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vlaiv

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About vlaiv

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    Novi Sad, Serbia

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  1. Simple: narrow band filters go by them selves in front of the sensor. If shooting LRGB, use L, R, G or B (depending what you are shooting) in front of the sensor and LPR filter before that. I use it at 2" nose piece (actually I'm switching to threaded connection for both RC and APO, so I'm using M63 adapter / rotator that has both male and female 2" thread - male on camera side, female on scope side - this is where I put LPR filter). I'm in red zone (white not far away), with mag 18-18.5 skies. Hutech IDAS LPS P2 is my filter of choice. Remember there are two things that you need to know when shooting in LP. First - your SNR will suffer (a lot with broad band) - you need to throw more imaging time at image to get the same results as in dark skies. Second - there will be nasty gradients due to uneven sky illumination. Larger the FOV - it will be more noticeable. There is a way to deal with this - either correct your subs prior to stacking, or do some sort of background removal after stacking. I prefer the first choice - because camera FOV rotates in respect to LP gradient - stacked image will tend to have non linear gradient. If you deal with individual frames - LP gradient (is much harder to notice by eye) will be more linear (although with different orientation in different frames). I use frame normalization that can deal with such cases and orient LP gradient the same on each frame (my algorithm). I usually select the least affected frame to be the reference frame (all others have their background "oriented" like that) - the one in zenith, or just below it to the east (the least LP affected part of sky). On the matter of exposure, I recommend the following for ASI1600: - broadband - 1 minute / Gain 139 (unity) / offset - choose one you like and stick with it, and make it over 40 or so (default with latest drivers is 50 - good value). - narrow band - 4 minutes / other settings same as above. My flats and flat darks are extremely short exposure because I have very strong flat panel - less than 10ms for broad band (2-3ms for L) and it works fine. I also recommend very large number of calibration frames. I use 256 of each (darks, flats and flat darks - darks I record and use that master for quite a long time, flats and flat darks I do after every session, or filter change - mobile setup, so I tear down after imaging - no guarantee that it will be the same next time, and it almost never is).
  2. vlaiv

    M17--The Swan?

    At eye piece I usually only see "Nike" sign, very rarely omega shape. This almost 150 years old sketch sums it up nicely:
  3. vlaiv

    M17--The Swan?

