ZWO ASI1600MM-cool – UV imaging

One other thing I was interested to test out on the ASI1600MM-cool is its UV-imaging potential. This could be particularly useful bearing in mind its slightly larger sensor, for documenting the UV-pattern of flowers or of the human skin (bruises, sun-damage etc). I picked a Buttercup flower from my back garden and used this as my subject for testing its UV-imaging capability.

For this test, I mounted the camera with my Coastal Optics 60mm F4 APO and Baader U filter. Although the Xagyl filter wheel is mounted on, it was set to an empty slot with no mounted filter. But the larger sensor meant that I could place the camera closer to the subject than I could with my ASI178MM-cool.

Mounted for UV imaging
Mounted for UV imaging

I took an image of the Buttercup first of all using my full-spectrum Olympus OMD EM5, to show what it looks like in colour. The first image was shot using a burst of light from my Quantum X4D flash (1/2 power, ISO 200, F11).

Buttercup - OMD EM5 Baader U with Quantum X4D flash
Buttercup – OMD EM5 Baader U with Quantum X4D flash 1/2 power

Next image was using the Fluorescent tubes from my Light studio as the UV light source. It did require about 8 secs exposure in order to grab enough light (at ISO 800 & F5.6 if I remember correctly)

Buttercup OMD EM5 - UV from fluorescent tube (8 sec)
Buttercup OMD EM5 – UV from fluorescent tube (8 sec)

Next are the images taken using the ASI1600MM-cool. Clearly the image is overexposed even with 1/4 power burst from the Quantum X4D – showing how sensitive this camera is to the UV (at Gain 0). Clearly the Bayer Matrix of the OMD EM5 reduces its sensitivity to UV very significantly.

Buttercup - ASI1600MMC, Gain 0 with 1/2power burst from Quantum X4D
Buttercup – ASI1600MMC, Gain 0 with 1/4 power burst from Quantum X4D

Next is the image of the Buttercup using the Fluorescent tube as light source. I used a Gain of 500 as I wanted to show how grainy the single image is.

ASI1600MMC - single shot at Gain 500 (fluorescent tube as UV light source)
ASI1600MMC – single shot at Gain 500 (fluorescent tube as UV light source)

The reason is that I wanted to test out Livestacking of the UV image using Sharpcap, to compare the result of stacking many images to reduce the noise in the final stacked image; this is a feature to allow stacking of multiple short-exposure astro images to produce a better image like video astronomy. Now I could not get Sharpcap to align the images for me, as there were no stars to detect in my image, so had to stack without this feature. But as you can see below that stacking 106 frames helped to reduce the noise in the image significantly. But clearly the subject needs to be absolutely still so as not to introduce image blur.

Live stacked image from 106 frames at Gain 500
Live stacked image from 106 frames at Gain 500

I also tried stacking with Gain set at its maximum of 600. And clearly stacking does reduce the grain in the image significantly.

Live stacking of 62 frames at Gain 600
Live stacking of 62 frames at Gain 600

In conclusion, I have to say that the sensor of the ASI1600MM-cool is highly suited for UV-imaging. And if solely using for UV-imaging where the ability to cool the sensor is not as important, then perhaps even the ASI1600MM or ASI1600MC cameras would be suitable, bearing in mind the coloured version will have a Bayer matrix which reduces the sensitivity of the camera to UV.

And if I do get the chance, I would like to try an LRGB imaging approach for UV, using Baader U as the Luminance channel, 330AF20 as green channel (covering 320-340nm), U360 with BG39 as blue channel (covering 340 to 390nm), and either leaving the red channel blank (as the red dye of the Bayer matrix normally blocks the UV in colour cameras) or using a combination of both filters to generate a synthetic red channel to create a more aesthetically pleasing image. That experiment should prove interesting, but whether I can align the images and stack them together is probably the main challenge.

Boon