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This manual is for beginner- to intermediate-level CCD observers interested in learning what photometry is and why it is important, through how to perform variable star photometry and submit data to the AAVSO. Advanced CCD users who have not done any photometry may also find this helpful. Cambridge, MA 02138. If I were to try my hand at photometry and measuring light curves, what equipment would I need to change. In looking at the CHOICE training, it seems that CMOS cameras are undesirable, CCD cameras being their focus. What are some examples of lower-end cameras that will get the job done. Seems like I would need to purchase a different set of filters for photometry, different from AP filters, correct. Also, does CA and SA in a simple achromatic telescope matter, or does photometry require an ED or apo telescope? Thanks for your inputs. However, you will be more than fine making your first steps with a ZWO ASI 290mm. It's a bit smaller in chip size than you would ideally want, but still ok. I think you don't have to be a prophet to predict that in the not too distant future, CCD astro-cameras will anyway completely vanish from the low- to mid-price segment, and maybe even from most of the high-end market as some manufacturers are ramping down or even terminating their CCD sensor production in favor of CMOS. There can be no doubt that CMOS and photometry go together just fine. Photometry is not about making super sharp images, it's more about throwing photons in the general direction of a sensor:-), Indeed we sometimes defocus intentionally to spread light over more than one pixel (you might have seen that part already in the CHOICE material). Good luck and if you run into problems, don't hesitate to ask the good people at AAVSO.org for help:-) We can discuss all your questions there. Cambridge, MA 02138. If you are a first-time beginner- to intermediate-level DSLR observer, this manual is for you.http://macautemple.com/userfiles/cetis-9600-manual.xml
Even if you are an advanced observer, you may find the content useful. Software and camera-specific discussions are in the AAVSO DSLR forum. The manual was inspired by the great interest in DSLR photometry during AAVSO’s Citizen Sky program. Cambridge, MA 02138. Taking into account the community’s comments, I have assembled a task force of volunteer observers, who have been working on a new VSX data submission manual, which can be found here: As such, we would like to invite you, our community, to read it, test it and send us feedback.We are looking forward to your comments. Cambridge, MA 02138. These are intense courses taught over a limited time and require daily commitment of time from each student to complete successfully. This includes time taking images as well as participation in discussion questions on the forum. Students will be asked to share their results with the instructor and with other students. An “open book” final test is required from each student. Please consult the syllabus to ensure you have the time commitment to take these courses. Tags: CHOICE Courses ccd photometry DSLR Photometry Thanks you two! Tags: photometry observing manuals observer resources DSLR Photometry ccd photometry ccd manual Cambridge, MA 02138. Comments are welcome. Although it is aimed at the photoelectric audience, it has much of interest to any new photometrist. Those who have seen earlier drafts may want to pick up this version.A very readable but thorough treatment. I encourage others to provide comments, after which this manual will be added to the web pages. Jim Cambridge, MA 02138. A better definition would be something like: Considered in this context (where no pixels can be filled or half filled) it is clear that the parameter represents the width or extent of the function at a certain point, namely, half its maximum value. We will include it in the next update. Suggestions on how best to define that in a clear simple way are welcome.http://www.jurymarudeejay.com/public/blog/cetakan-plastik-manual.xml
The different views about wording is a strong clue that words alone won't be enough. To me the figure seems quite clear, but a couple of beginning students in the CCD1 class still had trouble with the concept. I now think they may have been confused by the definition in the text. I'm just concerned at the possibility of confusing measurements across an image or part of it, with measurments across a mathematical function describing the profile of that image. How about something like: Would that ease your concern? The seeing disk is the image of the star, not the profile of the ADUs from the image. Therefore I don't believe the seeing disk itself can have a FWHM. Here is a modification of what I suggested: Cambridge, MA 02138. Please help by suggesting more resources and reading material. Tags: observers-ccd CCD resources ccd observing CCD observers Cambridge, MA 02138. No-one (as of a few minutes ago) has yet posted a resonse to it. I have been inspired and am selling some stuff off so that I can get a CMOS mono (an ASI 183 Pro cooled) and join the crowd as we march towards the CMOS universe of photometry. Ray, your word doc is a great start. I would urge all to stick to this goal, it’s easy to get diverted. How do I achieve the greatest dynamic range with a 12-bit camera? For example, I plan on using the 183 with an 80mm refractor for bright variables. How about my C11 edge at 1950mm FL.But if you informed CMOS photometrists will use this forum to gather them together a guide might emerge. The ASI178MM-Pro, which has the same pixel size, full well capacity, similar read noise, quantum efficiency, is a 14-bit (though the 183 has 20MP resolution vs the 6.3MP of the 178). Unfortunately, ZWO no longer sells the cooled version of that camera. However, QHY does sell a camera with the same sensor. This post is a response to some of the quetions posed in that document. It allows binning, and yes, various gain settings can be used, both with binned and unbinned images.