    Maybe this helps ...
  4. Not necessarily bad, but more like quirky, have a look at few reviews (some have issues, some report goto fine, but almost all mentioned that manual movement is harder than non goto version), see what other people think, it maybe helpful for your decision. http://www.iceinspace.com.au/forum/showthread.php?t=78143 http://www.iceinspace.com.au/forum/showthread.php?t=72836&highlight=GOTO https://www.skynews.ca/review-sky-watcher-synscan-goto-dobsonian/ http://scopeviews.co.uk/SW400P.htm
  5. Yes, I get what you mean, but I've read couple of reviews of this goto and tracking feature, and while everyone agrees that tracking is good, not everyone is impressed with goto, its precision and the work involved to properly set it up. Also, at least once it has been mentioned that manual motion of goto version is not as smooth as regular/manual version. Using external power source seems to cause problems, because scope always uses quickest route between objects - sometimes causing power cable to get wrapped around the base. It is recommended to use power tank / battery type of power supply and to put it on base (inside), so it rotates with the scope. I do like the idea of having both tracking and hand controller that you can use to center/slew while looking at the eyepiece. My concern is whether it is sound option given cost premium and mixed reports. I would like to have "no glitches" option at affordable price - then it would be my first choice - but if I look at it this way: questionable goto or eq platform for tracking and manual finding things with a bit more aperture (for the same amount) - I must say I prefer other option.
  6. Don't think you need to do precise alignment with EQ platform, usual compass + latitude of observation site would be quite enough (actually many platforms don't even have latitude adjustment, they are pre build for specific range). Sure you would not be able to do long duration precise tracking, and you might need to nudge the scope every 5 to 10 minutes to recenter the target, but that is much better than having to nudge it every minute or less. You would also need to "rewind" the platform every hour or so. As for flatness of the base and ground - I guess if one can put dob on ground, then EQ platform is much the same. Just make sure you have some sort of adjustable legs so you can go relatively level - that is more than I've got on my 8" dob - it has only 3 small rubber legs.
  7. 12" ? Dobs are about aperture, so if you have the funds (and storage space).... Personally I would probably go for non goto version. Tracking is something that I would want, especially for planetary but goto does not appeal to me in terms of finding stuff. That is something I like to do on my own (star hopping, or alternative - looking at the sky, pointing in general direction of target and hoping for the best - you would be surprised how often this turns out to work well). That 12" is really F/5 so no much different in terms of coma. It is quoted to be F/4.9 because they used 12" measure for primary, while staying in metric for focal length - 12" is 305mm rather than 300mm and focal length is 1500mm, so actual "speed" of telescope is F/4.92. However I believe that there is some variance in real focal lengths of mirrors produced - couple of mm at least, and maybe even full cm +/-, so again for all purposes it can be said it is F/5 scope. Depending if you want the goto or just tracking - I think there could be cheaper alternative (I would go for that for example, over goto): Regular non tracking 12" SW dob - still F/4.92 (or maybe GSO one, cheaper still and lighter), everything the same except: 1. Weight No much real gain, but it "disassembles" in 3 rather than 2 pieces - probably easier to carry (each piece, and rocker box is somewhat lighter). 2. Price Price cut would be from goto / non goto version + eq platform, something like this: https://www.teleskop-express.de/shop/product_info.php/info/p10564_TS-Optics-EQ-Dobsonian-Telescop2-Drive----Platform-for-45--N-S.html Since "barebone" version is about 66% of goto version, and EQ platform is less than 1/3 of goto version (there are even DIY plans floating around the web).
  8. Yes indeed, all of these methods are approximate since all rely on either making certain assumptions, or having exact data that you don't have before imaging. Don't think we need to focus as much on precise figures, but use both approaches as guidelines. If I'm after a target for which very good sub duration would be 1 minute and 40s, and I already have dark library built for 1 minute, I'm not going to obsess and redo my dark library and have exactly 1:40 subs, I'm more than happy to have 1m subs in that case. On the other hand having target where it benefits to go close or above 4 minutes - I'll use 4 minute exposure and build such master dark. One might ask, where did you come up 1 and 4 minutes with? Well this sort of calculations let you choose couple of exposure lengths for different scenarios based on your setup, prepare for those exposures (build darks, check that your mount / guiding is up to it, etc ...) and just use them. What is important in both approaches (or maybe a third one) is understanding why they work (and under which restrictions) - this will give one ability to say, yes I know why 1 minute exposure is going to work well enough, or I need to try to sort my guiding as 10 minute exposures are going to give me much better results. BTW, on the matter of spreadsheet - it has been created with Libre/Open office software, so if you to examine it - no need for latest Microsoft Office suite, just download and install one of those (both are open source and free).
  9. I did not go over the math, but I'll present my view (again no math, but I can do if you want me to), it is quite simple. Only thing differentiating stack of subs vs one single exposure in terms of SNR is read noise of the camera. If there were no read noise, it would be perfectly ok to have subs of any length. Other noise components all depend on time, read noise is only one with "fixed amount per sub" (target shot noise - depends on time, LP shot noise - depends on time, dark current noise - again depends on time). Short subs do have advantage over single long sub, and I'm all for short subs. Gust of wind? Airplane / satellite in frame? Poor guiding/seeing for couple of seconds? Just discard that single sub, and if you operate on short subs you still have 99% of data left. There are drawbacks as well. Time it takes to download sub (very improved with CMOS), full well capacity - we don't want to saturate the signal, dithering (less time spent on longer subs if expressed as percentage of total imaging time), amount of data to store and stack - larger with short subs. So it is balancing act. We want to find the shortest sub length that will not have very large impact on final SNR (all above included, so FW and guiding, and data considerations ...). I agree with above analysis in terms of graphs - there is a point at which read noise starts to be dominant component and sub duration vs number of subs starts impacting SNR heavily, and equivalently there is point of diminishing returns where using a five (or more, put any number in) times as long subs will yield less than 1% SNR improvement. So far this is pretty much in line with linked text. Where we disagree is having optimum sub duration. There is no such thing as optimum duration of sub in general. There is decision where point of diminishing returns starts for particular parameters. It is not the same if imaging very bright target vs very faint target. For bright target one might conclude that last 1% of SNR is not important and arrive at exposure length of only 15 seconds. For very faint target it might well turn out that to reach 1% of SNR loss you need 10 minute subs. This calculation depends not only on target brightness but on "aperture at resolution", quantum efficiency of sensor, read noise, dark noise and LP noise. For example, people doing planetary work don't even think about LP and they are using exposures in milliseconds. Why? Because of target brightness, it just swamps everything else. Maybe our two views on subject differ because intended use? I was under the impression that above calculations are related to live stacking and determining optimum exposure in that case, without having prior knowledge about total imaging time, while my approach starts at set imaging time and looks how to best divide that imaging time in number of subs. To get the idea how sub length vs number of subs behaves for specific conditions - just use spreadsheet that I've attached - I made it precisely for that reason, to try to estimate SNR in relation of total imaging time and number of subs. Unfortunately it does not work for narrow band quite, but you might be able to "adopt" it for that use. You just need to figure out two things: Target surface brightness in darkest parts in band you are interested in (for example Ha) and what would be photon count in that particular band for 0mag source, you already have QE for different bands, and you have sky values for those bands (everything else is the same). I just had no luck in trying to find narrow band magnitude data for common emission nebulae anywhere - only thing to do is to do measurements when imaging and publish results. For galaxies it is easy - faintest parts usually have 22-24 mag surface brightness, depending on galaxy type for large and near galaxies and it can go down to mag 28-30 for very small and far ellipticals (well for most targets to be imaged).
  10. Ah, I see, not sure that I agree though ...
  11. What method for determining sub duration did you use? I did not do any particular math, but figured that 4 minute subs were good place to be for NB with ASI1600.
  12. vlaiv

