F-Ratio: Not as important as you may think..

[@lukomatico]

Hope you enjoyed this one! - I just want to reiterate that F-ratio is still important, as we show here - but it's not a very useful indicator of speed by itself, it can only be useful when taken into context with sampling ratio and aperture. It's possible that someone imaging with, for example, a huge F10 SCT and a camera with big pixels could capture a great image of something that fits their FOV in less time than someone imaging the same target with a small F4 newtonian paired with a camera with small pixels - the F ratio is only part of the equation 🙂 It's a lot to go into, but all the same I hope this was interesting! Now... don't try and image the Squid nebula at F50 and say lukomatico told you to!!! :-P

[@swagonman]

Once you are at the same sampling scale, it is just about the light gathering of the aperture opening in area. It is not about the focal ratio. So the advantage is (280mm/120mm)^2 = 5.44 times more photons collected and put down to the pixel. Does that match your results? Note that since it is a ratio of aperture areas, I’ve already cancelled out the pi and the 0.5 (for diameter to radius conversion) from both the numerator and denominator. We should also remove the area that is consumed by the central obstruction, so 5.44x will reduce a bit more. You can measure your camera diameter and do that math. And then you also have the differing QE and read noise for each camera. But in theory, at the same sampling scale, it is the ratio of the aperture areas that are collecting the light. It’s intuitive if you think about it.

[@DreBo99]

Yeah, it's not a myth nor a total lie, IT'S THE APERTURE! Thanks for clearing this up. Your comment should appear at the top and the video should get a part 2

[@lukomatico]

That's spot on! :-)

[@lukomatico]

Re: the title, - that's just what you've got to do to succeed on KZbin these days, a little provocative, a little questioning!

[@DreBo99]

@@lukomatico I know and I don't mind. Totally fine to do that. I was just hoping that this would have been cleared up more in the end once you captured everyone with a catchy title. You could have used the analogy you made in the beginning. With a photo lens you increase the aperture by reducing the F-number. With a telescope you reduce the F-number by reducing the focal length (i.e increase the field of view, reduce the resolution) with a reducer. You will not capture more light from a DSO (that fits into both field of views) because the aperture is left unchanged.

[@swagonman]

I think you can make a general formula using focal ratios and pixel areas. But I’m not 100% sure on this, so correct me if I’m wrong. I think it would be (7.1/2.2)^2 * (2.9/3.76)^2 = 6.2 times faster. As mentioned in my other comment, it still ignores the loss of photons blocked by the central obstruction.

[@valeryo78]

This serie of videos is pure gold!!!! It really helps to undestand how different scopes and cameras behave trying ot keep the most similar setup. And it also scales down differences in speed to more "realistic" values. Great work, if I had to change the scope now I'd use these videos as references!!! 👍👍👍

[@lukomatico]

Glad you enjoyed it my friend! Clear skies! :-D 👍👍

[@FredLombardo]

As someone who is used to shooting at F2 and now diving into F7, I found this to be most informative. I briefly tried F10 with my Edge HD8, my only large scope but that proved to be a non-starter due to seeing conditions. Just bought the 0.7 reducer and will be trying that out when the skies clear. Excellent video. I like the fact that you experiment to try to get different perspectives. Well done.

[@lukomatico]

Thank you Fred, I really appreciate that mate 👍👍

[@FredLombardo]

@@lukomatico you’re welcome. I ran a test on Sunday night and, thankfully, the reducer helped. I was dodging clouds but was able to get PA, Plate Solve. Guiding was not great but I ordered a OAG focuser which should help. Managed to get a few subs of the Bubble Nebula that actually stacked (unlike what happened at F10).

[@Jcorban08]

Thanks for another excellent, fascinating video, Luke! I learn so much from your videos. And the comments are interesting too. Looking forward to the next one! CS

[@lukomatico]

Thank you mate! Clear skies! :-D

[@croysk]

Great video. The photography rule applies to most situations as the sensor, focal length, etc remain the same, and the focal ratio is changed by changing the aperture. So in that case it is valid. For astrophotography we care about sampling scale and aperture. Others have posted great comments explaining how to get down to the ~4 ratio. I’d add to those that the Strehl ratio is another factor. Assuming perfect construction, the RASA has a fairly large obstruction so its Strehl is like to be 0.88 or so at most, with the refractor being more like 0.99. In practice, I’d expect the Strehl of the RASA to be more like 0.79 and the refactor 0.93. The RASA would lose out slightly more in extended objects as the light thrown out from the air disk into the rings reduces contrast as well.

[@GarnettLeary]

Fantastic video. Great topic. I have encouraged people for years to use what they have, ignoring the continued misinformation of speed importance. The community by large discourages F12 instruments for example. I personally encourage slower instruments to beginners because they are typically easier to focus and have far less aberrations typically. Camera sensitivity has bridged a huge gap in equipment. It would be a shame for a person not to get their feet wet simply because they’ve been talked out of using a less expensive slower scope.

[@lukomatico]

Thank you so much my friend! Great points, as always 👍👍 Hope you're doing well!

