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Totally agree mate nicevideo

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.

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.

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

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)

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.

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

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.

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

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

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😉

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.

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.

Interesting and I think a very valid conclusion 👍

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.

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.

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.

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.

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

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

Always love your videos. Thanks for making this one!

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

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!

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 ...

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

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.

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.

My experiences differ.

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

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. 😂

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

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

@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.

Dude! Just 👍

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

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.

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

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.

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.

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.

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.

F/ratio alone is not a really good indicator, but all things being equal aperture and f/ratio are still the two biggest factors. Astronomy tools has a calculator that is based on aperture. I usually use the calculator in polar align pro which calculates the difference in f/ratio then subtract the two. It gives you a much better estimate. A good example is my fra500, with the reducer at f/4 it’s only .8 x the speed of a 130 phq with the same reducer. So if I spent 20 hrs on the 130, I could get the same exposure in 16 hrs on the 500. Not a big difference but it’s still the difference between 2 nights and 3. It depends on what your after but in the end I still believe nothing beats aperture. I also run an 8” Newtonian reduced to f/3 and there is no way my 500 can keep pace with that. From that rig I can easily get enough signal in one night of 8hrs.

Anybody live streaming 2 stars going nova tonight?

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

Another great vid!

That is wonderful news Luke. I am really happy for you and Chloe. You really deserve all good things coming your way just because you are... well you!

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?

Baby and astrophotography are mutually exclusive, all the best!

He’ll Yeah!!!!! Congratulations guys! Best thing my wife and I ever did. The good thing is sleepless nights with the babies gave me lots of imaging time!