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collimation: reflection concentricity lost when aligning the secondary to the primary

collimation reflector beginner
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#1 pez_espada

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Posted 24 February 2020 - 09:30 AM

Dear all,

 

I am just a beginner coming from Refractorland. I have bought a small 130PDS (5.1" F5 Newtonian) for wide field variable star work and as a RFT (with a coma corrector). The little Newtonian serves also as my introduction to Reflectorlland and the art of collimation (that I am eager to learn well). Well I have read some guides on the topic (i.e. Don Pensak's, "Astro-Baby's", Gary Seronik's, etc) and am trying to master the technique using a sight tube/ Cheshire and collimation cap only. I am not attracted by lasers but getting interested in autocollimator tools such as the Catseyes and Fairpoint.

 

I follow the receipt of firstly center the secondary under the focuser by making the circles produced by the the bottom of the focuser, the secondary and the primary reflection within the secondary all concentric as seen through the peephole of the collimation cap attached to the focuser. I can achieve this (more or less) by playing with both the center bolt of the secondary and the three small collimation screws. I rotate by reaching the support of the secondary, then I adjust  the central bolt and collimation screws until I get as concentric circles as I can. I tight all screws and central bolt.

 

Then I try to get the secondary aligned with the primary by making the cross-hair of the sight tube to coincide with the primary's  small donut, by only playing with the three collimation screws of the secondary

 

Once done this, I proceed to align back the primary with the focuser's optical axis by placing the dark dot (produce by the top end of the Cheshire tool) inside the primary's donut by adjusting the primary's three collimation screws only.

 

When I finish all these steps, I go back to step one to check the centering of the secondary under the focuser to find out that the initial concentricity of the multiple images is lost I guess by the further adjusting of the secondary done in step 2.

 

I understand by a recent post that keeping concentricity of the reflections and images in the first step (aligning the secondary under the focuser) ensures full field illumination. Even when I am not planing at the moment tu use the 130PDS for AP when visually estimating variable star magnitudes is extremely important to have an illuminated full field. That's one of the reason I was atracted to the 130PDS being desined for AP and able to porduce full illumination due to its relatively large secondary (37% CO).

 

So what am I doing wrong? How can I collimate the scope to ensure the best illuminated filed of view possible? The attached pic shows the loss of concentricicty of the primary reflection within the secondary mirror after the collimation procedure described above, even when I made them concentric in the first step.

 

Many thanks

 

C

 

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#2 ww321q

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Posted 24 February 2020 - 10:33 AM

My new refractor came with a collimating eyepiece. I've been using it on my reflector. One of the things I had to do was shim my focuser to get it to point dead center. I started playing with telescopes just 1 1/2 years ago. I think my cheap laser collimator is almost useless. Something may be slightly out of alignment that you haven't caught yet. Sometimes I just push on parts to see the change. Stick with it you'll figure out what you're missing. Have fun with it. :) 


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#3 Asbytec

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Posted 24 February 2020 - 10:52 AM

Gosh, your primary reflection looks pretty close to centered under the focuser draw tube. That's good, so you're pretty close to collimated at this point. A tiny bit off (short maroon line upper left). You probably picked up that error when you tightened your center bolt pulling the secondary aft and away from the primary. Once the secondary is centered, again, fore and aft there's no need to adjust the center bolt. Loosen one tilt screw to rotate the secondary. We can fix that error. The secondary is obviously rotated as evidenced by the reflected offset pointing somewhere (green arrow) other than toward the primary (red arrow). And the secondary looks tilted toward the "bottom left" of the focuser draw tube. We can fix that, too, so the three collimation signatures (focuser draw tube, secondary mirror, and primary reflection) remain in tact.

Collimation.png

I suspect what is happening is two of the secondary adjustment screws are tilting and rotating the secondary into this position as you are attempting to collimate the cross hair with the primary center donut. That is why your are nearly collimated, but with some residual tilt and rotation error. But, I would not use a collimation cap for this, a collimation cap is a primary axis alignment tool. Use your site tube or focuser draw tube, instead. Focus down until you're almost all the way down or the site tube looks just a little larger than the secondary mirror. I see you're using white paper background. Good. So, go back to your centered concentric circles as a starting point then try again. For now, ignore the primary mirror reflection, just get the secondary centered under the site tube (preferably) or draw tube if necessary. Use your eyepiece locking screws to hold the site tube because we're going to collimate to the site tube registration so the eyepieces will register likewise.

I can achieve this (more or less) by playing with both the center bolt of the secondary and the three small collimation screws.

