Cone Error or Sensor Tilt or Sensor Shift or Collimation or ... what?


wbelhaven
 

Hi,

Wondering if you folks could help me solve a mystery. Upon polar aligning to what SharpCap Pro thinks is within single digit arc-seconds of the NCP, I can never get the NCP (per a plate solve) "more centered" in my camera than what's shown in the first image attached.

When I slew in the DEC axis (up button or down button on AP 1200 GT hand controller or ASCOM equivalent), the NCP moves up or down in that image, and, because the polar alignment is so good, it doesn't move at all when I slew in the RA axis (left or right button, etc.). See the image for the 'compass' of how the camera is aligned. (Right = Zenith, Left = Northern Horizon). In other words, the camera seems to be pointing above the NCP when the DEC axis is slewed to "the closest thing to +90 I can get".

My immediate thought when seeing this was "cone error".

But then I started looking at the Sky Flats and the actual Light frames I've taken, the former of which (second attached image) seems to show a slight off-centered-ness of the illumination pattern, and a deep inspection of the corners of the image of Light frames seems to show slightly better correction on one side of the chip than the other (essentially identically matching what you'd guess by looking at the Sky Flats) with the center point of best correction, as confirmed by CCD Inspector, about where the peak intensity of that Sky Flat is ... although, I have to admit, the effect is pretty subtle and it would take a pretty OCD person like me to notice.

When I correlate the first data point ("pointing above the NCP") with the second and third data points (flats and lights that aren't 'homed' to the center of the chip) I start to think more like (1) a chip that's slightly tilted, (2) a chip that's slightly off center, or (3) lens cell not perfectly aligned with tube and focuser, i.e., collimation.

What's your best guess at the culprit here? I suppose it could be some combination or all of the above.

I'm inclined not to mess with anything because it's pretty darn close to being inconsequential to the resulting images. But would like to know what's going on anyway, being a curious sort of fellow.

In case it matters, here's the imaging train: AP160EDF > AP 4" Focuser Extension (two thumbscrews pointed 180 degrees from the two thumbscrews on the focuser draw tube) > AP 160FF > 2 x 1mm AP extension rings > Precise Parts 160FF thread to M68 thread adapter > ZWO M68 OAG > ZWO 2" EFW > ZWO ASI6200MM-Pro Camera.

The only things that aren't threaded or screwed together are the two sides of the 4" focuser extension (I can't reach focus without it).

Stats, in case this matters:
Eff Focal Length = 1265mm (per plate solve)
Chip = 36mm x 24mm, 3.76 um pixels, 0.61 "/px

Thanks,
WB


ROBERT WYNNE
 

I would try this same test with identical set up on another subject less prone to light train error and see if it repeats. If not, then the finger points to the CCD. Better than I would know of a better solution or test to challenge the entire system. -Best, Robert

On 05/21/2021 3:28 PM wbelhaven via groups.io <wbelhaven@...> wrote:
 
 
Hi,

Wondering if you folks could help me solve a mystery. Upon polar aligning to what SharpCap Pro thinks is within single digit arc-seconds of the NCP, I can never get the NCP (per a plate solve) "more centered" in my camera than what's shown in the first image attached.

When I slew in the DEC axis (up button or down button on AP 1200 GT hand controller or ASCOM equivalent), the NCP moves up or down in that image, and, because the polar alignment is so good, it doesn't move at all when I slew in the RA axis (left or right button, etc.). See the image for the 'compass' of how the camera is aligned. (Right = Zenith, Left = Northern Horizon). In other words, the camera seems to be pointing above the NCP when the DEC axis is slewed to "the closest thing to +90 I can get".

My immediate thought when seeing this was "cone error".

But then I started looking at the Sky Flats and the actual Light frames I've taken, the former of which (second attached image) seems to show a slight off-centered-ness of the illumination pattern, and a deep inspection of the corners of the image of Light frames seems to show slightly better correction on one side of the chip than the other (essentially identically matching what you'd guess by looking at the Sky Flats) with the center point of best correction, as confirmed by CCD Inspector, about where the peak intensity of that Sky Flat is ... although, I have to admit, the effect is pretty subtle and it would take a pretty OCD person like me to notice.

When I correlate the first data point ("pointing above the NCP") with the second and third data points (flats and lights that aren't 'homed' to the center of the chip) I start to think more like (1) a chip that's slightly tilted, (2) a chip that's slightly off center, or (3) lens cell not perfectly aligned with tube and focuser, i.e., collimation.

