More Fast Focal Ratio Mythconceptions

[@comeraczy2483]

Very impressed to see this, particularly on KZbin! Thanks a lot for taking the time (and the risk) of addressing this strongly entrenched issue so effectively. I believe that the f-ratio mythconceptions (I love the word) are widespread in the astrophotography - and more generally the photography - communities because human beings love to repeat what our elders love to repeat. The concept of f-ratio was critical for film photography because, as you demonstrated, the f-ratio defines the intensity of the light projected on the sensor. With film, people had to blindly decide what film sensitivity and what exposure time to use, and the f-ratio was a critical factor in the decisions. As these were very important (and frequently made) decisions, they were discussed a lot. With digital sensors, the whole concept of selecting film sensitivity completely disappeared, exposure time can be adjusted on the spot, but the discussion points stayed anchored around the f-ratio, strengthening the idea that it is important by itself.

[@Astro75mm]

this is why i love your channel, this info goes right in the face of all the youtube channels, peddling the redcat 51 and all its derivatives as if its alpha and omega of astro photography

[@SKYST0RY]

I decided a long time ago I was going to follow the science, not the trends. Though I have nothing against the Redcat. But narrow aperture is something that can't be worked around. The resolving capacity of aperture is a physical property of every telescope.

[@chrzanik666]

Hence my first proper telescope was 6 inch reflector 🪞 and love every imperfections and need to faf with collimation and all.

[@astrofromhome]

Great video! This is something that I have realized also that focal ratio is not the main driver if you get a good image. With my 8" RC I can see that images build up as quickly as with my 70mm Apo even though the focal ratios are f/8 against f/6 meaning that the Apo should be 2/3 quicker from the focal ratio but in fact there is no difference. The small Apo was for long time my most used and most liked telescope but I am changing most of my imaging to larger aperture telescopes. I just realize that I have to use it again. Not that I don't like my small Apo anymore as it has its use-cases. So each of my telescopes is for a specific case except two which are very similar in focal length. I would not try to capture the Spaghetti Nebula with my 8" RC, my 6" Newtonian or 4" Apo. I would use the 70mm Apo as it is the use-case for such small telescope. Same with the Rosette or Seagull. Sure I could capture details with the others, do masaics but having the right scope for the target size is always more convenient. The limited number of nights with clear skies is the other downside why it does not make much sense to start a project like the Rosette in 9 or 12 panels. So have the right telescope for the use case if you are limited with clear nights. Don't look too much at the focal ratio, choose what fits your needs of the framing that you like to achieve. A larger aperture with a reducer is able to gather more photons because of is size than a smaller scope with just a flattner.

[@SKYST0RY]

Very good points, and clear nights are perhaps the most precious resource for many people. A clear night with good seeing, low turbulence and ideally free of moonlight--they are hard to get. For myself, I decided a long time ago that images will just take as long as they take to get. Though I do intend to compensate soon by building a second observatory.

[@TevisC]

SNR and Image scale! Have a long - slow focal length big scope? Pair it with a large pixel camera or bin 2x2. Have a fast large telescope, Pair it with smaller pixels.. An F10 8" SCT and an 8" F4 Newt can image at the same speed and image scale if the pixels sizes are adjusted accordingly. They would also have the same SNR. It really comes down to camera choice-preference. I prefer my ASI533mm to other camera choices. Thus, my chosen scope would be an 8" F4 newt (or 10" F4) to image at 1 arc second (or a bit smaller)..

[@SKYST0RY]

A good point. A bit down the line, I plan to go more in depth into that aspect of things.

[@moxican7]

Thanks for the upload. I must admit I was somewhat under the impression of the F-ratio mythconception as well. There is one thing I'm not sure I understand though. When you talk about energy of light being spread across a larger area with a longer focal ratio, thus image getting less bright, does that mean one needs longer exposure to compensate in order to bury read noise? Would one need longer exposure to compensate for the energy loss of higher focal ratio optical system? Because if the answer is yes, then the mythconception is a good to know information but will not change much in the field of astrophotography.

[@SKYST0RY]

To clarify: Light is spread across a larger area due to focal length. Focal ratio changes nothing; it is only the the expression of the consistent relationship between aperture and focal length. The value of longer focal length is the ability to obtain more detail, the same way you can see more detail of a distant site if you look at it through binoculars than with the naked eye. The cost of longer focal length is more spread out light, which is dimmer per square area but equivalent energy within the larger image circle produced by a longer focal length. Both are superseded by aperture, which is the great detail resolver. This means the astrophotographer has options. One option is to choose between detail vs exposure time. Another option is to get a camera with a larger sensor to cover the larger image circle a longer focal length will create. Yet another option is to get a telescope with a larger aperture. There are yet other options.

[@Stefan-Astro-Art]

How many hours did you expose with both telescopes? I have the C8HD with x0.7 a F7 and the william optics pleiades 68mm F3.8. Both got Asi294mcpro. I did both on the Crescent for 3h with a dualband HA-OII filter. And i have more detail with my F3.8 than the C8. I need more than 12h to get a good detail in the Crescent

[@SKYST0RY]

The refractor version of the Bubble Nebula has about 720 minutes of integration. It has a focal ratio of 5.5 with its reducer. The SCT version of the Bubble Nebula has about 750 minutes of integration. It has a focal ratio of 6.1 with its reducer.

