In the past I’ve gone way out of focus and tweaked the primary much the same as the way you adjust the secondary on an SCT.

#21 Vic Menard

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Posted 24 October 2020 - 06:09 PM

...In the past I’ve gone way out of focus and tweaked the primary much the same as the way you adjust the secondary on an SCT.

Way out of focus, an SCT is still concentric. Look at the offset of your secondary mirror in post #12--that's what it will look like way out of focus with a centered star. Worse yet, you can force it to look concentric with that "way out of focus" star, but it won't be collimated. 

 

I still think the best way to star collimate is to start with a good Cheshire alignment. Nine times out of ten, you'll likely find that the star collimation verifies your Cheshire alignment (and probably better than 9 out 10 if you use a calibrated Cheshire and make it "near" perfect).


Edited by Vic Menard, 24 October 2020 - 06:23 PM.


#22 Asbytec

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Posted 24 October 2020 - 07:06 PM

Way out of focus, an SCT is still concentric. Look at the offset of your secondary mirror in post #12--that's what it will look like way out of focus with a centered star. Worse yet, you can force it to look concentric with that "way out of focus" star, but it won't be collimated.

Yes, when we collimate on a star, we do not collimate on shadows of the secondary and spider vanes. We collimate on diffraction caused by the aperture and obstruction (the latter is responsible for the Poisson spot at the center). If it gets to the point you defocus and begin to see an shadow image of the obstruction, including the vanes, you are too far from focus. In fact, the Poisson spot will be slightly offset, too, in the center of the offset secondary shadow. Close to focus, however, that offset is absolutely miniscule.

Edited by Asbytec, 24 October 2020 - 07:25 PM.

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#23 Lucky 777

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Posted 24 October 2020 - 09:02 PM

I’m sure glad I made that comment, got a real education.  I’m a tinkerer and a little OCD by nature so I will get it as good as possible.  Thanks again for sharing your knowledge and experience. 




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