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12dstring

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Everything posted by 12dstring

  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 wavelength
  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 necess
  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
  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 ligh
  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);
  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..
  15. If there's enough interest then it might be worth doing a group buy from Mouser, that's where I got mine from (along with some other sensors and bits - free postage above £50). Should come to about £3.50 each delivered.
  16. The TSL237 may be older, but it's a lot more sensitive. I've built a few SQMs with the TSL237, and my main 'visual' one outputs 0.3Hz under 20 mag/"^2 skies. Knowing the responsivity of 2.3kHz/uW/cm^2 that means ~0.13nW/cm^2 falling on the sensor. The TCS3200 has a responsivity of 386Hz/uW/cm^2 meaning under the same irradiance it outputs a frequency of 0.05Hz (20 seconds per cycle). Factor in also that the typical dark frequency is 2Hz for the TCS3200 (40x your real signal) and 0.1Hz (0.33x your real signal) for the TSL237 (the ones I've tested are even lower) and there's going to be a big di
  17. The newer version of the FOV calculator displays the area too. For reference you just multiple the width by the height, so for your setup: 16.5' x 12.38' = 204 square arcmins
  18. Looks fantastic out there at the moment, as does most of England. I wish I could say the same about Friday's forecast, but you never know. It was forecast cloud for the past two nights but we've seen stars every night this week. It's a great place to work, but the hours can get to you sometimes. Summer is quiet time when we do all the equipment installs and upgrades, and then in october all the students arrive and it suddenly gets very busy every night until April. I work mostly as a technician, in particular I'm working on the robotic telescopes at the moment. I also teach the undergraduates
  19. Hi Michael, I work there, so hopefully can give you some idea.. The main attraction naturally is the 7 large telescopes, between 14-inch and 20-inch. They'll all be open with a knowledgeable PhD/undergraduate student inside. Some of them are robotic so will have cameras on taking images, the others will have eyepieces and be pointing at different targets for you to look through. Our largest one has been refurbished over the summer, and is still just used visually at the moment (just need to collimate it for next friday!) If we have the manpower there will also be some smaller portable telescop
  20. Are the dX and dY offsets and angles calculated all fairly small/similar? Perhaps something is throwing off the alignment. The stars disappearing towards the left and the dark pattern on the left look like it hasn't been stacked evenly with all of the subs aligned.
  21. You may have come across a field of view calculator I wrote over 5 years. It's by far the most popular thing I've written, and gets quite a lot of links from various SGL threads. It originally was just a simple calculator, but was expanded to something of a fov simulator. I've been working on a new version on-and-off for some time now, and it's finally at a stage where it does something useful so would love to get some feedback. If you'd like a play, it'd be great to get any feedback on the interface/useability or any suggestions for features you might find useful: Field of View Calculator v2
  22. I should have mentioned I stretched the images seperately so that the background and nebula were around the same brightness, perhaps not the best way of doing it but it shows the signal strength above the noise. The stock camera should be around 16x less sensitive to H-alpha than the debayered one, as the IR filter block 75% of that wavelength, and the CFA blocks another 75%. Of course the dark and bias noise are not affected by these mods so should be pretty much the same. The one with just the IR filter changed might actually be less noisy than the others as it has a cold finger attached (co
  23. Some great progress here guys I finally managed to get a few test shots last night. Unfortunately I wasn't set up for bulb exposures so was limited to 30 seconds. Here's a comparison of M57 with a stock 350D, the 'normal' IR filter mod (Baader ACF replacement), and then the sensor with CFA removed. Shot with an Astronomik 1.25" 12nm H-alpha filter on an f/10 LX200, 4 x 30s, no calibration frames. Also got a quick M31, 10 x 30s with 4 darks, using a 2" Baader neodymium filter: Looks like no flat issues here. I still haven't finished cleaning up the sensor and it's looking pretty good at f/10
  24. The scratches on the lamp post still only show 1 in 4 pixels, they're just brighter. They could actually be a slither of microlenses that are still there. Searching around though there are a couple of small spots where the filters have been removed.
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