symmetal

Offset is just a fixed DC voltage added to the analogue value read from the sensor pixel to avoid an analogue value of zero being fed to the ADC (Analogue-Digital Converter). Depending on the gain value applied to the signal, by the analogue amplifier also before the ADC the minimum offset required to avoid a zero value also changes. Having too high an offset limits the dynamic range available from the ADC but in reality this change in dynamic range is negligible. It's much more simple to just use an offset that avoids zero value ADC outputs at any gain setting you use as blinky says. I use a value of offset 56 on my ASI1600 at gain 139 (unity) just to be doubly sure of avoiding zero values. You using a value of 50 for all your gain settings should be fine. Alan

Happy to help Alan. Locking the focus seemed to have produced the biggest improvement and the flat blue field indicates no tilt and no curvature worth mentioning. Normally blue is the most critical for getting a flat field. You could always run a 'shrink stars' photoshop action (or similar) on just the red channel to reduce the red halos on the composite image as the red focus 'bloating' is similar over the whole field. Good luck with the L3 filter or new camera if you get them. I like widefield images too but they are more hassle to get good over the whole frame. Alan

Here's the results of your latest image through CCDI. I don't know if they're more of a help or a hinderance. Looking at each channel at full size in Photoshop or similar I think tells you more than CCDI does. The curvature and 3D plots implies large large focus variation top to bottom in Red, but in reality the variation is not that great. CCDI scales best to worst in black to pink so a large or small variation can result in the same graph shape. Between black and dark blue the difference is slight. The blue channel is showing fairly consistant focus over the whole frame though. The small variation in FWHM indicates this. All the stars are very slightly oval so I think there was a slight tracking/polar alignment error causing the rather high aspect scores overall. Blue has uniform aspect ratio stars over the whole frame. As you say getting the narrower UV/IR bandwith filter could well help with the red. Your focusing seems to be optomising for blue, so maybe offset the focus in a direction which helps the red without upsetting the blue too much. trial and error will be needed to do this. Curvature 3D Curvature Star Aspect Ratio Hope you get your full power back again soon. Alan

Hi Alan, Hope you're keeping safe and haven't had any damage yourself. We're due to have a storm hit over the weekend, but no snow forecast for me this far south (yet). On a quick check your latest image is much better with good star shapes all over. The elongated red stars on the right of your previous image are now much improved. The red channel is still more out of focus compared to the green and blue but again better than the previous image. I'll do a CCDI analysis later and post the results. Alan

Just to clarify the 3D plots from CCD Inspector. The R and B seem to show the curvature in opposite directions but they may actually be in the same direction. It's probably more likely they are in the same direction. CCDI has no way of knowing whether the out of focus/shape stars are in front of, or behind the plane of focus so it just displays these poorer stars in the 'up' Z direction, the worse the star the higher value of Z. For each plot the best focus stars are given the lowest Z value and all other stars are plotted as having a higher Z value. If the best stars are in the centre the curvature will generally be a 'bowl' shape like the B image above. If the best stars are towards the edges the curvature will generally be a 'dome' shape like the R or composite image above. The colours used (black to pink) are relative indicators for that plot only. So a pink area in one plot doesn't mean it's as bad as a pink area in another plot. All the images shown go from black (good) to pink (bad), but the Green channel 'bad' is a lot better than the Red channel 'bad'. I believe the curvature and 3D plots just take the star FWHM values. If you look at just the Red channel on Alan's image the stars are very elongated on the right hand side but CCDI gives these the best 'scores' on the curvature, and 3D plots (lowest Z value) while the circular but out of focus stars nearer the centre are given a poor score (high Z value). If you view the star aspect ratio plot as shown below it does show the Red channel as being worse on the right so in reality all the various plots have to be looked at to show what's actually happening and not just rely on one plot. CCDI displays problems as percentages with the higher the percentage the worse the problem, but a percentage of what I don't kinow. What does 100% curvature, tilt or aspect look like. Alan

Yes, no problem Alan Hopefully your focus lock has improved things. Alan

Actually, thinking again, the CCDI results could be seen as more useful with OSC in that they do show where your focus problems are for each colour which you couldn't do if it was a mono camera with a Luminance filter. This would just give effectively the composite image as Billy posted and you may try correcting tilt etc. when it's seems to be primarily a Red focus problem. Alan

Yes, you're right, I meant to put L3 I don't think CCDI is so useful with OSC cameras. Here's the results on the separate R, G and B channels along with the composite. Note the large differences in tilt and curvature (and in different directions) depending on what colour you're looking at. Alan

Looking at the individual colour channels the red is significantly out of focus over the whole image compared to the green and blue. The green shows the best focus with the blue almost as good. Also on the right side of the image the green and blue channels are still pretty good with the elongated stars being caused by the red channel. As it's a OSC camera it would seem to be too much IR getting through. The L2 Astronomic filter has a wider passband compared to the L1 and may not be so suitable for this scope but I'm surprised it's causing this much blur on the red channel. Alan

The 17-32 image is rotated 180 compared to 1-16. Did a flip occur between 16 and 17? This would imply light leakage getting in somewhere. Do you perhaps have a tilt adjuster in the train? I always put black tape around the edge of the adjuster as it can be a route for light to leak in. Alan

You could drill holes in the existing base and fix the steel rods in the holes with 2-part concrete anchoring adhesive. You could then spread concrete bonding agent on the existing concrete before pouring the new concrete pillar on top. Alan

In the situation where you had a significant load on both batteries you may end up with battery 1 not reaching full charge and if you had that set to 100% priority then battery 2 would never get charged. 90/10 priority ensures battery 2 would at least get some charge. Once one battery is fully charged the other one gets the 100% charge anyway from the solar panel. If the load on both batteries is similar then a 50/50 priority may be best to avoid battery 2 not receiving enough charge on dull/short days. If the solar panel puts out enough to comfortably charge both batteries then the priority is not really important. Alan

It looks like the default priority of 9 will be like you assumed. If your solar panel delivers 1A for example, then battery 1 will get 900mA and battery 2 will get 100mA. When battery 1 is fully charged battery 2 will then get all the current. If battery 1 starts discharging then it will go back to the priority setting currents again. It doesn't say what happens to battery 2 when both are fully charged. Hopefully it stops charging like battery 1 does when it is fully charged. If the normal load on battery 2 is higher than battery 1 then you can lower the priority number so that battery 2 gets a bigger share of the solar panel current. If you set it to priority 5 they will each get half the solar panel current. All four of your batteries need to be of the same type. I assume they are from your previous post. Just different capacities. Battery 1 can be your 2 60Ah in parallel and Battery 2 can be your 2 50Ah in parallel. Or vice-versa. Alan

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

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

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

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

Thanks Han, that looks like it will be very useful. 😀 Alan

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

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

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

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

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

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

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