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Not really. Since you are over sampled - you have more than you need information in that sense. If binning gets you back at close to proper sampling rate - then you won't loose any precision. Star centroid algorithms work very well. Even with single star you can get precision down to 1/16th of a single pixel - like in guiding. In average astro image - software will be aligning based on about hundred stars - that additionally improves precision by factor of 10. Binning "messes" up by factor of 2 (even when properly sampled) if at all. In any case - we are talking about fractions of pixel that are one simply can't perceive in the final image (we would be hard pressed to notice shift by half a pixel let alone 1/10th of it). Is the scaling down equally good as binning? Let's do some more tests. What if I need to scale down by .75 to get to proper sampling rate, or if I need to do it by 0.33 (equivalent of x3 bin)? Here is a little experiment: Again, I've created test image with noise of 1.0 and then scaled that to 0.9, 0.8, 0.7, 0.6 and 0.5 of it's original size. Scaling down does not produce nice continuous improvement in SNR - in fact scaling by 0.6 is worse than scaling with 0.9. Now lets try 0.33 and see if that works like bin x3: Oh, no, it's worse that bin x2 in terms of SNR improvement. While method with scale 0.5 works to produce OK results - we should not assume it is proper solution and it will work in general. Btw - there is a way to do fractional binning as well - but it is complex topic.

There is this: http://www.darkskiesawareness.org/img/sky-brightness-nomogram.gif (click to get image since http images are no longer displayed embedded) Taken from this page: http://www.darkskiesawareness.org/nomogram.php It is a rough guide but can serve purpose. Bortle 1 and 2 could be considered truly dark skies and I think that they start at SQM 21.5. Difference between 21.6 and 21.2 in bortle scale is significant - Bortle 2 vs Bortle 4 (well on edge with Bortle 3 - but still Bortle 4). There is this page as well that lists approximate SQM ranges: https://en.wikipedia.org/wiki/Bortle_scale

Most people use reducer for point 2 or for point 3. Point 3 would be - at some point with larger sensor you need field flattener. These come in two flavors - without reduction and with reduction. Most people opt for one with reduction because of point 2. It does become faster. We can define speed in simple terms to be: Aperture at resolution. When you add focal reducer, aperture obviously does not change - but resolution does change as you change the focal length of system. Pixels stay the same size - as you are using the same camera - but due to focal length decrease - each of them covers more of the sky. They become "bigger" in relative terms (cover the same amount of sky as would bigger pixels on the scope without reducer). That improves the speed as each pixel receives signal from more sky (same aperture) and hence gets more photons which equals signal increase in given time. SNR increases. Nope - it is other way around - you can have the same impact if you get camera with larger sensor not smaller. Larger sensor covers more sky - reducer lets your current camera cover more sky. Once you get camera with larger sensor - you need to make sure you are also sampling at changed sampling rate - otherwise you will not see speed improvement. Benefit of larger sensor is that you can use it on larger scope to get the same FOV - and larger scope means larger aperture. In the end - just be careful. There is something called corrected and illuminated circle for telescope. You can't get good stars past that boundary (and with many scopes quality of stars is questionable near that boundary). If you wonder why most reducers are in range of 0.75-0.8 and not below - well, for that reason. You design reducer for APS-C sensor size - which has 28mm diagonal and you have x0.8 reducer - then original imaging circle needs to be 28 / 0.8 = 35mm. Majority of scopes won't have good stars and illumination past that diameter. Same goes for sensor size. It makes sense to use FF sensor only on scope that is capable of illuminating 45mm diagonal. I would argue that a) you are not under sampling with SpaceCat. If you take sampling rate at face value - it would look like you are under sampling. You are close to 4"/px with ASI071 - that must be under sampling, right? Well - SpaceCat has 50mm of aperture and 50mm of aperture has Airy disk diameter of 5.13" in green light - larger than single pixel. Add a bit of seeing and tracking error to it - and I'd say 4"/px is really not that under sampling b) without reducer you are over sampling with TSA120. With that aperture in normal conditions - 1.5" seeing and 1" RMS guiding - you can expect star FWHM to be around 3" and corresponding sampling rate to be ~1.8"/px (FWHM/1.6). With better guiding you'll get down to ~1.5"/px range (FWHM of 2.5").