[@BadYossa]

Totally agree and support this comment. This dude seriously knows his stuff! 🙂

[@AndreasWiederin]

You don't just gather more signal, you also gather more noise, but not as fast. If you image x times longer or with an x times faster optical system, the noise will increase by a factor sqrt(x) and so will your signal to noise ratio. So if you estimated visually that the 10x faster system looks to be maybe 3x faster in realtity that sounds about right. Image scale on each pixel of course also plays a huge role. When someone uses a RASA11 with a 585 sensor at 2.9micrometers in bin 1 (1.1'' per pixel) and I use my c11 reduced to f6.3 with a imx 571 sensor in bin 2 (0.8''/pixel - bin 1 only makes sense with great seeing and very good guiding at 1760mm fl), then the speed gain from the optics (x10 faster) for the rasa and the speed losses due to the 7x smaller pixel area almost cancel each other out - especially when only the square root of that difference actually is relevant for the signal to noise ratio. Instead of 10x faster optics when ignoring pixel size you get a factor of ~1.49 for the light gathering and 1.22 for the signal to noise ratio in that scenario in favor of the rasa. You might think it's an unfair comparison but I have had this very discussion with a hyperstar user who was thinking about a smaller pixel sized camera like the 585 instead of using the back of the sct for imaging smaller targets. Oh and the huge difference in aperture has already been commented on.

[@lukomatico]

This is all correct, so you can agree that f-ratio by itself isn't a perfect determinant of speed

[@HeavenlyBackyardAstronomy]

I agree, Luke. My school of thought has always been aperture size. The more light gathered, the better off you will be. F-ratio is important to me because of the size of the field of view. Good comparison.

[@lukomatico]

Thanks so much Pat!! I'm certainly a fan of big aperture where possible, thanks for weighing in mate! 👍👍

[@ferenc-x7p]

As someone who has been using fast Newtonians , there is always a cost, that may not compensate for the speed they have. People overlook this easily. In astrophotography there is always a con for everything! That cost is that around F2-F4 is, that every tiny issue someone may have, like a small sensor tilt or a small backfocus issue will be severely enlarged. Add or remove 0.05mm backfocus and your stars are out of shape. This didn't matter on your F6 refractor, but it matters hugely with the F3 scope. Change in temperature and the focus point changes by wider margin. Having a tiny sensor tilt that didn't matter on an F6 scope is now a much bigger issue with the F3. A slight focuser sag or moving focus will ruin all the shots. At higher speed, everything little problem is also 5-6X magnified compared to a scope with half the speed. I won't even mention filter compatibility, because there is a reason that for example Baader makes "fast optics" narrowband filters and they cost somewhat more than just the regular one, but even if they weren't, now you gotta buy a set of filters only to be use with a certain type of telescope. There are just filters, that may work on 98% of all the telescopes, but NOT on your F3 or F4 telescope. Totally possible! I'm just saying- fast scopes are fun and work well, but be prepared with all the cons attached to them. You might not only be using a different scope out of many, but you might have to compensate to it with the entire gear and they require far more precision (backfocus, tilt etc) and yet you might still not going to like what you get, despite the extra photons advantage, especially if someone has been using a well corrected, optically excellent - but slower refractor.

[@brianhayward8240]

Using a smaller sensor than the telescope can illuminate is basically like using a focal extender on that telescope. Great way to make a F3 scope behave more like an F6. It's basically throwing away aperture and resolution.

[@capturethephotons2078]

A lower Fstop collects light faster if that is considered Important in this demonstration. Also it might be important when figuring out a camera to match your ota for "proper" sampling.

[@stevenholt824]

Its seems tha aperture of the rasa v the askar is the thing thats making the main difference in exposure.

[@Snow.2040]

You are ignoring the role pixel size has on the speed of light collection by using two different sensors with different pixel sizes. At a given sampling rate (in this case around 0.9 arc seconds) the telescope with the larger aperture will collect light faster, so to measure how much faster the RASA is going to be you have to take in account the aperture (not the f/ratio in your comparison) in which case the calculation will be like this: (pi)*140^2 / (pi)*60^2 = ~5.44 times faster, But that is ignoring the central obstruction in the RASA which has a diameter of 114mm (so radius of 57), so to account for that: (pi)*140^2 - (pi)*57^2 = 51,368 mm^2 , now we will plug that back into the other equation: 51,368 / (pi)*60^2 = ~4.54 times faster. Now accounting for the fact that you have to quadruple the integration time (or the speed) to double the signal to noise ratio, so 15 minutes with the RASA will have a bit more than double the snr of 15 minutes with the askar, your results start to make sense.

[@lukomatico]

I'm not ignoring that fact, I'm deliberately utilising it for the demonstration :-) Your math supports the results in the video though as I'm sure you know, so that's good to see! Clear skies!

[@Snow.2040]

⁠​⁠@@lukomatico I understand that you deliberately matched the sampling rate, but you then used the “10 times faster” figure from just the focal ratio difference when in reality the focal ratio was irrelevant in the experiment you set (which I feel like you should have mentioned that), if you for example compared something like a RASA 11 and an EdgeHD 11 while matching the sampling rate you would have gotten the same snr for the RASA 11 and EdgeHD 11 images despite the difference between f/2.2 and f/10 being 20 times because they have the same aperture. You are comparing the aperture, not the focal ratio. Clear skies!

[@mark-h911]

Brilliant video 👍You can’t beat real world results from real world experimentation. Well done, Luke 😃👍

[@lukomatico]

Cheers mate!! Agreed! 👍👍

[@grahamwhite75a]

Luke, your comparison videos are really interesting. Keep them coming. I must admit I was tending in the same direction on F ration. I had started to conclude a more important parameter is the focal length, that has more impact on the images.

