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There is no difference really - they are the same thing - ratio of aperture to focal length. It is the fact that: a) there is plenty of light and noise is not as dominant b) different F/ratio lenses are used on same camera body that can simplify things so that "photography F/stop" can be be used as indicator of speed - the way it is used in regular photography. In astrophotograpy - things can't be simplified like that because light is scarce and certain scope can be paired with many different cameras and pixel sizes. For this reason in astrophotography it is better to think in terms of aperture at resolution rather than only F/ratio. That is because you need to think of FOV in terms of focal length rather than aperture. It is the focal length and sensor size that determine field of view. Take any telescope / camera combination - and simply "stop down" aperture using aperture mask - FOV won't change. When you stop down regular lens by using slower F/stop - do you change the FOV? No you don't. Why would then FOV be related to aperture size?

I use it to all things astro related as it is such a versatile tool. Even have couple of plugins that I wrote for it to do various things like background removal and such.

Very strange thing those OIII flats. Same pattern appears in both Ha and Ha flats, and Oiii and Oiii flats - yet Ha calibrates fine while Oiii does not. Here are uncalibrated OIII and Ha subs binned to oblivion (x16) to raise SNR and show pattern Here is calibrated Oiii: Flats over correct. I suspect that there was some sort of light leak when shooting OIII flats. Both Ha and OIII do have very strange flats indeed - but I would not worry too much as long as it calibrates out properly.

Other than above remark - pretty much yes.

It's not artifact. At current moment in time - object can indeed be 20 billion light years away and no, that does not mean that it is some sort of paradox in terms of age. Don't confuse distance with time. Only way they are related is in moment of observation. If we are seeing now - at this moment object that is 10Bly away (calculated by red shift and varying Hubble constant across the time) - it only means that distance traveled by light from emission moment up to now is 10Bly and that object is at least 10By old. If object is now at 20Bly away from us (but we don't see it there now) - well it does not mean that it is 20 By old, but it will be at least 20By old at the time in the future when observer on the Earth sees it as 20Bly away (we don't see it now but we will see it in the future once light emitted now reaches us). We can't see objects that are older than 13.7By - not ones that are further than 13.7By - first is due to age of universe and later is because light did not have enough time to reach us. That does not mean that there are no objects that are further than that at this moment, nor does it mean that there are no objects that were farther than that at the beginning just 1By after big bang for example. In the end - there are object that we will never see - since there is accelerated expansion of universe, light that left those objects will never reach us as universe will expand more than the distance light can travel in given amount of time and there will always be "more to travel" before it reaches us.

Are you completely sure about this? I just checked import tariffs for Egypt and while there are goods with very high tariffs, I've found following quote: I'm asking because for that kind of money - 8850 EGP you can basically get this sort of scope (which is probably better): https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-200p-dobsonian.html (when calculating cost subtract UK VAT from the price as item will be exported from UK). Maybe you can instead see if you can import from some neighboring country? Egypt is member of League of Arab states and I think that means some sort of free trade or similar? By the way, for other members to see what sort of telescope that 160 / 1300 is there is video posted on facebook: https://www.facebook.com/Voyager.Telescope/videos/vr-160-1300/2300772490238792/

Hi and welcome to SGL. Indeed. You can think of celestial sphere being fixed and Sun/Earth system embedded in center. In Spring earth is on one side of the sun with respect to celestial sphere and in autumn it is on the other side. That is why there are "seasonal constellations" - like Orion that is observed in winter and Cygnus that is observed in Summer.

@Astro Noodles Maybe have a look at this youtube channel: https://www.youtube.com/c/pbsspacetime/playlists There are bunch of short videos in nice format that deal with host of questions about cosmology (and more). Not overly complex and gives nice introductory overview of what physics says about it all. For example, this video is on Hubble tension / Crisis in cosmology: https://www.youtube.com/watch?v=dsCjRjA4O7Y

You can really say the same about general relativity or quantum mechanics / quantum field theory. In fact, cosmology is just the two above applied on observations that we make of the universe around us on many levels. My friend noted very interesting thing that I'll paraphrase like this: "It's now been almost 100 years that we have two of the most successful physics theories - GR and QM, yet 99.999% of people know nothing about them and have serious trouble comprehending concepts needed to understand those theories". That is indeed very interesting observation and I can only assume it is so because of the level of mathematics needed to formulate those theories. By contrast - people don't seem to have any problem with Newtonian mechanics and gravity. Don't assume that cosmology is more "philosophical" than other physics just because of the math involved. I don't think so. I don't believe that our perception has anything to do with it. We are well equipped mentally to make a measurement as it is comparison of two values. We know that speed of light in vacuum is a constant regardless of frame of reference - because we measured it and we always get the same result. We know that galaxies are receding from us because again - we measured it, or rather we measured shift in light spectrum that happens due to this. And so on ... Gravitational waves have already been used to measure Hubble constant and so have many different methods - have a look at the list on wiki page: https://en.wikipedia.org/wiki/Hubble's_law#Measured_values_of_the_Hubble_constant

In DSS it is apparently called - rational for some reason (although rational can be fixed point as well - but that is technical stuff - just use 32bit rational).

