Welcome to Photometry!

Hello DocArne,I read the page on photometry. I would like to ask you whether I could help using my digi cam (not a DSLR) which has a max. exposure time of 15 seconds. Its a Canon G9 to be clear.Thank You in AdvanceSicerely Yours,BOB

Hi Bob, You can use most any CCD camera for photometry. Check: http://www.hposoft.com/EAur09/CCD/EAurCCD.html The use of 10 second intergrations is recommended, but to prevent getting out of hte linear area of the CCD much shorter exposures should be used and then stacked to make a set with at least 10 seconds worth of data. Also, check the Workshop information after 7 August 2009. Jeff

I just checked the link you included...is it dead? I didn't get anything but a File Not Found message. Is there another website you recommend?

Try copying and pasting the link. It works that way. I get an error too if I click on it here. http://www.hposoft.com/EAur09/CCD/EAurCCD.html

Thanks, that worked.

I have a Canon Rebel XTI and two telescopes...a 9.25 SCT and a 80mm Orion ST...the inexpensive achromat. I wanted to get some starter pictures of Epsilon Aurigae before the eclipse starts...suggestions on getting started? Exposure time, ISO, file type to save to (TIFF, RAW, JPEG)? I thought since we are essentially doing a light curve ( i assume that's what it is) that I should get a few pics before the event starts.

First, I do not recommend using the 9.25" telescope. The star system is too bright. You could try using the 80 mm scope. You will need software that allows you to see the max/peak pixel values. Keep them under 40,000 ADUcounts by adjusting your exposure time.Then you want at least a total of 10 seconds worth of images. If each exposure is say 0.5 second, then take at least 20 exposures (20 X 0.5 = 10.0) and stack them. Save the files as RAW, ideally ,fits. Take 3 sets and determine 3 magnitudes. Average and do a SD on them. Report the average and SD. Experiment some. You will need to develop a technique that works well for you. The major portion of work will be in the processing. Let me know if you have questions. Jeff

I use the software that came with the camera...the canon program...can't recall the name of it at the moment. What are ADU counts? How can I find them? How do I determine the magnitudes?

I am not familiar with the Canon Software. Explore it and see if you can analyze images (see max/peak pixel ADUcounts). You may need to get other software that allows camera control and pre-processing. Note:AIP4WIN does not control cameras. You may find some freeware on the Internet. Maxim DL might work, but it is $500- $600. ADU counts are Analog to Digital Unit counts. Each pixel on the CCD is like a well that fills with electrons as photons knock them into the well. After a given time (exposure time) the number of electrons in the well is counted (sometimes the number is multiplied by an amplifier) and then the well is emptied ready for the next exposure/filling. The electron count is the ADUcount. The 40,000 ADU max is important because once over that the CCD become non-linear. For photometry you must stay in the linear region. To be sure, you should do a test of the camera to see at what point the linearity deviates. Some CCDs may deviate below 40,000 counts. If all of this is new and confusing, I suggest you get AIP4WIN ($100) because it comes with a large book on image processing that explains most to of all this. You will need to master the image taking, ADUcounts and linearity before you proceed future with DSLRphotometry. To determine magnitudes you will need to use software like AIP4WIN. Using the G plane image you create circles around the stars of interest then sum up all the pixel counts in the circle. A second circle (annulus) is created around the star and area around the star. A sum of the pixels in an area of that circle represents the sky reading. The Star - Sky ADU counts represent the final star reading. Once you have that you can determine the Raw or instrumental magnitude with m= -2,5 log (ADU counts). Jeff

Yes, this sounds much more complicated than anything I have ever done and way out of my league. I have done lots of imaging before with my DSLR, darks, some flats, etc, but I've never heard of most of the terms you just said. Not in Chicago yet, and I already feel lost and overwhelmed...wow. I mean, I understand what you mean by well count...but my camera has a CMOS chip, not a CCD. No, EOS Utility can not show max/peak pixel ADU counts or analyze images. Here is what I currently have available...DeepSkyStacker, Registax, Photoshop, and I use the EOS Utility software that comes with the camera to image/control the camera.Can DSLR Focus do these things?

