The Admiral

Thinks! Would I be right in saying that if you 'restore' it'll offer a number of previous stages to select from? I haven't ST handy to check at the mo. Ian Edit. I've also added to a feature request on the ST Forum to automate the log file actions. Any support there would be helpful! http://forum.startools.org/viewtopic.php?f=6&t=1039

Very nice Jon, good to see a complete arc and the Running Man as well. True, an APO does give you one less hassle. It would be interesting to see how much difference the filter actually makes. And to think that you weren't sure about taking up AP with your gear! I hope that you find the experience encouraging. Ian

You're not alone Nige! It's a complicated technical subject and I feel I'm only just scratching the surface and even then it's a bit of a challenge. I've just been looking at a book called "The Astrophotography Manual" and there is a short section in that which I think at least sets the scene. I haven't bought it . It covers lots of things I'm not interested in at the moment, but Google has this book on free to view, so if you are interested have a look at the section "Image Capture", starting on p118, here I think*: https://books.google.co.uk/books?id=r5vlCAAAQBAJ&pg=PA154&lpg=PA154&dq=the+astrophotography+manual&source=bl&ots=xLb-rLghv2&sig=VOeTf6KznNpecy_LmKq3BvirofM&hl=en&sa=X&ved=0ahUKEwiXt5qq4aLQAhVKFMAKHeZeClEQ6AEIRDAH#v=twopage&q&f=true I'd say it's probably describing the situation in the least technical way. There's a little on filters and binning as well. The Photonstophotos site is rather deep, somewhat out of my depth currently, but I'm determined to make some headway! Hope that helps! Ian *Edit. No, it takes you to a PixInsight page, but you can switch pages on the top right.

Well, I really need to brush up on all this , but just to throw in some extra bits. The reason I said that the read noise would halve was because there would be half the number of subs, but OK, of course the read noise for each sub will now be different because of the different ISO. I haven't been able to track down the read noise in e terms, but I have come across this site: http://www.photonstophotos.net/ where I can compare the results in terms of DN (whatever that is?) with other cameras. Comparing your EOS10d, EOS450D, and my XT-1 (http://www.photonstophotos.net/Charts/RN_ADU.htm#Canon EOS 10D_12,Canon EOS 450D_14,Fujifilm X-T1_14) Clearly, these cameras behave in a very different way! Comparing the new XT-2 sensor with the XT-1, that is different again (http://www.photonstophotos.net/Charts/RN_ADU.htm#Fujifilm X-T1_14,Fujifilm X-T2_14) Apparently, the XT-2 uses a dual amplifier system before the ADC, and this results in the stepped read noise response with ISO. In other words, there isn't really a simple answer to this, as all cameras have some differences in their characteristics. Ian Edit. Ah, I see you have the Nikon D520. That isn't on the list on this site, unfortunately. But some cameras show some amazingly complicated variations with ISO.

My take would be that as it's a nice bright target I expect the ZS66 would get away with it, and save you blue bloat. It will have just over 3x less light gathering power, so you'll need to up the number of subs. Not advice with any practical experience behind it mind, just a gut feeling. At best we can expect seeing worse than 2", so as you say you are more than likely to be oversampling a little. Try it and see, but I sympathise that this weather doesn't exactly encourage experimenting! Ian

Sorry, I don't understand, why would you need to double the number of subs? OK, let's start from the premise that you want to record for a given total amount of time, and collect the same number of photons, so that we are comparing like with like. You have a choice of imaging at either 1600 or 800. If you image at 800 you will need subs twice as long in duration as at 1600 to get the same result, but only half as many. On the plus side of using 800 compared to 1600, there will be: half the read noise each pixel will accrue twice as many photons and create a signal at source twice as large. Even though this is amplified with a gain half that of the 1600 case, surely this must be better statistically will the sensor noise be doubled? If we are following the sensor with half the gain, why would the sensor noise double? Wouldn't the effects of reduced gain and increased sub length balance out? the signal is comprised of wanted signal and skyglow signal. Both will double but the difference between the two components will double so would be statistically easier to separate. On the down side, the sensor will not operating sub-optimally. So at risk of conflating all these effects incorrectly, wouldn't we be better to use 800 rather than 1600 overall? After all, isn't that why EQ imagers like to have subs of many minutes in duration, presumably with the gain on their astro-cameras turned down (cf. ISO) to keep sky-glow in check? Well, I'm trying to work it out in my addled brain as I write. I think I need to get back to first principles but you have my initial reaction Ian