What do you mean by throwing away information? You don't throw away any information with this process. I blinked the two and indeed - difference is minimal. I think binned version is very slightly better in terms of SNR - but honestly, without direct comparison - I don't think anyone could tell. I did comparison once - 10-20% more noise is very hard to perceive visually. It is there but really on threshold of detection and depends on stretch applied. You can do it by resampling down as long as you are happy with SNR gains you are getting. I've found that best way to compare things is "split screen" approach. Register both stack to the same reference frame and after stacking and prior to any other processing - do a copy/paste of half of one stack - over the other. Then proceed to process such image. Any difference between the two should be obvious after processing.

Will do!

Not yet, but I do intend to try - mostly EEVA style - both with phone camera and with dedicated astro camera (still need to get c mount lens for that). I have this idea that Mak102 is great fast imaging scope when "properly" used - in this case afocal method like images I posted above. Not going to use it for serious imaging but I do want to try out the approach - I think it is viable option for people just starting out and wanting to get their feet wet - low cost / all around scopes that can do both visual and imaging. Hope that phone adapter / afocal method and proper software support will deal with EEVA / basic imaging requirements.

I think that you can actually attempt to make it better yourself for minimal cost. It is about spacing between two lens. Maybe try changing thickness of spacers (replacing them) +/- 10% and see what sort of results you get that way.

Don't think so - but you can open multiple files at once (open image sequence, no need to select them - you can specify what files to open by name containing text, limit, skip sort of thing) - bin all of them with single click and then save again as image sequence. I usually open in batches as subs tend to be large and I only open a number of subs in single go that will fit in my ram (to avoid swapping and such). I specify part of capture name, use limit and skip number of files that I already binned.

Why don't you try following: - Calibrate your data in APP without aligning / stacking it (if that is an option) - Use ImageJ to bin it to wanted size (2x2 or 3x3) - Stack them in APP without calibration - just align and stack Would that be possible?

Just an observation that could be relevant to this discussion. Often I hear phrase: such and such 4" APO can soak up magnification and image does not break down at x300 or x400. I always wondered about that. Can't be that my 4" Mak is so much poorer than these premium scopes? Price difference does suggest that it might be the case. Then I realized important thing - telescope and eyepiece are not only optical elements. We have another lens in optical chain - our eyes. Theory suggests that 20/20 vision needs at max x1.2 magnification per millimeter of aperture. 4" telescope really needs only ~ x120 times for someone with average vision to show all there is to be seen. Adding magnification only makes things easier to see as it makes them bigger. At some point, things are big enough for blur to be obvious - let's say that is "breaking" point of telescope. I've also realized that about 1/4 people actually have worse vision than 20/20. This is with inclusion of corrective measures - glasses or surgery. A Quarter of us simply don't see as sharp as rest. There is very small portion of population that has sharper than 20/20 vision. For example, if person has 20/10 vision - they would only need x60 magnification to see all there is to be seen with 4" telescope. It also means that for them - image is going to "break down" sooner than for the rest of us. Eyesight starts to deteriorate with age and we tend to loose sharpness after age of 60 - regardless of glasses we might be wearing. It is not uncommon that young people can read distant signs without trouble (sharp vision) while elderly struggle even with glasses on. For someone that does not have sharp vision - image will not start to break up until very high magnification - it is much like needing larger font on your computer to see it clearly - to someone with sharp vision, that font will be too large but you'll feel very comfortable with that font size as smaller fonts strain your eyes. In light of all of that - I came to realization. We should not easily "recommend maximum useful magnification" - as that is very personal thing and depends on visual acuity of person in question. Some might find that 4" gives sharpest views at about x120 while others will happily use x250 for relaxed observing. It further suggests that we should not take "image breakdown" as criterion of scope performance as that is heavily dependent on visual acuity of observer regardless of actual differences between scopes. Third and most interesting thing - someone with poor eyesight might characterize performance of good and average scope as being the same - or scope of smaller and scope of large aperture - because they need much more magnification before they start noticing the difference.