What's your best guess at the culprit here? I suppose it could be some combination or all of the above.

I'm inclined not to mess with anything because it's pretty darn close to being inconsequential to the resulting images. But would like to know what's going on anyway, being a curious sort of fellow.

In case it matters, here's the imaging train: AP160EDF > AP 4" Focuser Extension (two thumbscrews pointed 180 degrees from the two thumbscrews on the focuser draw tube) > AP 160FF > 2 x 1mm AP extension rings > Precise Parts 160FF thread to M68 thread adapter > ZWO M68 OAG > ZWO 2" EFW > ZWO ASI6200MM-Pro Camera.

The only things that aren't threaded or screwed together are the two sides of the 4" focuser extension (I can't reach focus without it).

Stats, in case this matters:
Eff Focal Length = 1265mm (per plate solve)
Chip = 36mm x 24mm, 3.76 um pixels, 0.61 "/px

Thanks,
WB


wbelhaven
 

Could you please clarify that Robert? Not sure I'm following your suggestion. FYI, so far with the new toys I've imaged M51, M101, M13, and the Leo Triplet, which runs a decent range of DECs, and all of which show slightly better correction on one side of the chip than the other.


Roland Christen
 

That sure is confusing, isn't it. A couple of points of information:

1) You can tilt the lens all you want, it will not affect the flat shadows.
2) To see if the focuser is tilted, you can use a collimation laser and shine the light up the focuser toward the front lens and look to see if the laser exits the front lens in the exact middle of the lens. However, in order for the focuser to affect the flat image even a small amount, it would need to be tilted a HUGE amount, and you would never get stars to focus across the field.

So, the above two items are red herrings.

3) when you slew the RA axis left or right anywhere near the pole, the image won't move, rather it will slowly rotate on the chip.
4) a non-orthogonal telescope will never be able to point directly to the pole if the mount is properly polar aligned. You would need to shim the scope rings to get it to point to the pole. However, that would do nothing for how your Sky Flat is centered. The two items are unrelated.

Rolando

-----Original Message-----
From: wbelhaven via groups.io <wbelhaven@...>
To: main@ap-ug.groups.io
Sent: Fri, May 21, 2021 5:28 pm
Subject: [ap-ug] Cone Error or Sensor Tilt or Sensor Shift or Collimation or ... what?

Hi,

Wondering if you folks could help me solve a mystery. Upon polar aligning to what SharpCap Pro thinks is within single digit arc-seconds of the NCP, I can never get the NCP (per a plate solve) "more centered" in my camera than what's shown in the first image attached.

When I slew in the DEC axis (up button or down button on AP 1200 GT hand controller or ASCOM equivalent), the NCP moves up or down in that image, and, because the polar alignment is so good, it doesn't move at all when I slew in the RA axis (left or right button, etc.). See the image for the 'compass' of how the camera is aligned. (Right = Zenith, Left = Northern Horizon). In other words, the camera seems to be pointing above the NCP when the DEC axis is slewed to "the closest thing to +90 I can get".

My immediate thought when seeing this was "cone error".

But then I started looking at the Sky Flats and the actual Light frames I've taken, the former of which (second attached image) seems to show a slight off-centered-ness of the illumination pattern, and a deep inspection of the corners of the image of Light frames seems to show slightly better correction on one side of the chip than the other (essentially identically matching what you'd guess by looking at the Sky Flats) with the center point of best correction, as confirmed by CCD Inspector, about where the peak intensity of that Sky Flat is ... although, I have to admit, the effect is pretty subtle and it would take a pretty OCD person like me to notice.

When I correlate the first data point ("pointing above the NCP") with the second and third data points (flats and lights that aren't 'homed' to the center of the chip) I start to think more like (1) a chip that's slightly tilted, (2) a chip that's slightly off center, or (3) lens cell not perfectly aligned with tube and focuser, i.e., collimation.

What's your best guess at the culprit here? I suppose it could be some combination or all of the above.

I'm inclined not to mess with anything because it's pretty darn close to being inconsequential to the resulting images. But would like to know what's going on anyway, being a curious sort of fellow.

In case it matters, here's the imaging train: AP160EDF > AP 4" Focuser Extension (two thumbscrews pointed 180 degrees from the two thumbscrews on the focuser draw tube) > AP 160FF > 2 x 1mm AP extension rings > Precise Parts 160FF thread to M68 thread adapter > ZWO M68 OAG > ZWO 2" EFW > ZWO ASI6200MM-Pro Camera.