My understanding (and I am not a CMOS expert, merely a user) is that binning in this camera (and at least some other CMOS cameras) is done with software after the pixels are read out. Note that the line profiles in the attached PowerPoint (which I'll explain) show maximum counts above 60,000. For 12 bit ADC, the maximum count per pixel should be 4096. The ASI 1600MM software, in RAW16 mode, multiplies the actual counts by 16 to give the numbers you see. Curiously and enigmatically, in both unbinned and binned 2x2 mode, the sensor saturates at 65,504 counts per pixel, as seen after taking a series of flats at various exposures for linearity testing, and AstroimageJ to analyse the FITS images. It shows the field of FR Cet, a 6th mag variable of uncertain type, and non-transformed V mag photometry of the var and the check star. The 7th mag star is a spectroscope binary, and is variable, although variability has not been reported to my knowledge. Should be self explanatory on careful study. The variable is enigmatic, hence the reason I started to study it. There is a big difference between (1) having general, well-established CMOS Photometry Best Practices on which to base a CMOS Photometry Guide, and (2) writing that Guide. Without them, I'm having trouble seeing the point of attempting to write a Guide until they are agreed. Perhaps the first step is to get specific agreement between CMOS Photometry leaders on what those Best Practices are. Before I bought my ASI1600 camera last year, I scoured the internet for information on the use of CMOS cameras for photometry. I found very little definitive about the practitcal matters I was looking for - apart from testing for linearity! Within the AAVSO, I can't think of a better way than to have a group of people who have experience in photometric studies of variable stars come together to pool their expertise and experiences.
If there is another way to arrive successfully at a description of best practice for CMOS photometry I'd be happy to support it. At least for some CMOS software, the ADU count for each pixel is averaged when they're binned. If four pixels have an ADU count of 3000, 2000, 2000, and 1000 (assuming the Gain is set so the ADU count at full well capacity is 4095), the ADU count of the 2x2 binned pixels would be 2000. It just looks like a loss of data along with running the risk of one of the binned pixels being saturated and not knowning it rather than getting an extra 2 bits of bit depth. Or better yet, in the case of a 14-bit CMOS camera, just add the ADU count fo the four pixels and storing the data for the binned pixels as a.FIT file without multiplying the ADU count to fit 16-bits. It would make it easier for everyone just to amend the CCD Photometry book to include CMOS cameras. I still believe that the best way forward would be for the Board to set up a formal process with a group of experienced observers, someone to lead the group and perhaps HQ representation. Thus, at unity gain, saturation is 65,504 counts both for unbinned and binned 2x2 images. I think that the idea of data being lost, or the reisk of one of the pixels being saturated is not anything to worry about. Testing provides data on linear range, and line profiles and other statistics provide information on the pixels containing signal and the surrounding sky. It would be apparent if there is any saturation. It might be better to say that precision is loss. It can be tested, and my preliminary tests support my opinion. Since a lot of common info is present in these guides, it is felt that it would make most sense to use them as a basis for a CMOS guide which would add the technical details and different procedures needed to properly conduct CMOS photometry. In fact, two BSM telescopes now utilize a ZWO ASI183 camera for photometry with good success.
