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Collimation of fast newton (self built)

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#1 AndreaL

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Posted 25 October 2020 - 04:49 PM

Hello,

I've built a fast corrected newton, 780 mm focal lenght,  f3.9, corrected with a Big Paracorr.

Spider is 45 deg vs focuser.

secondary mirror has 4.7 mm offset and it has been glued with offset.

I am now trying to collimate it, but I think I have issues with the positioning/rotation of the secondary mirror.

 

I am attaching a picture taken through the sight hole and hope someone can analyze it and give my some analysis and how to best fix it.

 

I think it is decently collimated but probably need to be rotated.

 

 

my procedure and my results:

 

1- I've tried to square the focuser using the tilt base of the focuser.

2- adjusted rotation and placement with a sight tube (very difficult)

3- collimated the secondary using a laser

4- collimated the primary with sight tube and barlowed laser, verified with cheshire

 

Collimation was ok in my opinion but the secondary was displaced up in the sight tube so I decided to correct the focuser tilt up/down

I repeated the above process and ended up with a situation in which the secondary was displaced down.

I readjusted the tilt in the opposite direction, collimated and the final result is the one attached.

 

In my opinion there is still at least a rotation error.

 

It would be nice to have an evaluation to understand if I am interpreting the results correctly.

 

Thanks a lot,

Andrea

Attached Thumbnails

  • prova 1.jpg

Edited by AndreaL, 25 October 2020 - 04:51 PM.


#2 cuzimthedad

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Posted 25 October 2020 - 04:56 PM

Hi Andrea and welcome to Cloudy Nights. Someone will be chiming in soon to help you find the answers you're looking for, but I just wanted to take a moment to say hi and welcome you to the site.



#3 Couder

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Posted 25 October 2020 - 05:00 PM

The quadrant closest to the bottom of the picture looks smaller than the top one, which makes me think the secondary is not centered..



#4 Vic Menard

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Posted 25 October 2020 - 05:02 PM

From what I can see, your axial alignments are good. Unfortunately, I can't see the actual edge of the secondary mirror to determine the optimal placement. If you could rack the focuser out a bit, the secondary mirror should begin to reveal itself. If you put a piece of white paper behind the secondary mirror against the inside of the tube wall (opposite the focuser) it will make the secondary mirror stand out. Don't cover the primary mirror, we still need to see the reflections and the primary mirror center marker/HotSpot.

 

And welcome to Cloudy Nights!


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#5 AndreaL

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Posted 25 October 2020 - 05:17 PM

Thank you for the welcome and super quick reply.

Vic, I hope to get new pictures in one day or two.

 

Thanks a lot,

Andrea



#6 rkinnett

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Posted 25 October 2020 - 05:23 PM

The image of the secondary reflected back from the primary looks pretty far off center due to tilt of the primary.  Tilting the primary into alignment is generally the easiest step (esp. if you have a laser collimator with a side-view target or a camera looking into the focuser), but you may have to iterate.  It looks like axial alignment is dead on now, but it may shift a bit as you tilt your primary.  Tilt your primary to make the image of your secondary appear centered, then you may need to readjust your secondary again, and repeat if necessary.

 

https://www.youtube....h?v=8ROvNH5uwDo



#7 Vic Menard

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Posted 25 October 2020 - 05:32 PM

OK. For what it's worth, assuming the focuser is off in the direction of the green arrow and the primary mirror is in the direction of the red arrow, I agree with you that your rotation isn't quite corrected yet. Also, assuming this is an imaging scope, you may have to use a focuser extension to get the Cheshire/sight tube pupil closer to (or above) the focal plane. You'll also want to reduce the primary mirror clips protrusion into the face of the primary mirror (they make interesting dark diffraction spikes on brighter stars).

 

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  • post-279919-0-55291400-1603662295_thumb.jpg


#8 Vic Menard

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Posted 25 October 2020 - 05:39 PM

...Tilt your primary to make the image of your secondary appear centered, then you may need to readjust your secondary again, and repeat if necessary.

This is an 8-inch f/3.9 with an oversized secondary mirror and Paracorr to illuminate a big sensor. The silhouette reflection of the secondary mirror surrounding the underside of the big (3-inch?) focuser will appear offset toward the primary mirror end of the tube assembly.


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#9 AndreaL

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Posted 27 October 2020 - 10:35 AM

Hi Vic and others,
got the point on the mirror clips: I will take care of them after I tune the telescope.
 
I spent several hours trying to improve the positioning of the secondary but with no much luck:
 
-I feel like the system is very sensitive to secondary rotation : a minimum amount of rotation changes the shape significantly
-the initial positioning is lost as soon as I start collimating the secondary: the mirror support tends to rotate and shift up/down when collimating the secondary.
This is probably due to the design ( I can share on this) and the fact that the mirror is very heavy: it probably bends down due to weight at rest when collimating screws are not engaged. 
 