The only things that aren't threaded or screwed together are the two sides of the 4" focuser extension (I can't reach focus without it).

Stats, in case this matters:
Eff Focal Length = 1265mm (per plate solve)
Chip = 36mm x 24mm, 3.76 um pixels, 0.61 "/px

Thanks,
WB


Roland Christen
 

Based on this, you probably have sensor tilt.
Cone error has zero to do with better correction on one side versus the other.

Lens tilt will affect all stars the same way - all will have slight comatic shape from one corner to the other across the entire chip. Lens tilt will not cause flats to be de-centered.

Focuser tilt can cause slightly better correction from one side to the other, same as chip tilt or interconnection part tilt.

Rolando

-----Original Message-----
From: wbelhaven via groups.io <wbelhaven@...>
To: main@ap-ug.groups.io
Sent: Fri, May 21, 2021 6:18 pm
Subject: Re: [ap-ug] Cone Error or Sensor Tilt or Sensor Shift or Collimation or ... what?

Could you please clarify that Robert? Not sure I'm following your suggestion. FYI, so far with the new toys I've imaged M51, M101, M13, and the Leo Triplet, which runs a decent range of DECs, and all of which show slightly better correction on one side of the chip than the other.


Jeffc
 

> all of which show slightly better correction on one side of the chip than the other

I would think if you rotate the camera 180 degrees (and not rotate the focuser if possible) and the "better correction" moves to the other side then it is "sensor tilt".
If the "better correction" stays in the same place, then it is something else like focuser tilt.



Roland Christen
 

If the better correction stays put, it is sensor tilt.
If it moves to the other side it is focuser tilt.

Rolando

-----Original Message-----
From: Jeffc <jeffcrilly@...>
To: main@ap-ug.groups.io
Sent: Fri, May 21, 2021 7:16 pm
Subject: Re: [ap-ug] Cone Error or Sensor Tilt or Sensor Shift or Collimation or ... what?

> all of which show slightly better correction on one side of the chip than the other

I would think if you rotate the camera 180 degrees (and not rotate the focuser if possible) and the "better correction" moves to the other side then it is "sensor tilt".
If the "better correction" stays in the same place, then it is something else like focuser tilt.



ROBERT WYNNE
 

Roland has said what I attempted to say. -Best, Robert

On 05/21/2021 4:39 PM Roland Christen via groups.io <chris1011@...> wrote:
 
 
Based o n this, you probably have sensor tilt.
Cone error has zero to do with better correction on one side versus the other.
 
Lens tilt will affect all stars the same way - all will have slight comatic shape from one corner to the other across the entire chip. Lens tilt will not cause flats to be de-centered.
 
Focuser tilt can cause slightly better correction from one side to the other, same as chip tilt or interconnection part tilt.
 
Rolando
 
-----Original Message-----
From: wbelhaven via groups.io <wbelhaven@...>
To: main@ap-ug.groups.io
Sent: Fri, May 21, 2021 6:18 pm
Subject: Re: [ap-ug] Cone Error or Sensor Tilt or Sensor Shift or Collimation or ... what?

Could you please clarify that Robert? Not sure I'm following your suggestion. FYI, so far with the new toys I've imaged M51, M101, M13, and the Leo Triplet, which runs a decent range of DECs, and all of which show slightly better correction on one side of the chip than the other.


wbelhaven
 

Thanks everyone, really appreciate the analysis. To summarize, the current hypothesis is some sensor tilt (possibly focuser tilt too) and likely also some cone error. Adjusting the sensor tilt and focuser are difficult (inconvenient might be a better word) and out of my league, respectively. The cone error is pretty easy to fix and might give slightly better pointing and tracking (and thus guiding) but is probably not worth messing with either. I may try the "rotate the camera" test to see what happens, as that's really easy to do.

If anyone's curious to see the full res, uncropped, images I've captured so far with this setup, here's a link: https://drive.google.com/drive/folders/1U4sjCDcXa0xUFLCvdm9b9-1gV9zu5Xea?usp=sharing

The seeing was really good on the nights I captured M101, M13, and the Leo Triplet ... not so much on M51.


John Jennings
 

Based on the stars in a couple of your pics, it looks like you have a bunch of astigmatism due to the current flattener spacing. For me its always been difficult to correct tilt with that much error in spacing. You end up chasing the tilt. I would get the spacing closer then optimize the tilt afterward. Just my 2c.


