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symmetal

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symmetal last won the day on December 21 2019

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About symmetal

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  1. CCD astro cameras give a 16 bit output so a pixel value can be one of 65536 values (2^16 = 65536). These values are called ADU (Analogue Digital Unit). The histograms on capture software display these ADU values, 0 is on the left up to 65535 on the right. Ideally you want the histogram of your important image data to be within these values. To see the faint areas of the image you have to expose long enough that the sky background is above ADU value 0 which means the left end of your histogram is not clipped off at the left hand edge. Bright stars will then invariably be too bright to be correctly exposed and will be clipped at the right edge of the histogram with an ADU value of 65535. Clipped values are lost forever and can't be recovered. CMOS cameras generally have fewer than 16 bit Analogue to Digital converters, 12 or 14 bit is common. The astro CMOS camera will normally add extra bits of value zero to create a 16 bit value output so that the capture software histogram display still shows 16 bit ADU values. DSLR cameras, I believe, output a 14 bit value in their raw format so the capture software adds 2 zero value bits to make 16 bits again. This means histogram displays can be compared no matter what the camera bit depth is. If your flat frame peak is in the middle of the capture software histogram it has a value around 32000 ADU. Usually you can hover your mouse over your captured image and the captute software will display the ADU value under the cursor. So when people talk about a certain ADU value, it has the same meaning independant of the camera producing it. Your DSLR most likely doesn't have a linear display histogram as mentioned previously, so a value of 32000 ADU as displayed in the middle of the capture software histogram will likely display more over to the right on the camera display. David likes the flat frame peak around this 32000 ADU value, while I'm happy as long as the edges of the bump are not clipped at the sides of the histogram. Alan
  2. David, your first reply reply confused me as you said 'half the full well depth of the sensor'. Your 6D has a full well depth of around 80,000 electrons at its lowest ISO. If this is ISO 100, then at ISO 1600 the ADC saturates at about 5000 electrons. I think you meant to say half the saturation value of the ADC. The DSLR camera histogram display is often not linear (like the default linear histogram on most capture software) and stretches the dark half more. So half ADC saturation is more towards the right hand side on the camera display. However, the flat frame calibration maths is Calibrated (x,y) = Image (x,y) / Flat (x,y) * (Average Flat pixel value) As long as the flats histogram is not left or right clipped you get the same calibrated output values irrespective of the actual ADU values of the flat, assuming a linear response of the sensor. If the sensor response is not linear it would actually be more accurate for the average flat exposure ADU value to be more similar to the actual image average ADU value. We assume a linear sensor response to make things easier and so just need the flats to be not clipped. Putting them around the middle of the histogram is convenient to ensure this but is not necessary. In the Craig Stark presentation around the 50 min mark be says something like as long as the flat exposures aren't 'banging into the endstops' they'll be fine. I would say yes you are. Good luck! To avoid the camera shutter or a 'flickering' flat light panel possibly causing exposure problems just ensure the exposure is at least around a second or so. I don't see any reason not to use a lower ISO for the flats in order to get a longer exposure more easily, as long as you take a separate set of 'bias' frames at that ISO for the flats calibration. Alan
  3. As long as the histogram peaks are on a linear portion of the sensor transfer characteristic is all that matters I would have thought. With modern cameras this is pretty much anywhere as long as it's not black or white clipped. Also of course not clipped by the ADC, so away from the edges of the histogram satisfies both requirements . I use my ASI1600 at unity gain always so ADC full well is 4096 e- (electrons). To get half sensor full well of 10000 e- it would be white clipped, unless I switched to minimum gain on the camera. I just take flats at the same gain (or ISO on a DSLR) as the lights. Saves having too many variables. With Anthony just starting out using flat frames this should make it easier. Alan
  4. Where it is is fine. As long as the peaks are well away from the left and right edges is all you need to do. Alan
  5. Thanks Han, that looks like it will be very useful. Alan
  6. As James said, the pinout shown in the manual is looking into the socket from the outside. The tab on the RJ12 here is on top. Many RJ plug pinout drawings show the tab on the bottom which is why the numbering looks reversed on the diagram. If you have a standard FTDI USB to RS-232 cable (TTL or 3.3V level version) with six wires coloured Black, Brown, Red, Orange, Yellow, Green then connect as follows FTDI Black (GND) to RJ12 pin 5 GND) FTDI Orange (TXD) to RJ12 pin 4 (RXD) FTDI Yellow (RXD) to RJ12 pin 2 (TXD) I find the FTDI cable wires are a little to wide to easily fit into the crimping channels in the RJ12 or RJ45 connectors so I usually splice the FTDI cable to a ready made RJ12 or RJ45 cable with one end cut off. See how you go. Alan
