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I am trying to work out which camera would be better to image a faint tail with, such as a comet’s tail or in particular Mercury’s illusive sodium tail.
Using a narrow band filter which only transmits 589nm (sodium), the logic would follow to use my mono (ZWO 174MM) camera. But my colour camera (ZWO 585MC) has smaller pixels and more of them and deeper wells.
The sampling rates of the two with my fast short tube refractor are: 3.5″/px and 1.73″/px respectively.
Three of the four pixels (green, green and red) in the Bayer group on the 585MC have a QE of 60% at 589nm compared with all pixels on the 174MM having a QE of 65% at 589nm. Both are 12 bit ADC, 0.8e read noise, but 174MM has well depth of 37.8k and the 585MC well depth of 47k.
So my question, which camera will be a) most sensitive to the faint tail, b) show the most detail, c) have the best dynamic range / contrast? Anything else I should consider?
Thanks for any help.
James
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I think they will be very similar. The QE doesn’t seem that relevant as you will be imaging against a bright sky I suppose. Pixel size doesn’t seem important either when imaging such a nebulous thing. The larger well depth of the colour camera may give a slight advantage in pulling out a fine discrimination of brightness levels.
This sounds like a rather similar problem to the twilight imaging of the dark side of Venus, where the results are critically sensitive to altitude and sky darkness. Martin Lewis did a lot of work in quantifying the effect of noise in those observations. He gave a talk on it to a BAA meeting a few years back, which is in the YouTube archive.
Hi James,
The first challenge is to capture the sodium tail then later attempt to pull out detail if you can. I would go for the 174 based camera to maximise the SNR as it has the larger pixels and even employ a focal reducer to further improve SNR. I believe you need long accumulated exposures.The main issue with the 174 is the relatively high read noise so another possibility is to use the 585 and bin the pixels 2×2. The 585 has a much lower read noise and with 2×2 binning the pixel size is almost exactly the same as the 174 (5.8um v 5.86um).
The expert on this is the BAA’s own Chris Hooker and I would seek advice from him – maybe you and he will be at the Winchester Weekend. There is a short section on imaging the ion tail in the BAA Mercury handbook but I think Chris who wrote the handbook has gathered much more experience since writing that section; https://britastro.org/wp-content/uploads/2020/04/Guide-to-observing-Mercury-v2.6.pdf
Good luck
MartinMartin, thank you.
Yes, poor Chris Hooker has been helping me, dragging me along really as I’m not that bright when it comes to solar physics, magnetospheres, energy states, wavelengths etc…! But good fun for me anyway. I’ve just seen there is a 585 mono camera which appears to have a QE of just under 90% at 589nm. I may have to do some overtime. The binning is an interesting idea I’d not thought of.
Regards
James
That’s good you are talking to Chris, he pratical expertise on sodium tail imaging is second to none,
I have the QE for the ASI585MC at about 90% for the red when I read this from ZWO; https://i.zwoastro.com/zwo-website/manuals/ASI585_Manual_EN_V1.0.pdfThe mono IMX585 sensor cameras are just coming out and Player One and QHY have them on the market with ZWO coming soon. They will be significantly better QE for this sort of application as you will get signal on all pixels instead of primarily the red and some signal on the green (and almost none on the blue).
Martin
Martin, thanks for that link. It is interesting, depending where you look, the transmission data varies… Surely the colour IMX585 should have a near identical transmission profile whatever camera it is in? Here are three plots, one from your link, one from the ZWO website (https://www.zwoastro.com/product/asi585mc-mm-pro/) and one from the PlayerOne website (https://player-one-astronomy.com/product/uranus-c-usb3-0-color-camera-imx585/).
James
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Yes, James, you would at lest think that the two originating from ZWO would be the same!
The Player One plot is not absolute QE but relative QE which is an equal energy reponse curve. That can’t really be compared so easily with the absolute plots as it tends to elevate the red relative to the blue. It factors in the photon energy and also normalises to the highest value.
I recently came across a link which explained how you convert between absolute and relative curves but can’t find it right now.
Regards
Martin
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