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I'm trying to model the view thru telescope - not only size of objects vs field of view (most common simulation type) but also contrast and what would be visible given sky conditions (transparency, light pollution), type of scope and target position in the sky. Quite a bit of "science" / number crunching went into this, and images that I'm going to present were in no way processed other than by means of number crunching (no brightness / contrast adjustments or curves in photoshop or such). Overall, I'm pleased with how images turned to be, and I believe they are quite a bit close to what one can expect from telescope view, but I would like confirmation from other observers. In your experience do these images match what you would expect to see under given circumstances? These simulate best possible view - meaning target at zenith, best ever transparency (actually transparency and target position were not taken into account, so these images are slight over estimates what can be possibly seen but difference is small), with x60 magnification (20mm plossl) and 8" standard dob (26% CO, 94% mirror coatings) under: a) SQM 18.5 (which is red zone closer to white, Bortle 7 for those not familiar with SQM readings) b) SQM 21 (green zone almost bordering on yellow, Bortle 4): Above are best viewed under normal lighting conditions (accurate color reproduction in sRGB color space requires regular illumination - dark room will show brighter view), also your monitor needs to be calibrated properly. Values used for simulation were from my experience, and images match very well indeed. I think they are slightly over estimating visibility, but I'm now looking them under lower illumination levels than advised (and I know when I'm doing astrophotography processing - image looks a bit dimmer during the day in daylight than it does with soft dim illumination at night). In any case - first image shows hint of spiral structure - something that I never saw from my LP back yard - but just two cores and number of stars is remarkably accurate as is contrast between sky and field stop (I did not do color but blue tint should be more accurate on background sky). With second image, fact that you can spot the bridge and darkening in it matches what I saw from SQM 21.3 site (according to light pollution info, but could be closer to SQM 21 in reality). Extent of galaxies also matches and so do stars. In any case, I would appreciate feedback if these two match the reality and your experience.

It looks like my above calculation was pretty good Thanks to @Martin Meredith for pointing out what to look for in the thread I posted about magnitude difference (and that is Weber's Law and JND - just noticeable difference), I was able to find that difference in intensity of light that we can distinguish is something like 1%-3% according to this: https://www.sciencedirect.com/topics/engineering/just-noticeable-difference That is fairly close to 4.3% that I got from assuming that our brightness perception needs to be considered in mag/arcmin^2 rather than mag/arcsec^2 when comparing to star brightness (as we can't resolve past 1 arc minute) and from Bortle scale data on limiting stellar magnitude under certain SQM reading. We now have a tool to do simulation of how the DSO will look like under different circumstances: SQM, alt, elevation, AOD, telescope/eyepiece combination. That is something I wanted to do for a long time - make image of what can be expected to be seen at eyepiece for given conditions - but never really knew how to go about it.