The shift up/down is the major issue at the present, I think: the only way I can think to correct it is by changing the focuser tilt (up/down): 
the idea is to correct this focuser tilt at the end, then readjusting collimation and may be iterate. what do you think?
 
note: checking the focuser tilt at the beginning using a rod and a laser produced the worst results.I think because of the bending due to weight, that I corrected with tilt and that I should not. 
 
Thank you in advance for your help
Andrea

Attached Thumbnails

  • collimation 27 10.jpg

Edited by AndreaL, 27 October 2020 - 10:36 AM.


#10 Vic Menard

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Posted 27 October 2020 - 01:44 PM

...The shift up/down is the major issue at the present, I think: the only way I can think to correct it is by changing the focuser tilt (up/down): the idea is to correct this focuser tilt at the end, then readjusting collimation and may be iterate. what do you think?

Usually, there is a (larger) center mounting screw and three tilt adjustment screws behind the secondary mirror. Your offset is pretty close (green circle). If you want to pull it back a millimeter or two (away from the primary mirror), you'll need to loosen all three of the tilt adjustment screws and tighten the center mounting screw. But I still see a small rotation error (silhouette reflection of the secondary mirror pointing in the direction of the red arrow when it should be pointing in the direction of the green arrow). I understand your concerns regarding the secondary mirror support/adjustment mechanicals, if yours are different, a picture may help. I would leave the focuser leveling alone for now, let's get tilt/rotation and tilt/offset sorted first.

 

note: checking the focuser tilt at the beginning using a rod and a laser produced the worst results.I think because of the bending due to weight, that I corrected with tilt and that I should not. 

I'm not sure what you mean here--was the rod in the center mounting hole for the secondary mirror and the laser in the focuser? Did you try to change the focuser leveling?

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  • crop10.jpg


#11 AndreaL

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Posted 27 October 2020 - 04:54 PM

Usually, there is a (larger) center mounting screw and three tilt adjustment screws behind the secondary mirror. Your offset is pretty close (green circle). If you want to pull it back a millimeter or two (away from the primary mirror), you'll need to loosen all three of the tilt adjustment screws and tighten the center mounting screw. But I still see a small rotation error (silhouette reflection of the secondary mirror pointing in the direction of the red arrow when it should be pointing in the direction of the green arrow). I understand your concerns regarding the secondary mirror support/adjustment mechanicals, if yours are different, a picture may help. I would leave the focuser leveling alone for now, let's get tilt/rotation and tilt/offset sorted first.

 

I'm not sure what you mean here--was the rod in the center mounting hole for the secondary mirror and the laser in the focuser? Did you try to change the focuser leveling?

Vic, thank you.

I found setting the rotation very difficult: it is really sensitive, in real terms I could be off 2-3 deg max but there is a small window in which I can't improve much. i can see the elliptical shape changing form one side to the other but can't find the nice spot.

Any idea is appreciated. probably working with more light could help. I have seen an eyepiece with concentric rings but I don't see an advantage vs teh sight tube.

My mechanicals are similar, but I have a steel plate with 3 small concaves: the collimation screws touch here.

the plate can rotate.

The problem is that as soon as I start putting pressure on the screws the plate rotates and the rotation is changed, but I think I have figured out how to prevent this.

I have also tried to have the screws pushing on the flat part of the plate but they tends to slip over the surface (it is stainless steel) and collimation is duable but complex. I am attaching a picture of the plate.

May be I could remove the plate, change the material with something softer or have a sharper screw.

 

>I'm not sure what you mean here--was the rod in the center mounting hole for the secondary mirror and the laser in the focuser? Did you try to change the focuser leveling? (sorry I don't know how to do multi quotes :-))

yes, using the rod shows that I need to correct a small up/down tilt, but I got the best results with a very small tilt if none.

 

last, I think my positioning is biased by the weight of the mirror: basically my mirror is already tilted down due to weight at the end of positioning (or, I am positioning the secondary while it is tilted down). I don't know if this can have an impact.

what I can do is to try to collimate with the OTA vertical, or add three springs on the collimatiion screws to prevent unwanted tilt whenthe screws are not engaged.

Attached Thumbnails

  • 2.jpg
  • 1.jpg


#12 Vic Menard

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Posted 27 October 2020 - 05:19 PM

I'm not sure what I'm seeing, it appears that you have 2 dowels that pass through the spider hub and the tilt plate and then engage the secondary mirror holder--I don't see how that can rotate? 

 

It looks like you have a single, center mounting screw that passes through the hub and the tilt plate and then threads in to the center of the secondary mirror holder--that's how you would set the position fore and aft. 



#13 AndreaL

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Posted 28 October 2020 - 05:04 AM

Hi Vic, 

I understand  that the image is confusing, sorry.

i wanted to show you the first focuser alignment and the mechanicals behind the hub.

you can find a better picture attached:

 

I have a central hub with a central hole for a pass through dowel. This threads in the secondary mirror support and provide the axial adjustment.

then I have the three collimating screws, that thread in the hub and push on the steel plate I mentioned before.

the plate should allow to rotate the mirror more easily. the plate is not tied to the mirror support, but the two bind together under pressure.