    Runcam Night Eagle Pro 2 (Astro edition)

    Pardon my ignorance, but what is the benefit of such camera over regular planetary CMOS sensor?
  13. vlaiv

    Which mono camera ?

    I would go with 178MM and filter wheel at native (no barlow). Pixel size on 178 in my view fits perfectly for maximum resolution at F/12, assuming it is SW Mak 150mm (others might disagree, and I've seen planetary imagers use much higher sampling rate as they say it makes post processing easier).
  14. I'm not sure about the noise but PI image looks like better aligned stack - faint stars are tighter in right image (zoomed / cropped part). That could be just down to centroid accuracy (better in PI so more precise alignment). Also I don't know what sort of algorithm is used for sub transform / align. Depending on type of filter - it might have impact as well.
  15. For prism I'm not surprised, some prisms polarize light in certain way - for example Herschel wedge / prism polarizes the light, so you can use single polarizing filter to tune down amount of light by turning eyepiece with polarizing filter attached. Mirror diagonal is probably dielectric kind (not regular silvered or aluminum) - that could explain strange behavior of light. Dielectric coatings ensure maximum reflection by employing interference between different layers of dielectric material. It is quite possible that some wavelengths that are reflected due to interference get polarized, and some don't which produces distinct "color filter" kind of effect when combined with polarizing filter in certain position. Other explanation would be that you hit an odd angle to the sun. I know that daylight is somewhat strangely polarized - you can see that by holding your polarizing filter (or polarized sun glasses) in different angles and looking at the different parts of the sky - you will notice that there are areas where sky gets darker and some areas don't seem to be effected. If Sun was at such an angle to produce polarized light in the part of the sky where Moon was, and if for some reason blue, due to atmospheric scatter was not polarized (not coming from sun directly but bouncing around so polarization has been randomized - regular day skyglow) - prism might not be affected because it also (re)polarizes the light before reaching polarizing filter and mirror just passes the light as it is. Now that I think about it - second explanation seems far less likely, but who knows ....
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