Here is an interesting question related to observing and sky brightness. When looking at different Bortle scales I noticed something interesting. Here is example from wiki page on Bortle scale, it says: Bortle 1 sky (let's say that is SQM 22) - by using 32mm (or 12.5") telescope, limiting magnitude is about 17 (on wiki page it says 17.5 but I think that is a stretch, as 12.5" aperture can at best deliver about x2090 more light than naked eye, and NELM is said to be mag7.6 - mag8, while x2090 represents difference of ~8.3 magnitudes - those two combined give 15.9 - 16.3 mag rather than 17.5). In any case - I selected mag17 as limit for ease of calculation. How come that we can't distinguish such star from background even if difference in brightness is whole 5 magnitudes - sky being mag 22 and star being mag 17? Maybe mag 17 star does not give off enough photons to be detected? I don't think so as mag 18.5 sky looks bluish/gray in the scope, and we can do calculation on how many photons does it produce. Mag 0 star will do something like 980,000 photons per second per cm^2. Mag 17.5 (again let's use this number as it is easier to calculate) has 10e-7 less photons than that, so it is 0.098p/s/cm^2. 12.5" scope has something like ~800cm^2, so it captures ~78.8 p/s. Now we have figure of 5-7photons being detectable by human eye (in single burst - will be seen as a flash) - having x10 as much every second or that much every 0.1s is going to be detectable by eye. In any case, we can do the same for stronger LP - Bortle 4 is SQM 20.4-21.3 and limiting mag in 12.5" scope is now 15.5 - again about 5 magnitudes difference (x100 intensity), but plenty of photons this time. So there is enough contrast - x100 but sky prevents us to see the star? Maybe issue is with surface vs point brightness. Surface is per arc second, and our eye can only resolve about one arc minute. Maybe it sees integrated brightness of sky over that one arc minute, so it sees mag 8.89 brighter sky than SQM says (in terms of photon count)? Let's go with that - and case of SQM 21 and limit magnitude of 15.5 and try to see if it "fits". 21 - 8.89 = ~12.1 and star is now 15.5. Sky is brighter 3.4 mags then star that is 4.3% difference in brightness (sky alone vs sky+star). That makes sense. Converted in magnitude difference (not sky to star, but contrast - sky to sky+star) that would be -2.5*log(1/1.043) = ~ 0.05 mag difference. I already posted question in variable /double star section of SQL - what is magnitude difference that human eye can see. I expected answer to be something like 0.1mag or similar - but 0.05 mag is not far off.

I'm trying to figure out some stuff related to how human vision works and this question is important part of the puzzle. Let's take average human and a telescope of some aperture (pretty sharp, low magnification - stars are still point like and airy disk is not resolved), two stars of very close magnitude. What needs to be magnitude difference between two stars (same spectral class, so color is the same, everything is the same except brightness) for most people to say - yes I can see that one is brighter (very slightly so) than the other? Does it vary with brightness? Maybe at mag 5 we can spot 0.2 mag difference while at mag 10 we can spot only 0.4 mag difference? How does aperture of telescope affect this (I'm guessing it will affect this if visible mag difference depends on brightness of stars)? Either personal experience or "general knowledge" are welcome answers.

That is to be expected - most of LP is in fact in lower parts of atmosphere where atmosphere is denser. Also - almost all source of LP is ground based. SQM can't distinguish if light coming from the sky is ground LP scattered in atmosphere or reflected of clouds, but we can be pretty sure there is same amount of LP originating from the ground.

I'm sure that quality varies between nights but on a single night with two or more repeated measurements in same direction - there probably is no much difference in obtained value?

CEM60 seemed like sensible upgrade path from HEQ5 for me - I also have belt modded heq5 and when I did my research - only thing that modded Heq5 lacked (apart from manufacturing precision and general fit and finish) was backlash control - something that magnetic/spring loaded worm handles very well. That gives better controlled/behaved mount for guiding as there will be less lag after issuing guide command (better responsiveness). Then there is manufacturing precision as well. Both seem to be present in CEM60 and from what I saw - guide logs confirm that. Because I actually want Mesu 200 level of precision, I decided to skip CEM60 step and eventually go for the mount that I really want. It means that I will be using HEQ5 for some time to come - but I'm fine with that.

I think that it would be good if you could tell us the rest of your imaging gear - mainly if you guide and what camera do you use as well as intended targets. My guess is that you would want something to go high resolution as 130PDS pretty much covers everything else (btw - depending on what coma corrector you have - it will not lag optically behind 80ED and might even be ahead - larger aperture, no color aberration issues - faster scope, etc ...), but for that you need to match your camera with the scope and have good guiding / tracking.

