The Admiral

Good capture Steve, and well done for having the fortitude for imaging at this time of year! Ian

Thanks for your comment Ken, and yes you are probably right. I always seem to have trouble with star colours and often get pink stars, and M57 should really go blue-cyan-orange and not blue-cyan-magenta. Might have another go tomorrow when I feel a bit less jaded after last night's extended day. Us olduns can't take these late nights like we used to . I hope the house move goes smoothly. Ian

Things have been a bit quiet around here; the truth is I find late nights difficult! Anyhow, I've just acquired a 0.79x reducer/flattener and I took the opportunity late last night to try it out and see if I could optimise the sensor spacing. In order to minimize the risk of trailing I used 15 second subs and only took 10 at each setting. That enabled me to find frames with minimal trailing and look at the distortion of the stars near the corners. Anyhow, as I was looking towards Lyra I also captured the Ring Nebula, so I wondered what I'd be able to get out of those 10 subs. Surprisingly, I thought it worked quite well, considering that the integrated exposure was only 2½ minutes! This is without the reducer/sensor spacing being optimised. Here is M57 enlarged. As usual, the set-up was an Altair Wave 102mm f7 SuperED APO, TS Photoline 2" 0.79x reducer/flattener, Fuji X-T1, Nexstar 6/8SE Alt-Az mount, 28 May 2016. No flats or darks used, though I did use bias frames (~50). Stacked in DSS, processed in StarTools, and finished off in Lightroom and PWPro. Cheers, Ian

I'd planned to dual-boot a Win 8.1 machine with Win 7, but now realise that the the former uses UEFI and the latter standard BIOS. I guess that you'll be confronted by this with dual booting Win10 and Linux, though I'm sure that there is help on the web, and I'm guessing you have more computer knowledge than me. I've got cold feet over my project, and I think I'll just swap drives and do a clean install of Win 7. I can happily do without Win 8.1 anyway. Ian

I agree Paul, I've dabbled a bit with Linux Mint but whenever one looks at blogs and support pages one is confronted with compu-speak, and that doesn't help me one bit (or should that be byte ). I think it is an obstacle for those who want to use Linux unfortunately, and will stifle uptake. One feels that one needs to become an expert just to enter the game. A pity, as I'm going off Windows and would like to have a back-up plan. Ian

A good plan, to the benefit of all concerned. Are you likely to get any total rejects? I presume if so, they will be returned to supplier. Ian

Thanks Neil. I'm not entirely sure what is being shown in your example - I'm not familiar with Paint. Actually, in answer to happy-kat's question about 90s exposure, I thought I'd simulate that in Capture 1. That 3x exposure is equivalent to 1.7 stops (or EV), so adjusting the exposure slider I get this. However, this time I enabled 'exposure warning', which will indicate where the value exceeds 255 by the red splotches. As you can see, the centre of M42 and other stars appear to be clipped. When I checked with the x1 setting, I still had clipping. As I'm sure that you are aware, RAW converters can 'fill in' for saturated channels in order to try to make a 'best guess' at what the colour should be, so caution is needed. Exporting the normal file as a tiff and looking at it in my favoured pixel manipulator, Picture Window Pro, using high histogram expansion, I get this: As you can see, the data is definitely piling up on the RHS, so I think in my case there is clipping of highlights. Ian

Thanks happy-kat. The longest exposure I've used is 30s because most of my imaging is of objects towards the south where rotation is at its worst, and because to go beyond that I'd have to buy an intervalometer. I should think the sky-fog peak would be well up the histogram with 3x the exposure. I'm not sure that there is much further to the right, because you'd expect to see the histogram piling up against the histogram RH edge, though admittedly with these low exposure levels that might be difficult to observe until it was extreme. I can't say whether a 5diii would be better or by how much. It would depend on the ISO it needed apart from anything else, and the Fuji does seem to have good photon efficiency down into the red. I don't think their performances are too far apart. Ian

