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simon hicks

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About simon hicks

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  1. Hi Olly, OK, you've got my interest up now! Lets use relationship 6 shown above to compare a Tak FSQ106+camera combination with a Borg+camera combination. Lets assume we are taking the Tak with the reducer, so its F3.6 and FL=381.6mm. And lets combine it with the Moravian G3-16200 which is a nice big 16.5MP camera. This combination gives us the following numbers; Camera: Moravian G3-16200 Pixel Number X 4540 Pixel Number Y 3640 Pixel Count (MP) 16.5 Pixel Size (um) 6.0 Sensor Size X (mm) 27.24 Sensor Size Y (mm) 21.84 QE (0 to 1) 0.56 Read Noise (e-) 10 Scope Tak FSQ106 plus Reducer Focal Length (mm) 381.6 Operating Focal Ratio 3.6 Aperture (mm) 106 FOV X (degrees) 4.09 FOV Y (degrees) 3.28 Pixel Res (arc sec per pixel) 3.24 Optimum SubExposureTime 0.00572 We will now pick a camera with smaller pixels, which will mean a smaller focal length Borg scope to achieve the same field of view. So we'll use the ZWO ASI 1600 again and combine it with a Borg BO6738 (F3.8, FL=255mm), and the numbers are; Camera: ZWO ASI 1600MM Pixel Number X 4656 Pixel Number Y 3520 Pixel Count (MP) 16.4 Pixel Size (um) 3.8 Sensor Size X (mm) 17.69 Sensor Size Y (mm) 13.38 QE (0 to 1) 0.77 Read Noise (e-) 3.5 Scope Borg BO6738 Focal Length (mm) 255 Operating Focal Ratio 3.8 Aperture (mm) 67 FOV X (degrees) 3.98 FOV Y (degrees) 3.01 Pixel Res (arc sec per pixel) 3.07 Optimum SubExposureTime 0.00354 Remember that both of the examples above give exactly the same FOV (within a gnat's whisker) and almost exactly the same pixel resolution (both are 16MP cameras), but the Borg/ZWO combination is 1.6 times faster than the Tak/Moravian combination. There are obviously other considerations here...like the Tak+Reducer+Moravian is about three times the price and 3 times the weight of the Borg/ZWO combination. And the Tak optics might out perform the Borg....I really don't know. But the point is that the above gives a way of considering one aspect of the two combinations, i.e. the relative optimum sub exposure time. That is of course if the relationships are correct! :-) And that's what I'm trying to understand. Cheers Simon
  2. Hi Olly, You are right that dynamic range is much more important than well depth. And dynamic range is -20log10(WellDepth/ReadNoise). What you find is that the newer smaller Sony type pixels are giving much smaller read noise than the larger older pixel sensors, and in fact the dynamic range can be just as high, and often higher, with the smaller pixel sensors. For example, the Moravian 16200 with its 6um pixels has a (relatively) massive well depth of 41,000 and a read noise of 10e-, giving a dynamic range of 72.26dB, whereas the ZWO ASI 1600 with its puny 3.8um pixels has a well depth of 20,000, but a read noise of 3.5e- giving a dynamic range of 75.14dB. Conclusion is that the latter has a bigger dynamic range than the former even though the pixels are smaller. The trend seems to be that the smaller pixel sensors are coming with lower read noise. Well yes, but this is only relevant if you only take one exposure length subs to capture the whole scene (in other words you're happy with saturated stars). If you take different length exposures and combine in an HDR image then the shorter time due to the higher QE is surely a benefit. Your longest subs will be the ones to try and capture the faintest details in the nebulosity, and this length will not depend on well depth, only QE and read noise (and sky background and the level of the target of course). And yes, I agree with your comments about the merits of a Hyperstar vs a Tak FSQ106.....the analysis above is really just about speed and can't really say anything about the quality of the optics. The point is that it is easy to compare the speed of say an F8 200mm aperture scope with an F5 130mm aperture scope for instance....but what about the speed comparison when you place two different cameras (different pixel sizes, QEs, ReadNoise, etc) on these scopes....what will the comparison on speed be then? A more interesting comparison would be something like a Tak FSQ106 on a 'larger pixel' camera, in order to give a FOV of say 2 degrees, vs a 'smaller pixel' camera with a shorter focal length Borg lens giving exactly the same FOV and exactly the same arc seconds per pixel. If we assume that the Tak and the Borg are of a similar quality in terms of their optics, then which combination is better (in terms of speed).....that's the point of the analysis. Cheers Simon