John Jennings
 

Not all pics exhibit the flattener spacing issue. Are all the pics with the same spacing, or did you disassemble and change the spacing? Tilt can change if you reorient the imaging train. I could never get my AP155 + flattener tilt adjusted until I added the gerd nueman Tilt Adjuster CTU.


wbelhaven
 

Thanks John. Good eye. I added a 1mm spacer at some point, but now I can't remember exactly when. Pretty sad, I admit. Here's what I know and what I'm not sure of, in the order of images captured:

  • M51: definitely before I added the spacer, but the seeing was lousy those nights so it might not reveal anything definitive
  • M101: likely changed horses mid stream -- some nights without, some nights with the 1mm spacer
  • M13: likely after adding the 1mm spacer
  • Leo Triplet: definitely after adding the 1mm spacer
  • M44: please ignore as it was taken with a different scope which was very much not properly spaced
I also added a "Brady Bunch" folder (smile) with collages that make it easy to simultaneously view the corners, the sides, and the center of each image, all on a single screen at full resolution.

To my eye, the Leo Triplet image is the best resolved and also an improvement over the others in the corners. I'm going to try another 1mm spacer as that's really easy to add. If it overshoots, I'll just remove it. I suspect there's tilt here, as the corners of this image don't look symmetric to me (the starts do not all flare radial or all azimuthal, respectively, in the corners).


John Jennings
 
Edited

I completely understand the capture issue. I've got tons of unlabeled collimation images floating around.

The imaging train I'm setting up right now consists of an AP155EDF, AP 67PF582 flattener + AP1260 bayonet adapter + Gerd Neumann CTU + Optec 2.7 to M54 adapter + QHYCCD CFW3L filter wheel + QHY410C camera: In that order.

I've been going ring around the rosy with my 155EDFS + flattener.  I purchased it and the flattener used 20 years ago and kept it in storage until recently.  I chased some large collimation errors with the 155 for some time until I finally placed my Takahashi collimation scope at the end of the focuser extension tube and quickly found the objective was very much un-collimated with the focuser. What a great device.  The Tak collimator is extremely sensitive and easy to use for my Mewlon 300 or refractors.

I will admit I recently placed my AP130GT 2.7" focuser on the AP155 with an adapter I recently purchased from AP. I upgraded my 130 GT to a  3.5 focuser with QUAD TCC. I'm upgrading the 155 when the QUAD TCC for it becomes available. With the Tak collimator, it took only 5 minutes to collimate the main objective with the push pull screws on the cell.

I then replaced the rest of the imaging train minus the camera, flattener and one filter in the filter wheel. I placed the collimator in place where the camera is normally attached. I then adjusted the tilt with a Gerd Neumann CTU to correct for the tilt in the adapters/filter wheel/camera interface, not touching the main objective collimation. You cannot collimate using the TAK collimator with any other optical surface in the imaging train except the objective. That's why I had to remove the flattener and filter.  I then replaced the Flattener and checked the tilt via a quick check through the clouds last week. It seems a lot better, but had some residual tilt. I measured the flattener's front and back surfaces for wedge and found a little wedge there. I then took apart the flattener and carefully re-seated the flattener cell / lens and retightened the Allen lock screws. If you look at the 67PF582 flattener, you realize all the weight of the imaging train is supported via the flattener's cell, and the cell housings can't be screwed down to the lens tightly or it will pinch the optics. It relies on the small allen set screws to keep everything locked in place.   There's no telling how my flattener had been treated  before I purchased it used.  I put it all back together and had another quick session through the clouds. CCD Inspector and visual inspection shows the tilt reduced and I think I'm finally close. I've yet to finish the flattener tweaks and tilt adjustments.

There are many places for something to be just slightly off and that adds to the cumulative error. I know for a fact the thread on Optec 2.7" to M54 adapter connected to the CFW3 doesn't seat perfectly square.  I had been shimming the camera only before I went back to basics and started re-collimation from scratch and ordered the Gerd Neumann CTU. I tried placing the CTU in front of the imaging train and flattener so I didn't have to deal with back focus issues, but with so many errors in different places in the imaging train, I simply couldn't come to a place where I was happy with the tilt. So I placed the CTU before the CFW3 and went about trying to correct the issues one at a time. I'm only relaying my experiences to illustrate how hard collimation can be at times with filter wheels and other junk in the imaging train.  

What a learning lesson. Don't give up. I haven't so far. I'm just trying to get completed before the Galaxy season is over.


