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Point and Shoot Camera?

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Jack144's picture
Jack144
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 Hello all,

   We would like to involve a beginning college astronomy class in obtaining light curves. Our plan is to order ten cameras and tripods which teams of three students would check out for the semester to obtain and plot data. My questions are: Is the use of a point and shoot camera (around $200 each) feasible? If so, what suggestions would you have on the model? Any suggestions would be welcome.

Thanks,

Jack 

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Roger Pieri
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Hi Jack, A good but not simple question ! I do it from time to time beside using my DSLR but with an high end DSC (Canon PowerShot "G9" ) Such high end cameras (today Canon G12 and Nikon P7000) have a larger sensor even if much smaller than DSLR, and important, they have full manual and RAW image modes. I don't know any other having such RAW capability, maybe there are ? Anyhow that class of DSC is expensive, in Europe this is about 450 Euros, probably same in $ in USA... (less taxes !) - The firstissue is to get enough electrons / photons for achieving a good SNR. The sensor being very small the lens pupil is small tooand get proportionnaly less photons. I do improve it using a large x2 teleconverter that increases the pupil surface by a factor 4, I also use the longest focal of the G9 that anyhow provides the largest pupil. The choice of the lens is important ! - A large defocus also help increasing the exposure duration without saturation. This is also an importantpoint: you need a camera you can control the focus, thereare few havingthat possibility ! - The DSC shall have a full"manual" mode to control the exposure up to 15 seconds, set the largest aperture, the ISO at the lowest. An "auto" mode would probably push ISO and exposure at max and saturate on the stars. - Live-View: Framing a star field is not easyon an LCDof limited number of pixels. There are cameras the rescalling"erase" the fine details and the stars. Only very bright stars keep visible. All show more or less the problem but there are large differences between cameras, chose a display having as much pixels as possible and then check if the rescalling do not erase too much the stars. - The last point is the RAW output, It seems only the "G" series and the "P7000"have that possibility. A $200 camera surely has only a JPEG output. The issue is not much the 8 bits and the JPEG compression if stars images are well defocused. The true issue is the fact the JPEG mode is not a linear representation of the photon-count but applies a "gamma" following the BT-709 standard. http://en.wikipedia.org/wiki/Gamma_correction This can be managed but with a specific processing, the best being to "degammatize" ( ! ) the data and then process it normally ( that means apply the opposite gamma law to recover the linear representation of the photon-count) It's possible to load JPEG images under IRIS and then apply a de-gamma using View>gamma adjustment... and set the gamma slider to 0.45 . Then you could process the RGB components as usual. Agood solution would be to buy second-hand cameras of the "G" series. The G9 works very well, the G7 should be ok, the older versions have a limited focus control if I remember well. But second-hand DSLR are probably available at similar price ? Clear Skies ! Roger


Hi Jack, I'm not aware of any off-the-shelf point-and-shoot cameras that would be very well suited to photometry because they often compress their images (as Roger mentioned above). You could, however, try out some modified firmware that will bipass this and save the data in RAW format! The Cannon Hack Development Kit (CHDK,http://chdk.wikia.com/wiki/CHDK) boots the camera up using different firmware that gives you access to the camera's low-level features, including RAW images in several (but not all) cases. From what I understand this doesn't overwrite the existing camera firmware, just uses something off of the SD card. The list of supported Cannon cameras is listed on the right side of the main page. I haven't tried using this before and I'd suggest reading the disclaimers on the main page. If it works though, you could get some cameras capable of RAW images for $50.00 on ebay! Best of luck and please do let us know how it goes, Brian

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Thanks Brian for the info on CHDK, I was not aware of it,I shouldreview this possibility too. But at afirst look I am a little bit surprised they try to provide a set of functions that already exist in cameras like the "G" series ! Also the "S" series I forgot to mention. The S have similar functions / technology / sensorthan the G of the samegeneration in a compactbody but at similar price ~ 400 Euros here. Thepresent S95 is superbe with a advanced manual focus like the G, including a magnifier with which it's very easy to adjust the defocused star imageat a precise size ! Clear Skies, Roger

