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What would ray diagram for my setup look like?

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#1 sbharrat

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Posted 03 December 2023 - 12:29 PM

I was blissfully unaware that I completely don't understand the light path through my setup until I was doing some debugging for a problem.

 

My setup is a TS PL130 -> M68 Flattener -> M69 Extension -> M69 Rotator -> M54 Filter Wheel -> asi2600MC

 

(Yes, this TS Flattener is a weird M68 on scope side and M69 on camera side)

 

Now the setup is advertized to have an image circle that fully covers the APS-C sensor of the asi2600. And it certains does -- in my images I have stars all the way to the edge. So when I saw a flat like the below

 

flat-pl130-lenh-180deg-3dec2023.JPG

 

I assumed that the bright center circle is from area in the center of the lens and that the light entering at the edge of the lens is at or beyond the edge of the image. 

 

Now I added some tape in front of one edge to verify orientation for my camera

 

IMG_2643.jpg

 

I expected to not really see the tape yet because it only covering the very edge of the lens which should correspond to beyond the edge of the sensor. But surprising (to me) was that this is the picture I got

 

flat-pl130-lenh-tape-3dec2023.JPG

 

Huh? I clearly don't understand how the light flows through here. If the edge of the lens appears to correspond to the fully lit circle, what's the path of the light that fills in beyond the brightly illuminated circle? 

 

Can someone who understands this please draw me a ray diagram that shows the path leading to the fully lit circle versus out to the full image circle? 

 

Appreciate the tutorial....

 



#2 sharkmelley

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Posted 04 December 2023 - 01:48 AM

I assume that this exposure has been incredibly stretched?  It will probably be more useful to see an actual raw flat frame (plus a raw bias to give us the actual bias level).


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#3 Drothgeb

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Posted 04 December 2023 - 12:01 PM

The flattener is the same as the Stelkarvue SFFX1. Not sure why it has 69mm threads instead of 68mm like the front. But it is an excellent flattener, that seems to work well on every scope I’ve used it on from 80mm - 150mm. Does a good job with FF on my Esprit 150. I liked mine so much I bought a second one.

 

I’m sitting here with an iPad and can’t do a ray diagram. But here’s a link that may help you. 
 

https://photographyl...t-is-vignetting.

 

If you want to check orientation in real time without tape, just stick your finger in front of the scope. It was cause a diffraction spike on the star images. It’s what I do when collimating my SCT, so I know which knob to adjust.


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#4 sbharrat

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Posted 04 December 2023 - 02:00 PM

I assume that this exposure has been incredibly stretched?  It will probably be more useful to see an actual raw flat frame (plus a raw bias to give us the actual bias level).

It is very stretched and I know to expect vignetting that shows this way when stretched.

 

This was more a question of my understanding the light flow. I thought (assumed) that the light entering the very edge of the objective eventually ended up at or just beyond the edge of the sensor (if the image circle is larger than the sensor). The fact that I see the effect of the tape at the very edge show up well within the image is what is confusing me. 

 

But I guess if I consider light from in between the entrance pupil and the extreme off-axis shown in the diagram pointed to by drothgeb, I suppose the effect of an obstruction on the edge of the objective can show up anywhere between the center and the edge of the image. 

 

vignette.jpg



#5 sbharrat

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Posted 04 December 2023 - 03:27 PM

I assume that this exposure has been incredibly stretched?  It will probably be more useful to see an actual raw flat frame (plus a raw bias to give us the actual bias level).

Incidentally, the light drop off is quite small according to PI...

 

statistics-center.jpg

 

statistics-corner.jpg

 

So it is about 3% difference between corner and center. 

 

My confusion was more about WHERE the (additional) dropoff seemed to show up rather than there being a drop off. 



#6 sharkmelley

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Posted 04 December 2023 - 03:37 PM

Incidentally, the light drop off is quite small according to PI...

 

So it is about 3% difference between corner and center.  

Is that flat bias subtracted?  You can't judge light fall off unless the bias is subtracted.



#7 sbharrat

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Posted 04 December 2023 - 05:05 PM

Is that flat bias subtracted?  You can't judge light fall off unless the bias is subtracted.

Right. Yes, I didn't do that correctly. But this is with 2600mc with really low dark current so results don't really change.

 

statistics-center.jpg

 

statistics-corner.jpg

 

Still about 3% drop off in right corner. About 4% in left corner. 




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