In the near future you will hear about the technical details of this work BUT it would be useful to hear from others about their procedures and results. So far no one has provided specific details about how they operate their CMOS cameras with respect to gain, binning, etc. Stand up and provide your comments. Don't worry about being uncertain or incorrect. Tell us how you run your CMOS camera. Let others poke holes in the technique OR support and confirm it. We can learn from each other. Describe your techniques, your settings, your results. Accept any critique or disbelief that may occur. Learn from it! Help the few people who are discussing the guide to develop the necessary methodology sections. But I thought I would get the words on the forum quickly so people could comment, critique or ask questions as wish. I'm eager to read your report on the photometric precision of the camera. One question: given the tiny minimum exposure, it is clear that there is no mechanical shutter, so I suppose that to take darks you need to use a black filter? I apologize in advance for the fact that not everything converted from InDesign to a Google Doc perfectly, especially the images and page breaks, but feel free to fix it. I am definitely no expert on formatting things in Word! Best regards. The attached 14 page document is much more fleshed out, and also includes screen shots, images of photometry fields, and other figures and tables of results. It does, however, describe how I use the camera, and some of its peculiarities. Of course, this is much better, and it includes a photometric precision evaluation which I find very valuable. So far only 2x2 binning is possible with the driver and MAXIM DL. The V0351 Dra varied between mag 12.5 and 13.1. I used no filter as I do not yet have a connection with the FLI filterwheel. With 15 s exposure the uncertainty using for Ref the 10.48 star and for check the 11.43 star from the AAVSO sequence was between 0.015 to 0.02.
No much time was invested into focussing. With 30 sec exposure the uncertainty dropped to 0.009 to 0.013. It was very humid, but clear with bright moon. File size is 30 MB. Another friend will write a PYthon program to do 2x2 binning on the images. We also tried to image the asteroid (33165) joschhambsch. It is about mag 18.9. MY friend succeeded with the double of the FL (about 1500mm) and a 20 cm refractor and ST10 to get it on images taken with subs of 2 min and in total 56 min exposure. But on the small refractor images due to a large FWHM I had (about 8 arcsec) I could not really identify the asteroid. The camera looks linear over a long range of ADUs. Best regards. Is this an issue with the current generation of CMOS chips. Best regards. I already have an Atik EFW2 filter wheel that should work. I look forward to learning how to use this camera and specifically picked it because the price break is likely to encourage new photometrists. Thanks to all of you for your productive posts, I have learned a lot and look forward to learning more. No need to reply and gum up the thread. If anyone is going to bin with a CMOS in software, then they should take a test image at 1x1 or not accept anything higher than the maximum linear ADU value divided by the number of pixels being binned. Hypothetically, if someone was to take images of a star at 2x2 binning in which the binned pixels have a reading of 65535, 40000, 40000, and 30000, the average ADU count would be 43,883.75. While the average is still below saturation and below the maximum value for linearity for most cameras, that one pixel would give a false reading and make the star appear dimmer that it really is and the person taking the image would have no way of knowing without taking an unbinned image prior to taking the binned images. So anyone using a ZWO camera that wants to submit to VPHOT will have to run the.