I'm quite happy with mag rating - single verifiable / measurable number. What is not to like about it? It suits me perfectly for imaging, and although it does not tell the whole story about observing site - unlike something like this: (btw anyone knows software that produces such image? Guide cam with all sky lens is enough to record needed data - it's just calibration and nice display after that) it is enough to do some rough calculations on what to expect from a night of imaging of particular target. On the other hand - I have no idea how to relate that to what to expect from observing. I think we have all the data needed to build decent model of what can be seen or rather what would average person expect an object to look like given: - scope, eyepiece - SQM reading - extinction (elevation, altitude, AOD) couple that with some facts about human vision - like ability to detect only several photons high signal (like 5-7 photons) and that there is 1000:1 contrast at any one time, I still don't have a clue how to put all of that together. I don't know the answer to a simple question - if SQM reading is mag20 and nebula brightness after extinction is mag23 - will contrast be enough to be detectable by human eye (I can tell you in percentage how brighter nebula will be compared to background but that tells me nothing if it will be visible by human eye)?

Any reason for not considering CEM60 (non EC version)?

Not sure if any one scope would justify upgrade over 130PDS for DSO imaging unless you have very specific need. Do you know why you are upgrading? Maybe better upgrade path would be other imaging related gear - like guide setup if you don't have one, or maybe dedicated astro camera if you are using DSLR ....

I just noticed that different people quote different SQM to Bortle mapping - very interesting. Bortle scale nomogram places natural unpolluted sky at about mag 21.75 - 21.8? It also looks that Bortle 1 begins there - here is image of this: Wiki page on Bortle scale lists these SQM values as Bortle 1, 2 and 3 skies: Bortle 1 - 21.7–22.0 Bortle 2 - 21.5–21.7 Bortle 3 - 21.3–21.5 This resource - https://www.handprint.com/ASTRO/bortle.html lists following SQM values: Bortle 1 22.00–21.99 Bortle 2 21.99–21.89 Bortle 3 21.89–21.69 What should we adapt as being correct scale?

I did mean to mention that - with age ability of eyes to do that goes down. One of the reasons people report different levels of field curvature when testing eyepieces / scopes.

It is feature of all simple scope designs and in general it is a function of focal length of telescope - shorter FL stronger curvature. With most scopes for visual and most eyepieces - you will not see it. Only when you have short focal length scope and wide field long focal length eyepiece (large fields stop that covers large area of focal plane) - you will see it at the edges. Amount of field curvature that one sees - depends on observer. It is much more visible when imaging because sensor can't adapt to small changes in focus position - while eyes can and we do that instinctively - as soon as you shift your central vision to a new object it adjusts focus without you doing anything. You only see out of focus when you have two objects near each other that are large distance away - like holding finger in front of a distant object - one of them is going to be blurry - either finger or distant object - but then you can switch your focus at will. If you observe center of the field and then shift your attention to edge - eyes will try to adjust automatically. Only if you observe both at the same time or difference in focus is large enough for eye to fail to adjust - you will see field curvature. Above two images explain what happens - imagine line going from center of the lens to focal plane - as you move from center and keep same focal length - you are starting to draw a circle instead of straight line. Longer focal length will draw larger circle (bottom image), which will be less curved - less deviate from straight surface.

I've got rather ambitious list to contrast achievements of this, soon to be past year (total of 0 imaging sessions, and perhaps 2-3 observing ones). - got myself a new rig recently (Mak102 on azgti) and want to try out (and enjoy some of them regularly) different activities with it: lunar grab&go, EEVA with eyepiece projection or reducer and small sensor cmos, DSO imaging in similar configuration and planetary and lunar imaging. Will need to purchase wedge and counter weight for azgti to put it into EQ mode. - that setup is almost complete, but I will need to fashion "proper" dew shield for it. I was hoping to make a firm one rather than flexible - maybe out of aluminum sheet (0.5mm) or plastic sheet and some flocking self adhesive material that I already have. - need to make two motor focusers (for each of my imaging scopes) with single controller. That will be DIY motor focuser - each scope will have on stepper with bracket, but there will be single arduino controller box as I don't use both scopes at the same time. - hope to get quark combo at some point to get some Ha solar observing for the first time in my life. - will look to add Riccardi FF/FR x0.75 for my imaging scopes - eyepiece or two is always nice addition (I'm now short of ES82 8.8mm to fill the gap between 6.7mm and 11mm) and so are other bits and bobs - will need ES tele extenders for solar work and planetary imaging for example. But far the most important new "astro" gear is moving to a new house Hopefully it will all go as planned and that will happen somewhere at the end of summer / start of autumn following year. This of course includes observatory. I've purchased the land, architect is doing calculations as we speak (well, not right now, it's holiday break) and request for building permit has been filed. It's 2 mags darker there @pete_l It took me this whole post to figure out the pun in title of this topic - all the time I wondered what peoples plans and wishes had to do with "perfect vision", and then it hit me - next year is 2020