[@BadYossa]

That was pretty damn interesting and some of the comments have also been pretty epic. I'm really enjoying this new style of work you are putting out. Love it. PS Congrats on the happy news! That's epic as well! x

[@lukomatico]

Thanks so much buddy!! Hope you've been doing well, it's good to see you back 👍👍

[@scottfairbairn6305]

Interesting video. I noticed that the stars look way better on the Askar than the 11". I'm not sure if that's from oversaturation or a consequence of design differences, but I like the refractor, which looks a lot better.

[@Azergoth]

Hi @lukomatico! Great video, as always. I think the easiest way to explain this would be to compare this to crop factor when comparing sensor size. Both cameras have different sensor sizes. Or pixel sizes, which is the same but scaled down. And there lies the link to the sampling rate you are referring to. Sampling rate which is calculated with pixel pitch (sensor size) and focal length yielding… f/stop 🙂 To formulate it differently: fstop is a good metric, IF applied to same sensor sizes, OR when crop factor is applied to the fstop. As a reminder, it’s the crop factor squared that should be multiplied with the fstop to compare exposure, or total gathered light. Whether it’s per pixel or for the entire sensor. Lastly, and there’s where using the sampling rate does make a lot sense, is that it doesn’t care about cropping an image. Important when we’re dealing with fixed focal lengths and fixed object sizes. (And we’re unable to “zoom with our feet”, obviously😉

[@siegfriednoet]

Very interesting video again Luke, I really like these kind of comparisons

[@lukomatico]

Glad you enjoyed it my friend 👍

[@OlliesSpace]

Hey Great video Luke - This makes a lot of sense. Even with my little experience with faster scopes, you'd think the improvement would hit you in the face, but it doesn't. Hope you're well pal.

[@lukomatico]

Thanks mate!! We're doing well bud, cheers ❤️ hope you're good too!

[@PhilPassmore]

I get the feeling that what you have effectively been testing, is not so much the 'myth' of the f ratio, which from a physics point of view is very hard to argue with, but rather our ability to extract great images whatever the handicap of equipment that we live with.

[@lukomatico]

I like that way of looking at things! 👍

[@astrophoto_andy3335]

I wonder what the difference might be with an IR cut filter on a broadband target? I'm generally happy with my refractors 😊

[@lukomatico]

Happy with your refractors is a good place to be mate!! I absolutely love em :-D Good question btw!! - I think we'd probably observe similar enough results really, but without doing the test I can't say for sure of course! :-)

[@AstroQuest1]

Great video Luke. I was hoping someone would do an experiment on F-ratios like this since Cuiv did a related experiment recently - I don't have the right equipment to do it. Cheers Kurt

[@lukomatico]

Thank you mate!! Glad you enjoyed it 👍👍

[@Wheeljack678]

Very interesting take, and really thoroughly experimented with. I like your thinking. I've often wondered whether or not faster gathering of light comes at the cost of SNR and where the cutoff is. I currently have an 80ED which with reducer/flattener comes in at 510mm at f/6.3 I've been looking at something like the Askar FRA600, which with a reducer will be 420mm at f/3.9 - but I've been asking myself is it really that much more efficient for a B5/6 location to justify the cost?

[@lukomatico]

Thanks mate!! - I think the Askar will feel faster for sure, but probably not dramatically so! - you'd really feel that wider FOV though I think, everything is a balancing act I guess! Cheers! :-)

[@filipgloria]

Theoretically, could it be that you’re using a smaller sensor and that makes the hardware more prone to noise? 🤔

[@lukomatico]

Smaller pixels can be noisier for certain 👍

[@cymrych79]

Thanks for this topic! After moving to a very light-polluted small city (Bortle 8-ish), I actually delayed getting my gear out of storage for over a year until I got a Hyperstar rig set up, thinking I absolutely needed that speed to even bother trying AP at home. And the Hyperstar does a great job, for certain. But I also travel extensively for work, and usually to places with much darker skies nearby (just got back from 2 months in South Dakota, where I had Bortle 2 skies within 20 minutes of the hotel), and lugging a C9.25 and CEM40 around in the work truck is fine, but kind of a pain. So I started experimenting a little with an old Orion Short-Tube 80 I had kicking around, as its 480 mm focal length is pretty comparable to the C9.25's 514 mm with Hyperstar, just to see what the real difference in needed integration time truly was. And as you've shown here, I really don't need 7 times the total exposure time with the short-tube 80 to reach a close SNR compared to the C9.25 Hyperstar. About half that time seemed about right. And couple that with better and better gradient removal tools, BlurX and NoiseX, it was hard to identify any deep differences in my tests (except for the obvious limitations of the ST80 in terms of basic optical aberations, of course... It was just some informal testing, but a better refractor would take care of all of that.) The only appreciable difference, I think, would be in the inherent advantages of larger aperature. In my tests with a 294MC Pro, this didn't amount to a significant impact on the final images since the Hyperstar C9.25 was pretty undersampled, but with a different camera with bettering sampling scale, the C9.25 would offer higher overall resolution than the ST80. For faint fuzzies, that increase in resolution might not matter much, but for globular clusters and smaller galaxies, I think I'd see a significant difference. So different tools for different target types of course still applies. Although the overlap in capabilities between those tools is definitely getting larger and larger, which is great for those of us who don't have the financial or storage capability for more than a couple good scopes.

