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Odd looking flatfield

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#1 mmnb

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Posted 19 September 2020 - 01:35 AM

Screen Shot 2020-09-18 at 11.31.07 PM.png This is the flat field I am seeing from my QHY 16200A on an AstroPhysics 6inch refractor.

Is the distinct offset in radial symmetry a sign of anything?  I tried rotating the spike-a-flat frame and didn't see any change in the pattern, so it isn't a defect in the panel.  I can 'see' the pattern in images (darker in the bottom corners, lighter at the top) but am just concerned it's a sign of a problem in the optical train?


Edited by mmnb, 19 September 2020 - 09:56 AM.


#2 futuneral

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Posted 19 September 2020 - 01:40 AM

there is no image



#3 mmnb

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Posted 19 September 2020 - 09:57 AM

there is no image

Doh! added image.



#4 Tapio

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Posted 19 September 2020 - 10:02 AM

How about if you rotate camera?

#5 mmnb

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Posted 19 September 2020 - 10:14 AM

How about if you rotate camera?

I didn't want to touch my camera unless there is good reason to; do you see any suggestion here that there is?



#6 sn2006gy

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Posted 19 September 2020 - 10:26 AM

I didn't want to touch my camera unless there is good reason to; do you see any suggestion here that there is?

looks like you're out of colimation.. vignette is usually pretty well centered when everything is colimated



#7 Tapio

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Posted 19 September 2020 - 11:06 AM

Collimation/tilt issue would be my guess, but as it's a AP 6" refractor, I hesitate.

Maybe it's better first check how actual star images look like, and how they are calibrated.



#8 mmnb

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Posted 19 September 2020 - 11:01 PM

It's an AP triplet apochromat oil spaced lens. The scope has been used optically ok.  My stars look ok from last night (FWHM 3.3 or so IIRC), but there is definitely streaking in the lower left corner of the field.  I tried a little poking (e.g. gentle push on the camera etc.) and the image does not change.

gaaaahhh. I'll take some defocused star images and see what I see. I'm seeing other threads on refractor collimation and I really hope this isn't the problem (I can't see what else it could be though...).  The only thing that sounds hopeful is this :
 

"I believe most people think of collimating a refractor as moving the objective as a whole to point the light straight into the eyepiece. This is easily done if the refractor has the usual three pairs of push pull screws in the lens cell. You are moving the cell in relation to the eyepiece. A simple Cheshire eyepiece clearly lets you see if you are in collimation. Yes, I've done it."
https://www.cloudyni...-apo/?p=1144783

If this situation is recognizable to anyone, would love advice.



#9 mmnb

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Posted 21 September 2020 - 07:52 PM

Astrophysics has advised that this likely a problem at the opposite end and not the lens.

Maybe be able to see by simply looking at the chip from the lens.  I am certain it isn't the filter since all the filters have the same flat profile.  I've asked QHY if it is recognizable in anyway, but hard to imagine it is something other than the assembly between the focuser and camera (though they seem quite rigid).



#10 decep

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Posted 21 September 2020 - 08:33 PM

It almost looks like the filter wheel is not aligned with the slot.



#11 mmnb

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Posted 21 September 2020 - 10:56 PM

It almost looks like the filter wheel is not aligned with the slot.

If that's the case, assuming I did not do anything unbelievably stupid (the filter wheel appears to work fine), I should expect to see this from other 16200A users (have not seen such a complaint).  I've already posted on the QHY board to see if they have any insight.



#12 decep

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Posted 27 September 2020 - 09:18 PM

I am making an assumption, but I would assume most filter wheels align themselves by shining a small LED (probably infrared) through a hole in the internal wheel.  If some trash or something blocks (or otherwise affects that light) it could cause the wheel to not be able to find alignment.

 

Or it could use a hall effect sensor (magnetic) and that could be detecting something that could cause it to become misaligned.

 

The short answer is you probably need to open up your filter wheel to find out.



#13 mmnb

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Posted 12 October 2020 - 12:16 PM

This was finally fixed today (it's a remote setup).  It was the way the camera was connected on to the electronic focuser....it was clearly crooked by eye and there was sticker on the focuser that was preventing flush alignment.  So relieving to see the flat look correct.

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