Stars are fine - it is image format used to store data that is causing trouble (or rather how different software interprets pixel values). It is best to save as 32bit floating point fits. You saved in 32bit integer TIF and 32bit integer can be treated as signed or unsigned - ImageJ and windows image viewer as shown by @wimvb treat TIF 32bit integer as signed integer and that means that very high values are treated as negative (up to half are positive - larger than half are negative - that is some sort of binary format stuff how to represent negative numbers). Just export data from DSS as 32bit floating point FITS (or even TIF - 32bit floating point has strict definition) and you won't have these issues.

Here is linear data in ImageJ Dark pixels in star core all have negative values while the rest of the image has regular values. I just realized what happened - signed / unsigned conversion. It is ImageJ import thing - it imported data as 32bit signed integer - but in reality it is 32bit unsigned integer - those negative values are in fact values higher than 2,147,483,647 "looped" to negative values".

Actually that is either stacking artifact or some error when importing TIF into ImageJ They were that way in linear stage and I did not bother to fix it - I just did basic processing on the image (in fact - I don't usually do more than that): 1. Background wipe 2. Stretch 3. Masked denoise First step is done in ImageJ and last two in Gimp. Mask is simply denoised layer used as transparency mask (inverted and stretched in opposite direction).

But if you look at this version - I think it's all fine: If you have such large mismatch in resolution between two images and you want to make compound image - well, you need to rescale one of them to match their scales. You can either loose detail in more resolved one - or simply make less resolved one look bad - without detail. Fact that these two telescopes have very different resolving power - order of magnitude different - makes it hard to match resolutions and hence - one source needs to suffer. Authors chose visual to be largely scaled up to match resolution of radio and that made visual channel look bad - but it is not bad itself when viewed at proper scale to itself.

Then fire away. Expanding universe is waiting for no one

Everything is moving away from everything else - regardless of position. That is rather easy to see why on simple example - but for some reason harder to extrapolate to whole universe Say you have following arrangement: A---B---C Three people (Alice, Bob and Charlize) that are in a straight line. Bob is standing still, Alice is moving to the left at steady pace and Charlize is doing the same to the right. In first instance they are separated by three bars of distance and in some future time they are separated by 5 bars of distance like this: A-----B-----C (A and C are moving away from B with steady speed of two bars per some time interval). Bob sees everyone moving away from him in every direction. So does Alice - but notice something important with Alice - it sees Bob moving with speed of two bars per interval - but what about Charlize? She is further away and moves with 10 - 6 = 4 bars of distance per same time interval. Alice sees Charlize being twice as far away as Bob and receding at twice the speed in comparison to Bob - Hubble's law in action in simple example. Everyone can see them selves as standing and everything else is moving away from them (choice of reference frame) and further someone is from them - faster it will move with respect to them - but in reality everyone will see the same speed at same distance - Hubble constant. Now all you need to do is extrapolate that to 3d case. Usual step used is to extrapolate that to 2d case - by use of muffin or balloon. Imagine balloon with spots on it being inflated: If you choose any two point on that balloon - their distance increases linearly with time (as balloon is being inflated) - but points at distance of "two" will move twice as fast from each other as Alice and Charlize did and so on ... Next step you have to do in your head as we can't show 3d analogy on 2d screen

Actual speed is about 73 (or 67) km/s/mpc. Speed of recession varies with distance. That last bit is mpc - or MegaParsec which is equivalent to about 3261560 Ly So at a distance of 3.26Mly average recession speed of objects is 73km/s - at twice that distance or 6.52Mly, recession speed is 146Km/s and so on... M31 that is 2.5Mly should have slightly lower recession speed that 73km/s - but it is in fact not moving away, it is approaching MW due to gravity interaction in local group. At small distances gravity is still dominant force and galaxies bound in local groups and small clusters - act like regular bodies in gravity bound system. M31 is actually moving at a speed of 110km/s towards us. Which means it will get here in about 4.5By.

Yes it does. It stretches both space "in front" of traveling light and "behind" it. Only stretched space in front of traveling light affects the time it takes for light to complete the journey, but stretching space affects total distance between two objects at the moment light reaches second one.