[@lukomatico]

Hey mate!! - It's really fantastic that you've done some testing on this, thank you so much for sharing! that's interesting! I agree with what you're saying mate, I think it's a nice mature take on things :-) Cheers & clear skies!

[@IronMan-2024]

Spacecraft properties Type Ritchey-Chrétien reflector Diameter 2.4 m (7 ft 10 in) Focal length 57.6 m (189 ft) Focal ratio f/24 Hubble

[@delleps]

Very nice video. I think your experiments are great for everyone to see the consequences of their choices in equipment and technique. But curiously, what I didn't see in your video was an apple-to-apple comparison of your 1-hour RASA image against a 1-hour Askar image. You compared a 15-min image with a 171-minute image at 6:42, a 15-min to 60-min at 8:08, and a 1-hr to 171-min at 9:19. What we were seeing were S/N-equalized image comparisons but perhaps that was your intent. Last summer I took my 20-year old C11 into Starizona in Tucson, Arizona for a tuning and walked out with a highly tuned, immaculately collimated C11. Not only that, it sported a brand new HyperStar4 on its corrector plate. Up until then I had been shooting with my RedCat51 and SpaceCat51 and enjoyed the wide-field images I was able to capture in OSC and LRGB. I could devote an evening or couple of evenings to focusing on a target for 5 to 10 hours and end up with some nice images. Once I had the C11/Hyperstar outfitted with a camera and balanced on my CEM40 mount and shot my first-light focusing frame of Cr99 the Rosette Nebula, I was absolutely blown away; the 15-second exposure I took to focus and frame the image was more glorious and impressive than any 3- or 5-minute integration I had ever taken with ANY telescope. I could hardly believe what a difference an f/1.9 focal ratio could make. So for the last year I've almost exclusively used the C11/HyperStar4 with the exception of using my RC8 for the brief Galaxy Season in the Spring. So in defense of f/1.9, As a rule, I'd much rather dedicate my long-exposure time to imaging with my Hyperstar-equipped C11 than my other platforms. The images I obtain hour-for-hour with my f/1.9 setup vs f/4.9 or f/8 clearly have a more buttery-smooth S/N profile and detail in nebulae I couldn't match, given equal integration time. In my humble opinion, f/1.9 was a game changer for me. Still, I'd strongly suggest people do their own tests with their own equipment (including display monitor) to make their own conclusions. Best, Jim Adams Tanque Verde Observatory Tucson, AZ USA

[@lukomatico]

Hey Jim! Thanks for the message and feedback within, I appreciate it! I agree with your conclusion, nothing beats actually just doing a test with your own equipment to figure these things out 👍

[@KingLoopie1]

Interesting video with this line of experimenting! Thanks Luke!

[@lukomatico]

Glad you liked it mate!! Cheers for watching!

[@meibergstrmandersen9181]

Hi Luke, interesting. Just some observations: the rasa might be f2.2 but generally there’s a 30-40% obstruction from the camera, right. Same thing on fast newtons. Question being the obstruction offsets some of the speed difference at least compared to a refractor. Also being new to the wonderful Astrophotography world I cannot help mentioning that the clear skies time is a scarce resource why speed is pretty important. Hence favouring fast telescopes. What do you think ? Br Kenneth, Copenhagen

[@lukomatico]

Hey Kenneth! I definitely feel fast scopes are still important and preferable where possible, but certainly it seems a little slower scope isn't too much of a hamstring 👍 Re: the RASA c/o, it amounts to the scope being around f2.45 or so when you work it out by area 👍 Hope that helps!

[@Hubaround1]

This backs up my own experience between my RASA 8 and C8 Edge at f/7. So, yep, it's not 10x but more like 3x-4x faster. Still, that's a lot faster and worth the effort to me with limited time for imaging.

[@jakublieder]

Hi Luko, it doesn't work like that. The problem is that you have the same resolution "/pix. You have to use the same camera (same pixel size) for both telescopes. The RASA has a smaller pixel so it gets correspondingly less light than the resulting 10x. The cameras have similar QE, but if it calculates, so the resulting speed is 5x faster for RASA, not 10x - that would be true if you had the same pixel size for both cameras, but there is central obstruction, so it would be 9x faster. It's a bit more complicated to understand. So the 4x result is pretty close to the 5x because there are other variables - camera QE, filter, transmittance, reflectivity ...

[@lukomatico]

Hi mate! - I wish I had two scopes of the exact same f/l to allow me to use the same camera for the comparison, but alas, I do not - the two cameras here were deliberate decisions to try and normalise the pixel scale while using two different focal lengths 👍 Cheers!

[@gianlucabelgrado3624]

It is better to consider the T value, and not the F value of the telescopes. The Rasa has the camera in front, which reduces the light. The T value indicates the amount of light that is transmitted by the lens. Use a flat field box with both telescopes, and measure the difference in exposure to bring the histogram to the same value. It's probably less than 10 times

[@xkrrrr]

Did you measure and compare the SNR in the different images?

[@lukomatico]

I didn't do that for this one Sven, possibly something to add in a future video! Cheers 👍

[@GrouchoDuke]

Great comparison again, Luke. 3-4x is still no joke with the RASA. I do love my ED127, but it's super tempting to get something faster. DeltaRho 350 maybe? Thanks for the great work, brother!

[@lukomatico]

Thanks so much mate!! I'd say if going faster scratches the itch, then do it!! 👍👍 Clear skies bro!

