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Ugly stars due to stressed lens cell?

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8 replies to this topic

#1 betelgeuse91

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Posted 07 April 2020 - 01:48 PM

Hi, I am wondering if stressed lens cell could cause those kind of star shapes.

 

I heard this somewhere but I am not sure if that is my case.

 

 

Here are my gears:

 

Telescope: at80edt (80mm triplet refractor form Astronomics)

Flattener: above refractor's dedicated reducer/flattener x 0.8

Camera: nikon d5300 full-spectrum modded (APS-C)

Filter: IDAS 2" light pollution suppression filter D1 

 

Image train looks like:

D5300 - T ring - flattener - 2" filter

 

I also guide with a separate guide scope. (so no diffraction from oag prism)

 

The flattener screws directly into the focuser, and the filter comes before the flattener.

 

Here are the star shapes I am concerned with. Please take a look at the bright stars near the corners. 

 

XZblM9xt.jpg

 

ih9fPxht.jpg

 

I don't have perfect back distance from the flattener, but the spike-like effects on stars seem like they are from some other cause. 

 

What do you think....?

 

I will really appreciate any ideas or advises. Thank you so much. 


Edited by betelgeuse91, 07 April 2020 - 01:52 PM.


#2 Jsquared

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Posted 07 April 2020 - 02:04 PM

I would remove the flattener and do a star test. I think you are asking if there are pinched optics. Google this. Should give a triangular star test.

#3 betelgeuse91

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Posted 07 April 2020 - 02:43 PM

I would remove the flattener and do a star test. I think you are asking if there are pinched optics. Google this. Should give a triangular star test.

It seems like that I will need a 2" nose piece to do that.

 

So if it persists even without the flattener, it could be the pinched optics?

 

If it goes away when flattener is removed, then will it be more like the distance to the flattener then..?



#4 Jsquared

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Posted 07 April 2020 - 06:47 PM

It seems like that I will need a 2" nose piece to do that.

So if it persists even without the flattener, it could be the pinched optics?

That’s my opinion

If it goes away when flattener is removed, then will it be more like the distance to the flattener then..?


Or a bad flattener

You need to remove the flattener and look again. Image better maybe bad optics in flatener or bad setup. If image still bad without flattener then you need to do a star test. This requires a high magnification eyepiece with focal length at or less than the F ratio of your scope. There is a procedure for doing this with a bright star moving the focus both forward and back out of focus. Pinched optics are easy to see with a star test. Google “how to do a star test”. Hope this helps.

There are many well seasoned people on cn who may be able to help. This is imho what to do.

#5 Jeff B

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Posted 07 April 2020 - 08:11 PM

I would take this to one of the imaging forums.  You'll get plenty of "help" there.


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#6 betelgeuse91

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Posted 07 April 2020 - 09:25 PM

Or a bad flattener

You need to remove the flattener and look again. Image better maybe bad optics in flatener or bad setup. If image still bad without flattener then you need to do a star test. This requires a high magnification eyepiece with focal length at or less than the F ratio of your scope. There is a procedure for doing this with a bright star moving the focus both forward and back out of focus. Pinched optics are easy to see with a star test. Google “how to do a star test”. Hope this helps.

There are many well seasoned people on cn who may be able to help. This is imho what to do.

I just found a 2" nose piece lying around so I will test it without flattener whenever sky clears out. I don't have any eyepiece though, but I can use camera & APT to do the star test I think..

 

For the star test, (moving in and out of focus) do I place the star in the middle of the sensor? 

Will pinched optics show effect even if the star is in the center of the sensor? In my pictures above, stars in the middle are perfectly fine... 

 

 

I would take this to one of the imaging forums.  You'll get plenty of "help" there.

I will try things suggested here and ask more there if problem persists... Thanks ..! 



#7 Jsquared

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Posted 08 April 2020 - 03:46 AM

I just found a 2" nose piece lying around so I will test it without flattener whenever sky clears out. I don't have any eyepiece though, but I can use camera & APT to do the star test I think..

For the star test, (moving in and out of focus) do I place the star in the middle of the sensor?
Will pinched optics show effect even if the star is in the center of the sensor? In my pictures above, stars in the middle are perfectly fine...


I will try things suggested here and ask more there if problem persists... Thanks ..!


I’m purely visual so I can’t answer your question. Google “pinched optics”


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#8 Jsquared

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Posted 23 April 2020 - 09:42 AM

So did you figure out things ??

#9 betelgeuse91

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Posted 01 May 2020 - 09:26 AM

So did you figure out things ??

 

I tried taking taking pictures without the flattener, but it was hard to diagnose. Without the flattener, there is field curvature pointing towards the center, but the effect of the weird stars is also radial and they might have just overlapped. 

 

Image taken without the flattener looks quite normal, but it's hard to tell if the stars in the corner contain the above effect or not. (also without the flattener, the camera-to-telescope connection is no longer threaded, and there was a tilt, making it even harder to diagnose the stars in the corners..)

 

But I contacted Astronomics and asked them what they think. They told me that the only thing they can think of is pinched optics and they told me how I can loosen the lens cell. So I followed their instruction and did it. 

 

Now I am just waiting for a clear night to test it out...


Edited by betelgeuse91, 01 May 2020 - 09:27 AM.



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