Yes that's a good one, but if you think it's still a bit dark try stretching even more and see what happens. Nothing ventured and all that. You might reveal the complete arc a bit more, which is certainly visible even now. Pity you've only got half of the 'running man'. If you did image it again it'd be worth seeing what a vertical format will do. Then again, if you use the '66 it'll have a wider FOV. I'm getting lost amongst all these filters! Which one's the fringe killer, and how does it differ from the semi-apo? Ian

Is that because at 100ISO the sensor is operating below its native ISO? Ian

OK, I can see that the 100ISO is far noisier, but the read noise surely won't stay the same because if you reduce ISO you will increase the sub-duration and therefore you'd need fewer of them. If you changed ISO from 1600 to say 800, you'd double the sub duration for the same result, and only need half the number of subs (i.e. same total duration). Consequently the read noise would also halve. And the signal will be twice as large. So too might be the dark noise, but that can be subtracted out. Or is my logic illogical? Ian

I guess Nige that with a Newt hanging off it a wider stance is likely to prove more stable. Ian

Not quite sure why that should be so, as it looks as though the semi-apo cuts more into the blue than the Baader Moon & Skyglow + UV/IR cut combined. Still, if that's what you found, so be it. Ian

I use my Nexstar with only about a foot of leg extension. I can't prove any benefit but it strikes me that it ought to be a little more rigid. Ian

But then this comes back to the question: "Do you gain more by keeping the exposure duration long but at a lower ISO, and so capturing more photons, than you lose by reducing ISO below optimum?" Ian

Ah, OK, so really this is the "expose to the right" principle you'd use in conventional photography, in order to get more out of the darks in the image and to minimize posterisation, yet not clip the highlights. For an object with a very large range, like M42, which might exceed the DR of the imaging system,one can additionally adopt the process of blending two or more images using different exposures. Or HDR as it is conventionally known. Ian

Could that be because RAW is linear, and ISO works on the RAW, whereas when we observe our image the data is first modified by the application of a gamma curve, I wonder? My brain won't compute! See for example http://www.cambridgeincolour.com/tutorials/gamma-correction.htm Ian

Hmm, hang on, I'm not sure it works like that does it? Surely bit-depth has nothing to do with dynamic range (DR)? If I understand correctly, increasing ISO will increase the (analogue) gain and will amplify the signal from the electrons in the wells. This is passed to an analogue-digital converter to digitize the output. The trouble is, ADCs have a maximum input before saturating, so as the ISO increases, this saturation limit is met by correspondingly smaller and smaller outputs from the sensor. In other words, the DR reduces as the ISO increases. A further complication is that cameras use digital gain at some ISO value rather than analogue amplification, and that point is camera specific. I also believe that the bit-depth relates to the conversion by the ADC, so whatever range of inputs and ISO, the ADC range will be covered by that number of bits, or levels. That's how I've understood the case, but am prepared to be educated Ian

The Nikon D3200 does have the blinky option. That is, flashing highlight warning in review and replay. See for example https://www.dpreview.com/reviews/nikon-d3200/6 Ken summed up the situation re. background level quite succinctly. You need to stretch more in ST to bring up the background, but don't fret too much if it looks noisy at that early stage in the processing sequence. Ian

2am!!! If only I agree with Ken, the background is a bit dark. If you stretch the image a bit more in ST in the develop (I use manual, not auto), you can bring up a bit more of the hidden detail. At the same time, you'll probably think that the background is too noisy, but go with it because by the time you've performed all the other actions you'll find it'll quieten down considerably. At the end of the day it is easy to adjust the black level in a photo application if you're not happy with it, certainly easier than trying to bring out otherwise hidden detail. And of course, the more you stretch the image the more stars you'll likely see, though this doesn't appear to be a particularly star rich field anyway. Still, an excellent image, but I suspect that there is more to be had from it. Ian Edit. It could be that your PC monitor is the correct one, especially if you've calibrated it at all in the past. I think these mobile devices tend to make images look more vivid as it's perceived that's what the general public want/expect. I find quite a difference with mine compared to my calibrated PC screen.