Loosen all the secondary adjustment screws so they are roughly equal and in a more neutral position. let teh secondary be where it will be. If it's not centered, then you can shim the focuser slightly to center the secondary under the site tube. Once the secondary is re-centered, avoid the urge to use the tilt screws at this point. They are tilting and rotating your secondary away from its careful placement winding up with the image you show above. That's not what you want. Use only fore and aft secondary movement, secondary rotation, and any focuser shim to achieve secondary centering. Tilt is not really for secondary placement unless you compensate for it, tilt is primarily a collimation process.

So, once the secondary is centered, now pay attention to the primary reflection. Avoid the urge, for now, to begin tilt collimation using the three adjustment screws. Rotate the secondary so the reflection of the primary is centered on the secondary major axis. Once you put the primary reflection on the major axis, the primary center marker (donut) will also, and necessarily so, be on the secondary major axis, too. When the secondary is well centered under the site tube, very importantly, the secondary major axis also resides along the focuser axis. So, as a result the focuser axis, as defined by the site tube cross hairs, is in line with the primary center mark. Now, locate only one secondary adjustment screw also in line with the focuser axis. You can see all of your collimation can now be done by tilting the secondary along only the one focuser axis (secondary major axis and primary center mark are all in line with the focuser axis and one collimation screw, which is a great starting point).

So, begin tilting only the one secondary adjustment screw that moves the cross hairs along the focuser axis and secondary mirror major axis. Turn the other two orthogonal adjustment screws in unison, in the same direction, and by the same amount to allow that in line secondary tilt. This tilt (in line with the cross hairs and primary center mark) will bring the secondary reflection to the center of the (well centered) secondary mirror. If the secondary rotation is nearly perfect, then the cross hairs will also be on the primary center mark and all three collimation signatures will fall into place. Part of the problem, I believe, you are experiencing is using the two orthogonal secondary adjustment screws independently trying to collimate from three different directions. Those two screws induce some needed tilt to collimate, but unwanted tilt moving the secondary off center. They also induce some unwanted rotation of the secondary resulting as your image shows. It's intuitive to use the three secondary adjustment screws independently, but I find they are best used in unison as described. When everything is on the focuser axis (and secondary major axis) we need tilt in only one direction, the other two screws are used in unison to allow that in line tilt to happen.

Next, focus outward to the apex where the secondary mirror and the primary reflection are the same apparent size. Any residual tilt and rotation can be seen using the mirror clips (which are a proxy for the edge of the primary reflection). Any residual rotation and secondary tilt can be easily seen against the collimated primary reflection (cross hairs on the center mark). If so, loosen that single adjustment screw in line with the focuser axis so the secondary can rotate (only). Recenter the primary reflection on the secondary major axis, then tighten that single adjustment screw to bring the cross hair back into the center mark. If you miss the center mark, back off, rotate (only) the secondary, and re-tighten the adjustment screw until you nail the cross hairs on the center mark. Or, in truth, once you're this close to being collimated, you can use the two orthogonal adjustment screws to finish the cross hairs onto the center mark.

Here's another thread discussing it. https://www.cloudyni...ally/?p=9979091

I know it's a bit confusing, so here's some illustrations. This is the method I used, it's a little different but I find this to be the best way to hold the collimation signatures without excessive tilt and rotation error your image above shows. This is also an easy and quick method to check you collimation at the apex prior to observing.

Focuser Collimation and Secondary Rotation.jpg

Focuser Collimation Final Approach.jpg

Edited by Asbytec, 24 February 2020 - 11:29 AM.

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#4 Jon Isaacs

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Posted 24 February 2020 - 11:22 AM

Gosh, your primary reflection looks pretty close to centered under the focuser draw tube

 

 

To my eye, the alignment of the primary and the focuser axis look good. They're the important things.

 

Depending on how the photo was taken, the secondary positioning may need adjustment but is much less critical in terms of on-axis performance.

 

Jon


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#5 Asbytec

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Posted 24 February 2020 - 11:26 AM

True, Jon.

#6 Jon Isaacs

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Posted 24 February 2020 - 01:37 PM

True, Jon.

 

I have messing around with collimating Newtonians for 20 plus years.

 

Intuitively I understood that what was most important was that the eyepiece/focuser was looking at the center of the mirror and that the axis of the mirror was aligned with the axis of the eyepiece/focuser.

 

But what was confusing is that most discussions spend far more time on the less important factors, the positioning of the secondary mirror.  While this has some effect on the quality of the view, it's much less important in most situations.

 

To my eye, Pez's scope is well collimated in terms of the two important factors and show provide excellent views without further fiddling.

 

There is a time for further fiddling but for now, it's time to enjoy the views.

