Topics

Field curvature with Flatteners and compressors


Roland Christen
 

Hello All,

The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".

Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.

Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.

Rolando


James Stone
 

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.

 

 

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors

 

Hello All,

 

The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".

 

Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.

 

Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.

 

Rolando

 


Roland Christen
 

In the case of most telescope optics (aside from Schmidt cameras), having a curved field would not help because of the sag/tan deviation which results in significant astigmatism (as shown in the image).

Rolando



-----Original Message-----
From: James Stone <jrs7r@...>
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Sent: Tue, Dec 29, 2020 1:48 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.
 
 
From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors
 
Hello All,
 
The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".
 
Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.
 
Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.
 
Rolando
 


James Stone
 

Great, thank you for the additional explanation.

 

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 3:01 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

 

In the case of most telescope optics (aside from Schmidt cameras), having a curved field would not help because of the sag/tan deviation which results in significant astigmatism (as shown in the image).

 

Rolando

 

 

 

-----Original Message-----
From: James Stone <jrs7r@...>
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Sent: Tue, Dec 29, 2020 1:48 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.

 

 

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors

 

Hello All,

 

The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".

 

Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.

 

Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.

 

Rolando

 


Dale Ghent
 


On Dec 29, 2020, at 2:48 PM, James Stone <jrs7r@...> wrote:

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.

A few weeks ago I was wondering the same thing when I saw a photo of the detector array used by the Kepler space telescope, which led me to an article from the ESO on the topic. 


Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 


/dale

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors

 

Hello All,

 

The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".

 

Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.

 

Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.

 

Rolando

 

<image001.png>


James Stone
 

Thank you Dale, appreciate the additional links.

 

From: <main@ap-ug.groups.io> on behalf of Dale Ghent <daleg@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 3:04 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

 

 

On Dec 29, 2020, at 2:48 PM, James Stone <jrs7r@...> wrote:



Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.

 

A few weeks ago I was wondering the same thing when I saw a photo of the detector array used by the Kepler space telescope, which led me to an article from the ESO on the topic. 

 

 

Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 

 

 

/dale

 

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors

 

Hello All,

 

The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".

 

Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.

 

Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.

 

Rolando

 

<image001.png>


Roland Christen
 


Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 
A rubber sensor?

If you just want to compensate for field curvature, the camera manufacturer can simply provide a curved cover glass in front of the sensor which will redirect the rays to focus over the entire sensor. It has to be the right shape for any given field curvature (which I'm sure the camera manufacturer will be more than glad to custom produce for any focal length scope at the right price?). BUT will it produce pinpoint Airy Disc size stars? No, because of the sagittal/tangential deviation that produces astigmatism. No way to fix that at the focus point.

The reason why professional astronomers want to use curved sensors is because they don't want any glass optics in the optical path because glass does not transmit into the UV and deep IR. They are willing to put up with astigmatism that comes with a professional RC scope.

Rolando


-----Original Message-----
From: Dale Ghent <daleg@...>
To: main@ap-ug.groups.io
Sent: Tue, Dec 29, 2020 2:04 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors


On Dec 29, 2020, at 2:48 PM, James Stone <jrs7r@...> wrote:

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.

A few weeks ago I was wondering the same thing when I saw a photo of the detector array used by the Kepler space telescope, which led me to an article from the ESO on the topic. 


Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 


/dale

From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors
 
Hello All,
 
The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".
 
Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.
 
Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.
 
Rolando
 
<image001.png>


ucmarkb2
 

If I may briefly interrupt, I have a question that I trust THIS group will provide a reliable answer: 
I have a 20" RC with ion-figured sital optics that need re-coating.  Who's the most reliable coater out there?  MANY thanks in advance for your consideration,  patience and suggestions.




On Tuesday, December 29, 2020 Roland Christen via groups.io <main@ap-ug.groups.io> wrote:

In the case of most telescope optics (aside from Schmidt cameras), having a curved field would not help because of the sag/tan deviation which results in significant astigmatism (as shown in the image).

Rolando



-----Original Message-----
From: James Stone <jrs7r@...>
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Sent: Tue, Dec 29, 2020 1:48 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.
 
 
From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors
 
Hello All,
 
The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".
 
Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.
 
Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.
 
Rolando
 


Roland Christen
 

I have used Ostahowski for my large optics.

Rolando

-----Original Message-----
From: ucmarkb2 via groups.io <pandrolmb@...>
To: main@ap-ug.groups.io
Sent: Tue, Dec 29, 2020 4:05 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

If I may briefly interrupt, I have a question that I trust THIS group will provide a reliable answer: 
I have a 20" RC with ion-figured sital optics that need re-coating.  Who's the most reliable coater out there?  MANY thanks in advance for your consideration,  patience and suggestions.

On Tuesday, December 29, 2020 Roland Christen via groups.io <main@ap-ug.groups.io> wrote:
In the case of most telescope optics (aside from Schmidt cameras), having a curved field would not help because of the sag/tan deviation which results in significant astigmatism (as shown in the image).

Rolando



-----Original Message-----
From: James Stone <jrs7r@...>
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Sent: Tue, Dec 29, 2020 1:48 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.
 
