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These are the graphs for 100 and 800 iso, from images of a white sheet, uniform and constant illumination taken with nikon D3000
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Hi, Grant
It is actually not a good idea to do photometry with jpg formats. Unfortunately I have few resources and have to make do with those. However, with 256 levels you span a range of about 6 stellar magnitudes. And apart from the smallest variation still observable (sensitivity), 6 mag is not exactly small. For what you say, you are absolutely right, but I think that if, for example, I have to measure the height of a tree but don’t have a tape measure, what do I do? Do I leave it alone? No, I do it by eye: the tree is 8 m high + or – 2 m; I can tell at least that it is a cherry tree and not an olive tree. Also, PV-exposure linearity would be necessary to reduce PV measurements all at the same exposure time.
In my case, everything is more complicated. The attached graph shows PV-time for two iso values 100 and 800. I think I can solve the problem by observing that the two curves are similar to the charge function in the RC circuit. Of this it is not difficult to calculate the time constant, which is done for these two graphs. The awkward problem is having to apply the same mathematical algorithm to each photograph in order to use it. Robin’s graphs would be ideal for me, but only because the equation of a line goes in place of the charge law of a capacitor.
Of course, I will have to get the soft stuff you suggest.
Thank you.
Translated with DeepL.com (free version)Robin, the answer to my dilemma can be found in the AAVSO book you recommended, which I did not know.
The non-linearity between pixel value and exposure time is a peculiarity of the CCD sensor. My DSLR actually mounts a CCD.
“4.5.2 Exposure time, saturation, and non-linearity
Non-linearity is even more dangerous in photometry because it is less
obvious to detect than is saturation. Fortunately, DSLR cameras now exclusively use CMOS sensors that
do not have the problem of non-linearity that CCD sensors do.”
So, for me, three possibilities are proposed:
1- Abandon my observation programme;
2- Buy another camera with cmos;
3- Devise a mathematical procedure for the reduction to the same exposure.
I intend to rule out 1 a priori. I am heading towards the third, as the second is not for my pocket.
GreetingsRobin, today I promptly took RAW images (NEF nikon) of a uniformly lit white wall. As usual just varying the exposure time (from 1/50 to 1/3200); measured the Pixel Value on each RG&B channel with the soft AIP4Win. Unfortunately, the result is identical to the jpg: graph similar to the charge curve of a capacitor in RC; asymptote (maximum value) always 255. Probably my computer inexperience is deceiving me. You present a graph that is not only linear, but with values on the ordinates also much higher than my 255. I would ask you for as explicit an explanation as possible. Thank you
Robin, I am very pleased that you have been so attentive to my troubles. And for that I take the opportunity to learn more. I only point out that the PVs in my graph are extracted from the G-band of white surface images. With RAW I cannot operate, I have to learn how to do it. Now your graphic result unbalances me quite a bit. It is exactly what I would like to have with my nikon. For the last 5 years I have been observing the colours of sunrise and sunset by mathematically processing RGB images, but without the need to do photometry. Now, I intend to start systematic observations of the brightness of the sky at zenith in the 3 RG&B bands. This is why I need the numerical pixel values. Unfortunately, my problem is also English, which I know very little. Reading and understanding just a few pages of a manual is a real feat; if it is American English, I often have to give up. The translation into English of articles I write in Italian I always entrust to a professional translator, which is not easy to find here in Italy. My wife could help me, but you know how women are, especially wives lose patience immediately. Which is obviously not allowed to me, when there are jobs (especially difficult and heavy ones) to be done around the house: immediately and done right! Anyway, I will follow your instructions very carefully.
Sincerely regardsDear Fellow Astrophiles
thank you for providing me with information and links, which are valuable supports for better direction of my studies. I immediately read all the parts that are most relevant to my problems. However, the fundamental problem is not addressed: what is the general characteristic PixelValue- Exposure? The magnificent articles report precisely such graphs, but only for narrow shutter speed ranges. In that case, the general curve certainly can be confused with a straight line. Therefore, a PV-time proportionality relationship can be considered valid, and thus it is easy to reduce any photometric reading to a single shutter speed. However, when the shutter speeds are very different from each other (e.g. varying from 1/200s to 4 s), a curve emerges, virtually similar to the one in the figure. Reducing the PV readings on each frame, e.g. on those taken during a lunar eclipse, becomes difficult. Apparently no AA considers this problem.
If anyone has addressed this before I would be happy to exchange some thoughts with them.
Again, many thanks to you all.Translated with DeepL.com (free version)
Yes, they are PV read on jpg format images. The RAW format I have not tried, because matlab does not accept RAW so it takes a lot of fiddling. What is a soft that allows me to quickly read the PVs of a .RAW frame?