 

Jon


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#7 pez_espada

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Posted 24 February 2020 - 01:39 PM

Gosh, your primary reflection looks pretty close to centered under the focuser draw tube. That's good, so you're pretty close to collimated at this point. A tiny bit off (short maroon line upper left). You probably picked up that error when you tightened your center bolt pulling the secondary aft and away from the primary. Once the secondary is centered, again, fore and aft there's no need to adjust the center bolt. Loosen one tilt screw to rotate the secondary. We can fix that error. The secondary is obviously rotated as evidenced by the reflected offset pointing somewhere (green arrow) other than toward the primary (red arrow). And the secondary looks tilted toward the "bottom left" of the focuser draw tube. We can fix that, too, so the three collimation signatures (focuser draw tube, secondary mirror, and primary reflection) remain in tact.

attachicon.gifCollimation.png

I suspect what is happening is two of the secondary adjustment screws are tilting and rotating the secondary into this position as you are attempting to collimate the cross hair with the primary center donut. That is why your are nearly collimated, but with some residual tilt and rotation error. But, I would not use a collimation cap for this, a collimation cap is a primary axis alignment tool. Use your site tube or focuser draw tube, instead. Focus down until you're almost all the way down or the site tube looks just a little larger than the secondary mirror. I see you're using white paper background. Good. So, go back to your centered concentric circles as a starting point then try again. For now, ignore the primary mirror reflection, just get the secondary centered under the site tube (preferably) or draw tube if necessary. Use your eyepiece locking screws to hold the site tube because we're going to collimate to the site tube registration so the eyepieces will register likewise.
Loosen all the secondary adjustment screws so they are roughly equal and in a more neutral position. let teh secondary be where it will be. If it's not centered, then you can shim the focuser slightly to center the secondary under the site tube. Once the secondary is re-centered, avoid the urge to use the tilt screws at this point. They are tilting and rotating your secondary away from its careful placement winding up with the image you show above. That's not what you want. Use only fore and aft secondary movement, secondary rotation, and any focuser shim to achieve secondary centering. Tilt is not really for secondary placement unless you compensate for it, tilt is primarily a collimation process.

So, once the secondary is centered, now pay attention to the primary reflection. Avoid the urge, for now, to begin tilt collimation using the three adjustment screws. Rotate the secondary so the reflection of the primary is centered on the secondary major axis. Once you put the primary reflection on the major axis, the primary center marker (donut) will also, and necessarily so, be on the secondary major axis, too. When the secondary is well centered under the site tube, very importantly, the secondary major axis also resides along the focuser axis. So, as a result the focuser axis, as defined by the site tube cross hairs, is in line with the primary center mark. Now, locate only one secondary adjustment screw also in line with the focuser axis. You can see all of your collimation can now be done by tilting the secondary along only the one focuser axis (secondary major axis and primary center mark are all in line with the focuser axis and one collimation screw, which is a great starting point).

So, begin tilting only the one secondary adjustment screw that moves the cross hairs along the focuser axis and secondary mirror major axis. Turn the other two orthogonal adjustment screws in unison, in the same direction, and by the same amount to allow that in line secondary tilt. This tilt (in line with the cross hairs and primary center mark) will bring the secondary reflection to the center of the (well centered) secondary mirror. If the secondary rotation is nearly perfect, then the cross hairs will also be on the primary center mark and all three collimation signatures will fall into place. Part of the problem, I believe, you are experiencing is using the two orthogonal secondary adjustment screws independently trying to collimate from three different directions. Those two screws induce some needed tilt to collimate, but unwanted tilt moving the secondary off center. They also induce some unwanted rotation of the secondary resulting as your image shows. It's intuitive to use the three secondary adjustment screws independently, but I find they are best used in unison as described. When everything is on the focuser axis (and secondary major axis) we need tilt in only one direction, the other two screws are used in unison to allow that in line tilt to happen.

Next, focus outward to the apex where the secondary mirror and the primary reflection are the same apparent size. Any residual tilt and rotation can be seen using the mirror clips (which are a proxy for the edge of the primary reflection). Any residual rotation and secondary tilt can be easily seen against the collimated primary reflection (cross hairs on the center mark). If so, loosen that single adjustment screw in line with the focuser axis so the secondary can rotate (only). Recenter the primary reflection on the secondary major axis, then tighten that single adjustment screw to bring the cross hair back into the center mark. If you miss the center mark, back off, rotate (only) the secondary, and re-tighten the adjustment screw until you nail the cross hairs on the center mark. Or, in truth, once you're this close to being collimated, you can use the two orthogonal adjustment screws to finish the cross hairs onto the center mark.

