Opportunistic reflected UV with planetary astrocam

[otoien]

I recently aquired a planetary CMOS camera, the ZWO ASI678MC. Here is my planetary rig with 300mm f/4 PF and stacked 1.4 and 2x converters to provide an 840mm f/11 lens.

 

 

 

This has an IMX678c 7.7mm wide back illuminated chip in 4K resolution and 2um pixel pitch. It has an excellent IR response.

Example with an Antlia  685nm IR-pass filter and Nikon AF 20mm f.2.8 lens:

 

 

 

but the ZWO chart is a little less clear on what happens on the the UV side:

 

 

The sensor has an AR coated protective window. The curve above is with this window in place. A chart of this window's  transmission spectrum in the specs of another ZWO camera model gives a hint that deep UV responses cannot be expected:

 

 

 

However I happened to have a 20mm diameter 2mm thick ZWB1 filter, which by itself of course leaks IR, but found a QB39 IR cut filter, also in 20mm diameter 2mm thickness in ebay seller tangsinuo's store. The transmission is only given for the 1mm thickness:

 

 

 

Then I found a 1,25" "moon filter" from Agena Asto that had a donor cell with a wide rim. The 20mm filters had a loose fit in it, but worked with the ZBW1 closest to the sensor and  retaining ring that the slightly narrower QB39 would slip into; scotch tape was used on the edge as spacer between filters to avoid newton rings. 

 

 

 

Then I have a Cassarit 50mm f/2.8 lens that way back was given to me as part of a corroded camera in parts in a plastic bag by late Professor Krog in Oslo. As the lens did not show signs of water damage, just needed cleaning, my guess is that the camera succumbed to a humid tropical climate, possibly when he participated in the 1967 Alpha Helix Expedition in Amazonas. While it is not listed among the better classes of UV lenses, it is a triplet with old coatings.

 

 

 

Visual response with a  ZWO UVIR cut filter, all of these were captured as small videos with FireCapture and stacked in Autostakkert, Small versions are with unadjusted colors before using a gray control point on the upper right qwhite patch in the left panel. I try to keep aperture of the Cassarit at f/5.6. The UV-IR cut filter is designed for astro work and has an enhanced red respons up to above 656 nm. Light souce is my nightrider Lumen 900 bike light:

 

 

 

Then with ZWB1+QB39 filters only with my Convoy S2+ 365nm and Tank 007TK-566, both with what is likely ZBW2 filters on the front at 4m distance from the target. 200ms exposure and Gain 320. Some preliminary tests indicate that a Gain of 100 corresponds to ISO 100 sensitivity and from there approximately a dobling of ISO for every increase in gain of 50. So this would be about ISO2000:

 

 

Then the same combination in sunlight, 200ms gain 228:

 

 

It is notably different color responses, with yellow-green dominating with 365nm source and sunlight colder, which I assume is due to dominance of longer wavelengths in sunlight that records more in the blue channel. 

 

To test effectiveness of the filtering here is

IR pass 685 nm 1ms exposure, Gain 26:

 

 then  ZWB1+QB39, 192ms Gain 26:

 

Finally ZWB1+QB39+ IRpass 685nm, 1s Gain 405 - This would be about 10 EVs below the UV exposure if my calculations are correct:

[There is a light leak at lower right as the IR pass filter was mounted to a snout that was just held onto the front of the Cassarit.]

 

So I will conclude that for my limited practical purpose, IR leak is not a problem with this combination.

 

 A couple of days ago to my delight the first dandelions showed up outside our department at the university to allow a practical test. The initial try was handheld as my tripod was at home and it is recorded on my phone with the ZWO ASICap  app, so here is just a low res overview, ZWB1+QB39 :

 

 

I picked up one dandelion and brought home for a more proper test. The best result appeared with a cloud diffusing the light. 

 ZWB1+QB39, f/5.6, 517.5ms exposure, Gain 200 (corresponding to ISO400), 12 bit mode, 32 frame .ser raw video in Firecapture, stacked the 8 best, WB adjusted according to another capture on a Teflon tape wrapped plastic case where histograms were equalized. Gamma adjusted to 2.2 in irfanview (as the video is linear), with following increase in contrast and saturation (click for larger version):

 

 

Not bad for a lens that is possibly 60-70 years old and never designed for UV captures!