Really good result! Well done, congratulations.
I repeat that I have tried many times but with much less success than this. After a few weeks, halos of all colours always appeared on the mirrors and the Ag film started to peel off at the slightest attempt at cleaning. Just one question: how long does this silvering last?Hello everyone
I am stepping in to ask for some information. I receive the Journal, but up to a month late, some issues don’t reach me at all. This has been happening for a couple of years. Can anyone tell me if there is a problem with correspondence to Italy? Of course, I read everything online, but the fact is that I always wait for the paper version, I await it with greater anxiety than for my lover’s letters.
Thanks.Translated with DeepL.com (free version)
It must be admitted that Prof. McKim knows more than the devil. Thanks for the valuable information, Prof. I really didn’t know about the inverted mirror and other procedures you mention. Nor did I know about the possible effect of an atmosphere polluted by sulphur dioxide and hydrogen sulphide. And concerning this, the various attempts at silvering I made in the chemistry lab of the high school in the city where I teach (physics), a very industrialised city. I will not attempt to silver again, but I will at least theoretically explore the topics presented by McKim, to whom I extend my thanks. Thanks also to those who have considered my intervention.
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Good morning everyone. I am a bit surprised to find this topic, I didn’t think the problem of silvering the telescope mirror still existed. And even more I didn’t know about that silvering kit. The first mirror silvering I commissioned from a well-equipped glassmaker, who did a very good job. The only major problem was the minimal durability of the Ag deposit, which began to oxidise and yellow. After a few months (max 6-8 months) it had to be removed and re-done. Then I tried to work on it myself by buying Ag nitrate, ammonia, H2O and more. Very bad result! Longer life (up to a year) but silvering with dark spots, uneven deposit, minimal adhesion so cleaning impossible, and other defects. Everything has made me prefer mirror aluminisation. However, here in Italy, aluminisation is made by very few companies and costs a lot. So I gave up the resolution power of a 200 mm and opted for a smaller (90 mm) catadioptric that is very handy and inexpensive. Nevertheless, the D200mm dream still haunts me. I would kindly ask for your opinion on what I have reported. I am a self-made astrophile, and as a basis I have the manual: Procedures in Experimental Physics by John Strong.
Thank you and greetings to you allTranslated with DeepL.com (free version)
My enthusiasm for astronomy is ebbing away.
Concerning the weather, I think something is happening perhaps beyond what the researchers are reporting. Here, in central Italy, we have had a winter very different from previous ones. In my mountains (Gran Sasso d’Italia), snow was always abundant from the end of November to the end of April. On the Campo Imperatore plateau, at an altitude of 2100 m, where the astronomical observatory is located, at least 1 m of snow. By contrast, last winter was disastrous for skiers and for all tourist activity. The attached photographs are a comparison between the normal winter situation on the plateau and that of winter 2024. All taken in the first week of February. During the winters of the past years, we have always had frequent very clear and cold nights down to -10 C. Using the telescope on the moon and planets was a delight. This year, on the other hand, cloud cover was more frequent than usual and the sky was less transparent, so there were very few night frosts that we did not have to use antifreeze spray on our car windows. The month of May allowed only a few observations. This morning I went out with my two dogs and wore a padded jacket because of the black sky and cold air. Summer had already broken out in the past. All this together with light pollution (from my terrace you can read the newspaper in the middle of the night) is dampening my passion for observational astronomy. No more shopping for telescopes, ccd cameras, go to’s, eyepieces, mounts, filters, mechanical work and as many other accessories as we want.
Alan, I agree with you.
I can’t take it any more! I have decided two things:
1) I will devote myself only to the study of Astronomy;
2) better enjoy that money by inviting a beautiful woman to dinner. Just tell me I’m wrong.LIGHT POLLUTION PROBLEM? IT DOES NOT EXIST!
Here in the beautiful country (Italy), lights are scattered everywhere and directed almost randomly, even upwards. They mostly use illuminating globes and LED spotlights directed towards everything that can be illuminated. Not only are the neighbour’s lights dazzling and annoying, the public lighting is also bad. A 120×100 car park, 200m from my house is lit by 54 street lamps of 250 W each. In this climate, the absolute indifference of private individuals and institutions reigns. The anti-pollution law is not even in the administrators’ drawer. The motorist who forgets to dim his headlights is fined, but the headlight of the certain shop is ignored, or the headlights of the supermarket and the motorway that point towards the road and dazzle all night long. What if one appeals? You get a good chuckle. Summary: the problem of light pollution does NOT exist here.Alan, I agree with you, the effect caused by planes is certainly not minimal. I too have observed many aircraft contrails in the air. The phenomenon has become more frequent since about a year ago. And I will tell you that I am now doing research to find out two things: 1) the photodissociation time of the combustion products ejected by planes into the atmosphere; 2) the floating time of the same in the air. The vast majority of discharges occur at the upper limit of the troposphere (10-12 km altitude), where the atmosphere (close to the stratosphere) is on average rather static. Therefore, the ground deposition time of long-lived photodissociated products will certainly be of the order of months to a couple of years. Unfortunately, the vast majority of people do not understand this. As you say, it is enough for many to eat, drink and watch the stars (the stars as it were) on television. This thoughtlessness is leading us to an irreversible situation, and afterwards those who want Christ pray to Him.
