How Does Focal Length Change What Size Pixels a Camera Sensor Needs?

[@NikonJax]

Finally!!! Someone who can explain the situation without confusing me with the math...lol Thanks!

[@SKYST0RY]

I'm very much a visual person. I want to know what the math means in real life.

[@IanNoelPace]

Indeed. Earned a subscriber. Many many thanks

[@FernandoRodriguez-pj5uh]

But for planetary you want smaller pixels for large focal lengths because you need a larger image and planets are very bright.

[@SKYST0RY]

Yes and no. You do want to increase your FL as much as possible, and because local planets are very bright, they are not much impacted by the increase in F ratio. But the idea of needing smaller pixels is a guideline more than a rule and will be influenced by factors such as aperture and FL. Check out the enormous pixel size used by NASA to make some of the best planetary images ever shot.

[@FernandoRodriguez-pj5uh]

@@SKYST0RY can’t compare an 11 or 14 inch telescope vs a 2.4 meter telescope like Hubble.

[@JayArnow-tl8in]

Superb graphics and explanation. Applause.
Have been struggling to conceptualize this in a simple way . This video contribution does it.

[@shreyaskanetkar4697]

Awesome explanation!
When I have to explain this to someone, I usually go with beer pong analogy. The cups represent camera pixel and hence bigger the cup size, you can thrown the ball in from further distance. Not the best analogy but still works!

[@user-jl7up8qc8z]

Brilliant! ..Clear explanation=clear mental picture=clearer understanding for further reading.Many thanks

[@magedsyehia]

For years I've been reading on and off trying to understand over and under sampling but it never sticks to my mind. Now your few mins video made it easily understandable in few mins. Thanks a lot !

[@leoncorns1450]

Thank you for such a great explanation. Even I now understand why this is important and it helps when choosing my next astro camera.

[@brocksterification]

Excellent explanation thank you. I've been gravitating towards a larger mirror and shorter focal length because of the astro I like to do and in a roundabout way your explanation of optics and sensors reinforces what I've been thinking. Excellent. Such a great pursuit/ hobby and this kind of info driven guidance versus ego driven stuff is what we need.

[@RichNH]

That was pretty good at explaining the basic concepts involved. At least it helped me understand it more than reading the fairly technical description of what was going on. Thanks.

[@hotflashfoto]

I prefer the higher resolution of smaller pixels. If stuff seems blurry, I can bin them, turning 4 pixels into 1.
What you need to discuss is full-frame vs crop-sensor, since that changes the effective magnification but without any optics.

[@SKYST0RY]

Binning is a good option. I haven't gotten around to it yet, but I already covered crop factor a while back.
ua-cam.com/video/gva75QPxqcg/v-deo.html

[@trondasdam6443]

Brilliant explanation in a very well, produced video., thank you!

[@gee6607]

Excellent video that for some reason i could not find before i purchased my camera, tho i looked😂

[@TimRobertsen]

I had no idea! Great video:)

[@chrislee8886]

Nicely done!

[@Aerostar509]

Great graphics on a well explained subject.

[@scorpianspirit5124]

Wow, great explanation, really well done and now I understand something that was not clear to me before... Thank you ! Cheers... ;-)

[@stephenc1111]

Thank you. Now it makes sense.

[@gerardford1116]

Simply perfect!

[@mikehardy8247]

I understand a lot of science and optics. This subject was confusing. Much less so now. I'll be saving this to watch a few more times!

[@jpastroguy]

Excellent. Thanks!

[@TheDostergaard]

Excellent explanation! I understand much better now.
One question though. Binning is often given as a solution or alternative to physically increasing the pixel size. Wouldn't the ability to do binning during processing let you keep the option available to decide then, whether you need more resolution or more signal?

[@SKYST0RY]

That's a good question. I should note I never do binning except on autofocus routines, and only when focusing narrowband filters. I prefer to somewhat oversample which takes advantage of the approximately Bortle 1.5 skies where I live. Binning can be done before or after. If you do it before, you can shoot shorter exposures (provided you want to). Binning will raise your SNR but it doesn't have quite the same benefits on modern sensors as it used to.