For some reason I related use of the word "snaps" as meaning photography . I've only observed from Bortle 4 / mag 21.1 skies and was blown away by views (was rather transparent sky that particular evening), so I can only imagine what it looks like in mag 21.7 skies.

Do you intend to image at these sites? It is far easier to asses difference between LP levels on imaging than it is for visual. Above difference of 0.14 mags in LP levels translate into difference of ~x0.88 (same as 0.21/0.24), or if we take inverse of that 1.143 or 14.3% in signal level. Take square root of that and you will see that noise increase due to change in LP levels is only 6.9% - not sure that you will be able to tell the difference (maybe only threshold objects that have intensity of the same order as LP noise levels).

Here is a good article on that subject: https://www.skyandtelescope.com/astronomy-resources/transparency-and-atmospheric-extinction/ You need to account your altitude above sea level, position of the star but also AOD - aerosol optical depth which is general measure of atmospheric transparency. good AOD forecast can be seen on Copernicus https://atmosphere.copernicus.eu/charts/cams/aerosol-forecasts?facets=undefined&time=2019123000,3,2019123003&projection=classical_global&layer_name=composition_aod550 AOD has impact on light pollution levels as well - but I don't know exactly how to calculate any of it. Relationship is simple - more dust and particles in atmosphere - more light scatter there will be from both target but also from ground sources that make up light pollution. In perfect transparency we would have absolutely no issues with light pollution.

I'm talking of comparing two sensors based on field of view and matching to particular scope. Here is example: Yellow FOV is ASI1600 + 10" Newtonian scope by Skywatcher, while red FOV is Atik 414 + 130PDS (again Skywatcher imaging newtonian) - which is 5.1" scope. Now important thing to remember here is that each little bit of above FOVs (small surface making up picture - let's not call it pixel yet as it is not on sensor but rather on focal plane) will contain focused light from whole respective aperture. With use of binning, and especially fractional binning - you can match sampling resolution of two cameras, so we can convert our "small surface at focal plane" to actual pixel that is "equal in size" (or rather resolution "/px). We still have difference in aperture and light gathering so more light will end up in pixel of the image with ASI1600 + 10" scope than with Atik 414 + 5.1" scope but both images will show same thing - same FOV sampled at same pixel rate. How does that make ASI1600 inferior to Atik414 (as above numbers would suggest)?

For "normal" angles it is just 1/cos(angle) You can use short cut cos values for 30, 45 and 60 degrees and that will give you air mass at each angle as: 0 degrees = 1 30 degrees = 1/(sqrt(3)/2) = 2 / sqrt(3) = ~1.1547 45 degrees = 2/sqrt(2) = ~1.4142 60 degrees = 2 But as you start approaching horizon - above formula will give wrong results. It assumes flat earth scenario, while earth is of course curved. Different models give different results for horizon but most are about 38 air mass value. See here for different models: https://en.wikipedia.org/wiki/Air_mass_(astronomy)