[@craiglowery4427]

Great experiment. I believe that AI, high resolution cameras and tools like PixInsight have changed astrophotography. Luke what do you think is the ideal refractor size for planetary imaging? I've been thinking a 150mm Sky Watcher on a EQ6R Pro mount

[@lukomatico]

Hey Craig! Glad you enjoyed it mate 👍 Re: refractor for planetary, as big as possible (while maintaining quality) is the best option! :-) The Askar 185 apo would be ideal but it's quite expensive, the SW esprit 150 would also be good, but the evostar 150ed may suffer from some false colour fringing on bright stuff like planets/moon limb 👍 Hope that helps!

[@desbarry8414]

Thats why I bought a C925 edge hd and 0.7 reducer and hyperstar, i have the aperture/light gathering and resolution. All I need is to vary the pixel size, seeing permitting of course. I can image at f10 f7 or f2.2 lucky me lol Rasa is for me a one trick pony.

[@azzaqmb]

If you ever look at redoing this test I would go say go off single sub's to start with not stacked ones look at the SNR on the singles sub's, then I would stack the same amount and recheck the SNR on the stacked to see how much change is there. So if the SNR is the same at say 30sec and 4min sub's it should be about the same if you do a stack of x10 x100 so on.... and also if you vs in B&W and blink between the two images that my show the difference's a little better.

[@user-pi5xu7qs7n]

Agreed. I started with a Hyperstar C6 and have settle on Newtonians, Mak Newt, RC in the F/4 to F/6 range which give me a good balance of SNR and required exposure time to generate good data and as you pointed out these fast systems are complicated and can be expensive with filters, etc. I do enjoy my 135mm F2.0 Rokinon for really wide field shots, but have moved away from faster systems.

[@ws5246]

Thanks for this Luke, very interesting comparison. Puts a big question mark against whether its worth dropping a grand plus on a Hyperstar v's the £135 for a f/6.3 reducer for SCT owners on a budget.

[@-donkey_696_]

I have 12sct Meade, I love the 6.3 focal reducer ,makes images a little smaller ,but are brighter images for the same exposure length on my DSLR camera, BTW if have the celestron focal reducer

[@lukomatico]

Thanks mate!! good point :-)

[@CenturionGMU]

This may have put the brakes on my spiral looking at new gear. Thanks for the point of clarity Luko. Nice little video to watch while making coffee as I got to work this morning. Clear skies.

[@lukomatico]

Thanks for watching and sharing mate!! I hope it was interesting, f-ratio still matters of course but it's not the be-all & end-all for sure 👍👍

[@CenturionGMU]

@@lukomatico I just got my rig in a state that I’m comfortable with as my entry point. My brain wants me to keep chasing. Gotta get good with what I have first lmao

[@tostativerdk]

Very interesting video! But I do find it surprising that basic physics seems to be “wrong” or misleading. It could be interesting to do the same test using a Samyang 135, where you can keep everything constant, except for f-ratio. Then is I guess one would reproduce the results from your terrestrial test? Keep all this good stuff coming!

[@lukomatico]

Thanks mate!! We can both know that physics won't be broken/wrong of course, but I just thought it was interesting to demonstrate that F-ratio by itself when not taken in context with the rest of the system doesn't really mean that much 👍👍 cheers!

[@Snow.2040]

Basic physics isn’t wrong or misleading. At a given sampling rate (~0.9 arc seconds in this video) the larger aperture will collect more light, NOT the focal ratio. If he did a comparison between a Samyang 135mm and an EdgeHD 11 while matching the same sampling rate for both then the EdgeHD would be 17.2 times faster (280^2 / 67.5^2 = ~17.2) despite the difference between the focal ratios telling you that the Samyang would be 25 times faster.

[@Alan-vk6bk]

Interesting and I think a very valid conclusion 👍

[@lukomatico]

Thanks mate!! Glad you think so 👍👍

[@StarlancerAstro]

One problem I see when using a hyperstar (F2) is yes you can collect data faster but you also collect light pollution faster. It can work quite well under very dark skies but under my B7 skies I hardly find an advantage over my 5.6 refractor.

[@petesastrophotography]

Excellent comparison Luke. As we've discussed before there's no such thing as a free lunch, adding a reducer to a telescope only increases its speed due to the fact the field of view increases resulting in more photons hitting each pixel. However, this is at the expense of resolution, you can't have it both ways. It's down to each person to decide what they actually want. When you factor in pixel size, aperture and F-Ratio its perfectly possible for an SCT to be 'faster' than a small refractor however counter intuitive that may seem.

[@lukomatico]

Thanks so much Pete!! - Hope you enjoyed this one as I know it's something we've both thought about! :-) Thanks for the inspiration to put this one together! All the best mate!

[@BarryWilliams0]

Really interesting video, Luke. How much does the central obstruction in the RASA affect its 'real' aperture?

[@lukomatico]

Thanks Barry!! That means a lot, - I think the losses from C/O, mirror reflectivity, coatings and any dust/dirt based light loss will probably amount to around 15% at a quick guess 👍 the frac probably loses 1-2% total at a guess It's an interesting subject I think!! Thank you for watching

[@mattysastropics]

Loved the video Luke, but I think mixing telescopes, especially design, might have had a big impact on your test. What if you had a C8 and compared similar length shots at f2, f4, f7, and f10? Of course, there would still be optical differences introduced by the reducers themselves.