  7. Do you have very dark skies gorann? I reach the sky background 10xRN^2 level at around 60s for lum with my Bortle 3 skies. For gain 139, offset 50 as you use, the sky background10xRN^2 ADU level (16 bit) is 1290. If the median of your subs is much greater than this you can take more shorter subs and so reduce star bloating and get a bit more dynamic range. Alan
  8. Yes, I use unity gain (139) all the time for all filters. Big advantage is a lower collection of darks needed and you don't have to keep checking what the gain setting is. Alan
  9. Oops, you're right . I've gone back and changed it. Further to my previous post, increasing the amplifier gain increases the signal shot noise too, so the total noise contribution from the ADC decreases. Post 3 from this CN post explains it more fully. Alan
  10. The read noise is only expressed in electrons rather than micro volts etc., so that the graphs have the same units. The read noise from the ADC is the same, so the higher the signal input value to the ADC, the higher the S/N coming out. Alan
  11. The ADC is 12 bit as you say and the gain is in the amplifier before the ADC. To give a 16 bit output the 12 bit value is just multiplied by 16, so adding 4 least significant bits of zero value to the 12 bit value to give a 16 bit output. I need convincing as to whether using gain values less than unity has much benefit. The camera ADC can only determine 4096 levels so the 20,000 well capacity, along with its dynamic range can only be converted to digital by reducing the pre-ADC amplifier voltage gain to 1/5 of it's unity gain setting. One 5 min exp at gain 0 must give the same dynamic range as stacking five 1 min exp at unity gain. The only difference is 5 read noises at unity verses 1 read noise at 0 gain. The read noise at gain 0 is significantly higher than at unity as the bulk of the 'read noise' is caused by the ADC itself and not the amplifier or read out circuitry before it. If the read noise is swamped by the skyglow then it becomes insignificant anyway. The stacked read noise of 5 unity gain exposures I'm unsure about as this article states that the read noise value must be squared before adding it to the shot noise and dark current etc., and then taking the square root of the result as the overall noise. This implies the read noise is more significant. Hopefully vlaiv can explain this bit if he sees it. Alan
  12. Yes, I pay £9.98 a month for full Photoshop CC along with the extras Terry mentioned above. I don't mind paying that as I use it for a lot more than astro stuff. Alan
  13. Filter wheels are made such that the thread points towards the camera. Likewise when fitting a filter onto an eyepiece. The more reflective (mirrored) side should then be pointing towards the sky, which is what you want to minimize reflections in your images or views. As you say some imaging trains force you to mount them the other way way round. The 2" UV/IR cut in my ZS61 used for imaging with a OSC camera has to go thread towards the sky. However both sides of this filter seem to have the same reflectivity so it doesn't really matter which way round it is. For unmounted filters there should be an arrow on the rim of the filter (for those filters that matter) and the arrow should point towards the sky. Here's an answer from Baader-Planetarium on the subject. Alan
  14. Great new tutorial. I based my workflow on your old tutorial and was never really happy with the junction between the surface and proms after processing them separately. I like your dodge and burning method on the proms now and processing the whole image at once. I'll try it on some of my images I took last summer to see how they turn out. Alan
  15. I agree with kens post. Here's a chart I'd previously made showing the sky background level to achieve the read noise swamping values that kens mentions. I use unity gain all the time (laziness to avoid multiple darks). Also the offset used affects the sky background level required. I use a constant offset high enough to avoid black clipping at any gain setting. The sky background is pretty much the median value of your image, if the image doesn't have large amounts of nebula in it, or hover the curser over the darkest area of your image to get the actual sky background value. If you use different gain or offsets to those shown I can make a chart specific for you if you want. I did post the excel spreadsheet file on a previous post so you can fill in your own values on that if you have excel or OpenOffice. These are really only useful for LRGB imaging. For narrowband, you'll never achieve the required sky background level unless you're in a very light polluted area. I would need to expose well over an hour to achieve it with Ha so I just expose as long as convenient, normally 600 seconds. Half unity gain is actually 79 and twice unity is 199 but I've used the figures Zwo prefer. (6.0dB (or 2 x gain) is 60 ASI gain steps) With my bortle 3 skies I find 60s for L and 180s for RG and B gets me close to 10x RN^2 sky backgrounds. Alan
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