Here's another thread discussing it. https://www.cloudyni...ally/?p=9979091

I know it's a bit confusing, so here's some illustrations. This is the method I used, it's a little different but I find this to be the best way to hold the collimation signatures without excessive tilt and rotation error your image above shows. This is also an easy and quick method to check you collimation at the apex prior to observing.

attachicon.gifFocuser Collimation and Secondary Rotation.jpg

attachicon.gifFocuser Collimation Final Approach.jpg

Wow you guys at CN are awesome! Thanks A. for that detailed explanation and taking the time, I greatly appreciated. I have already read and tried to follow the directions in my head but I am looking forward to putting them into practice with the scope asap. I guess after all these steps you still need to align the main mirror to the focuser axis (e.g. placing the dark dot into the donut) with the main mirror collimation screws?

 

On the other hand there is something I have noticed about my secondary since the scope arrived. It seems to be offset with respect to its secondary holder. The secondary's axis does not seem inline with its holder's (ergo neither with the primary's axis?). It looks to be offset pointing towards 10:30h as shown in the picture's orientation. Of course the picture is not taken with a precise exact angle to show with certainty this effect but I tried to capture what I visually detect as best I could, and I am aware that angles and parallax can trick the eye.  But if this asymmetrical offset is there I guess this is the direction described by the green arrow you pointed out on top of the original picture. Is this normal? I heard about "secondary offset" (my God, too much to learn still!) which I understand it renders the secondary and its round holder to not appear concentric when seen from top (because some needed shift of the geometric center of the secondary for some reason I fail to understand yet). However I would expect to see this offset to be along the major axis of the secondary (i.e towards 9:00h in the pic), not towards 10:30h. Am I right about this? if so, can it be a production problem (QC) with my secondary orientation respect to its holder?

 

Lastly, seeing that the endeavor of the collimation of a Newtonian is more complex than I expected can some one point me to some definite resources for learning? There is a lot of comments, articles, receipts, tools, procedures, opinions, etc that some apparently point towards different paths so to get lost is very easy.

 

Many thanks!

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#8 pez_espada

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Posted 24 February 2020 - 01:51 PM

I have messing around with collimating Newtonians for 20 plus years.

 

Intuitively I understood that what was most important was that the eyepiece/focuser was looking at the center of the mirror and that the axis of the mirror was aligned with the axis of the eyepiece/focuser.

 

But what was confusing is that most discussions spend far more time on the less important factors, the positioning of the secondary mirror.  While this has some effect on the quality of the view, it's much less important in most situations.

 

To my eye, Pez's scope is well collimated in terms of the two important factors and show provide excellent views without further fiddling.

 

There is a time for further fiddling but for now, it's time to enjoy the views.

 

Jon

Jon is a relief to know that at least I am on the right track to tame the beast!

As I previously mentioned I got this small Newtonian to learn collimation and reflectors in general and as a RFT for visual estimating bright variables. For the latter I better have a fully illuminated field of view.  You think that the FOV would be reasonable well illuminated with my current collimation as shown in the picture?

 

I calculate that one needs an error not larger than 0.05 - .01 magnitude of difference across the FOV to feel safe when estimating magnitudes of variables by visual comparison. This is half to one order of magnitude lower than the .1 magnitude difference one is used to work with.

 

Thanks,

 

PS. I'm loving this CN comunity


Edited by pez_espada, 24 February 2020 - 01:54 PM.


#9 Vic Menard

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Posted 24 February 2020 - 02:07 PM

...As I previously mentioned I got this small Newtonian to learn collimation and reflectors in general and as a RFT for visual estimating bright variables. For the latter I better have a fully illuminated field of view.  You think that the FOV would be reasonable well illuminated with my current collimation as shown in the picture?

No.

 

Looking at the specs on your scope, I found 130mm primary, 47mm secondary mirror, and 50mm focuser extension for visual work (the scope is also intended for imaging). 

 

This means used as a RFT (low magnification, wide field of view), it's likely your edge of field illumination loss is going to be close to 0.5 magnitude with the secondary mirror optimally placed (0.2-magnitude drop in the center, so a 0.3 magnitude differential). 

 

(Using a 1.25-inch low magnification eyepiece and reducing the secondary mirror to focal plane distance to 6-inches will give you about a 0.02-magnitude differential--but that's dependent on the 6-inch separation. At 7-inches the edge drops 0.1-magnitude. See here:  https://www.bbastrod...om/diagonal.htm  )

 

(Edit: If you use millimeters instead of inches, the numbers are a little better, but still problematic if you want to use a 2-inch wide field eyepiece and the secondary mirror to focal plane distance is greater than 150mm.)