Hello
I am not too surprised by what you say about the frequency of cloudiness in the sky, I have always known that in the UK clouds and rain are frequent and inclement. However, even here in Italy some worsening variation seems to be happening, skies more often cloudy and above all less and less transparent. I am neither a visual observer nor do I take photographs, but for several years I have been observing, with a camera, the colours of the sky at sunset and sunrise. Well, for about a year now, the number of clear evenings (i.e. followed by clear nights) seems to have decreased; from an average of 6-8 clear nights per month, we seem to have fallen to 2-3.
Another thing I have noticed is the decreasing consistency of the snow in the Gran Sasso chain. Until a few years ago, one climbed the highest snow-covered peaks (over 2500 m altitude) with a boot and crampons, sinking a maximum of 10 cm. For a few years now, people have been sinking down to their knees and beyond, and the avalanches (and unfortunately also the victims) have increased. It’s the effect of climate change, agreed. But I can’t help but wonder if this is the only cause or if there is an external cause.I always hoped for their title.
https://www.youtube.com/watch?v=P8Xk2e3ApiA&ab_channel=JORGEC
What a beautiful sky, all black. Back then, the sky was filled with clusters of stars, which you could touch with your hands, even from the cities. Remember?
Unfortunately today our starless sky is all white. I see white, it’s over!
I have carefully viewed the articles posted by Mr Christopher. They are very interesting, well laid out and equally well developed topics. I am interested in their content, and for this I thank the author for making them available in this forum. For a few years now, I have been interested in sky transparency and observing sunrise and sunset light in order to detect the ‘optical depth’ parameter. I have a small archive of data from about 300 observations. I will try to identify the possible existence of a correlation between the theory Christopher applies and the one I use for my observations. I hope this will provide good guidance for further experiments and studies. Thank you Christopher.
The Astronomical Observatory at Campo Imperatore, today 30 minutes after sunset.
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I read these remarks of yours with great interest, and I am actually amazed that such educated and reasonable people exist. You speak of respect for animals even with enlightenment. How nice it would be if it came to that in my region too. Here, in Gran Sasso, wild animals of various kinds abound, but there is no consideration or respect for them. The fox is bad because it plunders hen houses, the wolf is the bad wolf from the Little Red Riding Hood tale. Not to mention the poor bear that even enters some villages in the national park and arouses keen interest among tourists. Two months ago, a bear with two cubs approached the fence of a house; the owner shot it twice in the back. The poor animal left the two cubs at the mercy of anyone, even wolves. Imagine if our administrators, our politicians, our technicians and how many others would dwell on blue light pollution and the effects on mammals (including humans) and insects. In my city, the old sodium-vapour street lamps have been replaced by LEDs. In an unused car park of 100X80 m2 there are 54 of them! Many point at 45°. Some lampposts also stand radiantly in the forecourt of the astronomical observatory at Campo Imperatore. Are we talking about a starry sky? For many it is something that does not exist. Faced with this marasmus, one should only do two things: be ashamed and correct oneself. But who does?
Unfortunately, when it comes to light pollution here things are getting worse by the day. Surely you are familiar with the cycling event called Giro d’Italia Giro d’Italia. You also know that the cyclist who wins a stage will wear the ‘maglia rosa’ (pink jersey). The finish of the stage on 11 May 2024 will be in Prati di Tivo, a ski resort at an altitude of 1600 m above sea level just below the slopes of Corno Grande. The picture is the Corno Grande seen from Prati di Tivo. Corno Grande belongs to the Gran Sasso d’Italia mountain chain, it is the highest peak (2912 m) on the peninsula, populated by foxes, chamois, bears, wolves, snakes, wild boars, mice even on the peak you can see, lynxes, a wide variety of birds (eagles, crows, buzzards etc.). Well, brilliant idea from the organisers, guess what it is? Illuminating the whole of Corno Grande in pink! If this is done, there is no doubt that it will only be the first timid step towards an even worse process of not only light pollution of the mountain. Are we stupid or reckless enough?
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