FIT files through software for editing FIT headers first or run it through third party software I know that from experience. To suggest that four adjacent pixels would have ADUs of 65535, 40000, 40000 and 30000 is not realistic. The attached defocussed unbinned image of an 8th magnitude star is from my ZWO ASI1600MM camera. The peak value is 62225. To get to 40000, you need to move about 11 pixels across the line profile in one direction, and even more in the other direction. I have carried out linearity tests on unbinned images and images binned 2x2, and performed photometry on both unbinned and binnned images. The last star I submitted to VPHOT was an 11.6 mag. Also, I try to keep the FWHM at 3-5 pixels. The FWHM of RS Gru from your ASI1600 was around 26-28 pixels. And IIRC, if the image is defocused too much, then sky brightness would create too much noise in the image. I don't know if I would want to defocus an 11 mag star by a FWHM of 26 pixels. This would be over exposed. I would buy a 14 bit or better a 16 bit cmos camera. So with a narrow starfield, faint compare stars can be used with no problems, because they have enaugh light gathered. The ZWO ASI software multiplies this by 16, hence the counts for full wells are more than 60,000. This is -not- the same as a 16 bit camera. I have worked out how to optimise the precision. Besides defocusing there are also CMOS cameras with smaller pixels. The pixel angular resolution would be 1.3 arc-seconds so a FWHM at 2 pixels for that camera and telescope. Now replace that camera with either a ZWO ASI178MM or ASI183MM. That would be an angular resolution of 0.348 arc-seconds per pixels and a FWHM of 7.48 pixels. IIRC, the recommended aperture radius is 1.5?FWHM. So the images taken with the CCD camera would have an aperture radius of 3 pixels. So that would be approximately 28 pixels in the aperature circle.Human reflexes aren't fast enough to accurately click a stop watch for a single pulse.
Rather, the number of pulses are counted over a duration of 15 or 30 seconds and multipled by 2 or 4, respectively. The indivdual pixels of most CMOS cameras lack the precision of a 16-bit CCD camera pixels but that could be compensated for by spreading the light from a star across more CMOS pixels and defocusing isn't necessary if the CMOS pixels are small enough. Defocussing and even smaller pixel size would both do this.See the attached data and graph, taken from a well-focussed image on a night when the seeing was about 3.8 arc secs. The FWHMs of well-focussed non-saturated images of stars do not vary with magnitude, whereas the seeing disk, the actual image itself, does increase in size with increasing brightness. The attached shows the actual number of pixels in the seeing disks of stars of various magnitudes from one of my images. I have been testing since May last year, have posted my methods and results here and elsewhere, and find good linearity, and precise and accurate photometry. Methods or you may be different. So yes, it depends. I used it as a start for me to understand these parameters in a CMOS camera. I placed my thoughts in the subsequent section. Comments? These pixels have a specific full well capacity depending on the sensor. This means you can bin by 2 in the vertical direction (transfer two rows into the serial register before readout) without losing any charge. This well is like an even bigger pixel, typically with 4x imaging pixel capacity. This means it can hold a bin-by-2 row binning into the serial register, followed by a bin-by-2 column binning of the serial register, without losing any charge. If the gain is adjusted, then you might sample with 10 electrons per count rather than 2.5 electrons per count. While that permits exposing to the true full well depth of each native pixel, it means that a count for a binned image is different than a count for an unbinned image.
Perhaps more important, since the gain changes, the bias frame may also change. You should take biases and darks for each binning situation. So we bin 2x2, and defocus to yield 1.5-2.5 pixels per fwhm. Binning helps for the 183 in many ways, including making the 12-bit ADC into effectively a 14-bit ADC, because for a CMOS detector like the 183, the binning is done in software. I won't talk about CMOS again in this post. Again, if you have a long focal length telescope, you may want to use a focal reducer to keep from oversampling with a given sensor. However, you are adding an optical element to the optical path. Focal reducers tend to only correct human-visible light; how well they perform form UV or NIR light is unknown. They also can increase the vignetting. They are a reasonable solution to the oversampling case, but you need to be aware of their potential interference when doing photometry. Usually regular CCD cameras like the SBIG line will have a straight line up to a certain count value, and then curve over, asymptotically approaching some value which may be considerably below the ADC maximum count. For an ST-7XME, for example, deviation from linearity may occur around 48K counts. If you see this kind of curve, then you are sampling to full well on your sensor, even in the 2x2 mode. If you instead see a straight line up to 65535, then most likely you are not sampling to full well on your sensor, and instead are being limited by the ADC. For most anti-blooming-gate (ABG) CCD sensors, because of their inherent nonlinear behavior near full well, vendors typically increase the gain so that the linearity curve is linear up to the full range of the ADC, and so this test for 2x2 gain doesn't work as well. It takes some effort to interpret how it determines how one should set gain for each type of camera on the basis of both binning and well depth. My interpretation is aided by an understanding that in CMOS cameras like the ZWO ASI183, larger binned pixels (e.g.
