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About 12dstring

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  1. The filter used in the Unihedron SQM is a Hoya CM-500, however I don't think it's a particularly good filter for the job. It claims to match the human eye response but passes light below 400nm which the human eye can't see, and will also cause the sensor plastic to yellow if exposed to the sun for long periods. You can pick up cheap IR/UV filters from China/Hong Kong. These are the ones I use and plan to measure the transmission curve properly at some point, but the specs say Tavg > 95% @ 440-620nm which will allow LED and Sodium light to pass fully whilst blocking invisible wavelengths. There's lot of LED lenses around, e.g. this one. The one in the SQM doesn't have a part number so not sure exactly which one it is. Note that the field of view given for an LED lens won't be the same as when using a light sensor, as they will be difference sizes (in the same way different sized cameras with the same lens will give different field of views). The main thing is to reduce the influence of light from nearer the horizon, and a lens concentrates the light from the zenith more than a simple blocking aperture would.
  2. I didn't mean to discourage trying the TSL2591. As I haven't tested it yet I can't comment on how it works in the real world, only on paper. I didn't want people to think that the i2c sensor is automatically an 'upgrade' as it's easier to make readings with, as there is a big sensitivity difference. Taking multiple readings to average will be the best method. Just like imaging, more light gives better signal-to-noise. The frequency output sensors naturally increase effective exposure time for lower light, 10s or more under the darkest sky. Calibration is an issue, and it's not necessarily solved by having a commercial SQM to compare with. Unless you have the same filter, lens and spacing, you won't be able to calibrate them properly if you want to make accurate comparisons. DIY SQMs are best used for relative measurements, i.e. is tonight darker or lighter than average for my site, is my site getting brighter..etc
  3. Another important thing to note is if these are in permanent installations (i.e. exposed to the sun all day) make sure you have a good UV (and IR) filter in place. If exposed to UV over long periods the transparent packaging of all of these sensors will go yellow. This will reduce sensitivity to blue light and ruin any long term light pollution measurements.
  4. On paper the TSL2591 is whole lot less sensitive in low light than the light-to-frequency sensors. The reason being that it output a number of counts proportional to light level, and at low light levels the difference between 1 or 2 counts is getting pretty big. Some rough calculations give a count of 1 equal to mag 22.5 and count of 2 equal to 21.8, so it would be impossible to measure reliably at low light levels. The maximum dark signal is also a maximum of 25 counts which is in the region of mag 19 (in reality it will be lower). It may work well enough in towns and cities, but below mag 19/20 it will struggle to give meaningful results. The TSL235 is a better alternative to the TSL237. The responsivity is lower so the output frequency will be ~4x lower. The TSL37 would have an output period of around 10s at mag 22, and the TSL235 around 40s. However the dark current of the TSL237 is about 15-20x greater, which will have a big impact at low light levels unless you can compensate for temperature. My experience has shown the dark current is often less than specified in these sensors though. Counting periods of seconds with an arduino gives excellent resolution, and actually increases in magnitude terms the fainter you get, whilst your resolution will decrease dramatically with the i2c sensors under darker skies. I probably have a bunch of TSL237s somewhere I can put up for individual sale if there's interest.
  5. Not significantly warmer than other dark materials. Simple black acrylic paint is about 5% reflective, and this material would only absorb a little over 5% more energy than that. You probably wouldn't really notice the temperature difference (and that's ignoring the increased thermal conductivity),
  6. A few things to consider with these maps, which are simulations based usually on either population or roads; Different types of lights have hugely different effects on sky brightness at different distances. For example the terribly designed low-pressure sodium lamps that grace our country tend to spill lots of light horizontally. An effect called forward scattering tends to keep this light going horizontally and so it pollutes over large distances. LED lamps and other full cut-off lights only contribute light reflected by the ground, this pollutes on a much more local scale. The different light scattering effects are also wavelength dependent, so low-pressure sodium orange is scattered differently to the blue-rich white light of LEDs. The surrounding environment will also have a big effect. Lights in crowded cities will have a lot of horizontal light blocked by buildings for example. So all in all it's very difficult to predict based on population/road data alone. The best way to produce a good map would be to actually survey the sky brightness from the ground at different locations. I know of a few such attempts in Italy, but nothing much in the UK.