Edited by Vic Menard, 24 February 2020 - 02:55 PM.


#10 pez_espada

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Posted 24 February 2020 - 02:18 PM

No.

 

Looking at the specs on your scope, I found 130mm primary, 47mm secondary mirror, and 50mm focuser extension for visual work (the scope is intended for imaging). 

 

This means used as a RFT (low magnification, wide field of view), it's likely your edge of field illumination loss is going to be close to 0.5 magnitude with the secondary mirror optimally placed. 

Vic, I thought imaging reflectors were optimized for as full  an illuminated FOV as it gets. That was the very reason I got this scope with that huge CO, in the first place.

 

So how to know the best secondary size  and placement to obtain a full illuminated field in that scope with a 2" eyepiece such as an ES82 30mm?

 

How did you calculate that number of .5 magnitude decay at the edge of the field? Where at the edge (distance from the axis) is this loss happening?

 

Many thanks,


Edited by pez_espada, 24 February 2020 - 02:53 PM.


#11 Vic Menard

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Posted 24 February 2020 - 02:39 PM

So how to know the best secondary size  and placement to obtain a full illuminated field in that scope with a 2" eyepiece such as an ES82 30mm?

 

How did you calculate that number of .5 magnitude decay at the edge of the field? Where at the edge (distance from the axis) is this loss happening?

I'm not sure what the field stop is on the ES82 30mm, but I'm guessing around 40mm. Using Bartels' calculator, plug in 130mm for the primary mirror diameter, 650mm for the focal length, 47mm for the secondary mirror,40mm for the eyepiece field diameter, and if you know the secondary mirror to focal plane distance (I don't know so I provided several), the calculator does the rest. You can determine the focal plane by stretching a piece of cellophane tape across the empty focuser extension and point the scope at the Moon. One you can see the focused image of the Moon on the tape, measure from the center mounting screw behind the secondary mirror to the top of the focuser drawtube--that's the actual secondary mirror to focal plane distance.

 

Once you've played with the calculator a while, you can try different secondary mirror minor axes, different secondary mirror to focal plane distances, and different eyepiece field stops to determine what will work best...


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#12 Vic Menard

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Posted 24 February 2020 - 02:47 PM

Vic, I thought imaging reflectors were optimized for as full  an illuminated FOV as it gets. That was the very reason I got this scope with that huge CO, for the first place.

It's all about scalability, which makes reflectors with 2-inch focusers difficult to optimize as they become smaller in aperture and shorter in focal length. With a 47mm minor axis secondary mirror, your 130 mm aperture already has a 36-percent obstruction. Of course, for imaging, it's not uncommon to push for even greater obstruction to illuminate those sensor corners, but this scope is straddling the fence, including a 50mm extension for visual work. And while an "acceptable magnitude loss" of 0.4-magnitude may be OK for general visual RFT observing, it's not that great for variable star work.


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#13 Vic Menard

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Posted 24 February 2020 - 03:08 PM

I am just a beginner coming from Refractorland. I have bought a small 130PDS (5.1" F5 Newtonian) for wide field variable star work and as a RFT (with a coma corrector).

This may be your salvation. It's my contention that field illumination changes when a coma corrector is used. Your scope is no longer a simple Newtonian--it's a catadioptric Newtonian. Since the field lens of the coma corrector is some inches inside the focal plane, it is my opinion that the illumination profile is changed.

 

You're in the perfect position to verify this using calibrated field charts for variable star work. I'm pretty sure you'll find the light loss at the edge of the field is less than the calculated light loss from Bartels' website. But you will need to fix your tilt/rotation error and get your secondary mirror placement sorted.


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#14 Jon Isaacs

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Posted 24 February 2020 - 04:30 PM

This may be your salvation. It's my contention that field illumination changes when a coma corrector is used. Your scope is no longer a simple Newtonian--it's a catadioptric Newtonian. Since the field lens of the coma corrector is some inches inside the focal plane, it is my opinion that the illumination profile is changed.

 

You're in the perfect position to verify this using calibrated field charts for variable star work. I'm pretty sure you'll find the light loss at the edge of the field is less than the calculated light loss from Bartels' website. But you will need to fix your tilt/rotation error and get your secondary mirror placement sorted.

 

Another source of information concerning variable star work with Newtonians is John Bortle who has contributed over 200,000 observations to the American Association of Variable Star Observers.

 

I believe he used a 16 inch Starfinder which has a 4 inch secondary and currently uses a 10 inch GSO.

 

Based on what I know of the 16 inch Starfinder, based on Mel Bartels calculator, it fully illuminates a 36 mm field of view.

 

The 10 inch GSO is about 16 mm for full illumination which is O.70 degrees.