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Thanks for the information! The CHDK software sounds like a great solution, but I am not sure it would be a good idea for the College cameras. I would hate to "brick" them. How difficult would it be? I was trying to keep the cost per camera to around $200. Could I get one at that price if I did use CHDK that would: 1.Produce images in a RAW data format. 2. Focus semi-manually.3.manually select a shutter speed/exposure time of several seconds? If so, an suggestions as to the model? Jack
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Roger Pieri
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Hi Jack, Sorry but I can't say anything about CHDK having no experience of it. I just did a check of what you could find on e-Bay as suggested by Brian. As expected usedDSC like CanonG or S known having the right functions are expensive,pricesnear such of new !!This is not very surprising as those products are very appreciated from expert amateurs. What I have seen as an interesting possibilityare five years old DSLR, in particular the Nikon D70. Many are proposed around $ 150 for the body and $ 200 with lens. Such DSLR were within the best at their time and have all needed for serious photometry.I have a D70s I keep using and with which I did photometry befor buying a 450D. The version "s", theD70s,has an improved sensor, a better choice.It has been used by many amateur astronomers a couple of years ago. The drawback from present DSLR is the fact it uses a past generation CCD instead of the present CMOS. The consequence isa "dark process" is mandatory for exposure of few~ten seconds (no need with present CMOS up to one minute or so) Thenext issue is general with Nikon but minor: the fact they don't use a system offset. This could result in a tricky estimate of the background level in case ofextremely dark background. Thisis probably not the case! Clear Skies ! Roger

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Jack144
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Again, many thanks for your help. I have been looking at many possibile solutions, and new SD940IS cameras with the CHDK hack may actually be the way to go for us. At $130 + a $50 tripod, it is certainly worth a closer look. I wonder if anyone has used this camera? Jack


Hi Jack, I've been thinking of setting up an all-sky monitoring system with several of point-and-shoot cameras using CHDK. I would pick up 5-6 cameras and mount them on a RA-only tracking/controllable mount under a spherical glass dome. I figure with such a method I could do all-sky DSLR photometry down to 8th mag for < $2000 fairly easily. This dissertation thing keeps getting in the way though. I'd be interested in hearing your review of CHDK. It seems quite cost-competitive and with the ability to write mini-programs controlling every aspect of the camera it's hard to beat. From what I understand CHDK uses an existing feature in the Cannon firmware to load itself off of the SD card (as if it were a firmware upgrade) so if you remove the card and power-cycle the camera all should be back to normal. You'll want to double-check this comment against their documentation though. Cheers, Brian

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Jack144
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Hi Brian, We are working toward a solution for the lab exercise on light curves. I am looking at various cameras supported by CHDK. I first heard about the hack in an astrophotography class, but it was only mentioned (quite favorably) as an aside because it was a DSLR class. I had forgotten about it until it was mentioned here. I think it only works with Canon. It may be a low-cost solution for us. I am looking at the Canon PowerShot SD940IS, 12.1MP cameraat less than $200 and will probably buy one to try out. If that’s the final choice, I’ll let you know how it works out.

Jack

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Hello Jack, I have used a Canon Powershot G3 for photometry with good success. It has all you need: RAW Images, full manual control, good quality. I got 2 of them on the used market, one for 70 the other for 90 euros almost new. The other early Powershot models up to G6 or so are also good choices and available used. Here you find an example of an observation of Algol minimum with this camera: http://home.pages.at/vollmann/algol.htm I have also a description of the camera and procedures used: http://members.aon.at/wolfgang.vollmann/var_digital/var_digital.htm Both pages are written in German but Google translate should help! Best regards Wolfgang


Hi Jack,CHDK is not something which is liable to "brick" a camera. I have installed it and enabled it without difficulty. Given that the cameras on which it has been tested keep getting cheaper and cheaper, I think that there is a real opportunity here.Cheers,Doug

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