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Image Tilt, Testing After Purchase?

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#1 Farzad_K

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Posted 27 September 2021 - 09:54 AM

Hell Folks.

 

It is apparent, given software developed to calculate its magnitude, that image tilt is a common phenomenon at least across the board of less expensive gear.

 

Image tilt can be caused by the optical tube, camera sensor misalignment, connection of the camera and OTA or all of the above. There are ways to determine if the camera is at fault or if it is the OTA. But in general, it seems something that we should do and we don't is to test the equipment that we purchase for fidelity if you will. This is especially problematic for online purchases.

 

Do you test your new equipment? And when you determine they are not straight, have you had success in returning and replacing the item? Do you think equipment should come with some sort of certification or "straightness" performance certification such as maybe how much tilt there is and what would be acceptable?

 

It is terrible that in some cases, depending on the field of view, some of the image needs to get cropped to get rid of structures that are elongated because of equipment issue. But again, and as I have read here and there, you don't have to pay that much attention to detail.

 

For testing of the equipment, are there methods that don't require imaging outdoors? Hard to believe so because the OTA cannot be brought into focus indoors although you can use a reflex lens to isolate the issue regarding the sensor tilt.

 

 

 

Happy Monday.

 

Farzad

 

UPDATE: found this discussion on CN.


Edited by Farzad_K, 27 September 2021 - 10:46 AM.


#2 matt_astro_tx

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Posted 27 September 2021 - 11:48 AM

Can't think of a practical way to check for tilt indoors.  My first test after I got my rig up and running was a star field test.  I found a nice rich field in Cygnus and took some exposures with which to check that my field was flat and to check for tilt.  I thought I had tilt but it ended up being a backspacing issue, which was easily corrected with a couple shims.

 

I don't think a manufacturer's certification for "straightness" is practical.  It would drive the cost of components up to add additional testing and tighter tolerances than they already provide.  And given the wealth of third party components we mate together it would be a difficult task to tell which component of an imaging train is the culprit for tilt.

 

I think most people have success with camera tilt adapters to fix the issue.



#3 Farzad_K

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Posted 27 September 2021 - 02:56 PM

Can't think of a practical way to check for tilt indoors.  My first test after I got my rig up and running was a star field test.  I found a nice rich field in Cygnus and took some exposures with which to check that my field was flat and to check for tilt.  I thought I had tilt but it ended up being a backspacing issue, which was easily corrected with a couple shims.

 

I don't think a manufacturer's certification for "straightness" is practical.  It would drive the cost of components up to add additional testing and tighter tolerances than they already provide.  And given the wealth of third party components we mate together it would be a difficult task to tell which component of an imaging train is the culprit for tilt.

 

I think most people have success with camera tilt adapters to fix the issue.

I agree, it is one of those things that cannot be manufactured to perfection. Sensors, especially, can be tilted by just a very tiny amount and it can cause image tilt.

 

What adaptors are out there that can help mitigate image tilt that is known to be caused by either the sensor or the OTA (not rings, spacers, etc.).



#4 AhBok

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Posted 27 September 2021 - 04:39 PM

Tilt is definitely a problem that exists with some bargain gear. I purchased one of the many inexpensive 8” F4 imaging newts expecting it to be a “project” scope and it has been. The good news is once I masked the primary mirror edge and replaced the brass compression ring on the focuser with nylon screws, I was able to get my stars round and the tilt under control.

My point that no amount of returning these scopes would fix components designed with loose tolerances. You can spend more for gear with higher tolerances or find workarounds, as I have, to fix these problems. Also, my OTA has pretty good optics, so for its low price, I was happy to accept the hassle of “fixing” the shortcomings. For those not wanting to do this, the answer is to buy more expensive gear.

#5 Poynting

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Posted 27 September 2021 - 07:49 PM

Can't think of a practical way to check for tilt indoors.  My first test after I got my rig up and running was a star field test.  I found a nice rich field in Cygnus and took some exposures with which to check that my field was flat and to check for tilt.  I thought I had tilt but it ended up being a backspacing issue, which was easily corrected with a couple shims.

 

I don't think a manufacturer's certification for "straightness" is practical.  It would drive the cost of components up to add additional testing and tighter tolerances than they already provide.  And given the wealth of third party components we mate together it would be a difficult task to tell which component of an imaging train is the culprit for tilt.

 

I think most people have success with camera tilt adapters to fix the issue.

See post 4 & 10 of this thread for a way to test for camera sensor tilt indoors.

 

CCDer described an apparatus that makes adjusting for sensor tilt indoors using a laser and some plexiglass.