[@vitalieBu]

Wow... your level of explanation is addictive! Very easy to understand and also very pleasant to listen.
I have a 12" LX200 SCT with a 3048mm of Focal Length...i got a good deal ( $75) on a ASI224MC ( 3.75 micron pixels) and a 5-element 3X Barlow lens. Do you think that's enough for planetary and lunar or I should probably consider ASI482MC with 5.8 micron pixels? Or I should rather keep the 224 and save for a decent cooled camera for dso ?
Thank you!

[@SKYST0RY]

I mostly only image DSOs which is a fairly different technique. As I understand it, for planets you generally want as high an FL as possible and small pixels. Since planets are often shot using video, the exposure times of each individual frame are so short that the effects of the moving light waves makes little difference. I have never used the ASI224 or ASI482, but I am pretty sure planetary photographers would advise using smaller pixels. I suspect that the pixel size issue is less important than is generally thought, though, since NASA and ESA use cameras with huge pixels to image planets from telescopes like the Hubble.

[@vitalieBu]

@@SKYST0RY Thanks a lot for your explanation! I was thinking that since you already have a SCT ... you might consider imaging the planets( Saturn, Mars and Jupiter are visible time time of the year) I'm sure you could apply your astrophotography expertise towards planetary imaging...I know it's different but something tells me your results will be as good as your dso results! Thanks again and clear skies!

[@SKYST0RY]

One of these days, I may setup a dedicated rig for such. In general, once I have everything working smoothly, though, I don't like to change it. But I do plan to build a second observatory soon.

[@komr323]

Thank you for the video. My question is we use larger focal length telescope for more resolution of the deep field image, but now we are using larger pixel camera to reduce the resolution. Then why don’t we just use a wider fov and faster telescope together with smaller sensor camera in the first place?

[@SKYST0RY]

It is a good question with no simple answer. The short n' sweet answer is resolving power. Focal length doesn't give resolution--it spreads an image out. For the spread image to be resolved, aperture and superb optics are required to improve resolving power. A telescope must have resolving power to make use of long focal length, but a sensor must have sufficient photosite size to contain diffraction patterns called airy disks, plus accommodate the magnified seeing challenges. The results is optics control brightness (f ratio), aperture and focal length, but a sensor needs to be matched to make use of the outcome. And the image train behind the telescope would have to have sufficient diameter to contain the field of view. This would make for an exceedingly expensive telescope and camera. It is theoretically possible to do this but not perfectly. Perhaps the best example of such a telescope/sensor combination is ESA's Euclid orbital telescope. These are topics that were too complex for this video but they will be covered in the future.

[@komr323]

@@SKYST0RY then it comes down to the max resolution of the earth seeing. I heard usually

[@SKYST0RY]

@@komr323 In a real sense, that is true. We have reached the point in technology where telescopes can generally resolve better than the atmosphere. It's also why for most persons, unless they live in exceptional conditions, also will tend to well with OSCs. Tech is moving on.

[@tlbernhard]

I think at 5:39 into the video you may have misspoken. You said "the image becomes under sampled", isn't it in fact oversampled?

[@SKYST0RY]

Yep, I think you're right.

[@johnpartee7629]

Can you please tell me what size pixels I should use for a C11?

[@SKYST0RY]

That's a big question, and I am not an optics expert. I just find it fascinating. But a basic look at the specs of a C11 on astronomy.tools indicates the C11 with 2800 mm FL would benefit from a camera with 8-10 micron pixels. That's pretty large but you could bin pixels from a camera with a larger sensor. Also, a little over sampling (pixels a bit too small) doesn't represent as much of a problem these days due to technological advancements.