 
From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors
 
Hello All,
 
The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".
 
Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.
 
Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.
 
Rolando
 


ROBERT WYNNE
 

I have to say this entire thread has been one of the most cogent and informative discussions since I joined the group. I look forward to  information that specifies optimal spacing and field flatteners for various A-P scopes. What I have yet to determine is what camera or pixel density is the ideal mate for each A-P scope. Is this coupling directly related to the size of the airy disc which likely varies inside and among A-P product lines?  Do professional astronomers have a complete set of spacers in .0001" increments to compliment whatever size ccd may be coupled to a scope?
 
JS- Re: CAT scanners & curved sensors. I always thought CAT scan X-ray detector data acquisition and subsequent attenuation of that data is performed through algos but only first after precise calibration with a water phantom? That is an X-ray is not focused but highly collimated and then manipulated into optimal focus by math. But my knowledge is at best dated and perhaps we now have curved self adjusting X ray sensor array technology?  -Best, Robert

On 12/29/2020 12:38 PM Roland Christen via groups.io <chris1011@...> wrote:
 
 
 
Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 
A rubber sensor?
 
If you just want to compensate for field curvature, the camera manufacturer can simply provide a curved cover glass in front of the sensor which will redirect the rays to focus over the entire sensor. It has to be the right shape for any given field curvature (which I'm sure the camera manufacturer will be more than glad to custom produce for any focal length scope at the right price?). BUT will it produce pinpoint Airy Disc size stars? No, because of the sagittal/tangential deviation that produces astigmatism. No way to fix that at the focus point.
 
The reason why professional astronomers want to use curved sensors is because they don't want any glass optics in the optical path because glass does not transmit into the UV and deep IR. They are willing to put up with astigmatism that comes with a professional RC scope.
 
Rolando
 
 
-----Original Message-----
From: Dale Ghent <daleg@...>
To: main@ap-ug.groups.io
Sent: Tue, Dec 29, 2020 2:04 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

 
On Dec 29, 2020, at 2:48 PM, James Stone <jrs7r@...> wrote:

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.
A few weeks ago I was wondering the same thing when I saw a photo of the detector array used by the Kepler space telescope, which led me to an article from the ESO on the topic. 
 
 
Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 
 
 
/dale
 
From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors
 
Hello All,
 
The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".
 
Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.
 
Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.
 
Rolando
 
 
<image001.png>
 
 


Edward Beshore
 

In the early 2010’s, the Air Force and MIT collaborated on the development of a 3.5 m Mersenne-Schmidt that had CCDs arranged in a curved focal plane.

https://en.wikipedia.org/wiki/Space_Surveillance_Telescope

used chiefly for tracking space debris and satellites, it would also be a potent hunter of near-earth asteroids. It was moved to Australia shortly after commissioning in NM, but has not been recommission according to the article. 


Ed Beshore


James Stone
 

Interesting link Ed. An f1.0 design that scans the entire night sky to a 20.5 magnitude in 6 nights. That’s impressive. I wonder how they dealt with the sagittal/tangential deviation Roland referred to in his previous communications or if that was even necessary given the purpose of the scope in tracking space debris. Also interesting to note that it has no built in tracking so it relies on exposure times of 2-3 seconds at a time. I wonder what their ultimate plan is with the system. Maybe this will show up on GovDeals one day....


On Dec 30, 2020, at 10:28 AM, Edward Beshore via groups.io <ebeshore@...> wrote:



In the early 2010’s, the Air Force and MIT collaborated on the development of a 3.5 m Mersenne-Schmidt that had CCDs arranged in a curved focal plane.

https://en.wikipedia.org/wiki/Space_Surveillance_Telescope

used chiefly for tracking space debris and satellites, it would also be a potent hunter of near-earth asteroids. It was moved to Australia shortly after commissioning in NM, but has not been recommission according to the article. 


Ed Beshore


Roland Christen
 


What I have yet to determine is what camera or pixel density is the ideal mate for each A-P scope.
For deep sky imaging rule of thumb is to match the pixel size to the F-Ratio of your scope. For an F6 scope use a 6 micron pixel. For an F9 scope use a 9 micron pixel. You can use a smaller pixel but the larger pixels will have higher speed and greater well depth. All that goes out the window with Cmos.

For planet imaging use a pixel size 1/3 of the F-Ratio in order to exceed the Nyquist criteria. Advance planetary imagers go even further than that.

Rolando


-----Original Message-----
From: ROBERT WYNNE <robert-wynne@...>
To: main@ap-ug.groups.io; Roland Christen via groups.io <chris1011@...>
Sent: Tue, Dec 29, 2020 7:18 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

I have to say this entire thread has been one of the most cogent and informative discussions since I joined the group. I look forward to  information that specifies optimal spacing and field flatteners for various A-P scopes. What I have yet to determine is what camera or pixel density is the ideal mate for each A-P scope. Is this coupling directly related to the size of the airy disc which likely varies inside and among A-P product lines?  Do professional astronomers have a complete set of spacers in .0001" increments to compliment whatever size ccd may be coupled to a scope?
 