 

I checked and corrected for tilt on a new camera indoors using a similar technique without needing to build an apparatus. It worked so well I will repeat it here so hopefully it may help others:

 

1.) I used a Glatter laser, though any red laser will do. Place the laser with ample space underneath a sturdy piece of flat and clear glass/plexiglass (glass table, small aquarium, etc). In my case I first used an empty aquarium flipped on its side with the laser standing inside pointing at the opposing wall above it, and later used a glass plate placed on a tall wire closet shelf with the laser at floor level pointing up to it.

 

2.) Align the laser beam such that the beam is reflected back onto the source. This can be done with any kind of shim, paper, or you can think of a better way to adjust tilt. In the end the laser beam just needs to hit the flat glass and fold back onto itself and stay there, so achieve this any way you can.

 

3.) Place the camera with spacers/oag/etc attached on top of the glass such that the beam passes through the glass and hits near the center of the sensor. Assuming your camera has a tilt plate you will be using it to adjust out the tilt, if not then you will need to use a tilt plate in the imaging train.

 

4.) Use a (silver) sharpie to draw a ring on the glass around the bottom of the imaging train pressing against the glass (spacer in my case) and a tick mark on the spacer and glass such that you can place the imaging train back down in the same spot after picking it up to adjust the tilt screws.

 

5.) Adjust the tilt plate until the return spot off of the sensor is reflected back onto the laser source. This part can be a little tedious with the push pull tilt plate but it is certainly much easier and convenient than spending a night outside in the dark iterating on star shapes or tilt models.

 

The return beam off the glass will have diffraction rings, and the return spot off of the sensor will be bloated, so you can only be so accurate, but it worked well enough to solve all tilt issues I was having at the time with that new camera.


Edited by Poynting, 27 September 2021 - 07:56 PM.

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#6 Farzad_K

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Posted 27 September 2021 - 08:34 PM

See post 4 & 10 of this thread for a way to test for camera sensor tilt indoors.

 

CCDer described an apparatus that makes adjusting for sensor tilt indoors using a laser and some plexiglass.

 

I checked and corrected for tilt on a new camera indoors using a similar technique without needing to build an apparatus. It worked so well I will repeat it here so hopefully it may help others:

 

1.) I used a Glatter laser, though any red laser will do. Place the laser with ample space underneath a sturdy piece of flat and clear glass/plexiglass (glass table, small aquarium, etc). In my case I first used an empty aquarium flipped on its side with the laser standing inside pointing at the opposing wall above it, and later used a glass plate placed on a tall wire closet shelf with the laser at floor level pointing up to it.

 

2.) Align the laser beam such that the beam is reflected back onto the source. This can be done with any kind of shim, paper, or you can think of a better way to adjust tilt. In the end the laser beam just needs to hit the flat glass and fold back onto itself and stay there, so achieve this any way you can.

 

3.) Place the camera with spacers/oag/etc attached on top of the glass such that the beam passes through the glass and hits near the center of the sensor. Assuming your camera has a tilt plate you will be using it to adjust out the tilt, if not then you will need to use a tilt plate in the imaging train.

 

4.) Use a (silver) sharpie to draw a ring on the glass around the bottom of the imaging train pressing against the glass (spacer in my case) and a tick mark on the spacer and glass such that you can place the imaging train back down in the same spot after picking it up to adjust the tilt screws.

 

5.) Adjust the tilt plate until the return spot off of the sensor is reflected back onto the laser source. This part can be a little tedious with the push pull tilt plate but it is certainly much easier and convenient than spending a night outside in the dark iterating on star shapes or tilt models.

 

The return beam off the glass will have diffraction rings, and the return spot off of the sensor will be bloated, so you can only be so accurate, but it worked well enough to solve all tilt issues I was having at the time with that new camera.

Interesting approach for determining if there is a hardware tilt issue. There are other ways, I am sure. I have an ED80T refractor that has had a tilt issue (apparently) since I put it in service. The reason I know that is because I have gone back to my initial images back when I knew nothing about these things, and compared images captured with a DSLR and then with two different ZWO ASI cameras. So, I know the issue in this case is with the refractor although I must admit there are still other possibilities which could be rare.

 

If I continue in this hobby, my next telescope is going to be around $4k, and my next camera is going to be about the same amount and the next mount even more expensive. I would be very unhappy if there was a tilt issue related to the telescope and or the camera, and there must be an exit strategy - a return policy, a way to remedy this situation. It would be nice if a business existed that could test your equipment for tilt so that each one of us would not have to build their own laboratory to measure the tilt that we can see exists. A major issue outdoors is that we need to test the OTA and the camera independent from each other, and out in the dark it can be tough to do. But still - after finding out, what is the remedy?

 

Maybe for higher end stuff there is less chance of it, but I don't see any camera data sheets that actually mentions how much the sensor might be out of tolerance. It almost looks like something that we might be able to remedy using the software - something like distortion control.