 

The problem is: if the three "concaves" on the plate are not in line with the screws, engaging the screws rotates the plate and the mirror support.

Except the plate I think it is a very common design.

 

I got your suggestion of dealing with focuser tilt at the end of the procedure, but if the secondary mirror refection is not centered it is more difficult to evaluate the rotation as the sight tube profile doesn't fit around. 

 

Thank you again,

Andrea

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  • IMG_4513.jpg

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#14 sixela

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Posted 28 October 2020 - 09:16 AM

If you have a small rotational error that's usually because the "concaves" aren't well aligned. Usually you would not force that plate's rotation this stringently (with the three central holes) -- you'd make the two slots for the uncentred bolts that go through slightly wider so that you could slightly rotate the plate and thus the "concaves".

But you can live with the small amount of rotation -- all that does is make the fully illuminated field slightly elliptical, i.e. more narrow along one dimension (it'll have the same shape as the secondary seen from the focuser).

Just use the centre bolt as a pull bolt and the three tilt screws as a push bolt to set offset to/from the primary, and then just tilt the focuser slightly to centre the secondary under the focuser (from the vantage point in which the primary's reflection is just smaller than the secondary).

For small adjustments to centre the secondary under the focuser it doesn't really matter much whether you tilt the focuser towards the current position of the secondary or move the secondary towards wherer the focuser currently points. At least not unless you're worried about having an optical axis that is parallel to the tube axis. People who build "low rider" Dobs tilt their focusers 15°-20° without too many ill effects!

Edited by sixela, 28 October 2020 - 09:23 AM.


#15 AndreaL

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Posted 28 October 2020 - 11:14 AM

If you have a small rotational error that's usually because the "concaves" aren't well aligned. Usually you would not force that plate's rotation this stringently (with the three central holes) -- you'd make the two slots for the uncentred bolts that go through slightly wider so that you could slightly rotate the plate and thus the "concaves".

But you can live with the small amount of rotation -- all that does is make the fully illuminated field slightly elliptical, i.e. more narrow along one dimension (it'll have the same shape as the secondary seen from the focuser).

Just use the centre bolt as a pull bolt and the three tilt screws as a push bolt to set offset to/from the primary, and then just tilt the focuser slightly to centre the secondary under the focuser (from the vantage point in which the primary's reflection is just smaller than the secondary).

For small adjustments to centre the secondary under the focuser it doesn't really matter much whether you tilt the focuser towards the current position of the secondary or move the secondary towards wherer the focuser currently points. At least not unless you're worried about having an optical axis that is parallel to the tube axis. People who build "low rider" Dobs tilt their focusers 15°-20° without too many ill effects!

Thank you Sixela,

the plate is not tied to the support, it can rotate, but when pressure is applied through collimation screws the two binds together due to drag.

To avoid the rotation I should position the support and ensure the concaves are aligned with the screws: this is my challenge at present.

 

This telescope will be used with a very big ccd, I suspect that I need a perfect illumination .

I am not sure to get your point here: "(from the vantage point in which the primary's reflection is just smaller than the secondary)."

are you just saying that, given my configuration, this is actually easier? am I getting right? edit: it is easer because I can also work on equalizing the black ring around?

 

Regarding the last point, my concern is only related to to my use of the telescope: I need to have a flat field of 26mm of radius, through a Big Paracorr.

What you say it makes perfectly sense to me.

 

Thank you

Andrea


Edited by AndreaL, 28 October 2020 - 01:46 PM.


#16 sixela

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Posted 28 October 2020 - 04:03 PM

I mean that when you centre the outline of the secondary under the focuser, that will only stay that way with the focuser racked in that way. Because the perspective effect that introduces an offset between the geometrical centre of the face of the secondary and what appears to be the centre as seen from the focuser, if you rack in or out it will slightly change: the reflection of the primary will stay put but the secondary will seem to "creep" left or right.

If you want to centre the fully illuminated field, ideally you should rack out the focuser from the focal plane (where hopefully the reflection of the primary fits in the secondary with some room) until the secondary seems to shrink and no longer appears a lot larger than the reflection of the primary, with just a sliver of black° between the reflection of the primary and the edge of the secondary's face.


--
°If it ain't black, you need to rethink the baffling of your scope. In your case it isn't, because you're looking at what is behind the scope. That's usually not very good, because any light from there will convert into a background veil that will increase the background surface brightness at the eyepiece. Some people use tarps on the ground to fix that, but if you're observing close to the horizon you'd need a really large tarp. Or a spectacularly dark site with no light sources close to the horizon. OTOH, it's not hard to put an annular baffle with flocking paper (or even matte black paint, since it's not illuminated grazingly) at the back.

Edited by sixela, 28 October 2020 - 04:05 PM.



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