Maybe best type of unit to be used is mag/arcsec^2, or simply magnitude (surface brightness - not to be confused with stellar magnitude although it is the same thing). Magnitudes are better suited to our visual perception than physical quantities because our eye/brain works on close to logarithmic scale. Surface brightness of targets is also given in these units (or mag/arcmin^2 and here it is simple conversion as add 8.89 to get mag/arcsec^2 - but these tend to be average brightness) and you can estimate visibility from that. According to SQM calculator page (found here), to convert from cd/m2 to mag/arcmin2 use: or rather opposite one in above case. We have 0.24 mcd/m2 and 0.21 mcd/m2, let's convert that to mag/arcsec2 -2.5*log(0.00024 / 108000) = 21.633 -2.5*log(0.00021 / 108000) = 21.778 How big a difference is 0.14 magnitudes? Not much - change in transparency from excellent to very good will make about 0.1 mag attenuation of targets. Single air mass has about mag 0.16 attenuation.

I'm not saying that one can't make a good image with long focal length scope if they know what they are doing. I'm just saying that if people are looking for advice on which camera will serve them best - saying that good rule is fl = pixel size x 200 - when that particular rule will lead to oversampling in 99% of amateur setups, or mentioning that it is ok to use even double focal length without explaining impact of such decision, or how it can be mitigated, will do disservice more often than not. It is also the fact that explaining all the intricacies of choosing particular sampling rate and processing workflow (like binning, etc ...) can similarly confuse people and do them disservice and should be done carefully as not to confuse or deter from understanding.

I think you are very much overestimating "best" focal length for a given pixel size. What is the criteria that you are using for this? Sure you can use 5um pixel camera with 1000mm focal length for effective sampling resolution of 1"/px - but most of amateur setups are simply not capable of delivering that sort of resolution. One needs 1.6" FWHM stars in their subs to do that. Hence in most circumstances that will be oversampling. Saying that you can equally use 2000mm of focal length with 5um pixel camera without mentioning how can you make it work is probably going to cause issues for most people that don't have deeper understanding of this topic. I can imagine someone choosing C8 over 8" F/5 newtonian based on that reasoning and wondering why their images are so poor without realizing that at 0.5"/px they are spreading photons too much over pixels and that they need x4 more imaging time to match SNR of 8" F/5 newtonian with same camera.

There is a lot more to consider when choosing suitable camera. I will give you two examples where simple formula like pixel_surface*QE can go so wrong. For ASI1600 you obtained figure of 8.5 - that presumably being 3.8 * 3.8 * 0.5= 7.22 (in my calculator, but maybe you used different QE figure there - I've found it to be around 50% in quotations on the net). Actual figure does not matter for what I'm about to say - why don't you repeat your calculation but this time take binned x2 pixel size? Instead of going for 3.8 * 3.8 * 0.5, go for 7.6 * 7.6 * 0.5 = 28.88! Now we are talking, right? It's clearly the best of all on your list above. But hold on, binning must be cheating right? No, it's not - it's legitimately used and indeed it has the same effect as using larger pixel size (for hardware binning, for software binning like with ASI1600 it acts as larger pixel size and larger read noise - by factor of 2 so it is 3.4e instead of 1.7e at unity gain - but like you said yourself - read noise is not as important because it can be sorted out with longer exposures up to a point). Here you go - simple example that your metric of pixel_surface * QE does no hold much sense. Let's go with another example. Atik 414Ex - it has something like 9mm x 6.7mm or 11.22mm diagonal and "figure of merit" equal to 22 vs again ASI1600 which has diagonal of 22.2mm (let's simplify to double that of atik - it's not quite but almost). Let's now take some common type of telescope - like F/5 newtonian and compare how will two sensor fare if we decide to shoot same size FOV with each mounted on suitable newtonian. It is clear that scope paired with ASI1600 needs to have twice focal length of that paired with Atik to provide same FOV. Since both scopes are F/5 - one paired with ASI1600 will have twice aperture as well - or x4 light gathering surface. Which sensor do you think is going to provide better image on same FOV in same time? I would guess - one collecting x4 the light.

Yet you swear by Mesu - both being friction based products