#7 bugbit

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Posted 27 September 2021 - 08:50 PM

Tilt is not due to $$$ spent. It is in half million dollar scopes with $30k cameras. To what degree you want to alleviate it and how easy will depend on what you want to spend.


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#8 AhBok

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Posted 27 September 2021 - 09:45 PM

That depends on where the tilt lies. Cheaper focusers with lots of slop are major sources of tilt. Unfortunately, these are usually the focusers with no adjustments for tilt. The same is sometimes true for cheaper filter drawers or filter wheels where tolerances can be loose and create tilt.

#9 Farzad_K

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Posted 27 September 2021 - 09:47 PM

Tilt is not due to $$$ spent. It is in half million dollar scopes with $30k cameras. To what degree you want to alleviate it and how easy will depend on what you want to spend.

I hear you say that tilt is a fact of life and it is independent from the money paid for the equipment and the money should be spent mitigating it rather than purchasing components that are free from it. I always imagined that with higher quality equipment/components there are less of a chance of tilt issues and tilt issues may not be as severe - at least it is intuitively so.

 

Next time I am in the market to get an expensive refractor I will discuss with the vendor. I tell you my next refractor will have smaller FoV and as such less cropping flexibility. I would hate to spend thousands of dollars and joyfully take the scope out in the dark and cold only to accept that tilt is a fact of life.

 

With tilt being a fact of life I am surprised with the lack of corrective hardware in this field where it looks like there is a corrector for almost everything.



#10 Farzad_K

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Posted 27 September 2021 - 09:49 PM

That depends on where the tilt lies. Cheaper focusers with lots of slop are major sources of tilt. Unfortunately, these are usually the focusers with no adjustments for tilt. The same is sometimes true for cheaper filter drawers or filter wheels where tolerances can be loose and create tilt.

I agree that a bunch of stuff in between the OTA and imaging sensor could be part of the problem, but those are usually not the main components and can more easily be replaced. The most important is to have optical tube and a camera that are as free from tilt as possible, and I am sure that in that area you definitely can get better quality if you can spend the dollars for.



#11 bugbit

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Posted 28 September 2021 - 12:02 AM



 

 

"With tilt being a fact of life I am surprised with the lack of corrective hardware in this field where it looks like there is a corrector for almost everything."

 

There are correctors available.

You also need to realize that every manufacturer has to deal with tolerances that are allowable and when tolerances are involved then issues like tilt appear.


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#12 Rasfahan

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Posted 30 September 2021 - 04:18 AM

I believe there are not many third-party correctors for tilt available for historical reasons:

- Previously, larger-format CCDs were very high-priced „full ecosystem“ solutions that included their own tilt adjustments (and probably have less tilt in the first place).

- Many CCDs actually had rather small chips where tilt would not surface as much (ICX694, many KAF-XXXX).

- CCDs with larger sensors had larger pixels that can hide imperfections

- Current small-pixeled CMOS have led to lower-FL, faster scopes being used, which have a smaller CFZ, so tilt issues will be more apparent

- Those CMOS are affordable to many more people than the CCDs, so before there really was no market



#13 Farzad_K

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Posted 30 September 2021 - 08:45 AM

I believe there are not many third-party correctors for tilt available for historical reasons:

- Previously, larger-format CCDs were very high-priced „full ecosystem“ solutions that included their own tilt adjustments (and probably have less tilt in the first place).

- Many CCDs actually had rather small chips where tilt would not surface as much (ICX694, many KAF-XXXX).

- CCDs with larger sensors had larger pixels that can hide imperfections

- Current small-pixeled CMOS have led to lower-FL, faster scopes being used, which have a smaller CFZ, so tilt issues will be more apparent

- Those CMOS are affordable to many more people than the CCDs, so before there really was no market

CFZ being Critical Focus Zone - what role does it play regarding tilt?



#14 Rasfahan

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Posted 30 September 2021 - 08:53 AM

Tilt is visible because stars on the sensor are not in the same plane with regard to the focus plane. So the smaller the CFZ the less tilt is needed to effect stars.
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#15 Farzad_K

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Posted 30 September 2021 - 09:16 AM

Tilt is visible because stars on the sensor are not in the same plane with regard to the focus plane. So the smaller the CFZ the less tilt is needed to effect stars.

I thought tilt effect was characterized by star elongation. Certainly I can see the sharper the focus is the more prevalent the star elongation would be and the more out of focus the image is the less obvious it will be. Is that the point with the CFZ?



#16 Rasfahan

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Posted 30 September 2021 - 10:05 AM

I don‘t think tilt should lead to elongation, at least not for the small amount we‘re talking about here. But you‘re moving part of the sensor out of the corrected field and with the defocus the aberrations become more visible,too.
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