 

I believe Vic is correct in his thinking. A negative lens in front of the focal plane does increase the illuminated field as it changes the effective focal ratio. 

 

The Paracorr is about 3 inches long. In 3 inches, an F/5 light cone converges 0.6 inches, an F/5.75 light cone converges 0.52 inches, that's a gain of 0.080 inches, not a lot.

 

Jon



#15 Vic Menard

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Posted 24 February 2020 - 04:37 PM

...I believe Vic is correct in his thinking. A negative lens in front of the focal plane does increase the illuminated field as it changes the effective focal ratio. 

 

The Paracorr is about 3 inches long. In 3 inches, an F/5 light cone converges 0.6 inches, an F/5.75 light cone converges 0.52 inches, that's a gain of 0.080 inches, not a lot.

I think it's more than that--much more.

 

Bartels' calculator reveals field illumination at the focal plane as defined by the eyepiece field lens/field stop (40mm).

 

I see the Paracorr field lens as the field stop for the primary mirror in the catadioptric configuration. When you look at the illumination profile that much closer to the secondary mirror, the difference is significant. Especially when the Paracorr internal lens configuration has been optimized to reduce vignetting.

 

(Edit: I'm not sure that the OP is using a Paracorr, but whatever he's using, the experiment with a calibrated field chart should be "telling".)


Edited by Vic Menard, 24 February 2020 - 04:41 PM.


#16 pez_espada

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Posted 24 February 2020 - 05:06 PM

I think it's more than that--much more.

 

Bartels' calculator reveals field illumination at the focal plane as defined by the eyepiece field lens/field stop (40mm).

 

I see the Paracorr field lens as the field stop for the primary mirror in the catadioptric configuration. When you look at the illumination profile that much closer to the secondary mirror, the difference is significant. Especially when the Paracorr internal lens configuration has been optimized to reduce vignetting.

 

(Edit: I'm not sure that the OP is using a Paracorr, but whatever he's using, the experiment with a calibrated field chart should be "telling".)

The corrector I own is the Explore Scientific HR (https://explorescien...69f96ef1a&_ss=r), with a 1.06X mag (vs the 1.15x factor of the Paracorr-2).

 

I will use a calibrated star chart  to test the loss of illumination towards the edge of my particular reflector-coma corrector-eyepiece. I guess by calibrated field chart  you mean a chart with stars with known magnitudes, these are provided by the AAVSO and are the bread and butter for visual estimation.


Edited by pez_espada, 24 February 2020 - 05:07 PM.


#17 Jon Isaacs

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Posted 24 February 2020 - 05:17 PM

I see the Paracorr field lens as the field stop for the primary mirror in the catadioptric configuration. When you look at the illumination profile that much closer to the secondary mirror, the difference is significant.

 

 

You'd really have to measure it. But the Paracorr functions as a negative lens and my calculation basically looks at the size of the illuminated field 3 inches from  the focal plane.

 

The size of the field lens of the Paracorr is also an issue, the further from the focal plane it is, the larger it has to be. I measure it at 43.5 mm.

 

With a narrow illuminated field, that wouldn't matter.

 

And then there's this. When i first got my Paracorr 2, I noticed the 31 mm Nagler significantly vignetted at the very edge. I went back and forth between the type 1 and type 2 and only saw it in the type 2.

 

I called TeleVue and talked to Paul D. about it. He said yes, it was there.. these days I just accept it, it doesn't bother me.

 

Jon



#18 Asbytec

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Posted 24 February 2020 - 06:17 PM

"I guess after all these steps you still need to align the main mirror to the focuser axis (e.g. placing the dark dot into the donut) with the main mirror collimation screws?"

Yes. As Jon said above, your scope looks collimated, the rest is field illumination. I was just trying to help you keep the concetric signatures per your OP. The collimation looks "ugly" when the concentric circles are off, but axial alignment is achieved when the cross hair is on the center mark, and the center mark is centered in the Cheshire. I haven't read all the replies, yet, but I know your getting good advice.

"But if this asymmetrical offset is there I guess this is the direction described by the green arrow you pointed out on top of the original picture. Is this normal? I heard about "secondary offset..."

Well, secondary offset is "normal" and appears to be built into your secondary. The appearance of the rotated offset bulge (green arrow), however, is a result of a rotated secondary. The important thing is axial alignment.

The best single peice of advise I got while fighting with my secondary rotation and tilt, like you, while also trying to hold the three circles was to center the primary reflection on the secondary major axis (using rotation) prior to collimation. That's when collimation confusion finally went away and it became easy. All the advise you're getting above is appropriate to achieve axial alignment and nice star images. The rest is window dressing so it "looks" nice with better field illumination.

