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L filter messing stars at the corners

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#1 betelgeuse91

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Posted 12 June 2019 - 07:07 PM

I am using ZWO 2" IR/UV cut filter in the following order and the filter causes this shooting star like effect at the corners...

 

< dslr - t ring - flattener - L filter - refractor >

 

The filter is placed after the flattener so it does not effect the distance between the flattener and the sensor. 

 

I don't have perfect back focus distance so there is some coma-like effect on the stars when taken without the filter.

 

But with the filter, not only there is shooting star effect at the corners, there is field curvature at the lower left and right corners. Every time!

 

With other filters I have, there is some curvature like the L filter, but never the shooting stars. 

 

I have contacted the store where I bought the filter but they couldn't help, and I e-mailed ZWO but haven't heard back for months...

 

Please help...

 

gqyWwPZm.jpg

 

stKietTm.jpg

 

The first image is the iris nebula taken without any filter, and the second is taken with the L filter. Thank you. 



#2 PirateMike

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Posted 12 June 2019 - 09:22 PM

When you put a filter in the light path it will move the flat field distance a bit. The thicker the glass the bigger the difference.

 

You will have to adjust the spacing between the flattener and the chip until you reach the flat field.

 

 

Miguel   8-)

 

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#3 betelgeuse91

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Posted 12 June 2019 - 09:29 PM

When you put a filter in the light path it will move the flat field distance a bit. The thicker the glass the bigger the difference.

 

You will have to adjust the spacing between the flattener and the chip until you reach the flat field.

 

 

Miguel   8-)

 

.

I thread the filter in front of the flattener though...



#4 Der_Pit

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Posted 13 June 2019 - 09:42 AM

If there are optical errors like chromatism in the system, the L filter will always suffer most from all filters, as it sees the combined errors of all wavelengths.  For less broad filters you can often compensate partially by refocussing, but for L that doesn't help.

 

I cannot tell if the filter in front of the filter is changing the correction of the flattener.  Could do so, if it is vignetting it (i.e., substantialy smaller in aperture, or, at similar aperture, too far away).



#5 betelgeuse91

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Posted 13 June 2019 - 12:07 PM

If there are optical errors like chromatism in the system, the L filter will always suffer most from all filters, as it sees the combined errors of all wavelengths.  For less broad filters you can often compensate partially by refocussing, but for L that doesn't help.

 

I cannot tell if the filter in front of the filter is changing the correction of the flattener.  Could do so, if it is vignetting it (i.e., substantialy smaller in aperture, or, at similar aperture, too far away).

I see... Well, as it shows in the above images, I don't think the filter is vignetting much... 




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