, 2x2) are NOT created on the CMOS camera. (They may be on a CCD camera.) Each native (single) pixel on a CMOS camera is always read out and amplified (gain) before it is converted to a digital count in the ADC chip and before it is summed with adjacent pixel counts IF binning is requested.This helps reduce the random noise which adds some random bits to the real adu count of a star. In this case, the CCD camera appears to have slightly less effective bit depth than a CMOS camera and they become more similar in regard to effective bit depth.Tell us if that’s how you set your gain.Since we are talking about a 12 bit camera, it will (in my experience) be necessary to defocus the images, so degree of defocus is another parameter to be set. The third parameter is length of exposure. Thus, in my experience, one needs to image the brightest star of those being studied, take trial exposures (perhaps based on experience) then tune the exposure length, degree of defocus and gain setting until the ADU's per pixel are as close to the upper limit of the linear range as you can get, based on your own linearity testing. Re-setting the gain and thus being able to lengthen the exposures will under these particular circumstances increase the precision. (This purely empirical procedure is based on my own testing to optimize precision in a field where 6th and 8th mag stars are to be measured through a 120mm refractor.I have never tried to match the pixel size and seeing as described. I guess this is beause most of my photometry has until recently been with a DSLR camera, for which all images for photometry are defocussed. This helps reduce the random noise which adds some random bits to the real adu count of a star. In this case, the CCD camera appears to have slightly less effective bit depth than a CMOS camera and they become more similar in regard to effective bit depth.
However, if I understand correctly (and it is possible that I do not), with dark skies (low background counts) and faint targets read noise can predominate over shot noise. In contrast, at an observing site near a city with bright background skies, shot noise predominates. My only point (valid or not, I don't know) is that your comment may need to be considered differently under the two circumstances. My other hope is that it is not misleading. If that is found to be true, the gain setting should be slightly different from 3.66. It almost certainly will be? I think this is true for ccd cameras as well. It keeps from being undersampled or oversampled with the resulting increase in noise. It does not need to be exact. In fact, since one can only bin in whole numbers, it would be 1x1 or 2x2 or 3x3, etc. One choice will be close enough. Since most cmos pixels are so small, software binning more than 1x1 is usually needed. That is, you want to get close to the nyquist value (2) when whole number binning doesn't get there itself. So, you may defocus to spread out the star profile over more pixels.I think this is done after the above settings are selected. IMO, the best way to reduce this noise is to increase the number of images taken and that can be stacked later. CMOS cameras with their electronic shutters can image at really short exposures. At some point less than 5 secs (or so), scintillation gets so significant that stacking is necessary. They are all important to consider and to utilize under the full range of conditions we run into. I propose that one should not just try to vary all of the possible settings noted above in some random order. I think the settings should be made in the order listed. At least most of the time. Nothing is ever perfect!;-) I need to do more testing based on a traditional approach to imaging for photometry. The settings arrived at will depend on the brightness of the star.
