Can an f/11 Telescope Beat an f/2 Astrograph?
Got to love that Celestron RASA or SkyWatcher HAC125DX telescope! Fast astrographs are unbeatable - but are they really? In this video, we go beyond focal ratio myths and finally a...
The Space Koala Astrophotography by Luca Bartek
Excellent video! You have very good understanding, and I have more to learn as well!
Thank you very much for the video !!!
I solved this problem by dual-mounting both an EdgeHD 8" and a RASA 8 on my Avalon Instruments M-Due mount. When I want to image a small target I use the EdgeHD, and when I want to image a large target I use the RASA. Because the EdgeHD had an OAG on it, when I'm imaging with the RASA the EdgeHD is my guide scope 🙂
"when I'm imaging with the RASA the EdgeHD is my guide scope" - love it! :D
Thank you very much for making this video. The telescope comparison tool is really neat! It's awesome that you published it for us to use.
I have two f10 telescopes, one is 8 inches and the other is 12.
Great video! Thanks for sharing. One other way to summarize it: get the largest aperture money can buy, and don’t assume that a smaller F-ratio helps in all cases. Agree?
that is right! the smaller F ratio will help with wide angle shots but not with small objects
Hi! I come late on this one which is, as always, a very nice analysis made "simple" as You are very capable of! So my compliments to You and Your approach to things! I would rather like to give my two cents on the whole question which is quite counterintuitive if seen from the "usual" perspective of "acquiring systems capabilities". I would like to point out some subtle mistakes that are very common and try to finally change the point of view on the same old questions. First You start with the term "resolution" but this could be misleading, in fact We need to differentiate spatial resolution from raster resolution. The first is crucial for any comparison we try to make and is the angle of sky that we are acquiring. Then You explained the binning is a method to imrpove SNR and that's true but why? Than there is the F-Number and the real concept of light gathering area, the first (F-Number) is not a real factor to improve SNR of any subject that has an angle and changing the "F-Number" considering the obstruction is something clever but not "practical" since the true F-number doesn't change but the collecting area does. With this I mean that the considered angle that falls on each pixel doesn't change. You then exclude the under/over sampling factor for a better comparison but this is a crucial point! Not for the convolutions due to the atmosphere but for the Sky-Angle that we are considering for the comparison which is key. For easier understanding You use the words "brighter image" but this leads to a common misunderestanding point, How "bright" (which only means is more right side on the istogram) an image is does not means it has more SNR which is the real point. In fact on linear acquisition devices, the position on the histogram of the datas doesn't matter, You can represent the data with the same "luminosity" while they are not clipped. This drives us to another concept which is "exposure" that is also not a true "factor" in this story. Exposure is only related to an accomodation necessity for standardizing a specific value or tone of gray in the "classic" photography, we should care about the grade of confidence we need to achive for a measure. We should no more think in terms of "taking a picture" but "make the best possible measurement" in digital. Short Vs. Long FL, the image is brighter in a short FL? I underestand Your point but that's misleading, the image is not brighter but the considered angle per pixel is very different. That drives to other interesting considerations about the time needed to obtain the same "pixel sizes" images using mosaics or drizzle but why? What I'm trying to say is to change the point of view of the "problem", You've made a lot of exemples with different scopes but the same camera, some has 0.97"/pix and other with 1.94"/pix sampling scale and that is the real key point! We are constantly reversing the thruth which is the Photometry! We are trying to measure how much energy is present, in form of photon flux, in the most accurate way, in fact less noise means lower uncertanty of the measure in digital and not graininess which is a consequence of the rendering of this uncertanty. Consider a photon flux that comes from a subject that has an angle in the sky, all the energy is coming from that angle, the bigger is the angle the more is the energy. If You have a "perfect" system with no noise at all, no convolution and perfectly linear You can take a picture of 1, 2, 4 seonds i.e. and the result should be exactly the same all the time, just doubling the measured energy right? In reality is not true! That's becouse the photon flux is not linear but is poisson distributed and so, each exposure will measure different quantities over time, the longer the exposure is (while remaining in the full well of the sensor) the more similar it gets becouse we are naturally averaging the cont of photons. That's why when we "bin" a sensor we get more "signal" becouse we are considering an angle that is 4 times bigger and THERE IS more energy. If You have an F1 or an F10 system measuring the same angle do You think the F1 system will give a different result than the F10? No becouse, the F number can't change the photon flux that is present in that angle! It will be more on the right side of the istogram for the F1 system but the relationship between the datas is exactly the same and this is the SNR, a relationship not an absolute value, You can show these data exactly the same just changing the position on the histogram and the image will be the same. So there is not a "faster" telescope, there is a wider field of view telescope but, whenever You sample the same angle enlarging the pixel sizes on the "slower" scope and reducing the pixel sizes on the "faster" scope, they will get exactly the same SNR in the same exposure time IF THEY HAVE the same COLLECTING AREA. So, if You want to compare different FL or F-number systems, You can only do that matching the sampling angle, only in this way You will acquire exactly the same probability to make the same measurement for the same energy that is present, any other methods will drive to errors since we do not have the same initial quantities to measure. In the end my intention was absolutely not to diminish in any way Your brilliant work but to help everyone, including me, to better underestand what we are really doing. This is an amazing hobby and this is an amazing channel to follow and inspire and be inspired. Sorry for the very long post and keep going! Your work here is very important!