Hola, excelente vídeo. ¿Podrías decirme si un reductor de focal es mejor con el mismo diámetro? ¿Puedo hacer zoom en la fotografía final sin perder calidad?

[@SKYST0RY]

A focal reducer will cost a percentage in focal length to gain some brightness by reducing the image circle. The loss of focal length is tantamount to exchanging some resolution for faster integration time. Of course, reducers often have useful flattening effects, too.

@@SKYST0RY perdón, creo que no me ha respondido. ¿Usar reductor disminuye la resolución? Gracias

[@yangyunbo1]

this is best explanation about the f ratio i have ever seen .

[@blaircolliver5194]

Another great instruction video! Thank you.

[@shivatof1209]

hello, very good video. how do you make the tracing stars on the photo? thank you

[@SKYST0RY]

I get asked that a lot. I capture video of moving through the stars in a virtual planetarium called Space Engine, then screen composite the stars over the image in my video editor, Davinci Resolve.

[@shivatof1209]

@@SKYST0RY merci 😊

[@michaelklemm-abraham7298]

Still thinking about the information you presented in this and the last couple of videos. To me it makes more sense to think about the unobstructed light gathering area of a telescope. Because yes resolution is by physics depending on aperture but any obstruction (secondary mirror) leads to little light loss. It also doesn’t make sense to me that two telescopes with different aperture but same light collecting area should achieve different brightness (lets say a 5 inch apochromatic refractor and a 6 inch schmidt cassegrain that have different aperture but same unobstructed area).

[@SKYST0RY]

Unobstructed area (or T ratio) can play a role but often less than one would think due to the fact area of light collection is squared, so its relationship to diameter (aperture) is not linear. For example, in your proposed situation of a 5" refractor vs a 6" SCT, even presuming the SCT had a 1 inch secondary, the SCT would still have more light gathering area. The refractor has 19.6 sq in of area, while the refractor has 28.2. Subtract 0.79 sq in for the secondary and the SCT still comes out on top by a substantial margin. Even if the SCT had a huge 2" or 3" central obstruction, quite large for the scale and it wouldn't have this unless it had a very low (fast) focal ratio, it still comes out with more light collecting area. Regardless of the central obstruction--unless it becomes absurdly huge--the wider diameter will also still resolve more resolution. However, please note in the illustrations given in the video I avoided the additional complexities of working with T ratio by not designating any example telescope as a reflector.

[@michaelklemm-abraham7298]

@@SKYST0RY I didn’t have the time to calculate it through. Of course I am aware that obstruction usually has a surprisingly small effect. I just noticed with an other channel were in the original video (he corrected his statements) compared a 6“ Hyperstar to a 4“ apochromatic refractor because he had around two inches of diameter on both sides of the camera. A quick calculation of the area in square millimetres showed that the Hyperstar still collects something in the region of 70-80% more light (this is why the only comparison I could imagine on top of my head was 5-5“ comparison). I only know T-stops from photography. Never heard about it being used in astronomy. Again, the resolution aspect was clear to me since resolution ist only dependent on wavelength and diameter.

[@SKYST0RY]

@@michaelklemm-abraham7298 Interesting. I'd love to see that video. What was it's name? Insofar as I know, T stops in photography are the same thing as how t ratio is used in astrophotography. It's just a more accurate way of calculating light passage through a telescope and accounts for issues such as obstructions.

[@michaelklemm-abraham7298]

@@SKYST0RY I think it was this video: kzbin.info/www/bejne/rquxo62Lasmcmbcsi=Han75rrewv7OjeOr I‘ve read from one of the comments he corrected the video since I came across it so it might be completely different now.

[@SKYST0RY]

@@michaelklemm-abraham7298 Oh, yes, he's put out some decent videos. I don't fault him for the mistake. There are so many technical variables involved in making one of these videos it's easy to make an honest mistake. It happens. I've found most of his information to be very interesting.

[@robvandenwijngaart1970]

Refreshing explanatoin. Some 40 years a go, had a lesson in school about this stuf. Strange how all is still in your head and that is comes to the surface when you watch a video that stirs in your brain ;) (I have a Altair Starwave 110ED-R F7)

[@SKYST0RY]

To my knowledge, short of brain damage we don't really forget things. They just get lost in "filing" lol I had to look up that telescope. Looks nice.

[@robvandenwijngaart1970]

@@SKYST0RY Thanks, it's my first and only on. Have it for 8 years now. But first 4 years in the netherlands don't use it much. Now in Spain everytime I get the chace.

[@FrankSD76]

Thanks! Now I don’t feel like a bad person for succumbing to aperture fever. Anyone interested in buying a slightly used “fast” refractor? 😂

[@SKYST0RY]

Thank you! Professional telescopes used for research frequently have f ratios from 20 to 40. Professionals know a lot of other things are more important. For amateur use, I tend to aim for between f/5 to f/10. Well, it's better to say I focus on focal length, then aperture. That simply results in whatever focal ratio I get.

