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Originally Posted by Shiraz
Thanks Brett.
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Thanks Ray.
You're welcome, Ray. I'm glad you took the time to review and edit your original post because I was quite surprised by some of the sweeping statements that you made. Would I be correct in saying that you no longer hold the view that Well Depth is simply a marketing gimmick?
If so, I'd be interested to know what changed your view overnight.
If not, I encourage you to do a quick search on the Cornell University archives
(arXiv), where you will find an abundance of peer-reviewed papers which make heavy use of this important metric. Many of these originate from NASA/JPL. Given their experience in this field, I remain unconvinced that they would fall victim to mere marketing hogwash.
If I'm not mistaken, Ray -- and please by all means tell me if I am -- you made similar comments in the past with regards to pixel size, without having so much as an iota of data to back up your claims.
Quote:
Originally Posted by Shiraz
I measured the RMS variability of the ZWO dark and it is ~12 electrons due mainly to the dark current of about 0.5e/s/p (I had to make of few assumptions here - ZWO has not published a dark current number). That is almost exactly equivalent to a 5 minute dark from your QHY12, which also has an RMS variability of 12 electrons due to read noise. Your darks will not have the pedestal of the ZWO dark, but, when that is subtracted, the total noise will be ~identical for the two cameras. For anything shorter than 5 minutes, the ZWO will be better, but the QHY will always have 12 electrons read noise. At a sub length of 5 seconds (say), the ZWO could have under 2 electrons RMS total noise - this is very close to zero noise and I know of no other affordable camera that even gets close.
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Yesterday, you made mention of the ZWO dark and how it somehow compared to an SX ICX649-based camera (which is an excellent instrument) running at -10C. I'm not sure what you were trying to achieve there (given the difference in setpoint). Today, you say you've measured the ZWO dark variability and you're comparing it against... what?
Now I realize that comparing the QHY12 to the 1600MM is not an apples-to-apples comparison (forgive me, I don't have a 16Mpx sensor on hand), but a side-by-side visual inspection of 5 minute darks at -25C in no way suggests that the Panasonic sensor has less noise than the former -- much to the contrary. Look
closely at the ZWO-supplied darks -- you'll find what many would consider a serious row defect consisting of almost 10 contiguous pixels. In my relatively limited experience, I've never seen a defect of that magnitude on any sensor, even at room temperature.
If you received a camera from nearly any other vendor with that sort of defect, you'd not only get a heartfelt apology, you'd more than likely be promptly provided with a replacement camera (especially if it were a company like SX, whose sensor you made mention of in your previous post).
Quote:
Originally Posted by Shiraz
ZWO is going to have a problem though, because people are still thinking in CCD terms, where you must have long subs to get over the top of the read noise and that means that you must have deep wells to get around saturation on bright stars. The new CMOS chips will work best with short subs (where the older CCDs are hopeless) and that brings all sorts of advantages including drastically reduced mount requirements, no need for guiding, better resolution and almost unlimited well depth. However, it requires a complete rethink of how one uses a sensor - there will be people who will try to soak the new chips with 20 minute subs and f10 telescopes (like they used to do) and they will not get good results - expect a barrage of "this thing has bad dark current and saturates easily" as people drastically misuse the new cameras.
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So you're suggesting that, in an age where we are apparently moving away from CCD, folks can look forward to taking images of DSO's with the help of hundreds of 5 second exposures, just to circumvent the so-called well-depth limitation/marketing ploy and read noise?
Not this camera -- it simply doesn't have the sensitivity for the pipe-dream that you're describing. The saturation is real, Ray -- which is why well depth matters.
Let's look at the maths. The download time for a 16Mpx image, the storage required (admittedly of lesser concern in 2016) and the eventual processing power and thus the time involved is significant. Going by your 5 second suggestion, that's 120 images per minute and 7200 images per hour? Running that through PixInsight or Maxim? For a typical 3-4 hour session of imaging time (28,000 or so images?), you're looking at the better part of a day to fully integrate and stack all of the resulting data, even on an 8 or 12 thread system. No thanks
It sounds to me as though you're trying to weasel your way out of the fact that this is a noisy CMOS sensor with highly limited wells.
This works on something like an ICX825 -- which many folks here (including myself) have. It might be a smaller sensor than the one in the 1600MM but it is far and away much more sensitive and for most astronomical targets, the ideal sensor for the imaging technique to which you are inferring. That's why it ships with software that does it.
Quote:
Originally Posted by Shiraz
CCDs still have a place - the old methods still work well. The CMOS cameras also will likely not do quite as well at narrow band imaging, where the higher read noise will be more of a problem. They will also possibly do best with relatively fast scopes (haven't done that analysis yet). However, the new chips offer some very exciting possibilities for anyone willing to experiment with radically new ways of doing things (which will not be appealing to everyone). This really is shaping up to be a revolutionary development, as Emil's extraordinary image shows (see post #18)  |
Sorry to bring this up, Ray, but you in your previous post you asserted that the Panasonic sensor used in the 1600MM is a "dedicated astronomy sensor". This is untrue. I'm not sure where you sourced this information from (Cloudy Nights perhaps?) but few CCD manufacturers have developed a sensor specifically for astronomy usage and Panasonic is certainly not one of them, nor have they made any such claims about the intended market for this sensor.
What happened to your claims that the available QE data was useless/unhelpful/unreliable due to the fact that it is expressed in relative terms as opposed to the absolute? Has your stance changed on this point? Most of us here are using Sony sensors and are used to comparing such sensors using the relative data available on their whitepapers. Would you mind explaining how much difference there would be in having the absolute sensitivity characteristics available, when it's quite a simple matter to compare sensors using the available relative data, pixel size, read noise, well depth and other characteristics?
I'm not attempting to discredit cameras based on this sensor, just to be clear. I have one on backorder, but I intend to use it for guiding.