Can you be more specific about what you are asking? If you see the light from object that you perceive as being 10 billion light years away - then that light started a journey 10 billion years ago and it traveled thru space - the distance of 10 billion light years. We say that object that we are looking at is 10 billion light years away - but that is not actual distance and meaning of "actual" distance to that object is rather ambiguous. For sake of simplicity - let's assume that object we are talking about is at rest with respect to us in terms of local speed. Any speed that we discuss here will be due to expansion of universe. 1. You can ask - what was the distance between us and that object at the time the light was emitted. It was less than 10 Bly. 2. You can ask - what was the distance traveled by light since it departed object until it reached us - that is exactly 10Bly 3. You can ask - what is the distance between us and that object now - it is more than 10Bly This is nothing strange really - analogy to this can be made by the two of us using simple ball. Imagine that we are 10 meters apart and we are running away from each other. I throw the the ball at one point towards you. At that time we are say 10 meters apart. Before ball reaches you - you'll move a bit further away from the point you were at when the ball was thrown - so ball will travel a bit more than 10 meters. At the time you catch the ball - I'll also move a bit in opposite direction so I'll be a bit further still. So we again have three distances in ascending order. Only difference between this case and above case is that here the two of us are running away - but in case of distant galaxy - it is space that is expanding in between - and that makes things as if we are moving away from that galaxy - it looks like galaxy has some speed with respect to us. As far as distances involved - well we can calculate all of those by just knowing one - 10Bly that took the light to reach us. We can calculate what was the distance between us and that object - before 10By and we can calculate what is the distance now. In order to do that - and in fact to know that something is 10Bly away - we must rely on our cosmological model and solve it for observed data. In fact - physicist did that - and we are actually in a bit of a pickle there. We have same model calculated from two different sets of data and results don't quite agree. This can mean that either one or both sets of data contain systematic error - or our model is flawed. When I say model - I mean LambdaCDM model - currently accepted cosmological model. It connects things like age of universe, Hubble's constant, ratio of matter to radiation to dark energy in universe and so on. When someone says that object is 10Bly away - they actually mean that there is certain red shift in spectrum of light coming from that source and we can use that red shift to determine how much space expanded from the moment light was emitted up until now. Since we know how space expanded over time - we can tell how long ago light was emitted and so on and so on. Problem is that we currently have two different values for present day Hubble constant - 67km/s/mpc vs 73km/s/mpc. On one hand - that is extraordinary match since these two values are calculated from "data separated by 13.7Bly" - one is from cosmic microwave background radiation - light from about 377700 years from big bang, while other is from current measurement of how fast are galaxies around us receding from us as we speak vs their distance. On the other hand - it is problem that these two values don't match better - as that points to an error somewhere - to quote wiki: Depending on which set of data you take - you'll get slightly different answer to actual distance in light years - that is why astronomers often use red shift value rather than actual distance.

Actually - being under sampled helps the quality of your image (or rather SNR) so don't worry about that.

One of postulates of Special Relativity is that speed of light in vacuum is constant in any frame of reference - so yes, in that sense it is universal constant - however, light does not always travel at that speed - namely in different medium like in air, or glass or other - it will travel slower than in vacuum. If something is traveling at very large speed and shines light in your direction - several things will happen: 1. Person that shins light in your direction will measure that light travel at light speed 2. You will also measure that light passing you by at light speed 3. Light will be red shifted due to relativistic Doppler effect 4. That light will reach you in future at time it takes for light to travel the distance from point in your reference system at which origin shone the light all the way to you. Also note that following things will happen: 1. Person shining the light will not see the time as you do - it can't know what is your local time at which light was sent 2. Person shining the light will see distance between them and you - differently then you'll see distance between you and them 3. Person shining the light will appear to have their time slowed down in comparison to your clock etc ... In the end, I'd like to point out that we did not include effects of expanding universe which is very much relevant at such large distances (but above will be true even in case of light days). Expansion of universe adds another dimension to all of that and although you are not effectively moving against the source in regular terms - space between you is expanding and you are getting further apart - and that affects how long it takes for light to reach you and it also causes light to stretch with expanding space so it ends up further red shifted due to that (longer wavelengths as if it has been stretched).

I think that this can be easily "measured". You can for example - post linear stack in 32bit fits format and people can then process that data. You can then compare your processing to see if it can be improved. I agree with above - sky quality plays significant role for image quality - as does telescope used with camera (aperture at resolution). So does total integration time. However, this is narrow band image and I don't know how much of light pollution gets thru. Maybe people whose images you look at - have good denoising skills? That lets them push data further without revealing too much of artifacts.

Will this convince you?

Yeah, don't need those at all. You don't need graphs at all - there is stats panel for each sub you make. You should have frame and focus type of sub (just few seconds long) that you can take - or even loop. Stats window has basic statistics of the sub - number of stars detected, mean, median, min, max and so on - and HFR. Note HFR number and try to make it as small as possible by tweaking focuser. Workflow is - adjust focuser just a tad - take exposure. Don't adjust focus while you are exposing - but between exposures. Compare HFR to previous and adjust - (if tweaking focus makes number larger - change direction). There is often graph of HFR history between subs. Above bar graph looks like that - but I can't be 100% certain as I'm not using NINA. If you have such graph - you'll have visual representation of current HFR and its relation to past values. SharpCap has this: It shows graph with bars moving to the left to make room for new exposures and also best and last reading (either FWHM or HFR).