You obviously seem to have a good handle on the way your sensor is performing, so I'm wary of teaching you to suck eggs here. I'm interpreting what you say, perhaps incorrectly, that you are concerned with clipping at the low end of the histogram, which is important not to do, so I understand. But of course what is important here is whether the stars are clipped at the top end, and I find it well night impossible to tell from my own histogram display whether that's occurring because they are such small peaks (though switching on blinkies might help). The only way I've checked this is to put the files through a RAW developer and inspected the centres of the stars. I find some are, some aren't, when I use 30s at ISO1600. This discussion has been useful because it's made me think a bit about how I use my ISO. When I started imaging I used 100 x 15s lights on M42. M42 is very bright, and from that perspective is easy to image, but there is also a wide brightness range that needs to be captured. For my next attempt later in the year I had planned on using a mix of short exposure and long exposure subs, but now I'm minded to use the same maximum exposure length and reduce the ISO for a set. That way it should (a) increase the dynamic range of the sensor, and (b) allow more photons to be recorded per sub which should improve the SNR. If it works that way! I have to say that my sky background isn't horrendous, so 30s at 1600ISO doesn't put the peak of the histogram too far up anyway. Note also that I use the Fuji-X sensor which has its own proprietary colour filter array, and as such there is no readily available data as can be found for Canon and Nikon sensors. Ian

I'm in a bit of a quandary over this. Assuming that the target is small enough, should I use the native FL of 715mm, and crop the resultant image, or use my 0.79x reducer and gain a little bit of speed, but need to crop rather more? I guess it rather depends on how much resolution on the target I'm going to achieve. Without the reducer and with my 16Mp APS sensor I shall get about 1.4"/pixel, whereas with the reducer it'll be about 1.7"/pixel. Both are below the recognised value of 2" for the seeing in this country, so I imagine on that basis it won't make any difference to the final image and I might as well go with the reducer to get as many photons on each pixel as I can. The deficiencies in mount tracking will be more evident in the image without the reducer, but as the image with the reducer will need to be cropped more, this will cancel out (assuming the target ends up the same size in both images). Is there any other factor at play here that I should consider? Ian

Try adjusting the colour bias sliders using the "Max RGB" option. You may still not be able to get rid of the green, but you can get an overall impact of the colour changes you are making. See page 42 of the manual. Ian

I like to think of a Barlow as spreading the photons over a larger area of the sensor, and therefore giving rise to magnification, but each pixel will receive fewer photons and hence the exposure will need to be longer. I don't know what percentage of photons are 'lost' in the optics, but with good coatings I shouldn't think it will amount to much. Or am I missing something here? Ian Edit. Ah, perhaps you mean that the exposure times you can get out of the mount when using a Barlow are shorter!

Jeez, I've not tried drizzle for the very reason you give, mammoth file sizes from a DSLR! But it doesn't seem to be an easy target; although it's large it's not very bright and I think much competition from sky-glow doesn't help. I ended up taking over 300 subs and only stacking about 200, but even then I found it hard to process, to give at best a just about acceptable result. Still, I would guess that with the right manipulation of ST you could get a bit more out of that. Ian

That's a good start DorsetBlue. When I imaged M13, for one reason and another I only had 26 x 30s subs from a 102mm 'scope to stack, but that still gave me a reasonable result. I think that your 10 subs need to be increased somewhat. But experimenting is what it's all about (or would be if only we had a lot more usable nights!) Ian

A nice rendition there Matt. That's one advantage of imaging stars, they don't need such lengthy total exposures, and by keeping the sub exposure times short you run less risk of saturating them (though of course that will be more of a problem with DSLRs). Ian