Except to say, I find axial alignment easier and more accurate when you approach the center mark from a single direction along the focuser axis (and secondary major axis) using one tilt screw instead of three random directions using all three secondary tilt screws. It starts with a well centered secondary under the focuser and primary reflection centered on the secondary major axis.

It greatly reduces unpleasant looking tilt and rotation "error", too, because the primary center mark is on or close to that same focuser axis of tilt we're trying to achieve. And the rotated offset bulge (green arrow) magically points toward the primary as a result because the secondary is rotated properly to begin with and stays that way during collimation. That was when collimation (including field illumination) clicked for me. So I share that same good advice hoping it becomes easier for you and others, too.

Edited by Asbytec, 24 February 2020 - 06:41 PM.

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#19 Vic Menard

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Posted 24 February 2020 - 06:18 PM

You'd really have to measure it. But the Paracorr functions as a negative lens and my calculation basically looks at the size of the illuminated field 3 inches from  the focal plane.

Exactly. And closer to the secondary mirror means the Paracorr is seeing more of the center of the cone (which is fully illuminated) vs the partially illuminated outer edge of the cone (which is effectively "stopped").

 

...And then there's this. When I first got my Paracorr 2, I noticed the 31 mm Nagler significantly vignetted at the very edge. I went back and forth between the type 1 and type 2 and only saw it in the type 2.

 

My early Paracorr 1 was terrible with my 31mm Nagler, lots of vignetting, especially when my eye was even slightly decentered, too close, or too far away relative to the exit pupil. But then the tunable top showed up with bigger lenses and the 31mm vignetting seemed to be corrected. Although I still have the 31mm Nagler, I rarely use it because of astigmatism in my eye. That said, I don't recall seeing the kind of vignetting I had with my first Paracorr. The OP is using an ES corrector, which I know nothing about. But I'm still interested in hearing what he finds with his variable star charts (a half magnitude light loss should be obvious).



#20 pez_espada

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Posted 25 February 2020 - 09:18 AM

"I guess after all these steps you still need to align the main mirror to the focuser axis (e.g. placing the dark dot into the donut) with the main mirror collimation screws?"

Yes. As Jon said above, your scope looks collimated, the rest is field illumination. I was just trying to help you keep the concetric signatures per your OP. The collimation looks "ugly" when the concentric circles are off, but axial alignment is achieved when the cross hair is on the center mark, and the center mark is centered in the Cheshire. I haven't read all the replies, yet, but I know your getting good advice.

"But if this asymmetrical offset is there I guess this is the direction described by the green arrow you pointed out on top of the original picture. Is this normal? I heard about "secondary offset..."

Well, secondary offset is "normal" and appears to be built into your secondary. The appearance of the rotated offset bulge (green arrow), however, is a result of a rotated secondary. The important thing is axial alignment.

The best single peice of advise I got while fighting with my secondary rotation and tilt, like you, while also trying to hold the three circles was to center the primary reflection on the secondary major axis (using rotation) prior to collimation. That's when collimation confusion finally went away and it became easy. All the advise you're getting above is appropriate to achieve axial alignment and nice star images. The rest is window dressing so it "looks" nice with better field illumination.

Except to say, I find axial alignment easier and more accurate when you approach the center mark from a single direction along the focuser axis (and secondary major axis) using one tilt screw instead of three random directions using all three secondary tilt screws. It starts with a well centered secondary under the focuser and primary reflection centered on the secondary major axis.

It greatly reduces unpleasant looking tilt and rotation "error", too, because the primary center mark is on or close to that same focuser axis of tilt we're trying to achieve. And the rotated offset bulge (green arrow) magically points toward the primary as a result because the secondary is rotated properly to begin with and stays that way during collimation. That was when collimation (including field illumination) clicked for me. So I share that same good advice hoping it becomes easier for you and others, too.

Hey Asbytec,

 

I was following your detailed directions as best as I could and I think I have ended up with a much better collimation for my small Newtonian this time (or I think so). I am aware that is not perfect but I do think the circles and reflections do looks much more concentric now. And the offset bulge is more or less pointing to the primary too. Below some pictures on how it looks now, both from the collimation cap and the sight tube/Cheshire with the hair-cross. Let me know what you think..

 

Many thanks for that truly enlightening piece of advice. I have been fighting this since two months I have the reflector and I will be re-reading your post as there are still some things I am not fully grasping and I need to digest. But I think my collimation is much better now after simply following the directions. I guess I will have now  at least a more homogeneous illuminated field of view. But I will check what Vic says about the coma corrector having an effect of it. And check Mel Bartels's ATM page for the calculations of secondary sizes and illumination  of the FOV.