[@jesuspineiro1622]

The explanation is very good, but the simplification with a two-lens telescope is not the best. For optical calculations, the simplification is a single lens and the sensor, nothing else is required and your explanation would be clearer graphically. It is about image scale (proportional to focal length) vs pixel resolution (proportional to pixel size).

[@SKYST0RY]

True, but I wanted a concept people could relate to more broadly. I made this video about 5 times and kept getting bogged down in technical detail and over complication. I wanted to make this video an elegant but accurate visual illustration of the concept.

[@Astronurd]

Apart from planetary imaging.

[@MrYvano]

Hi, I have just cheked out your pictures in AstroBin and I am really surprised as you are using a C8 (like me but I use a Canon 600D no modified for the moment) and get such incredible pictures. Congratulations. But how do you do to get round stars on the edges ? Because all mine are alongated and I have been trying to solve it for many months with no succes,. What focal-reducer do you use ? How did you manage to get the right focus ? Oh, I would have so many questions to ask you for having that results. Clear skies from Spain. Yvan

[@SKYST0RY]

Hola, Yvan. Part of your issue may be the large sensor size of your camera. As the sensor gets closer to the edges of the image circle, you will get more distortion with most any telescope. The Player One Ares-M I am using with the C8 has a 1" sensor which has good clearance from the edge of the image circle. This also helps the OAG camera to get better stars for guiding. I use RC Astro's BlurXterminator to correct and then deconvolve the stars, and then do simple star reduction by constricting the light curve with the curve tool after separation of the star plates. When I put the stars back into the image, I duplicate the star place and screen composite in one plate and partial color dodge composite the other plate back in. An electronic focuser keeps the images precision focused through the night. Focus changes through the night as temperature changes so it is important to update that regularly. I really do not recommend using DSLRs or mirrorless cameras for astrophotography. They can work but they bring many challenges. A dedicated astrocamera will be easier to use and give better results. It's a complex process, but I have dozens of videos on these procedures.

[@MrYvano]

@@SKYST0RY The Player One Ares M?? But when I enter the information on Astronomy tool with my C8 + Focal reducer Celestron 6.3 (well Ares-M model is not in the list so I took the ASI533), it says it is not suitable. except in Binnng 2. For what reason did you buy it ? Sorry to ask for it, just want to uderstand why this camera should be good when Astronomy tool says no. Thanks once again.

[@SKYST0RY]

@@MrYvano Take the information you get from Astronomy Tools with a grain of salt. Well, a cupful of salt, really. It is an old tool going by old standards. Guiding has become better and tech has moved on. I find a being a little oversampled to be fine these days.

[@yvangarcia3535]

@@SKYST0RY If we can not anymore count on Astronomy Tool, life is going to be harder, jejejeje. OK I took well note of that. At the end, there is no better advice - answer than someone like you using it and having that background on that camera. Thanks again.

[@SKYST0RY]

​@@yvangarcia3535 Every image on my Astrobin from the start of 2024 was taken with the Ares-M and Celestron C8 combination. They work well together.

[@JeffSmithbureau13]

That all makes sense to me, but here's where I get confused: I have a C8, so about 2000mm focal length, f/10. Yet when I was buying a planetary camera, I got one with a pick size of only 2um (asi678) because for avg to good seeing, that size meant I didn't need a Barlow.... Which would actually increase focal length. I don't see how this aligns with longer focal lengths needing bigger pixels?

[@SKYST0RY]

There is a myth in astrophotography that a smaller sensor gives higher focal length. It doesn't. It gives a greater crop factor. You still need that Barlow to get actual magnification. It will improve your planetary images. I covered this in another video, linked:
ua-cam.com/video/gva75QPxqcg/v-deo.htmlsi=1x0o0ziEIByOw_xS

[@Guido_XL]