[@jostbogataj2404]

I agree with what you are saying in this video, but i think why people like telescopes with fast focal ratio is because you improve SNR quicker. As you said, the light is less spread out, so each pixel on the sensor gathers more light in a given amount of time, resulting in less noisy images.

[@SKYST0RY]

You may well be right, and it may be that this is much more of an issue under light polluted skies. I honestly don't know. Living in the Canadian backwoods, I have effectively no experience dealing with manmade light pollution. The video isn't against low focal ratio telescopes, though. Rather, it attempts to rectify the idea that focal ratio is what really matters. For example: Imagine two telescopes with the same focal ratio. Double the aperture on one. That telescope will now gather information four times faster and double the SNR (if I remember my SNR math correctly).

[@evastronomy8048]

This is totally true, in those terms, but a bigger aperture telescope requires a bigger eq mount and more precise guiding system and longer exposure times. That's a reason why most amateur astrophotographers opt to add an accessory to reduce the focal distance, a very expensive accessory like the hyperstar lens, and that is the real deal.

[@SKYST0RY]

The Hyperstar is good but absurdly expensive. Almost the cost of the telescope. But when you think about it, it's actually cheaper to buy a mount capable of handling a long focal length telescope than buy a long FL telescope plus Hyperstar. The C8 is about $2000 in Canada, plus another $1400 for the Hyperstar. But there is only a $400 price difference between a EQ5 mount (probably good enough for a shorter focal length) and an EQ6R which can handle quite a local focal length scope very well.

[@evastronomy8048]

@SKYST0RY is absolutely right, but in terms of speed, as the video is about, astrophotographers rely on refractors despite of the small aperture, just because they are faster, wider field of view and better and easier on focusing. For planetary and the moon sct are the best for sure.

[@ricardocernic5471]

Very didactic! Thanks for sharing

[@johnsmith-qc8ud]

Great debunking of misconceptions. Every astronomy fan from the times, when there were no digital cameras, knows that both the resolving power in seconds and the limiting magnitude of the telescope depend only on the entry diameter of the scope and nothing else. So it would worth mentioning not only the resolution formula, but also limiting magnitude formula which is 5*logD + const, where const depends on the receiver, whether it is basically an eye or photo sensor.

[@SKYST0RY]

I should delve into limiting magnitude some time.

[@MazzifLOL]

My "slow" Explore Scientific 127ED F/7.5 produces some of the best images for me.

[@SKYST0RY]

I'm not surprised. I find f/5.5 to f/10 to be a sweet spot. Professional telescopes may have focal ratios in excess of 20.

[@iamjessieray]

​@SKYST0RY all you have to do to prove that is look at the focal ratios the HST and JWST. I'm no professional, but I'm seriously starting to think that for the small objects I am interested in I really need to get myself an 8 inch f/10 and just roll with the native focal ratio. Might to get a camera with larger pixels to go with it though

[@SKYST0RY]

@@iamjessieray I remember the very first time I ever looked at an image I had shot with a 203 mm SCT compared to my 81 mm refractor. It was eye opening. I realized that day I was pretty much done with refractors. That say, "Aperture is king," is entirely true.

[@MazzifLOL]

@@iamjessieray I have a local astro buddy who loves to image with his f/13 102mm mak. Crazy. But he makes good results!

[@iamjessieray]

@@MazzifLOL I have seen some INCREDIBLE results with 5" maks so I'm starting to thing that f-ratio is really irrelevant if you are willing to put some more time in and can fit your object of interest in your FoV

[@MatthewHolevinski]

I would rather have an off axis f/20 than an f/2 hyperstar

[@PilotinMSFSonly]

🤯

[@hunterborg4409]

I think the main reason people think and like to say a low f ratio is faster and therefore better than a higher f ratio is to make it simpler for beginners.

[@SKYST0RY]

Possibly. The idea has become entrenched.

[@MatthewHolevinski]

more troll food?

[@browi1978]

I also just love spreading misinformation 👍

[@jml7916]

Not sure what you are saying, this video is accurate enough with explanations where simplifications are made. If you doubt it, do the etendue calculations and test it in your yard if you can. It will all make sense eventually if you do. This doesn't mean that a small telescope isn't useful, but under the right conditions a large scope will out perform a small one with a million caveats of course. Etendue shows that on a per pixel level an 8" scope is an 8" scope if you match the sampling in arc seconds.

[@evastronomy8048]

This is totally true, in those terms, but a bigger aperture telescope requires a bigger eq mount and more precise guiding system and longer exposure times. That's a reason why most amateur astrophotographers opt to add an accessory to reduce the focal distance, a very expensive accessory like the hyperstar lens, and that is the real deal.

[@SKYST0RY]

Sort of . . . but not, too. A bigger aperture doesn't require more or less exposure time. It depends on the focal length to aperture ratio. But modern mounts and guiding easily compensate, especially when combined with good shooting technique. I am quite happy with the results even if my guiding is 1.5. The old days when we needed low focal length to compensate for poor mounts and poor tracking are pretty much over.