  3. Hi Hugh, Thanks for the reply. Yes, you are correct, the analysis doesn't take the fully illuminated image circle into account. That's one of the (many) things that are in that 'catch-all' of quality, flatness, price, etc, i.e. things you would need to consider separately. However, I would point out that both of the lens/scopes in the example above are rated at 'full-frame' so both should be fine with either of the sensors in the two cameras. This does raise the interesting question of whether (all other things being equal) if the smaller sensor would be better....because its only using the central area of an image circle and therefore should inherently give a 'flatter' image than say a full-frame sensor that is putting a lot more demands on the off-axis performance of the lens. Remember that both combinations are giving the same image FOV and pixel resolution....so the old adage of 'bigger sensor=bigger FOV' isn't applicable here. Cheers Simon
  4. Hi Julian, Yup, thanks for the link. familiar with this site. Unfortunately it just calculates FOV and pixel resolution etc. It doesn't say anything about which combination would be better than another. For instance I could have two different cameras with greatly differing pixel and sensor sizes, and I would couple each of these to two completely different focal length scopes to give exactly the same FOV and pixel resolution. But which one performs better.....that's the point of the analysis. Cheers Simon
  5. Hi all, I have been trying to come up with mathematical relations to compare the performance of various camera/scope combinations. Essentially asking the question “For a given field of view is it better to have camera A combined with scope X or is it better to have camera B combined with scope Y?”. The criteria for me is the field of view as the starting point, i.e. I have a given field of view that I want. I am therefore trying to compare different camera/scope combinations that achieve that given field of view and see which camera/scope combination gives me the best performance. Here I am thinking of performance in terms of getting the shortest optimum exposure time to achieve a given S/N for faint nebulae. This analysis doesn’t consider which camera is the best, or which scope optics are best, flatness of image etc, it is purely related to getting the shortest optimum exposure time for a given field of view. I started with Steve Cannistra’s web pages and his equations relating focal length, aperture and optimum exposure time. ( http://www.starrywonders.com/fratio.html ) Basically, they show that 1. ExposureTime is proportional to F#^2 / Aperture^2 This basically describes the way the optics work, i.e. the more aperture you have then the more signal you collect, and the lower the f# the more you compress that light into a smaller image circle, i.e. you increase the density of light in a given area. Obviously, for a reflector Aperture^2 becomes PrimaryDiameter^2 – SecondaryDiameter^2, but lets keep it simple and stick with refractors for the time being. So the above allows us to compare say an f5 8 inch scope to an f4 6 inch scope. But the comparison is only valid for the same sensor, it basically says nothing about the camera. So to include the sensor in the comparison I have to include more terms. The first thing we can add into the mix is the pixel size. The above assumes we are creating an image circle collected from an area of the sky and thrown onto an area at the image plane, some of that area is covered by the sensor, and a small percentage of the sensor is made up of each pixel. If the pixel is huge then it gathers a larger percentage of the image circle, and if it is smaller then a smaller region. So it seems that the relationship for exposure time for a single pixel would be; 2. ExposureTime is proportional to 1/PixelArea And as most pixels are square, we can write; 3. ExposureTime is proportional to 1/PixelSize^2 We can now consider the sensitivity of each pixel, or the QE. The bigger the QE the more signal we get from each unit of light falling on the pixel resulting in a smaller exposure time. So here; 4. ExposureTime is proportional to 1/QE The other thing that Steve Cannistra found here http://www.starrywonders.com/snr.html is that for a given camera and optics, and for the imaging of faint nebula, and for systems where the dark noise is insignificant (cooled), then the optimum subexposure time to reach a given S/N is determined by the read noise (RN) of the camera by the relation; 5. SubExposureTime is proportional to RN^2 [Note that Chuck Anstey has done a more detailed analysis of optimum sub-exposure time that includes the signal level of the faint nebula features that you are trying to capture here…http://www.cloudynights.com/page/articles/cat/articles/astrophotography/finding-the-optimal-sub-frame-exposure-r1571 . However, in this analysis I am assuming