Planetary imaging is quite different from deep sky imaging.
A planet like Jupiter, needs to be magnified a lot, as it appears only as a tiny dot in the sky. Saturn is even smaller. That is why you always get the recommendation to do planetary photography with a large focal length, even using a Barlow, if the normal focal length doesn't suffice.
The problem with a tiny dot in the sky is that atmospheric seeing is even more of an issue than in the case of large deep sky objects, where you don't have to magnify that much. In fact, we did not have proper photos of the planets, apart from space probes that actually went to the planets, before "lucky imaging" appeared as an option. What you as a hobbyist also can now, is to use a planetary camera and put it on a telescope with a large focal length. Typically, a Cassegrain, like a Schmidt or a Maksutov, or even a Ritchey-Cassegrain. I am using a 190/1000 Maksutov-Newtonian hybrid. Not perfect at all, due to its limited focal length, but it gets the job done, with a small planetary camera attached to it.
The idea is to make video images of your planet on that small camera sensor with rather small pixels. One single exposure of Jupiter or Saturn looks like something you found on the beach. Almost useless. The seeing distorts the image decisively. You can see as how the planet's disk-shaped image is bouncing to and thro on the sensor. However, you will make video captures, gathering thousands and thousands of still images. Then, you put those video files into freeware software that will analyze and stack all of these images, skipping the bad ones. Software like PIPP and AutoStackert! are made for this. They turn your video captures of a planet into a single image that really looks stunning, after you processed that image with software, like Registax. The image that the stacking process produced, is still a bit fuzzy, but smart software can enhance contrast and color distribution to a level that you will love the result, created by your telescope and supported by a series of software.
The reason as to why that planetary sensor contains small pixels is that your planetary image is also small. It's tiny. Large pixels would not produce anything useful here. You can afford to have such small pixels, photon-wise, as the planet is still quite bright, compared to deep sky objects. So, the problem is not a lack of photons, but a lack of magnification and optical sharpness. Software allows you to process many thousands of still images and stack them into a wonderful final image.
Stacking is also done with deep sky images, but here, the issue is a lack of photons per exposure, combined with a bit of uncertainty in the alignment of all exposures, if you put them all over each other. You don't lack photons in case of a planet though. So, you can afford to have small pixels. They will be filled quite fast. In fact, you have to fill them quite fast, as otherwise, the seeing will make just a smudged image of each exposure, which not even the best software in the world can restore. Typically, your still exposures last only a few milliseconds. That is also the reason as to why your planetary camera needs to have a fast USB-connection: you want a lot of images in your video, pumped quickly towards your laptop or PC. The quicker, the better. You want a maximum rate of images in your video, so that the seeing effect is minimal and the total amount of images is maximal, offering lots of images to the software to turn them into a beautiful final image.

[@mikehardy8247]

​This answers my question. There seemed a paradox. It's sort of clear now.

[@JeffSmithbureau13]

Makes perfect sense, thank you

[@CriticalThinker-42]

Good basic explanation! But it doesn't address Binning (effectively making your pixels 2, 3, 4X their physical size), that planetary photographers prefer smaller pixels with more magnification, or that most if not all Terrestrial Cameras are under-sampled but still produce sharp images... and that Post Processing corrects for many deficiencies in the Captured Image... Especially as more and More AI improvements are introduced. 🍺🍻

[@SKYST0RY]

True, but going into those things would have made this video at least a couple hours long. More advanced topics for another time.

[@Lab00Rat]

I follow, but when we linear stretch the final image, does that not negate the smaller pixel, slower scope dim image? Otherwise, would binning make a small pixel camera into a big pixel camera?

[@SKYST0RY]

You can do a linear or nonlinear stretch, but it has nothing to do with this. It merely pulls the information into a wider histogram to make a dark image brighter. You can bin pixels at great cost to megapixel count. Ex. binning a 26 MP camera a mere 2x2 reduces it to a 6.5 MP camera in exchange for virtually doubling your pixel size. But that could work in some circumstances. But it introduces other issues, like do you want to mess with aligning a large frame camera on your scope, and will that leave room for an OAG (if you use one). It's why I prefer to avoid full frame and even APS-C sized sensors.