Maybe best option for such scope would be sort of modified Petzval design. There are FPL-53 doublets that 6" and F/8 and have excellent performance - like SW 150ED (not sure which glass that one uses) or TS Photoline 150 F/8 doublet (advertised as FPL53 + lanthanum element). There is also fast F/5 doublet in "quadruplet" configuration: Bresser 127 and 152 F/5 scopes have modified Petzval configuration 2+2 elements. I guess it is just a matter of time before someone makes decent 6", color free, flat field F/5 ED quadruplet scope

Not sure if that is correct. Binning simply averages (or sums) 2x2 (or 3x3, 4x4) groups of adjacent pixels. Nearest neighbor - copies pixel value from nearest sample point - it does not do interpolation - it is exact opposite form binning - and resampling with lowest SNR improvement - none. Here are results with addition of few other interpolation methods: I've just added few other interpolation methods: We now have - again the same as before - original, binned x2 and Quintic B-Spline (first row) Nearest neighbor (stddev - unchanged ~1.0), Bilinear interpolation (same result as binning) (second row) Qubic B-Spline and Cubic-O-moms in third row (stddev of ~0.75 and ~0.8). As you can see - binning in this particular example is the same as bilinear interpolation. This only holds for 2x2 and for x0.5 reduction case. Things get more complicated in general case although math is based on similar premise. Nearest neighbor leaves things unchanged - it does not improve SNR at all - it does not manipulate with pixel values - it just copies closest value over. This leads to terrible results in general interpolation - aliasing effects and distortion. In general - more sophisticated interpolation method - less pixel to pixel correlation there is and less blur and artifacts it introduces into the image - but also noise is kept at the same level. When choosing method for aligning frames - choose the best interpolation algorithm - one that preserves noise the best. This might sound like counter intuitive proposition but it actually helps with noise statistics and stacking then does a good job. Using bilinear or bicubic interpolation when aligning frames leads to coarse grain noise instead of fine grained noise in the image. It looks artificial. Binning is different than that - since one resulting pixel depend only on group of original pixels and that group does not contribute to any other destination pixel but one discussed - there is no pixel to pixel correlation - there is no such sort of blurring and yet SNR is improved by precisely know factor. There is different way (we could argue better way) of binning - and that is splitting image into "sub fields" and then stacking those. That is method that I recommended above. It just splits pixels of original image into groups and forms smaller images out of those - no pixel values are changed and then you can stack those. If you "bin" 2x2 - that will result in 4 new subs from one original sub (each having x2 smaller dimensions or x4 smaller total pixel count) - and you know that stacking N subs improves SNR by factor of sqrt(N) - so we now have 4 subs - and we improve SNR by factor of x2 - exactly the same as binning - but we did not do any math on pixels - we did not change them in any way - just split them into different groups.

Maybe look into eyepieces for terrestrial spotting scopes then? Or maybe not - just looked at prices of those - way too pricey to be useful.

I was not aware it is simple 1-1 attachment to microscope as well. It also uses 1.25" barrel size? I guess that is to be expected. My first guess as a good microscope eyepiece in that focal length would be this one: https://www.teleskop-express.de/shop/product_info.php/info/p8860_Fujiyama-1-25--HD-Ortho-Eyepiece-25-mm---made-in-Japan.html but you would not get any improvement in field of view. Price is also not suitable, especially if you need 2 or 3 of them. If you want max AFOV in that exact focal length of 25mm - then there is no much choice. 60°-62° is really the maximum. Pay attention that you might not quite want astronomical eyepieces. Astronomical eyepieces have certain geometric distortion that is preferred in astronomical use to that of terrestrial (and for that matter microscopic). You can't have perfect eyepiece - especially wider field of view one. It will either have rectilinear distortion or angular magnification distortion. For terrestrial use - zero rectilinear distortion is desirable. Straight lines need to stay straight. Look at above image that I posted - first one. There is a wall. Wall is really a straight wall - but in image it looks slightly bent. Such eyepiece magnifies objects that are away from the center of the FOV. Not desirable in astronomy as planet would "inflate" when nearing the edge of the FOV. Here is a good comparison between the two: Left is what astronomical eyepieces do - right is what terrestrial eyepieces often do. Notice how in right image - man standing on the left seems larger than woman standing in the middle of the image. By the way - there is almost no difference between the two optimizations up to about 42° AFOV. That is why very sharp and very precise eyepieces stop at about that FOV size.