[@lukomatico]

That's an interesting way to look at it mate! With that experiment, you'd see a large difference for sure if you used the same camera throughout 👍 What I wanted to do here was simply show that F-ratio by itself is a poor indicator of speed, that's why I chose to normalise the pixel scale by using two different cams for the scopes 👍 Thanks for watching mate!

[@southbronxny5727]

I've come to appreciate the tighter stars from the slower scopes. Good video.

[@lukomatico]

I hear you, my favourite stars so far are from my Edge HD 11 at F10! :-)

[@ferenc-x7p]

As far as star shapes, My F5 Newtonian (the cheaper one - worth to mention) has far better shaped stars than my F4 Newtonian. It is also more tolerant of which coma corrector I use.

[@nathanbarnard5997]

It’s like you can read my mind! I have been considering getting an f/4 600mm Newtonian because it is much faster than my current f/7 600mm refractor. After seeing this I’m not so sure that’s a worthwhile investment. Great video as always ✌️

[@MrGeert1972]

I hav3 that SW 150p Quattro F4 Newtonian and with the included coma corrector it becomes F3.45 @ 518 I stopt using the coma corrector so I got my 600mm back. And i can shoot and clean it up with BlurX. Still using my Nikon D5600 but my Astro camera is ordered and waiting on delivery

[@lukomatico]

Thanks mate!! - I'm really glad to hear it's gave you something to consider, I think you'll definitely feel an improvement in speed, but perhaps not as dramatic as the marketing might have you expect! at a guess, it'll probably feel about twice as fast (very roughly) Hope that helps bud!

[@ScottDavies]

Using my EdgeHD8 at its native focal length now, did the test at the end of Galaxy season and am not going back!

[@ferdigrafie]

Now the laws of physiks apply to camera lenses exactly like to telescopes. There is no difference, once you do the math right. The most important thing people forget is to multiply the F-Ratio by the EFFECTIVE Crop-Factor. Lets say you are looking at the Wizard Nebula with a field of view of 0.68° x 0.68°. To reach that field of view you are using 7.4 x 7.4 mm sensor area on your RASA and 10 x 10 mm sensor area on your Askar. Those numbers correspond to a crop-factor (compared to fullframe - important when dealing with f-ratios) of 4.13 (RASA) and 3.06 (Askar). That means the RASA becomes from F2.43 (calculated WITH the obstruction) to a F10.05 and the Askar becomes a F21.42. Between those numbers there are a bit more than 2 full stops, that means a bit more than 4 times more light for the RASA which matches perfectly your experiences. Like others already pointed out you can calculate these numbers much easer by just comparing the area of aperture of the telescope, since we are looking at the exact same portion of the sky. But it is important to know, that the numbers will be EXACTLY the same, when doing the math right. Now what can we learn from this? Aperture is as important as sensor area. If you want to take full advantage of a fast scope, you have to use the biggest sensor possible and (this is CRITICAL!) do not crop your image in post. Thats why i dont like the RASA8 - it cannot use fullframe sensors. And once calculated with obstruction+crop factor it isn´t more powerful anymore like a standard APO paired with a fullframe camera.

[@physmc1]

It’s not quite a myth, but it is based on keeping variable constant ( same aperture but different focal length, same focal length but different aperture). At the end of the day the light collection power depends on aperture and angular sampling

[@lukomatico]

Agreed, with the addition of pixel size and sensor quantum efficiency that gives a clearer picture of expected performance 👍

[@desmcmorrow2978]

Thanks for an interesting video. However, it is very well established - please see for example discussions on CN - that one needs to consider the etendue when comparing the efficiency of an optical system. This roughly translates as the square of the aperture times the plate scale. Since the latter is inversely proportional to the focal length the etendue under certain circumstances can be written in terms of the F ratio. A video on the etendue is long overdue - I suggested that Cuiv produce one following his video comparing an RC51 and a 5” Newt! So it is indeed not the F ratio that is the primary characteristic of the speed of an optical system and indeed no one in a position to know ever claimed it was. Cheers, Des

[@lukomatico]

Cheers Des! - I agree for sure. I've been down the rabbit hole of reading such threads and the information (and sometimes misinformation) within, - back when I'd just started out I placed a lot of importance on f-ratio, which led to disappointment at times, my hope with the video was to highlight that f-ratio by itself isn't the be-all & end all 👍 Cheers!

[@IronMan-2024]

Perhaps the terminology is misleading. If we compare Hubble with James Webb it can take images much “faster” because it has a much larger mirror and better sensors. It would be interesting to compare the focal lengths.

[@lukomatico]

I definitely think the connotation attached to the terminology is misleading, it's really just a description of the scopes optical geometry! 👍👍 Cheers!

[@deep_space_dave]

I bought into the whole F-ratio hype and purchased a RASA 8" but as I matured in this hobby, I realize that F-ratio definitely does not work the same when it comes to telescopes. The thing that really matters is aperture for max resolution minus seeing and critical sampling. For example I have done an experiment where I used both a IMX533 camera with 3.76um pixels and a IMX511 microscope camera with 1.12um pixels. The RASA 8" with the IMX511 camera basically became an EdgeHD 8 with a 0.7 focal reducer and IMX533 camera in terms of photon capture rate. So if you got a scope with a long focal length, you don't need a focal reducer or faster F ratio, just get bigger pixels or bin! Or if you want a wider FOV, then just get a focal reducer or camera with more pixels. Hope all that makes sense and mostly correct 🙂

[@lukomatico]

Thanks Dave mate!! - I hear you for sure, I think the important point is that F-ratio by itself means very little, it's just a descriptor of geometry really! it's only when taken into context with sampling ratio & overall aperture that it becomes more useful! :-D Thanks for sharing dude!!