that when comparing CameraA+ScopeX with CameraB+ScopeY then the target is the same, and therefore becomes irrelevant in the comparison] Now this is where I try to combine everything together. I am basically assuming that the optimum sub exposure time of relation 5 will also be determined by the other relationships 1, 3 and 4. So this suggests that; 6. SubExposureTime is proportional to (F#^2 x RN^2) / (Aperture^2 x QE x PixelSize^2) This would seem to allow me to compare CameraA+OpticsX with CameraB+OpticsY, at least to some level where we are concerned soley about the optimum exposure time required to capture a single sub of a given FOV of a faint nebula and getting a given level of S/N. Is the above valid? Are there more terms that should be in there? I’m really not sure, so I would welcome some input. The dimensions don’t quite seem right for a start. If I assume that the above is valid, then as an example of its use I could compare a couple of different camera/scope combinations. Let’s assume that my starting point is that I want a 5.5 degree FOV. I have picked a couple of camera scope combinations that allow me to achieve that….other combinations are out there and will vary in terms of price, weight, size, quality, etc….these are just examples. Lets start with the QSI1620 camera combined with a Rokinon 135mm lens, a combination that gives a FOV of 5.59 degrees along it long side. The list below shows the numbers for this combination; Camera: QSI1620 Pixel Number X 4250 Pixel Number Y 2838 Pixel Count (MP) 12.1 Pixel Size (um) 3.1 Sensor Size X (mm) 13.18 Sensor Size Y (mm) 8.80 QE (0 to 1) 0.77 Read Noise (e-) 3.5 Scope Rokinon 135mm F2 Focal Length (mm) 135 Operating Focal Ratio 2.8 Aperture (mm) 48.2 FOV X (degrees) 5.59 FOV Y (degrees) 3.73 Pixel Res (arc sec per pixel) 4.74 Optimum SubExposureTime 0.00559 Note in the above that the optimum sub exposure time (calculated from relation 6) is not in units of seconds or minutes, its just a number that we can compare with another camera/scope combination. So the second camera scope combination is the Moravian G3-16200 combined with a Borg BO7139, which has a focal length of 280mm. Note the longer focal length is needed because the Moravian camera has a larger sensor. So the numbers here are; Camera: Moravian G3-16200 Pixel Number X 4540 Pixel Number Y 3640 Pixel Count (MP) 16.5 Pixel Size (um) 6.00 Sensor Size X (mm) 27.24 Sensor Size Y (mm) 21.84 QE (0 to 1) 0.56 Read Noise (e-) 10 Scope Borg BO7139 Focal Length (mm) 280 Operating Focal Ratio 3.9 Aperture (mm) 71 FOV X (degrees) 5.57 FOV Y (degrees) 4.47 Pixel Res (arc sec per pixel) 4.42 Optimum SubExposureTime 0.01497 Note that both camera scope combinations give roughly the same FOV and roughly the same pixel resolution (that’s why they were chosen). However, the relationships suggest that the Moravian/Borg combination would require sub exposure times of around 2.7 times longer than the QSI1620/Rokinon combination. Is this a valid analysis? Or have I gone wrong somewhere in the logic or missed out some important terms? Has this all been done before more rigorously? (Please point me in the right direction!) It would be good to have a way to compare these things....and I'm beyond the limit of my ken. :-) Cheers Simon
  6. Thanks Olly for the details and thoughts....lots to think about there. I am coming to the conclusion (maybe wrong?) that if I have a given field of view in mind (lets say 5x5 degrees) and I want a chip with a decent number of pixels (lets say 4000 x 3000), then I can either get big pixels and put a long focal length scope on it, or I can go for small pixels and put a small focal length scope on it....they add up to the same field of view and the same arcseconds per pixel. And in both cases I'm massively undersampling anyway (or do I mean oversampling....can't think which way round!). And here's the bit I hadn't said in the first question, and that is that I'm trying to make up a lightweight portable rig, which would suggest that the smaller pixels and smaller focal length would be better. A really good 300mm focal length lens is heavy and costly, whereas a really good 135mm focal length lens is a lot lighter and cheaper. I think I read somewhere that the 16MP sensors on our smart phones use 1.3um pixels. By my reckoning, I could get a 5x5 degree FOV with a 50mm lens rather than a 135mm lens (at the same arcseconds per pixel). It'll come! :-)