So far, I have found no improvement in precision with binning 2x2 over unbinned images, but further testing can be done. It is not physical binning. I thought that setting gain to get maximum dynamic range, up front, before looking at the magnitude range of your targets is not appropriate. There is no reason to do that with my system for, say, a 9th or 10th magnitude star. With my small aperture and fainter stars, I would be throwing away useful signal if I decreased gain. Maybe that will allow photometry on more tightly focussed images. If so, it will be interesting to see if the actual precision is better with that approach than with my original one. That's fine for tagets with long periods where you can take multiple exposures during one imaging session. Amost all of my targetst are short period variables (delta Scuti stars) with periods of 1 to a few hours, and I take time series. Therefore, stacking would be a pain. The alternative would be rolling averages, not so much of a pain, but that would drop the peaks and raise the troughs of the light curve. I prefer to avoid that. It's not going to clear substantially for a while yet. As soon as it does, I'll try the above, and post the results. I think it is the BEST way to do things. However, my hope is that we come up with a methodology that is both technically sound and practical so that others do NOT need to go through the same pain. Keep in mind that I do strongly feel that a little experimentation is also the best way to LEARN rather than just depending on what someone tells you. It means you have to expose longer. So, should we use one gain extreme or the other or some intermediate compromise. I think I have made my choice and you have made another. Just exposure is changed. If you use VPhot, stacking is quite easy. Alternatively, you could just aggregate groups of images (e.g., groups of 5) to average out the random noise. You do need to be careful to avoid averaging out real fluctuations.
BTW, it is hoped that in the near future, that MaximDL will allow real-time stacking of images during the observing run so you are not forced to store a large number of short exposure, very large images (e.g., 50MB) on your hard drive. This is great! Cambridge, MA 02138. Is this an issue in fact.In CCD, a bad pixel would cause a column defect.I presume many folks use scopes with focal lengths longer than those whose scopes are used for astrophotography. Are the downsides of binning and oversampling small? This would not be a problem with CMOS per se since large chips are available in both. However, large format CCD chips are VERY expensive. Given the the lower cost of CMOS, more folks might start using full size chips. Over about 45 arc-minute to 60 arc-minute field of view, might air mass extinction start to become an issue from one side of the field to the other at 30 degrees altitude and focal lengths under 2000mm for high precision photometry? (Though how often do we approach millimag precision?) I would think those using DSLRs have thought through this issue. This will be very helpful for the period of time when both types of astronomical cameras are available. Best regards. For precision photometric applications, yet another disadvantage is that no two manufacturers use precisely the same filters; there are even differences within the product lines of individual manufacturers. If you take that path, be sure to pick someone whose process includes replacing the passivation layer on the chip. Photometric software performs primary extinction corrections across fields of view of several degrees extent, even at relatively high air masses. Full frame chips, now widely available in mid-range to high end DSLRs, permit even larger FOVs in principle, but one needs to beware of excessive vignetting. It's not so much that the vignetting cannot be corrected as part of the field flattening process, but rather that one loses SNR that flat fielding does nothing to restore.
As for astronomy cameras, those CMOS sensors are used in consumer product lines like the popular ZWO ASI cameras for some time now, and in the low to medium price segment mostly. Only lately you can find popular CMOS (astro) cameras in the higher price range. With my narrow FOV, I do not need to correct for differential airmass. However, this situation with wide FOV. The DSLR manual suggests that anything over 0.5 degrees might need extinction correction, especially near 30 degrees elevation from the horizon. Thank you and best regards. It's not CCD that is the niche--they are still as sensitive, productive, response-linear, and widely used as they ever were. It's astronomy altogether that is the niche market, and astro photometry not even that. Consider who bought FLI, and why. Let's ask: is medical research or amateur astronomy a more promising future market? CCDs are and have long been routinely available in wide fields of view, so the within-fov extinction problem is far from new. Even so, no I do not know of any software that does differential extinction within fov. One could write one's own scripts, but it would have to be from scratch. For example I don't see extinction handling in astropy at all. Myself, I would worry more about good flat corrections (including inaccurately corrected vignetting) than about cross-fov extinction gradients at 30 degrees elevation. I am obviously not alone. Yes, this could well change. Whenever the camera vendors decide to actually make a CMOS camera that's fully photometry-ready right off the shelf (as CCD cameras have been for many years), I'll eagerly consider it. But the camera I choose will be as performant and as boring (tinkering-free) as possible. My camera is just one citizen of a complex scope rig at a remote location, and it will behave itself night after 400-image night or I chuck it out.