[@CDigata]

Interesting - I tend to run 4 hours f5 -f7 where id do an hour at f2.. but im just lazy with today's processing

[@lukomatico]

Thanks so much Chris mate!! I think that's an interesting point too, with today's tools we can take 'less' data further than ever before, so much is changing! Thank you for watching! 👍👍

[@PRASTRO]

Hi mate, excellent comparison. 100% agree with your deductions, nice work mate Thanks!!!

[@lukomatico]

Glad you enjoyed it mate!! 👍👍

[@astrofromhome]

Interesting test! I also don't think ten times more exposure time is needed. But what I see, also in your exposures, is that the shots with the faster F-Ratio have a clearer 3D effect compared to the slower scope. I will also just go with refrectors in future. I have one Newton telescope and on RC telescope and that's enough reflecting telescopes to ruin my nights. 😂

[@baz_astra]

Always love your videos. Thanks for making this one!

[@lukomatico]

Thank you for watching mate! 👍👍

[@maojiexu7435]

The f-ratio is only an accessory product, and the reducer's main function is to increase the field of view

[@synquanon2987]

I’m afraid you left out factors like light pollution/seeing and mount tracking capability. Not everyone can shoot very long exposures under dark skies. Also, comparing images visually is a rough indicator. It would be very nice to see SNR measurements used for comparison. Keep up the good work!

[@TheTabugman]

I'm thinking part of this is down to the old refractor vs reflector contrast debate. Recrators are just better in this respect.

[@radioparisment628]

Yes I agree, and the explanation is revealed when you compare the Airy discs !

[@celiaxel3448]

Great video but I believe your trial parameters are misleading. In the terrestrial trial, you use an identical focal length whereas in your astro-trial you compare the RASA11 with a FL of 620mm with the Askar120 with a FL of 840mm. The difference comes into play when you look at the light you received from the same object In addition, the RASA has a central obstruction that takes away 16% of your light compared to the Askar. Both parameters downplay the difference in focal ratio.

[@IronMan-2024]

James Webb Space Telescope Spacecraft properties Focal length 131.4 m (431 ft) Focal ratio f/20.2 Collecting area 25.4 m2 (273 sq ft) Wavelengths 0.6-28.3 μm (orange to mid-infrared)

[@nikaxstrophotography]

Totally agree mate nicevideo

[@cemoguz2786]

In my mind under f 3 big big problems does not worth it. higher f 8 is too too slow and only reason to do is going for niche targets. sweat spot is always around f4 to f6 with most equipments. I am not an expert but I tent to not go for new untested areas but just do what others do for years and years with alot of information.

[@ljkildsgardsbryggeri]

I feel you compare a apple to a pear here. In your photograpic test, you don’t alter the focal length hench you have the same fov. Even the f roatio is correct the focal length is way off and the apurture is too big on the Askar and will give you much better light gathering capabillity. And using integrated exposure time is not the same as total exposure time since integration is averaging the exposures not adding. A better way to do a test like this would be to use static focal length, same sensor, same integration time but different apurture and exposure time. Then I belive you would have seen some differences.

[@elbass0]

I agree with your observations

[@lukomatico]

Thank you for watching mate!! Hope it was interesting 👍

[@tichwykes]

I've gotta say mate there really isn't much difference, I think the RASA would be sharper when splitting stars as I think you proved that in a previous video other than that there doesn't seem to be a lot between them. Don't get me wrong like you I love my RASA but boy it can be a pain & its fought me all the way as it's not easy to use the accessories that come with it are pretty useless to, you'd think what you pay for them you'd get some better camera adapters. The Altair 102 EDR that I have which is only a doublet I might add I've enjoyed using a lot more as it's a case of plug & play & optically it's really good. Also the Newtonian is almost done now to I've managed to do a fix so fingers crossed it will be in the observatory soon so I'll be shooting at F4.8 the refractor with its reducer F5.6 so not much between them. Clear skies!!

[@lukomatico]

Awesome to hear mate, I think nothing can really prepare you for the 'RASA experience' quite like being, well..RASA'd! when they're working well, they're lovely scopes and as easy to use as just about anything else, but my goodness... when something goes wrong it can be such a pain in the neck to get things back on track lol. Thank you for watching mate, as always! Clear skies!

[@Darren_71]

F:2 is King. Collecting Photons Using an EdgeHD11 vs a RASA11 with the same camera is worlds apart. I ain't got no no time for no F:10/F:7 any more 😂 Compare your HD11 vs your RASA11.

[@ChristianProulx]

I completely agree with you Luke, the F-Ratio is more complex to understand than it seems. I have a friend who is convinced that his f/2.8 setup is 4X faster than my Newton f/4. Unfortunately, I don't think that's the case. The SNR is more important than the F-Ratio, if my friend captures only 20 subs of 3 minutes f/2.8 and I capture 80 subs of 3 minutes f/4. Our WBPP or APP applications during Integration make an average of all the data and the SNR of my system will be better with my 80 captures, that's for sure.

[@lukomatico]

Thank you so much Christian!! I totally agree with you, too - what you say makes sense, I'm certain you'd have more actual data! 👍 Thank you for your support!