 

I committed  myself not to moving to a larger reflector (probably a 10") before I could really learn collimation first, and other optimizations for Newtonians. Coming from refractors  I am used to refactor-like views, but wanting more aperture. That's why I have become obsessed in the last two moths  with as perfect collimation as I can get. And that is why that as soon as I ordered this small "benchmark testing" Newton I also ordered a coma corrector at once costing more than the scope itself (an ES HR2 which is not Paracorr but with nice reviews though).

 

I think your post should be pinned up for others to see and learn.

 

Again many thanks,

Attached Thumbnails

  • collimation_after_01.jpg
  • collimation_after_02.jpg

Edited by pez_espada, 25 February 2020 - 09:21 AM.

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#21 Jon Isaacs

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Posted 25 February 2020 - 11:28 AM

My early Paracorr 1 was terrible with my 31mm Nagler, lots of vignetting, especially when my eye was even slightly decentered, too close, or too far away relative to the exit pupil. But then the tunable top showed up with bigger lenses and the 31mm vignetting seemed to be corrected. Although I still have the 31mm Nagler, I rarely use it because of astigmatism in my eye. That said, I don't recall seeing the kind of vignetting I had with my first Paracorr.

 

 

The very first Paracorr did vignette significantly with the 31 mm, the model with the tuneable top did not, the type 2 does but only at the edge where it's quite apparent.

 

Jon



#22 pez_espada

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Posted 25 February 2020 - 11:38 AM

The very first Paracorr did vignette significantly with the 31 mm, the model with the tuneable top did not, the type 2 does but only at the edge where it's quite apparent.

 

Jon

since you guys are going off-topic, would you mind to discuss vignetting of the ES HRCC instead (as that's the one I mention as OP), if you have any experience with it?


Edited by pez_espada, 25 February 2020 - 12:23 PM.

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#23 Asbytec

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Posted 25 February 2020 - 01:48 PM

Pez, I'm on my phone and can't do much, but your collimation signatures look much improved. Well done. Of course axial collimation is most important. More so with a paracorr, but I'd defer to those who know better than me.

I fiddled with it, as do many of us I presume, for a few weekends, too, only becoming frustrated. Tilting this way, pulling the spider vane that way, etc. trying to hold the signatures. I could align the axes, but there was always that uncomfortable nagging feeling it didn't "look" right even though it may have been.

What I realized, to my way of thinking, is just because the secondary looked centered does not mean it's well "placed" during step one. I was not really finished with step one for secondary placement which includes refined secondary rotation with the primary reflection as a reference. I began tilt collimation, step two, long before I was done with step one, secondary placement. (I mean, duh! Right?) Finish step one before doing step two with better rotation so the primary reflection is on the secondary centerline. Once it was well rotated and all the important peices were in line, then collimate the axes. Axial collimation became so much easier and the collimation signatures fell into place. It looked right, felt good, and collimation became enjoyable.

At the Apex, you can always reiterate the process to get the signatures as close as you like. And do so as the cross hair lands on the primary center mark. It's a beautiful thing to see happen. Much better than fighting with it during collimation using the three screws independently of each other, as it is intuitive to do, by centering the secondary reflection before you begin the important axial alignment during step two.

I was a noob suffering the woes of collimation not long ago, too, and this simple rotation of the secondary to finish placement, prior to attempting tilt collimation in step two, made all the difference for me. Collimation began to make sense and became easy because the moving parts were better aligned with each other to start with. And those beautiful circles held. :)

Finish collimation step one, secondary placement (and rotation), before starting step two, axial alignment. How simple is that? I think many of us noobs miss that part and start down the rabbit hole too early. We wind up on CN posting pictures like yours and mine with the real experts explaining it to us. :)

Edited by Asbytec, 25 February 2020 - 02:08 PM.

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#24 pez_espada

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Posted 25 February 2020 - 02:12 PM

Finish collimation step one, secondary placement (and rotation), before starting step two, axial alignment. How simple is that?

I think I will remember that. A gem.

 

Thanks!


Edited by pez_espada, 25 February 2020 - 02:13 PM.


#25 Vic Menard

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Posted 25 February 2020 - 04:22 PM

You're pretty close. You still need to clean up the focuser axial alignment (blue cross hairs)--your allowable error tolerance (for high magnification) with a coma corrector is about 0.7mm, maybe 1.5mm at RFT magnification. Your secondary mirror placement is much better (green circle is optimal). You'll have a better idea when you clean up the focuser axis.  waytogo.gif

 

 

Attached Thumbnails

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