[@Aerostar509]

Stretching is just multiplication, zero times 1,000 is still zero. So a dim object times 1,000 (as an example) is still less than a bright object times 1,000.

[@mikeforsyth2058]

What about binning, making a group of four adjacent 'pixels 2 x 2 into a one larger pixel ?

[@MERLE1593]

That works. I do it all the time with my ASI183M and 1300mm telescope.

[@SKYST0RY]

You can bin and get some benefits, though binning is less beneficial with modern sensors.

[@tlbernhard]

Isn't it more about F ratio than focal length. A Newtonian scope will have a longer focal length and a lower F ratio than a shorter refractor due to its wider aperture.

[@SKYST0RY]

F ratio is simply a mathematical function: Focal length divided by aperture. The model of telescope doesn't change that, though some telescope designs lend themselves to certain characteristics. Newts are great for low F ratio due to the lower cost of making wide mirrors. However, F ratio itself is widely misunderstood. A low F ratio telescope won't necessarily collect light faster than a telescope with a higher F ratio because F ratio is merely a way of comparing an optical tube to itself. What really matters is another variable. I'll cover it soon.

[@Aerostar509]

Boy, you open yourself up for a can of worms from all the good comments!

[@tbardoni5065]

Try taking a planetary picture with big pixels and see how it works out for ya.

[@UncleKennysPlace]

"Large" and "small" are relative terms. The only real disadvantages to tiny pixels are lower light sensitivity and a larger image size. So "correctly sized" would be a better description. There is no magic.

[@HeadbuttWarlock]

@@UncleKennysPlace And Planets are super bright in comparison to DSO's, so losing the brightness gained by large pixels isn't an issue. Not to mention planetary is done via lucky imaging with videos, light wobbling onto other pixels at high focal lengths can be averaged out via stacking.

[@SKYST0RY]

The physics will be the same, though the very short exposures used for planetary images negates much of the pixel size concern.

[@tbardoni5065]

@@SKYST0RY if I do planetary with my EDGE 11”, with an ASI071, 4.78um pixel, the detail is utterly terrible and its a blobby mess. If I use the smaller pixel camera, ASI178, 2.4um, the image is detailed, sharp, and great.
Is this not the opposite result that you suggest in this video?

[@SKYST0RY]

​@@tbardoni5065 The video is a basic introduction to FL/pixel size relationships. It doesn't cover complicating effects of photographic technique nor factors such as Dawes and Raleigh Limits, binning effects, etc. It would take the equivalent dozens of videos to cover all those topics in depth. However, pixel size is less a factor than how it is applied, and the only fixed rule in photographing anything is there are no fixed rules. There are many ways to get the same job done. For example: The link goes to the image of Jupiter shot a few years ago by the Hubble Telescope using Wide Field Camera 3. The WFC3 has a pixel size of 18 microns.
esahubble.org/images/heic2017a/
In other words, pixel size is a variable. Change enough variables and you can change what works, even making an 18 micron sensor take one of the best images of Jupiter from Earth ever recorded.

[@earthling-fh2mg]

I still don’t understand how pixels work. If you take a shot of the moon let’s say, you will have an image of the moon and the rest of it being dark background on, let’s say, a rectangular APS-c sensor. Does this mean that the central pixels are only picking up the photons from the central section of the moon and your 24MP sensor is dividing the entire scene by 24 million? If so, how does a circular lens produce a rectangular image without losing any data your telescope lens combination gathers? A rectangular sensor must end up losing some of the “image” produced by the light cone reaching it.