[@mattysastropics]

His math is wrong. F2.8 to F4 is one stop. One stop is 2x difference not 4x. F2 to F4 or f2.8 to f5.6 would be 2 stops difference and 4x faster

[@ChristianProulx]

@@mattysastropics With the Inverse Square Law it take 4X more light to fill 1 stop of Focal Ratio. But I see exactly what you mean.

[@mattysastropics]

@@ChristianProulx I am referring to what is commonly known as the Exposure Triangle. Basic photography - if you drop the aperture one stop (from f2 to f2.8), you have to either increase the exposure time (shutter speed) by 2x (a 1-stop increase), double the ISO (a 1-stop increase), or a combination of both that leads to a 1-stop increase. Your buddy is saying that his 1-stop faster setup equates to cutting his imaging time by 1/4 is simply not correct. It is cutting the exposure time by 1/2 according to the exposure triangle. But unless you are comparing the f2.8 vs f4 with the same type of scope, same aperture scope, etc., I'm not sure if its not a true apples to apples comparison. I have also not seen anywhere how camera gain affects imaging time in terms of stops in the same way ISO values do. It would be really helpful if manufacturers put out charts that showed that (for example, Gain 0 vs Gain 100 is a 1-stop increase, Gain 100 vs Gain 300 is a 1-stop increase, therefore Gain 0 vs to Gain 300 is a 2 stop increase, thus a 4x reduction in exposure time).

[@SterrenKijkenInDeStad]

I think the opening frame/thumbnail of this video is misleading and too much click-bait. There is nothing mythical about f-ratio, it is physics. And when towards the end of the video you start about reducers and not believing how they are in fact speeding up imaging, I feel you missed the mark. Sure, at fixed image scale it might be so. The fact that in your experiment you change BOTH the focal length AND the camera (pixel size) already shows you are doing a particular experiment. But for most people starting their astrophotography journey, it should be clearly stated that when changing the focal length for a fixed aperture and hence the focal ratio, one indeed gathers more light per unit time, at the expense of imaging resolution. The light cone is steepened, meaning the light is not spread out as much on the sensor and hence the light hitting the same surface area on the chip per second is increased, but with less resolution. Maybe you can consider changing this thumbnail? "When imaging at fixed resolution, it is not the F-ratio that determines light gathering power, but the aperture" would be much more to the point.

[@LeonHulstroem]

F-Stop: The need of speed vs depth of field ???

[@lukomatico]

For terrestrial photography, yes! :-D

[@thiagoprado7540]

For the theory, the only thing that Will change IS the snr ratio.

[@bogdanbacila9024]

The way I've always looked at this is that the only way to reduce you total integration time, for the "same" result, is by using a larger aperture, which also has the benefit of providing a better resolution for those finer filaments... But, I've never considered F ratio to be the factor having an influence on speed. It will influence the final filed of view, through the focal length, but never the speed or integration time. Larger aperture means a larger amount of light collected by the telescope (varies with the square of the radius), meaning the telescope diameter is the only variable influencing how much light your telescope actually collects, thus having an influence in the integration time, for an expected result. Two different F-ratios for the same aperture, will not have any impact on the light being collected, but will impact the field of view. At least, with the equipment we use, ranging from F/10 to F/2, in this interval, I feel that F-ratio will never have an impact on my integration time. Aperture will.

[@mick_hyde]

Too much noise reduction IMHO.

[@lukomatico]

I show the data with and without NR mate 👍

[@user-ze2ie8wc6l]

Another great vid!

[@lukomatico]

Glad you enjoyed mate!!

[@Chiclets1]

Dude! Just 👍

[@AmatureAstronomer]

My experiences differ.

[@lukomatico]

As do mine! Haha :-D Clear skies!

[@user-ku4yk4ut4z]

While your videos can be interesting they are not scientific ...Because the use of different cameras DOES not make an equal test....all parameters in a test have to be equal, except the thing your testing against ....so the only real thing you would be testing is F ratio...... and because your tests are often never equal..... this is not your first and sadly last bad example of testing .......you wont know anything unless you have real data from a real test! Making videos with Nonsense tests and dramatic captions only makes you look bad!

[@lukomatico]

I'm sorry to hear you feel that way mate, regardless of how you want to frame it or try to knock me as a person, it's simply a comparison of two rigs and the results speak for themselves regardless of what you or I think. I invite you to do better, go ahead and make your own videos to entertain, provoke thought and educate, good luck!

[@Cheapass-tro]

You've put some time into this Luke, for sure. Just one thing bothering me though... Do you still get time to do astrophotography just to create an image any more? Surely you don't get many more clear nights than me to be gathering all this data just for these experimental purposes? At least you said you've enjoyed doing this and you're not just sacrificing all clear nights for 'the job'.

[@lukomatico]

Thanks so much mate!! - That's really kind of you to be considerate of my enjoyment! you're absolutely spot-on, I do spend a lot of clear nights on testing stuff and experiments, but it's genuinely a lot of fun for me and helps feed my interest so it all works out in the end! :-) Cheers mate! All the very best, Luke

[@marcinb493]

@lukomatico Hey - should I receive any confirmation when buy BXT with your affiliate link? I haven't received any information but i'm almost sure i bought it through your link. Btw thanks for your content.

[@lukomatico]

Thanks so much for your support my friend, that's kind of you!! 👍👍 Re: confirmation, you won't receive any as far as I'm aware - but, if you used the link then it'll have worked! :-) Thank you again for helping me like that!