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Is a true flat field really possible when using an ASI-183?

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#1 PirateMike

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Posted 22 July 2019 - 12:05 AM

Is a true flat field really possible when using an ASI-183?

 

I ask this because of the simple fact that the center of the chip on an ASI-183MM Pro in not in the center of the light path.

 

I like my stars round, and it seems that the design of the ASI-183MM Pro is not helping me to produce round stars. In fact, the design of the camera is doing the exact opposite.

 

Am I right or am I wrong?

 

 

 

Miguel   8-)

 

.


Edited by PirateMike, 22 July 2019 - 12:10 AM.


#2 vdb

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Posted 22 July 2019 - 12:09 AM

One word, YES ...

I used the ASI on both FSQ and RC, flat fielding is no issue.

/Yves


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#3 Jon Rista

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Posted 22 July 2019 - 12:13 AM

Why wouldn't the sensor be in the center of the light path?

 

I seem to get pretty round stars myself:

 

ZosZJpW.jpg


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#4 Jon Rista

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Posted 22 July 2019 - 12:14 AM

cjzgpXR.jpg


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#5 PirateMike

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Posted 22 July 2019 - 01:43 AM

Hello John,

 

Just take a look at your camera. Is the center of the chip in the center of the camera's circular attachment point?

 

Mine isn't. The chip is centered along the short dimension but not the long. 

 

 

Miguel   8-)

 

.


Edited by PirateMike, 22 July 2019 - 01:44 AM.


#6 PirateMike

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Posted 22 July 2019 - 01:46 AM

Maybe I'm overly concerned on the "flat field" issue, but my chip is obviously not centered.

 

 

Miguel   8-)

 

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#7 james7ca

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Posted 22 July 2019 - 01:50 AM

Yes, Jon's recent results seem to show that you can indeed produce good looking stars right out to the corners of the IMX183. But, seeing conditions can help to make stars appear rounder. As I recall,  his FWHM was just under 2 arc seconds, which is pretty good but not great for a narrow-band, H-alpha image (in my experience, while imaging at 0.75 arc seconds per pixel with either the IMX178 or IMX183).

 

Also, I think Jon and I agree on the observation that as the FWHM goes down the eccentricity seems to go up (the latter meaning less round-looking stars). Strictly from a somewhat arm-chair analysis, that latter behavior seems to make sense. So, as your actual resolution goes up (indicated by smaller FWHM) it should be more likely that you will begin to see more defects caused by your optics and guiding.

 

And then there is the issue of the guiding itself, since as the image scale increases (more pixels per arc second) the more likely it is that you will see problem with your guiding (meaning non-round stars).


Edited by james7ca, 22 July 2019 - 02:03 AM.

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#8 PirateMike

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Posted 22 July 2019 - 02:55 AM

Also, I think Jon and I agree on the observation that as the FWHM goes down the eccentricity seems to go up (the latter meaning less round-looking stars).

I think what is being seen in this case is the tracking error becomes greater than the seeing error.

 

With earthbound seeing in play (you probably already know this) the star appear to move around and it moves in a random manner. This causes long exposures to show round (due to random movement) but large stars in the image. When the seeing gets very good, the stars should appear even rounder and even smaller because they are not "moving around" as much.

 

Now lets assume the extreme... Seeing is perfect, the stars do not move around at all (like we were in outer space), they just smoothly cross the sky with no "bouncing around" at all. What will you see in your long exposure images? Not round stars, you will see stars that are out of round by the same amount as you mounts tracking error.

 

This all boils down to the "not so great seeing" hiding the mounts tracking errors.

 

If I remember correctly, the human eye can not tell the difference between an oblong and a perfect circle if the eccentricity of the oblong is at or below 0.46, no matter how big or how small the object. So an eccentricity of 0.46 or lower is a perfect circle visually as far as we humans are concerned.

 

 

That's just my take. I could be wrong!

 

 

Miguel   8-)

 

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#9 PirateMike

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Posted 22 July 2019 - 02:58 AM

John, your stars are pretty round, but I can see that they are not less than 0.47 eccentricity.

 

Anyways,very excellent stars just the same. waytogo.gif

 

 

Miguel   8-)

 

.


Edited by PirateMike, 22 July 2019 - 02:58 AM.


#10 PirateMike

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Posted 22 July 2019 - 03:24 AM

Here is what I see with my camera. Warning... my poor drawing skills are very evident. wink.gif

 

Chip Not Centered.jpg

 

 

Miguel   8-)

 

.