Having looked at Samir's article, it set we pondering, and this is my take on what goes on! When we image, the light photons liberate electronic charge in each of the pixels of the sensor; the more photons, the more charge liberated. At the end of the exposure, the charge in each pixel is measured in an analogue way, but for the purposes of producing a digital RAW file it has to be converted into a digital signal. This is the job of the analogue-to-digital converter, or ADC. At 'base' or 'native' ISO, this is a straight conversion of the analogue signal. In order to give a range of sensitivities, as measured on an increasing ISO scale, the analogue signal is amplified before being passed to the ADC, so that weaker signals have a greater amplitude before measurement. There is a downside to this, however, because at some point the ADC will saturate, and this means that pixels which have received a high number of photons, and therefore contain a large liberated charge, will be above the measurement abilities of the ADC. The net affect of this is that the dynamic range is reduced, more so as the ISO values are increased. There is a further complication, because manufacturers introduce digital gain as well as analogue gain, at higher ISOs, and are often very guarded about the processes that they use, so it isn't always obvious what the native ISO of the sensor is and at what point they introduce digital gain. We don't want to operate in the digital gain region. I managed to find this information for my camera from http://www.photonstophotos.net/Charts/RN_ADU.htm Read noise increases linearly with ISO but above ISO1600 it is clear that the operation of the sensor moves into a different regime. Curves for other cameras can be quite different, according to the strategies adopted by the camera manufacturers. So what about the histogram? The histogram is a graphical display of the distribution of light intensity recorded by the sensor. It is not taken directly from the sensor, but rather from a jpeg processed from the RAW data. In producing the jpeg, a colour calibration curve and a response curve is applied in order to make the linear data recorded by the sensor appear more as the eye would perceive the scene. These curves are specific to camera models. Typically, the x-coordinate of the histogram would be linearly scaled from 0 to 256, so that no signal would be zero, and the maximum signal before saturation, or clipping, occurs would be 256. Some cameras show the histograms for each RGB colour, but others just a combined luminance curve. Because the histogram is taken from the jpeg, it should be clear that the display will vary according to the ISO setting, so that a source giving a single peak near the bottom end would have a peak which shifts progressively to the right as the ISO setting used to image that source is raised. Therefore, any recommendation to use, say 10% or 30%, for the placing of the sky-fog peak seems to me to be somewhat erroneous. I guess the main thing is to ensure that there is a gap between the peak and the zero of the histogram, but not one so wide that the rest of the curve is squashed to the right. Up until now I've not taken a lot of notice of the histogram, but I've now gone through all my images just to find out what I've been getting. The following are typical: This is M64, at 1600ISO, single frame of 30s and this is M42, at 1600ISO, single frame of 15s. Notice that in both of these one can make out a small component of the curve at the far right hand end of the histogram, and although this isn't the case for all my images, it is so in by far the majority of cases. So even with an exposure of 15s, I am still at risk of saturating the brightest stars. What I am not sure of though, is whether this really means that the sensor is saturated as well, but if it is, then it is likely that colour information will be lost for these particular objects. With this particular image, I guess I had the option of dropping the ISO to try to recapture some of the dynamic range, perhaps to as low as 400ISO, as there is plenty of room to the left of the sky-fog peak. That is predicated on being able to see the distribution well enough on the camera itself of course, as the histograms presented here are taken from my RAW processor. I could reduce the exposure time, but that implies a reduction in the number of photons captured. That is unacceptable as I want to maximise the signal-to-noise ratio to get the cleanest possible image. Here endeth the lesson Ian

No I don't think that's right, but I'm away and can't check it out. Just ignore me Ian

The x-axis on the histogram is effectively exposure value, i.e. f stop, so log to the base 2 would be correct I think. Ian

My experience with PI was not so positive, when I tried it on an M106 image (see this thread, 23rd Feb, and https://stargazerslounge.com/topic/264074-early-days-with-pixinsight-problem/). I found it very difficult to get any colour out of it, but it is probably down to my ham-fisted attempts. I came to the conclusion that it was quite hard work to get a decent output when the data is poor, unlike ST. Good luck with honing your skills. Ian

I'm never that sure about DSS scores (or the scores in Autostakkert either, for that matter), and I sort of agree with you, except that if they are relative to within a session, why isn't the maximum normalised to, say, 100%? Also, the DSS manual says: What is the score, and what is its meaning? The score is a measure of the picture quality. To put it simply, the higher the score, the more round and not too big stars were found. and Is the score a measure of the absolute quality of a picture? No. The score is a relative measure that is only used to sort the pictures of roughly the same area in order to keep only the best pictures for stacking. If the raw development process settings are modified you must register the pictures again and a new score will be computed. Ambiguous? Ian

Here's a plot I made of angular rotation rate with azimuth and altitude. Towards the South (or North) the rotation is maximised, whereas East and West it is theoretically zero. It also increases as altitude is increased (as shown by the different lines). Exposures are typically 1 - 2 hours I suppose. Ian

They are very good scores Nige. I usually find the top score is between 1000 and 2000, with a tail that goes down to single figures, and a handful with no score. Varies of course. Ian

Or perhaps worse still, the anomalies will be amplified! Ian

Thanks Nige. So for just ~10% of the total exposure time being from 90s subs, it makes that much difference. I can't get my head around that! Ian

That's a startling difference for a 12% increase in total exposure. Now my foundations are getting a bit shaken up, which I admit are a bit shaky anyway, because I keep coming back to this post, "To stack or not to stack: 30 x 1s = 1 x 30s?" (https://stargazerslounge.com/topic/245183-to-stack-or-not-to-stack-30-x-1s-1-x-30s/#comment-2668145 ). May be it has a lot to do with camera noise. I realise that longer subs do allow one to dig out the fainter objects, but in this case it's not as though you've a lot of longer subs in the mix. Puzzling. Can I ask a few questions please? When you say "85 x 45s plus 5 x 90s", is that the actual number that DSS used for stacking, or what you loaded it with? What scores did DSS give to the 90s subs? If you were to stretch the bottom image more, do you get nasties showing up? Ian

Moi! I use the X-T1 which isn't a Bayer array, and I don't fiddle with it in DSS, so notwithstanding DSS's hidden activity, I click the first box. Ian

Nige, do you know that ST keeps track of all its settings in the startools.log file, so you don't need to note them down? Don't know how it handles masks though. It's a pity you can't apply the settings instead of having to manually input them, but that's another story. Ian