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Yes, I have still verified that for large ranges of brightness, ordinary cameras are not very suitable. I am currently in the process of making a photometer with a photocell applied to the focus of a telephoto lens (really bad optics) with a diameter of 110 mm. However, the quality of the image matters little as I only need to measure the illuminance. I don’t remember if I mentioned that my programme is concomitant with the Perseid shower. It is a job that should be a novelty in the amateur field. Thank you Peter.
Greetings to all the friends who have listened to me and to whom I may have annoyed a little: the passion for Urania is strong.Translated with DeepL.com (free version)
Peter, thank you for your interest in my question. Yes, indeed everything is looking quite difficult, and as I would need to collect a lot of numerical data for my purpose, I have started to abandon the idea of proceeding with a chamber. I have just finished an observation conducted with a photoelectric cell placed on the focal plane of a telephoto lens. Everything seems to be much simpler. The only annoyance is having to write down the measured potential differences by hand, which if misread are no longer retrievable.
Everything is done in preparation for observing the Perseids. That is why I have also developed a theory on which I base my observations and process the collected data.Translated with DeepL.com (free version)
I learn this news with heartfelt regret. Meade has allowed me to do many things at reasonable prices.
The market for amateur telescopes and their accessories appears to be out of date. Telescope companies are suffocating themselves, especially as the stars have disappeared from the sky. The various online marketplaces as they are filled with gear (everything has become ballast) of all kinds still new put up for sale because they are superfluous. A few days ago, I took a Celestron D90mm refractor to one of the flea markets in my town. The manager didn’t want it, he said it wasn’t for sale. He only agreed to try because we have known each other for years.
In my country they say: only after you have bought it do you realise that you don’t need it.Translated with DeepL.com (free version)
Peter, yes Active D-Lighting is switched off.
Grant, that’s my problem, I don’t understand.
All I need is to set up a plan for observing the brightness of the sky at sunset and sunrise. Anyway, I have already started trying using the jpg format. The first results are quite satisfactory. We will talk about this as soon as they are available.Hello
Dear colleagues, there is another small (small so to speak) problem. The attached graph shows the pixel values of images of the usual white sheet, taken with a nikon D3000 camera and varying only the shutter speed. A few things emerge:
1 – the maximum PV value, corresponding to the minimum shutter speed, is 255. As if I had shot in jpg format;
2 – there is no proportionality between PV and shutter speed. The function is completely different from the one in Robin’s graph;
3 – the PV-lens surface graph is almost linear at each iso.
The sensor of the nikon is of the CCD type.I would ask for help in interpreting these at least apparent inconsistencies.
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Attachments:
Nick, please excuse me for not mentioning your intervention. It only appeared after the new graph was posted. Maybe it’s my PC showing signs of old age. I saw one of the links, it is a bit difficult for me, I will have to study it. Thank you.
Grant and Paul, you are very kind, I am not an expert in the soft stuff, however I will follow what you tell me tomorrow. I just hope I don’t do any damage to my system.
However, I see that the problem is not limited only to the jpg format I use, which gives little information. Certainly there remains the problem of reducing the PVs at the same shooting time. I enclose another graph that I constructed just today. In it, the PVs are shown as a function of the lens area (abscissa at the bottom) and the relative aperture ratio shown at the top, and for various ISOs. It is comforting that there is proportionality between PV variations for a given number of ISOs. It is certain then that PVs are a function of at least 2 variables: ISO and shutter speed. The mathematical function seems to be quite difficult to handle. I am looking for a good way to find it and then handle it as easily as possible.Attachments:
These are the graphs for 100 and 800 iso, from images of a white sheet, uniform and constant illumination taken with nikon D3000
Attachments:
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.