  7. Hi Gnomus, Thanks for the reply and the pointer towards the Monravian cameras. Your response has made me realise that when I say I'm interested in using focal lengths of around 135mm, what I actually mean is that I am interested in imaging fields of view of around 5 x 5 degrees or thereabouts. I had been looking at small pixel size sensors, which on a 135mm camera lens gives me the field of view I was looking for, but end up with a resolution of each pixel of around 5.6 arc seconds. However, as you point out, I could use a larger pixel size, on a large chip size....which would give me a much bigger field of view....which I could bring back to my original field of view by increasing the focal length of my lens. So in the case of the Moravian 16200 this would mean using a 300mm lens instead of a 135mm lens. The resultant pixel resolution is down to 4.1 arc seconds. OK, so there is more than one way to skin a cat as the saying goes. mmmmm....time to think. Cheers Simon
  8. Hi all, Years since I posted on here...trying to get back into the hobby and wanting to upgrade my kit. I am looking for advice on the best cooled camera for long exposure narrowband wide field imaging. Here wide field means with anything from 35mm FL lens to 300mm FL, but probably mainly working at 135mm. I am obviously trying to get a high resolution chip (many pixels) with a big array (bigger field), etc, etc. The problem is matching the pixel size to the resolution so I'm not too far oversampling. With seeing of say 3 arcsecs, imaging at 135mm FL suggests an ideal pixel size of around 1.5um....which seems incredibly small compared to what seems to be available. The closest I have seen are the Atik One 9 with pixels of 3.69um and the QSI 6120 with pixels of 3.1um. There are a few with pixels of around 3.5um, but with smaller array sizes. Dose anyone know of anything with a reasonable size array that has smaller pixels that would be better matched to my focal lengths? Do such things exist? Or should I just be going with one of the above and accept some level of oversampling as the price you pay for wanting very widefield images? PS...Please don't suggest doing mosaics....I live on the east coast of Scotland and its hard enough getting one image lets alone an array of them :-) Cheers Simon
  9. Hi Michael, Thanks for the suggestion. I disabled the on-access scanning of my virus software and then reinstalled the Core Software. Unfortunately when I try to open the AtemisCapture.pdf it still says it is corrupted. :-( Cheers Simon
  10. Hi Ronin, When you install the "Core Software" from the Atik site you get an "ArtemisCCD" item in your start menu and within that there's a "Documentation" folder. Inside the folder there's a pile of pdf manuals. The one I need is the one called "ArtemisCapture.pdf". Cheers Simon
  11. Hi, I've just got an Atik One 6.0 kit. Unfortunately the ArtemisCapture.pdf manual is fatally corrupted and won't load. I've tried downloading three times from the Atik site but still the same problem. All their other pdfs are loading fine. Can someone email me a copy of their ArtemisCapture.pdf file please. Much appreciated. Regards Simon
  12. Hi all, The astronomy society I am in has some astronomy equipment (scopes etc), and may buy some more. We are toying with the idea of the society insuring the equipment against damage whilst it is loaned out to members. This would cover repair or replacement in the case of accidental damage. Has anyone any experience of getting such an insurance...and if so, can you point me in the right direction please. Many thanks Simon
  13. Yes, it includes accommodation next to the observatory in what I think are spectacular surroundings.
  14. .....you forgot to mention the flood lights and the nuclear powerplant that will be needed! :-)Seriously though, your concerns are definitely valid but we have actually thought about them a lot. We are amatuer astronomers that love the dark skies there and would do nothing to ruin that....to do so would be a bit dumb. We all go to star parties with red light policies, etc. The site is right on the edge of the Galloway Forrest, the UK's first Dark Sky Park so the whole area is under quite strict light pollution regulation. The observatory and the route to it will of course be kept dark as will the whole area around. The local community are buying into the venture as well with B&Bs offering dark sky weekends etc. They are being informed of ways to reduce light pollution and the need to do it. The dark skies are actually their asset to protect. There's been a lot of thought put into this by a lot of people for a number of years. I personally intend to be using the grounds of the observatory for my own long exposure deep sky imaging....so there will definitely be no car lights ruining my evening.
  15. Brian Orme (of Renfrewshire Astro Soc) has taken some great pictures of the latest state of build of the observatory. The pictures were taken today (23rd June) and you can see them all here.... http://www.renfrewshireastro.com/forum/viewtopic.php?f=19&t=523
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