 



#11 james7ca

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Posted 22 July 2019 - 03:28 AM

Actually, I believe that the stated limit for round stars under PixInsight is an eccentricity of 0.42, not 0.46 or 0.47.

 

That said, eccentricity is just a measurement and YMMV. In fact, I've found that when you visually examine an image that eccentricity is only loosely associated with how round a star looks after processing. Meaning that a system that produced a median eccentricity of 0.48 may actually come out looking better than a different system that has a measured eccentricity of 0.44. Thus, IMO, eccentricity is best used as a measure of how good or bad your particular system is in comparison to its own typical performance. Also meaning that a particular eccentricity measurement (say 0.42) may look better on one system than it does on another.

 

In Jon's case, I think the stars look very good for being sampled at 0.93 arc seconds per pixel, even if the field does have a small area just above the 0.42 benchmark.

 

Of course, if the eccentricity is up in the middle to high 0.5 range (or worse) then it's usually clearly visible that the stars are not round.


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#12 PirateMike

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Posted 22 July 2019 - 03:32 AM

Yes, 0.42... my bad (memory) flowerred.gif

 

I may need to flush some number out of my head.

 

 

Miguel   8-)

 

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#13 chadrian84

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Posted 22 July 2019 - 05:54 AM

I just looked at my 183mm Pro. At first glance it looked off center. After looking more closely I think it’s centered correctly.
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#14 OldManSky

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Posted 22 July 2019 - 09:24 AM

Mike, you made me go look at mine, too.

With a ruler.

 

It sort of looks "off-center" like yours does, if you go from the outside of the camera body.

But if I measure from the inside of the T2 threaded circle, it's dead-on (within 1/2mm), both vertically and horizontally.

Perhaps the T2 threaded circle is offset from the center of the round camera body a bit?

 

At any rate, I've had nice round stars out to the edge of my field on a WO Z61 with the "old" standard FF.  With one exception:  at one particular orientation of the scope in RA/DEC, my focuser appears to sag just a bit, and I get very slightly oblong stars in one corner only.  I think I can get that out with a focuser adjustment -- at any rate it's not the camera's fault :)


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#15 JukkaP

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Posted 22 July 2019 - 09:45 AM

Just allmost about to open my system, but dont want dust :D. I have not noticed any problems. I get nice round stars across the field.

#16 Jon Rista

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Posted 22 July 2019 - 06:32 PM

John, your stars are pretty round, but I can see that they are not less than 0.47 eccentricity.

 

Anyways,very excellent stars just the same. waytogo.gif

 

 

Miguel   8-)

 

.

I shared the eccentricity map below the image. According to PI, the eccentricity throughout the frame is less than 0.47...

 

Also, note that there are some stars in the field that are pairs of stars (or maybe more) that can look elongated, but are not actually single stars. For single stars, from what I've seen across the subs for that night excluding a few I tossed, all the stars were pretty round. 


Edited by Jon Rista, 22 July 2019 - 06:35 PM.


#17 Jon Rista

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Posted 22 July 2019 - 06:40 PM

Here is what I see with my camera. Warning... my poor drawing skills are very evident. wink.gif

 

attachicon.gif Chip Not Centered.jpg

 

 

Miguel   8-)

 

.

So...how are you judging this? There IS a matter of perspective here. The AR window is set in front of the sensor by 6.5mm, so a bit over half a centimeter. That means that, if you look at the sensor from the front of the camera, the position of the sensor is going to shift depending on the angle of your head to the window and sensor. 

 

Unless you somehow manage to get your head EXACTLY centered directly over the center of the window, then it is likely that the sensor will appear to be off-center in one direction or another. 

 

A simple way to minimize this error is to put the camera at a distance, so that the distance between you and the camera is much greater than the distance between the sensor and the AR window. Then, even though there will still be a bit of parallax, it should be tiny... I'd be curious to know if you still think the sensor is off-center in this case.

 

That said, lets say the sensor is off of center a bit. Hmm, as long as it is FLAT to the image planer, then it being off center should not actually affect the shape of your stars much. Decenter may affect how shading (vignetting) distributes across the sensors, and in the extreme periphery you may notice some increased aberrations where diffraction at the corners of the window might affect those stars. But as long as the sensor is flat, it should just affect shading over most of the sensor, and your stars should otherwise be round.

 

If you are actually having star shape issues, then I would assume that you have tilt or collimation issues somewhere, rather than decentering. 


Edited by Jon Rista, 22 July 2019 - 06:40 PM.



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