  7. Swapping the sensor is fairly straightforward, it would take me a lot longer to debayer a colour sensor properly. The downsides are that sensors can be tricky to get hold of, and will probably cost more than the webcam, whereas debayering is essentially free.
  8. For reference this was my brief experience with webcams from the DSLR thread: http://stargazerslounge.com/topic/166334-debayering-a-dslrs-bayer-matrix/?p=1775129 Certainly the SPC900NC was a lot easier than the DSLR. The cover glass came off easily and the bayer filter was easier to remove. It does have microlenses so the net sensitivity isn't really improved in the visible range (but will be for UV/IR). The resolution noticably improved though. It's also easy to tweak the camera to output the raw b+w data.
  9. I've been using it on my 550D for over two years without a problem, and wouldn't go without it now. For astronomy there's some useful focus tools and the bulb ramping feature makes for some great automatic time lapses of sunsets, from daytime to milkyway. The audio and motion triggers are great fun too, useful for wildlife and lightning..etc
  10. I shouldn't think it would make much difference. Guiding software can measure the position of a star to better than a tenth of a pixel, which with your setup is less than the best seeing would allow (this will improve with the QHY5 over the LPI anyway as it has smaller pixels). In my opinion it would be more advantageous to have a larger fov for better star choice, and less light loss. Where I work we use 700mm fl guidescopes for 4064mm fl telescopes, and have never had a problem keeping stars round and small. Past a certain point you're at the mercy of the atmosphere.
  11. Here's a cheaper source of little IR/UV cut filters: http://www.ebay.co.uk/itm/371053668561 Can't go wrong for 88p each! I've just ordered a few and will measure a transmission curve when they arrive.
  12. I've the code for the last SQM I made (fitted with a GPS receiver and sits on the top of my car). It's written for a PIC, but might be useful for someone writing for Arduino. I'm sure it could be optimised, but it seems to work fine. #define TSL237 PIN_B0;int16 timermsb=0;int16 timermsbbuf=0;int16 timerlsbbuf=0;int16 i;int8 send=0;int8 bytes[5];int8 mult;#INT_TIMER0void TIMER0_isr(void){ //set_timer0(0); timermsb++;}#INT_RDA HIGHvoid RDA_isr(void){ int8 in; in=getc(PORT1); if(in=='S'){ if(send==1){ //ok to send fputc(bytes[0], PORT1); fputc(bytes[1], PORT1); fputc(bytes[2], PORT1); fputc(bytes[3], PORT1); fputc(bytes[4], PORT1); }else{ //not ok, buffer still writing fputc(255, PORT1); fputc(255, PORT1); fputc(255, PORT1); fputc(255, PORT1); fputc(255, PORT1); } }}void main(){ setup_timer_0(RTCC_INTERNAL|RTCC_DIV_2);//333ns resolution, 21.8 ms overflow enable_interrupts(INT_TIMER0); enable_interrupts(INT_RDA); enable_interrupts(GLOBAL); while(1){ restart_wdt(); enable_interrupts(INT_TIMER0); while(input(TSL237)); //wait for falling edge while(!input(TSL237));//wait for rising edge set_timer0(0); timermsb=0; while(input(TSL237)); while(!input(TSL237)); timermsbbuf=timermsb; timerlsbbuf=get_timer0(); mult=1; if(timermsbbuf==0){ //short period, repeat a few times (improves resolution in brighter conditions) while(input(TSL237)); while(!input(TSL237)); set_timer0(0); timermsbbuf=0; for(i=0; i<100; i++){ while(input(TSL237)); while(!input(TSL237)); mult++; if(timermsb>50){break;} } timermsbbuf=timermsb; timerlsbbuf=get_timer0(); } disable_interrupts(INT_TIMER0); send=0; bytes[0]=mult; bytes[1]=(timermsbbuf>>8) & 0xFF; bytes[2]=timermsbbuf & 0xFF; bytes[3]=(timerlsbbuf>>8) & 0xFF; bytes[4]=timerlsbbuf & 0xFF; send=1; }}
  13. The 237S is the standard three-lead package, the 237T is a smaller surface mount one. The 237T also has about half the responsivity and an output enable pin.
  14. I have the following and plan to do a comparison for measuring sky brightness at some point: TCS3200D TSL25911FN TSL237T TSL237S TSL238T TSL257T But for reasons stated above (120x less signal/dark noise) I don't expect the TCS3200 to really be sensitive enough under dark skies. Would be nice if I'm wrong though..
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