UV artifact rabbit hole

[dancingcat]

I am officially an idiot... well, maybe an idiot wannabe... :-)  I'm definitely a UV noob though.  I was fooling around with some Lantana camara and found UV bright straight lines in my image - no such straight lines in the same place in visible.  I immediately went down the horrible rabbit hole thinking I'd scratched my BaaderU filter, or had something go very wrong with my EM1-mk2 sensor (converted).  Well, after figuring out I hadn't scratched my BaaderU (same effect with a Hoya U-360/Schott S8612 stack which I almost never use and I know is clean), and that my sensor was clean, and my lens (Oly 30mm macro was clean), and there were no obvious online discussions of this that I could find, I looked more closely at my images.  Seems some critter made cobwebs in my flower, very fine, mostly dead straight (look like scratches in the image).  They were sometimes in the vis image and not in the uv, and vice-versa.  I attach two test images - you folks probably already know the effects of silk in UV of flowers, but I hadn't thought about it until now.  The images are 15-image Zerene stacks, but I didn't clean them up..the UV is a bit noisy.

 

There is a rabbit hole on spider silk UV reflectance here from research in Australia.

 

Visible iso 200 f/11 1", ambient room light, Kolari hotmirror 2.

 

Ultraviolet iso 640 f/13 15", BaaderU, Kolari UV LED.

[nfoto]

We learn by practical experience .... A scratched filter, however, will never show up by giving sharp lines in the image, as the incoming rays at the filter plane is way off being focused. If the filter is rear mounted, at worst one will have very diffuse shadows and even that occurs only if the filter is so badly damaged you cannot stand looking at it !! The rear element of non-retrofocus wide angle lenses can penetrate deep into the camera throat and in this case, one should be careful not to scratch the rear optical surface as the ray bundles here are nearly collimated, thus might give rise to ugly artifacts in the image,

 

A scratched cover glass over the sensor might give some artifacts but again mostly rendered not very sharp.

 

Cobweb remains in flowers are quite common and it usually takes the enhanced sharpness of the UV image to make them obvious.

[dabateman]

You need to read this about effect of dust, scratches or missing front element on the image quality: 

http://kurtmunger.com/dirty_lens_articleid35.html

 

On sensor,  you will need very small aperture,  like a pinhole to see whats on yhe sensor and it will still be blurry. 

[ulf]

An old trick to improve results from a lens with a scratch or nick that might introduce some image degradation is to cover the imperfection with some black paint like from a sharpie.

 

As the surface area of the damaged area often is very small compared to the undamaged area, you just have to cover the damaged area in a way that it will not radiate any light further into the lens that finally reaches the image sensor.

[dancingcat]

Thank you all… I was really surprised at how obvious the silk was in UV and not in visible.

[Nate]

When I first started astrophotography, just bought my 127mm Mak, and hooked my asi462 camera to it and was testing on the Mountain in the distance during the day, I saw specs in the pics. What's the first thing I do? Blow as much air as possible into the telescope body thinking dirt on the mirrors was causing it, which I actually was adding more dust. Yeah, it was on the camera sensor, cleaned and gone away. That dust on the inside mirrors wont cause issues.

 

Well there's my story on that.  I think the silk on flowers that become way more visible in UV is one of the reasons I enjoy photographing the non visible, there's a bit of mystery of what will show up before processing an image file. 

[Doug A]

@dancingcat your images do a great job illustrating the extra details of UV photography. I didn't realize webs would show up so well in UV. Fascinating.

 

Thanks for sharing,

Doug A