Question is - why is your master flat debayered? Calibration files should be left as they are and applied before any possible debayering. Also - check histogram of linear master flat and ADU values. Do you have any clipping? Filter that you are using aggressively cuts off light and there is a chance green channel is void from data. If you have offset set incorrectly that will cause clipping to the left on histogram. If you have any sort of automatic flat generation - software might be confused by lack of proper peaks and can over expose red - which would lead to clipping on the right of histogram.

As far as I know - that triplet has excellent color correction regardless of the fact that it uses FPL-51 glass. @Thalestris24 has both of mentioned scopes - 80mm F/6 and 115mm F/7 so she can possibly offer further insight into this.

Yes there is difference and in fact - using Lanczos-3 is one of the worst options for that (if you want SNR improvement). You don't need to capture binned data - capture it at native resolution and then bin your data after calibration. Binning and scaling down both reduce sampling rate and that is ok, but binning does better job of improving SNR. It has predictable and higher SNR improvement than any other scaling method. In fact - binning is one of the ways you can scale down your image (by integer fraction) and you can "extend" binning to include rational sizes and then it becomes very basic interpolation algorithm - linear interpolation. If you think about it - value half way between two value is just average of those two values. Mathematically there is no difference between bin x2 and linear resample + 0.5 pixel shift + x0.5 scale down (bin x3 or higher is not so simple). Other resampling methods introduce pixel to pixel correlation and actually try to preserve fine grained noise. For this reason they don't improve SNR as much. In fact, best resampling method would not change SNR at all. Here is an example: I generated simple 128x128 with pure Gaussian noise with sigma 1. Then I binned that image x2 and I resampled that image to 0.5 size with Quintic B-Spline (better than Lanczos3 but probably not Lanczos4). Original image shows that stddev is indeed 1. Binned image shows what we would expect from it - exact improvement of x2 because it was binned x2 (noise reduced to half), but scaled down image, has noise reduced to only 83% instead of 50%. You can see that binned image is smoother.

@Thalestris24 I can't help but wonder what are you doing exactly. Sorry if I'm being nosy, I'm just interested in "mechanics" of things - how do you plan to use equipment and to what effect. You say that you need good eye relief EP - to be used by spectacle wearer but then you talk about flat field and afocal method. My guess is that you are looking for all around EP for daytime use (digiscoping and observing of sorts) - but does it have to be single eyepiece? You say that you need flat field EP, and that is ok, however, most of field curvature comes from telescope itself. For this reason, I'm guessing that you are using 80mm F/6 with field flattener - which also happens to be reducer (from your sig). You now have F/4.8 scope. There are very few eyepieces that will be corrected to the edge in such fast system and they are expensive. For digiscoping - you just need certain field stop - focal length and AFOV are not as important as you can adjust things on camera side - by selection of lens on camera (or simply adjusting focal length in zoom lens). Maybe orthoscopic eyepiece would give best performance in this case even if it has narrow AFOV which might not be suitable for observation? 32mm GSO Plossl has as wide field stop as you can get in 1.25" format. Here is a test I did few days ago when I got my smart phone afocal adapter: Or this one: Both images are at 25% scale of original. Phone has 4000x3000 resolution but lens is such that it covers more than 50° of FOV so there is some black area in shot. Btw this is 32mm F/4 achromat finder/guider scope. It is incredible little lens. Plossl fares quite well regardless of the speed of the lens. Although this was hand held shot (I was holding complete contraption in my hand when I took this) - sharpness is still pretty good in the center of the field. Pidgin is a bit out of focus because it is a bit further away. I think phone focused on branches in front of it. Here is right edge - not looking bad at all Left edge is worse - but it might have something to do with phone centering - I did not pay much attention how centered it is - and then there is out of focus due to distance blur:

You don't need half of those settings. You are now using mono camera and it's using fits files. There is no debayering, white balancing and all that stuff any more. Just make sure you have this checkbox unchecked like so: Other things in this dialog box have no relevance (raw tab is for DSLR files anyway). If you are going to use Kappa Sigma rejection - you need to do background calibration. Especially if you combine data from multiple nights where you can have substantially different conditions like transparency and LP levels. That is good approach - just make sure you have proper flats. You can reuse flats between sessions only if you have permanent setup, but if you disassemble things between sessions - take flats for each session. Darks are ok as long as you keep parameters the same - temperature, gain/offset and exposure length (or have master dark for each combination you'll be using). That is what I saw as well - artifact that is more related to nearest neighbor interpolation depending on zoom level than actual pattern in the image. There is one thing that confuses me about that master flat. It is color image yet all there channels are the same. Why is that? No need to save color image for mono camera flat, and if you wanted to send all three channels as a single file - why are all channels the same? Even if there is a bit of pattern in flat because of pixel sensitivity distribution (that is factory process defect - I had checker board pattern on my ASI1600 in Ha wavelength but it calibrates out fine) it should calibrate out fine. Here is master flat how I see it. Not much of pattern, is there? When I measure angle of pattern in one M81 image - it is about 2.5 degrees: So there is difference of few degrees between some subs. Maybe pre / post meridian? I'm going to simulate this effect for you now - so you can see it that it happens under some circumstances: I just took gaussian noise and rotated it by 2.5 degrees using linear interpolation. Some of your images need to be rotated to be stacked and noise in them creates this pattern. What you could try: 1. How do you calibrate? 2. You could see if there is distinct difference in orientation between subs before and after meridian swap, if so stack only one group and see if there is effect 3. Use different stacking software that has advanced interpolation methods that don't produce this effect

Poor polar alignment combined with bilinear interpolation in DSS. First and last images are rotated by few degrees - you can check this in DSS as it solves for alignment - it displays rotation of each frame. When frames need very small angle of rotation and in combination with bilinear interpolation - that effect happens. What settings is StarTools recommending for DSS? Maybe we can tweak something to get better results, but in principle - try to avoid that slight field rotation over the course of the night. Btw, I'm not seeing the same effect in your flats.

It lists it at 16mm eye relief. Same usable eye relief as EF27mm. I have ES82 11mm which has 15.6mm eye relief by specification. I feel that 12mm Plossl is more comfortable to use although it has far less eye relief - only 8mm. Do you need 60° AFOV? Good plossl will offer better eye relief and excellent performance - with only "drawback" being 50° AFOV. https://www.365astronomy.com/25mm-GSO-Plossl-Eyepiece.html In stock and won't break the bank.

Actually - specs differ on different websites for some reason. BST 25mm has 16mm of eyerelief. TS for their version of Extra flat 27mm say it has 21mm of eye relief: but then again, they quote field stop of 28mm and I think they are pushing it since 1.25" filter thread is 28.5mm - I think that max field stop in 1.25" format is closer to 27mm. FLO on the other hand - lists even more eye relief: I did google translate of that link @Zermelo posted and it looks like FF27mm is not well suited for you. Ernest says that effective eye relief is more like 16mm since eye lens is recessed (it is 21mm from eye lens) Field stop is only 25mm and that gives AFOV of 50 degrees. I think plossl would work better in that focal length than this eyepiece. Maybe look into ES62 26mm if you have the budget for it. Check out this link: https://www.cloudynights.com/topic/592703-es-62-26mm-vs-panoptic-24mm/ and in particular post by russell23 as well as this brief description by @Nyctimene:

That highly depends on telescope you'll be using them in. In F/5 scope - you'll clearly see the difference between high and low end eyepiece. In F/12 - there will hardly be any difference if at all. Outer field performance is usually compared. Most eyepieces have quite decent performance in the center, but outer part of the field starts to show aberrations like astigmatism and such - which blurs things in daytime use (and distorts stars in night time). If you are worried about lateral color - some expensive and high end eyepieces also suffer from that. It is hard to get good lateral color performance in wide field eyepieces. That is something that might show up in daytime. Best cure - stick with eyepieces up to 60° or so. What will be intended use for this eyepiece? In those focal lengths (25-30mm) Plossl eyepieces have enough eye relief and are excellent performers. For example Vixen NPL 30mm: https://www.firstlightoptics.com/vixen-eyepieces/vixen-npl-eyepieces.html has 50° AFOV and 24mm of eye relief. If you really want wider FOV and decent eye relief, maybe this one: https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-62-series-ler-eyepieces.html 26mm focal length. It is said to be very good performer and it has max field stop for 1.25" barrel size.

Another thing that comes to mind - planetary images are often over sampled and there is no point in trying to restore frequencies that are not there. In order to account for that - you should be using some of standard kernels that emulate seeing / stacking induced blur (which is really gaussian because of central theorem) and optics PSF - which is Airy pattern with clear cutoff in frequency domain.