 

That is all for tonight, more to follow...

[otoien]

Back to the planetary rig with the 300mm f/4 PF +TC-14E+TC-20 EIII shown in the begiinning of this thread, which is used wide open to effectively form a 840mm f/11 lens. With this sensor size the angle of view is similar to a 4000mm lens on a full fram body. Note that for all solar exposures I used a Baader Astrosolar Saftery film ND 5.0 on the lens . Do not try solar exposures without such protection. First a visual two frame stitch with UV IR-cut filter, stack of  the best 5% of about 1500 frames captured at 44 fps in 8bit mode, as all of these wavelet sharpening applied in WaveSharp (click for possible larger versions) :

 

 

Then a detail captured at 106 fps in 12 bit mode, stack of the best 10% with 3x drizzle:

 

 

 

685nm IR pass filter, stack of the 5% best of 1000 frames:

 

 

 

Well UV is of course a problem with this rig. Just for fun  I initially tried the crazy experiment with a ZWB1+QB39 filters and was able to get a very dark and blue and noisy sun at 20ms exposure and gain 403 whichI somehow was able to white balance, but I suspect I mostly caught wavelengths at the border to blue/violet, just showing a small version:

 

However I have a 40+ year old Tokina 500mm f/8 mirror lens:

 

Initial tests indicated that it passed almost as much UV as the 50mm Cassarit, when effective aperture was considered, ZWB1 only (with  365nm UV sources, 100ms Gain 258):

 

 

 

 

50mm Cassarit also with only ZWB1,  100ms Gain 351, there could be variations in how well the UV sources were aimed. I believe aperture was set to f/8 or f/11 but not sure.

 

 

 

Solar UV capture with Tokina 500mm f/8 Astrosolar film, ZWB1+QB39, 2.5ms Gain 326, Stack of the best 2% of 2000 frames:

 

 

UV seems to show the solar surface structure better.

For comparison a similar capture with the 50mm Cassarit:

 

I tried UV on the moon but was not able to frame anything, even at very high gain. I am not sure if I was just troubling technically. IR is however excellent to do lunar mid-day captures that look a bit like night, 300mm f/4 PF +TC-14E+TC-20 EIII with 685nM IR pass filter with moon at about its highest point in daytime:

 

 

 

Finally I had to try for an IR Venus in daytime, high in the sky. Setup as the lunar capture with 685nm IR pass filter:

 

Perhaps there is a a slight indication of a cloud pattern?

 

A couple of visual daytime  captures with UV-IR cut filter, different colors are due to the processing. Venus is really a very bright juvel in the sky, even in daytime with the current lighting angle on it. With a little training I was able to spot it with my unaided eyes after initially locating it through my little 8x20  Zeiss binoculars.

UV is supposed to best show cloud patterns on Venus, so  I had to try a daytime capture with Tokina 500mm f/8, ZWB1+QB39, 20ms gain 350, only 49 frames per second - stacked the best 20% of about 8000 frames.

Seeing was not good but I am not sure if I want to repeat this last experiment...

 

 

[This image keeps repeating itself in spite of attempts to edit away the duplicate: ]

 

 

[dabateman]

Wow looks like lots of fun.

Inspiring. 

[ulf]

Thanks Otoien for an interesting post

 

7 hours ago, otoien said:

However I have a 40+ year old Tokina 500mm f/8 mirror lens:

 

Initial tests indicated that it passed almost as much UV as the 50mm Cassarit, when effective aperture was considered, ZWB1 only (with  365nm UV sources, 100ms Gain 258):

 

50mm Cassarit also with only ZWB1,  100ms Gain 351, there could be variations in how well the UV sources were aimed. I believe aperture was set to f/8 or f/11 but not sure.

 

I'd say that the Colour Checker is not ideal as a test taget for UV-transmission of lenses.

However it is a good idea to use 365nm UV sources.