My good, I'm in love... With the video, I mean 😊. I'm not even sure I understood it, as much as I wanted to, and may have to watch this 3-4 more times, but hands down the best YT video in quite a while. Well done on such a fantastic video. 🤩
I must say that I'm a bit confused 😅 Why would one buy a long focal length telescope and bin x2 or more and losing details? Comparison is interesting but maybe only applicable to specific cases where one would shoot M31 with a 2000mm focal length but still have an easily transferable image by reducing it's size through binning. And also why would you crop a drizzled image and throw away most of the signal collected? Maybe strange questions maybe I'm too new to this hobby to fully understand but my first feeling is like were comparing apples to bananas in the name of resolution which may not be such an important criteria in the end 😅
Your questions are valid - these operations suggested here were done to make a point of how you can get to the same image from one scope to the other - which is the logic one needs to understand to then generalize it and be able to reason while using the tool I presented.
This is completely AI, she is not a real person
lol please walk me through your reasoning as to why I am AI (I know the body outline with the green screen could use some work but come on)
Hi, thank you for the video, it is scientifically well made and well explained, I think. But as a physicist I have two questions (hopefully not too dumb 🤪) : 1) Maybe I missed it, does your online tool take into account inefficiencies linked to drizzling? 2) Regarding example 2 (with the same aperture), what about a third example, where both telescopes have to capture a wide field with high resolution? I think it would highlight an advantage of the faster system: no need to crop and therefore no wasted light, right? Which would mean the same comparison between the two system as example 2, question 2, meaning the fast system would be clearly better in that case (assuming computing power is a smaller issue than capturing signal). But again, there would be no "free lunch" because faster systems have the other drawbacks you have presented towards the end of the video...
hey, thanks for the questions! no the tool doesn't take into account anything with relation to drizzling. It only calculates the amount of light calculated per unit of sky area and is agnostic of whether you are going to drizzle the faster scope or downsample the slower one - or choose literally any sampling in between. Regarding the second question, you are right about a 3rd example (and actually a 4th one as well where you would want to capture the smaller FOV with a lower resolution). Yes the "faster" system will have an advantage in all cases where we want to capture a larger field of view, and it will have no advantage when it comes to narrower fields of view. This is kind of the whole message I'm trying to get across - that fast systems increase the field of view, they don't magically collect more light.
Thanks so much for this video! How do you feel about the Touptek M3G179C (2.4µm pixels) with the HAC125 (f/2)? It seems like you get the best of both worlds, but is that really the case?
you can only put 1.25" format cameras on that scope right? You'll get the best bang for buck if you put the largest possible sensor on it you can
​@the_space_koalathank youuuuu
I find that my Celestron C6-A XLT with a hyperstar can take a photo faster than my SvBony 503 70mm f/6 ED with an x.8 reducer/corrector, when both use an SvBony 605cc (IMX 533) camera. The C6 is an f/2 and the 70mm ED is an f/4.8. The C6 has a 300mm focal length and the 70mm ED has a 336mm focal length, both similar. Both have a similar field of view. If I photograph NGC 7000 (North American Nebula), using an SvBony 220 dual band filter, I can capture the same nebulosity in 15 minutes using the C6 that requires 1.5 hours with the 70mm f/6. So, is the C6 better?
what you described is perfectly in line with the content of the presentation and logic: comparing f numbers for speed makes sense only in case the focal length is the same (or in your case, very similar). That is because the f/number is only the ratio of aperture and FL and your FL is the same, so what you're effectively comparing is the physical aperture (diameter). Your 150mm C6 collects roughly 4 times as much light as your refractor
Interesting! Thanks for the video. One remark though, using small pixels with fast telescope is totally ok if you just want to shoot targets that have a small FOV 😅 I use the minicam8 with redcat51 with great results, it allows me to frame smaller targets compared to my apsc sensor. To produce the same image i would have to drizzle and crop with my aps c sensor and loose some photons, it ´s a cheap solution compared to purchasing a 400 mm quintuplet refractor 🎉
you wouldn't be losing any photons if you used the apsc. I personally go for the largest sensor I can and decide later if I want to crop, but I'm glad that this works for you so well
@the_space_koala yes! You are right… Also the aps c is OSC and miniam 8 is mono so there is also a gain of photons without the bayer matrix… Anyway thanks for your answer and CS
Conclusion: Aperture is still king.
always!
Did you dither the backround picture at 5:19 while talking about drizzling? If you did that on putpose, is a genious easter egg! 😂
I believe it's just a glitch... umm... I mean yeah it's totally an easter egg I did on purpose!! :D
I feel this is a dumb question, but why don't you make the object images the same size by just adjusting the magnification?
if you digitally change the magnification of an image you are not actually adding more detail (improving the resolution), but you're distributing the same amount of data over more pixels. You'll notice this if you take an image and you blow it up like 100x - the whole thing becomes pixelated. That is an exaggerated example of what happens everytime
I'm learning so much!!!
Nice explanation as always
This informative channel is so dedicated that every amateur astronomer must learn. I really appreciate your perfectly organized efforts.
Thanks for the kind words, I appreciate it