Oh, what do you mean by SD?

Photometry is a big journey, but you can take baby steps. The first thing you can do is simply take an image of the field getting both the variable and the comp stars in it. Then visually estimate its brightness from looking at the image on your computer screen. Report it as a "photographic estimate" (this is an option on the WebObs data submission form¹) with a "CV" bandpass (meaning the comp stars had a Vmagnitude). That's not officially a photometric measurement, but it gets you comfortable with the camera. This would be a good thing to try on the 10 training stars. The next thing you need to do is figure out how to get access to the various filtered images your camera takes. This may require special software. We'll write a guide to help you do this once we figure it out ourselves. We recently bought a DSLR at the AAVSO so we can test it and develop a tutorial. Once you have the various filtered images saved on your hard drive, then you do photometry on it like you would a CCDimage. A good introduction is here: http://www.aavso.org/observing/programs/ccd/manual/4.shtml Only read sections 4.1 and 4.2. You don't need to worry about 4.3 and onward right now. That's for more advanced folk. If you are coming to the Chicago meeting then that's perfect. The best way to learn about this is to sit around a table with a few others and just talk about it. :) It's really not hard at all, but there are lots of little steps to figure out. And it seems like to me you're already halfway there. Aaron ¹ WebObs is a web-based tool that existing AAVSOobservers use to submit their observations. It's available in the BlueGold section of our web site. We are going to program a much simpler interface for submitting visual observations for the Citizen Sky web site. Photometric observations will always need to be submitted via WebObs, though. Instructions will come shortly. If you want to get a head start, go here and register for a login/password.

SD is standard deviation. It's commonly used as an error term for calculations involving means. Most software that computes a mean will also give you an SD. (For example, Excel does it with the "=stdev()" macro.) If not, it's pretty easy to calculate yourself: 1. Calculate the average of the data set. 2. Subtract the average from each data point to find the difference. 3. Eliminate the negative signs and square the differences. 4. Add the squared differences together. 5. Divide the sum by the number of data points minus one. 6. Take the square root of the result. There are tons of ways of report error (a.k.a. "uncertainty") for photometric observations. SD is a good, simple way of doing it when you are working with many different frames. Most photometric software will provide you with error terms to report. So very few people have to do this manually. I include this info just FYI, and also because it may be a nice exercise to do it manually in the beginning. For details on SD, download the PDF of Chapter 10 of Variable Star Astronomy.

Yes it sounds complex, but if you are persistent you can learn and like many things, once you learn more it does not seem as complex. The idea of electronic photometry is to get some value, analog voltage, number of photons, number of electrons that represent the brightness of a star. Once you do that there are various stages of calibration to make the results more accurate. Whether you use a photon counter like I do, a PIN Diode detector like the Optec photometers, an astronomical CCD camera, web cam, modified web cam, video cam or DSLR camera, the goal is to get the value that represents the brightness of the star. You use the DSLR to take a color image of a star field. That image has three color planes, RGB. Each plane is a matrix of pixels. Each pixel in the matrix will have a different level of electrons and when read out will produce the ADUcount for that pixel. Normally you never worry about such things as the camera handles all that automatically. However, there is a great deal of information in the image, and you have to get down to the pixel level to extract it. Some software will allow you to display pixel maps. They have coordinates for each pixel and an ADU count. It's all really very interesting. At the Workshop I will be going over all this slower and in more detail. Your camera is actually an very complex instrument. The manufactures just set things up so operation and use seems easy. As far as I know, a CMOS detector works basically the same as the CCD detector. BTW, doing photometry requires some effort and you will not learn it in few minutes. At the Workshops I will also be giving a talk on spectroscopy and part of the talk is on using a DSLRfor it. Real spectroscopy can be done with a DSLR, but like photometry, there is a learning curve. Hang in there. Jeff