Moshen Chan
 

I've been using a method to square the camera sensor and imaging train that can be done indoors and might be useful for some in this thread. I'm not sure it works for all cameras but it works for my ZWO CMOS cameras. When you shine a laser into the sensor, you get back a reflected grid of dots (probably from the reflection pattern of the CMOS micro lens array). The center of this dot grid is the center of the sensor. You can place the camera and imaging train (OAG, filter wheel etc) on a flat piece of glass. When you shine the laser up into the sensor again you get reflected back the center grid dot from the sensor as well as the laser reflection from the glass. Then you can easily see if the sensor is parallel to the glass if both reflections don't match and meet at the laser aperture. Then you can adjust the tilt plate that's part of the imaging train appropriately.

There's more info from these Cloudynights posts:

In my case my ZWO 2600MC & 6200MM sensors were tilt free, but with the 6200MM once I added the ZWO 2" filter wheel I needed to shim roughly 0.15-0.2mm. This method let me make sure the camera to imaging train up to the flattener was tilt-free and much easier to do indoors than on the sky.

Moshen


On Sun, May 23, 2021 at 4:09 PM <johnrogerjennings@...> wrote:

[Edited Message Follows]

I completely understand the capture issue. I've got tons of unlabeled collimation images floating around.

The imaging train I'm setting up right now consists of an AP155EDF, AP 67PF582 flattener + AP1260 bayonet adapter + Gerd Neumann CTU + Optec 2.7 to M54 adapter + QHYCCD CFW3L filter wheel + QHY410C camera: In that order.

I've been going ring around the rosy with my 155EDFS + flattener.  I purchased it and the flattener used 20 years ago and kept it in storage until recently.  I chased some large collimation errors with the 155 for some time until I finally placed my Takahashi collimation scope at the end of the focuser extension tube and quickly found the objective was very much un-collimated with the focuser. What a great device.  The Tak collimator is extremely sensitive and easy to use for my Mewlon 300 or refractors.

I will admit I recently placed my AP130GT 2.7" focuser on the AP155 with an adapter I recently purchased from AP. I upgraded my 130 GT to a  3.5 focuser with QUAD TCC. I'm upgrading the 155 when the QUAD TCC for it becomes available. With the Tak collimator, it took only 5 minutes to collimate the main objective with the push pull screws on the cell.

I then replaced the rest of the imaging train minus the camera, flattener and one filter in the filter wheel. I placed the collimator in place where the camera is normally attached. I then adjusted the tilt with a Gerd Neumann CTU to correct for the tilt in the adapters/filter wheel/camera interface, not touching the main objective collimation. You cannot collimate using the TAK collimator with any other optical surface in the imaging train except the objective. That's why I had to remove the flattener and filter.  I then replaced the Flattener and checked the tilt via a quick check through the clouds last week. It seems a lot better, but had some residual tilt. I measured the flattener's front and back surfaces for wedge and found a little wedge there. I then took apart the flattener and carefully re-seated the flattener cell / lens and retightened the Allen lock screws. If you look at the 67PF582 flattener, you realize all the weight of the imaging train is supported via the flattener's cell, and the cell housings can't be screwed down to the lens tightly or it will pinch the optics. It relies on the small allen set screws to keep everything locked in place.   There's no telling how my flattener had been treated  before I purchased it used.  I put it all back together and had another quick session through the clouds. CCD Inspector and visual inspection shows the tilt reduced and I think I'm finally close. I've yet to finish the flattener tweaks and tilt adjustments.

There are many places for something to be just slightly off and that adds to the cumulative error. I know for a fact the thread on Optec 2.7" to M54 adapter connected to the CFW3 doesn't seat perfectly square.  I had been shimming the camera only before I went back to basics and started re-collimation from scratch and ordered the Gerd Neumann CTU. I tried placing the CTU in front of the imaging train and flattener so I didn't have to deal with back focus issues, but with so many errors in different places in the imaging train, I simply couldn't come to a place where I was happy with the tilt. So I placed the CTU before the CFW3 and went about trying to correct the issues one at a time. I'm only relaying my experiences to illustrate how hard collimation can be at times with filter wheels and other junk in the imaging train.  

What a learning lesson. Don't give up. I haven't so far. I'm just trying to get completed before the Galaxy season is over.


Chris Staargaard
 

Thanks for this tip Moshen. A daytime procedure for adjustments like these is always welcome here in Vancouver where clear nights and opportunities for imaging are at a high premium.

Regards,
Chris