That gives a very good hint of the more important upper UV-A range. If transmission is bad already here it will be really bad further down the spectra.

 

The best way of doing this kind of test is to set the lenses to the same aperture and compare the exposure level results.

The ideal reasonably cheap target is a sheet of matted virgin PTFE. I use grit 300-400 of wet sanding paper under flowing water carefully rubbing the PTFE in random directions, until the surface seam completely matted.

 

If the light source is not emitting much purple and the test is done in a dar room you might not need any filters.

 

With my mirrorless full frame camera I would take photos of the illuminated PTFE taget with the tested lenses to be compared with RAW.

A fixed distance between a test target and light source is extremely important.

Then I would compare the intensity of the images in the most bright areas.

 

I would use my FastRawViewer for the comparison directly in RAW before any RAW-conversion.

I assume there are other cheap or free tools that can do that too.

[otoien]

Thanks for the comments dabateman and Ulf. The UV source test with PTFE plate would be a bit tricky to carry out stringently with these two lenses as it would depend on very accurate and consistent aiming of the sources due to the narrow angle of view and gradients of the UV sources. 

 

If the original stacked .raw video (.ser files) captures of the sun with the mirror lens vs. the Cassarit is any indication (the samples above were sharpened and color corrected, so not representative in that respect, and also from different days ), the mirror lens was leaning more towards blue and was probably two EVs below the Cassarit, but I think this would need to be repeated as there were a few hours between the captures on that day, and I would also need to confirm that the Cassarit was stopped down to f/8 (or may be a little further with the effective aperture of a mirror lens typically being a bit lower than the nominal aperture).  It is easy to accidentally move that stop-down mechanism at the front of the Cassarit when refocusing the lens.

 

Anyway this is for me just an opportunistic way to play a bit with shallow reflected UV, with minimal extra spending. The initial test with the Color Checker target was a useful experience in that when working with fluorescence one can get the impression that these 356nm UV sources are relatively weak - usually used within 50cm distance. So it was surprising to see how much reflected UV response I got with this non-ideal lens setup and filtering at 4m distance, which would hardly show any fluorescence at all except on white paper targets.

[Akira]

The solar image in UV looks promising.  Hope to see more!

 

The monochrome sibling of ZWO seems to be much more sensitivity in 300-400nm range.

[ulf]

3 hours ago, otoien said:

The UV source test with PTFE plate would be a bit tricky to carry out stringently with these two lenses as it would depend on very accurate and consistent aiming of the sources due to the narrow angle of view and gradients of the UV sources. 

That is true, but if you mount the light sources in a fixed way instead of light painting, it would work fine. Then the distance will also be kept constant. 

When investigating the photos you have to look for the brightest area due to the gradients og the light sources.

 

A sheet of matted PTFE is a rather cheap and very useful tool in the UV-photography toolbox.

[otoien]

Thanks for the comments Akira and Ulf. Yes, my little compact flash container wrapped with Teflon tape will probably not do for this - the container itself shows fluorescence, and the tape is slightly shiny. Akira, If someone were brave enough one could possibly enhance the UV response by removing the AR-protective window. Some QHY models are even sold with a cell-less set of filters that are swapped in instead of the AR protective window. But one would of course not get yellow dandelions with a mono camera. 

[otoien]

A new controlled solar test today, as before Astrosolar film, ZWB1+QB39. These were not gamma corrected, using default WB coefficients (WBlue=99 WRed=62 and  only a very moderat wavelet sharpening applied, resized to same size. So there is basically no color correction applied

 

Cassarit 50mm f/2.8 @f/8, Gain 85, 20ms exposure

 

Tokina 500mm f/8, Gain 85, 40ms exposure (= +1EV)

 

 

80ms exposure with the 500mm was clearly overexposed. As gamma was not applied, exposure differences look more pronounced than they would have with gamma applied.

 

While one can conclude that there is likely only a little more than 1 EV less transmission for the 500mm lens with sunlight, the missing transmission appears more in the green and red channels. I interpret this as a shallower UV response. No PTFE sheet available yet, so the test with UV sources will have to wait.