[@SKYST0RY]

You are exactly correct. A telescope produces a circular image at the focal plane and a rectangular sensor loses whatever data doesn't fit in its space. It is an unavoidable loss. However, since that is data on the outside of the image where optics tend to be poorer, it's also an acceptable loss. You might like my video on crop factor that covers some of this: ua-cam.com/video/gva75QPxqcg/v-deo.htmlsi=1x0o0ziEIByOw_xS

[@earthling-fh2mg]

@@SKYST0RY Ok thanks but here’s the related issue I then have: in a schematic of a telescope, we are provided with a focal point at the very end of the cone of light created by the lens. That cannot be correct if the resulting image is spread over the sensor area. It would suggest that, if perfectly in focus, all the light collected at the objective lens then funnelled through the scope, would end up as a point image on the center of the sensor. To gather an image across the entire sensor area requires the sensor to be fully located somewhere further into the cone and, therefore not at the focal point. I know this is obviously not the case (you’d never believe I got 100% in my optics exam during my physics degree eons ago) but I find this highly confusing.

[@dave882]

Is this a poor ai voice ? Too much repetition

[@SKYST0RY]

I've often told my my admins to modify my voice but they say that since I am just an AI, I don't get a say.

[@thierrymartin8378]

This video is not the truth. The maths give solution from optical laws. the F/D ratio is the starting point and not only the Focal lenght. From the equation you get the theorical diffraction limits. 1.22x Lambda x F/D or the Airy spot. If your telescope gets all the optical remaining aberrations inside this spot over the full field of view (deteminating by the stop.) , you can start we this number. Of course with a photosite ( called in the video pixel) the minimum value is less than the Aity spot, because the optic at the front of the photosite and the very good quatum efficency of the detector well above human eye capacity. You have to consider for astro photography the air stability like virtual circles where inside the air is stable. Nothing to do with the animation There is not one star involve touching the photosite but the full field involving all the photosites of the camera. This is about a section of the sky or angle of view . A good seing is when the circle is 40cm or 16'' diametre. But usually the best in low altitude in the countryside is closed to 200mm dia,; meaning 0.6 Arscsec is the maximum resolution (the separation beween two stars) you can get. I mean this what the tracking resolution of a eqautorial mount should be able to manage too. You understand therefore this is not only the focal length but the diametre of the instrument is about. If you living in hight mountain with less atmosphere to cross, you get a better seing. The best in South Pole with 1m diamete of stable aire. Therefore small telescope like 80 120mm diametre are less prone to get oval stars if of course they are optically perfect . With the Rasa 8 this is starting to be more difficult even with 400mm focal lenght . You get in bonus fast saturation of the Cmos therefore small exposures which is statically better to minimise turbullences probability . But once again if the instrument is capable to reach the theorical diffraction limit which is not the case with Rasa due to a huge compromise with the corrector lenses. You may undestand why some refractors are so expensive for small diametre than Rass , just because the price of the lenses. But who cares about speed and diffraction limits amongs Rasa owners? Because some famous Made in Japan telescopes are reaching the difraction limit over a wide range of focus field fully corrected. whatever the color of the star, some of Rasa owners are selling their instruments to buy same focal length instrument giving sharpper stars. The video is not correct . And someone must tell it

[@stevenholt824]

I have to disagree slightly, a short tube sees more sky but the long tube sees less area but gets the same amount of light from that area !
imagine your eye at the aperture of a telescope, and it moves around so to cover the whole surface area collecting the light as it can see the star at all infinate points across the aperture then this light is focused down the tube to the collection lens and the sensor , the focal length wont matter , the field of view or 'magnification' is what changes with focal length. F/whatever is just a ratio . Its the aperture that governs light collection .
A photon thats traveled 2.5mly wont care if it goes down a 600mm tube or a 1200mm tube .......As long as it meets the correct sized pixel 😂😂😂

[@SKYST0RY]

Basically true, except a photon is also a wave. It is both at the same time. As a particle, it just travels down the tube. As a wave, it interacts with the tube. It's why I noted in the video the illustration is functional but imperfect because quantum phenomena are highly counter intuitive, and light is multiple things at once. In point of fact, a photon is actually a very long, very thin light that experiences no time while the universe around it does, but that is a whole other rabbit hole of fun to explore some time.

[@stevenholt824]

@@SKYST0RY these are the effects that cause differences in actual results v theory. If only there were no atmosphere then....... 😂