JS- Re: CAT scanners & curved sensors. I always thought CAT scan X-ray detector data acquisition and subsequent attenuation of that data is performed through algos but only first after precise calibration with a water phantom? That is an X-ray is not focused but highly collimated and then manipulated into optimal focus by math. But my knowledge is at best dated and perhaps we now have curved self adjusting X ray sensor array technology?  -Best, Robert
On 12/29/2020 12:38 PM Roland Christen via groups.io <chris1011@...> wrote:
 
 
 
Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 
A rubber sensor?
 
If you just want to compensate for field curvature, the camera manufacturer can simply provide a curved cover glass in front of the sensor which will redirect the rays to focus over the entire sensor. It has to be the right shape for any given field curvature (which I'm sure the camera manufacturer will be more than glad to custom produce for any focal length scope at the right price?). BUT will it produce pinpoint Airy Disc size stars? No, because of the sagittal/tangential deviation that produces astigmatism. No way to fix that at the focus point.
 
The reason why professional astronomers want to use curved sensors is because they don't want any glass optics in the optical path because glass does not transmit into the UV and deep IR. They are willing to put up with astigmatism that comes with a professional RC scope.
 
Rolando
 
 
-----Original Message-----
From: Dale Ghent <daleg@...>
To: main@ap-ug.groups.io
Sent: Tue, Dec 29, 2020 2:04 pm
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

 
On Dec 29, 2020, at 2:48 PM, James Stone <jrs7r@...> wrote:

Roland, thanks for this post. This is very helpful and a nice practical example. This may seem like an outlandish question, but are you aware if anyone has ever experimented with imaging detectors with individual elements placed along a curved arrays to manage issues with field curvature or has this always been addressed with an optical solution? In the radiological sciences, many of our tomography based machines consist of individual sensing elements positioned along curved arrays, but this is obviously a different application.
A few weeks ago I was wondering the same thing when I saw a photo of the detector array used by the Kepler space telescope, which led me to an article from the ESO on the topic. 
 
 
Needless to say, curved sensors are specific to the optics they sit in back of, but I’m sure with advances in materials sciences we might one day see sensors of relevant quality that can be adjusted to any curved field, within reason. 
 
 
/dale
 
From: <main@ap-ug.groups.io> on behalf of "Roland Christen via groups.io" <chris1011@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Tuesday, December 29, 2020 at 2:31 PM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>, "main@ap-gto.groups.io" <main@ap-gto.groups.io>
Subject: [ap-ug] Field curvature with Flatteners and compressors
 
Hello All,
 
The question is "what is the shape of the field curvature with and without flatteners and compressors, and how does it change with spacing".
 
Without any flatteners/compressors the typical field curvature of a refractor is approximately 38% of the focal length, inward curving and with astigmatism (oval stars shape pointing toward the center). Taking the example of our 140EDL refractors, it has a focal length of 1048mm and an inherent field curvature of 399mm. If you tried to take an image of a 2 degree field covering a 38mm circle, you would get the result shown in the picture below. For reference, the Airy Disc shown is about 10 microns diameter at 1048mm focal length.
 
Off-axis, the oval shape results from the difference in field curvature between the sagittal and tangential rays that come from the various parts of the lens. So the job of the flattener/compressor is to not only flatten the curvature, but also bring the sagittal and tangential rays together without introducing other defects such as coma, spherical and chromatic aberrations.
 
Rolando
 
 
<image001.png>
 
 


Edward Beshore
 

Hi James

The three-mirror design allows control of all of the principal aberrations, so those are controlled.

In the early 2000s, I ran the Catalina Sky Survey (looking for Near-Earth Objects), and we had inherited an old 0.7m f/1.8 Schmidt Camera (0.9 m primary). It used an aspheric corrector plate, but required a mandrel to stress photographic plate into a curved focal plane. We wanted to put in a 4096x4096 CCD, so we commissioned a over-the-CCD corrector that did a decent, but not perfect, job of allowing a flat focal plane over a relatively small area. Turned into quite the discovery machine for several years. We could hit 19th magnitude in 30 seconds.

Ed


James Stone
 

Fascinating Ed. Thanks for the additional information and perspective.

 

From: <main@ap-ug.groups.io> on behalf of "Edward Beshore via groups.io" <ebeshore@...>
Reply-To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Date: Wednesday, December 30, 2020 at 11:10 AM
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Subject: Re: [ap-ug] Field curvature with Flatteners and compressors

 

Hi James

The three-mirror design allows control of all of the principal aberrations, so those are controlled.

In the early 2000s, I ran the Catalina Sky Survey (looking for Near-Earth Objects), and we had inherited an old 0.7m f/1.8 Schmidt Camera (0.9 m primary). It used an aspheric corrector plate, but required a mandrel to stress photographic plate into a curved focal plane. We wanted to put in a 4096x4096 CCD, so we commissioned a over-the-CCD corrector that did a decent, but not perfect, job of allowing a flat focal plane over a relatively small area. Turned into quite the discovery machine for several years. We could hit 19th magnitude in 30 seconds.

Ed