Ok, so let me ask this...I obviously don't have the software to properly analyze the photons that the detector is picking up. Cost is a huge issue. Until Ican get the software/up there to Chicago, there has been a good stretch of weather I don't want to waste plus the fact that the eclipse is just getting underway. Since Ican't analyze yet...Ican at least get some images now and analyze them later after Ilearn how. Let's assume I can get it in focus. (although from what Iread on the AAVSO page, focus isn't always necessary) And from what was said earlier, I should get about 10 seconds total after stacking. What ISO should Iuse? At least then Ican have some stock images to bring with me to get help getting started. Secondly, 10 seconds worth of images seems rather short, although I do understand that this is a bright star...would multiple 10 second sets over an evening be preferable?

What I would recommend is using the shortest lens you have, maximum sensitivity and 0.5 second exposures. Take say 30 exposures and stack them.Consider that set 1. Repeat 2 more times so you have three images each stacked with 30 - 0.5 secondexposures. Then archive the images for later processing. Be sure to keep a notebook listing times and dates and file names. Just plan on these 3 images each night. If one set has problems, clouds or bumped tripod, take another set, but just three good images is all that is needed. Right now if you are planning to image epsilonAurigae, you will be fighting morning twilight so plan on starting with enough time. Time goes fast. The 0.5 second exposures should keep you in the linear area of the CCD. Since you are stacking hte images the tracking error might not show so you should plan on first focusing sharp and then backing off some. The some is something you will need to experiment with. Good luck. Jeff

Thank you.

Hello Jeff, I'm not sure if I clearlt aked it. but what I meant if I can use my compact digital camera to to take field pictures & not pictures through a telescope or optical instrument to contibute in photometry, or is it limited to specialized CCD ? Maybe I didn't get it clearly what you said about "CCD Camera". did you mean a camera with a ccd chip ? or a specialized ccd ? thank you

There is aspace at the end of the URL that you might have to delete manually.

Hi, I recently registered for this project and would like to try out photometry or spectroscopy methods. I am thinking of getting a DSLR camera soon (so far I only have Panasonic FZ28 digital camera). I have a general question at this point, which is whether workshop videos from Aug. 7 have been posted or not? I would like to watch them to get the basic ideas about both photometry and spectroscopy. Also, with my current camera (FZ28) and if I getAIP4WIN software, would it be possible to try it out? or would I need to get good DLSR to get any decent results? Thanks.

Hi Vlad, As far as I know Citizen Sky has not yet put the viedios of the Workshops on line. You can down load the Power Point and pdf file of my two Workshops. Go to the Campaign web site and scroll down toward the bottom for the links. http://www.hposoft.com/Campaign09.html Jeff Hopkins Phoenix Observatory Phoenix,Arizona USA phxjeff@hposoft.com

Hi Vlad, We're in the process of editing the videos. Part of the problem is in reducing the physical size of the digitized files, or in defining break-points so that the video can be downloaded easily. I would not expect the videos to be on-line soon, as we want to do a good job. However, Jeff's powerpoints, along with the other presentations, should be posted on the Citizen Sky site soon. As he says, you can download them immediately from his campaign web site (different than ours) if you don't want to wait. I've seen DSLR results from Canon and Nikon cameras, including the point-and-shoot varieties. If your camera outputs RAW format images that can be read by AIP4WIN, then you should be able to try it out. Why not? AIP is a great piece of software, and comes with that 1000-page book that will help you in the future even if the FZ28 doesn't work out. Arne

Hi Vlad, According to list of supported raw file formats http://www.willbell.com/aip4win/AIP4Win2.3%20Supported%20Cameras%2012%20... the FZ28 is indeed supported by AIP4Win !! (You will want to upgrade the version that's included with the book, tho, to the latest version). CS HBE

Is the SD calculated using only the three magnitude determinations? -Kevin

Thank you all for your responses! I just ordered theAIP4WINI guess I will go from there. I will let you know if and when I will have questions as I go along.

I am buying this too!