Perfect example and clarity of describing focal lengths. New to the game and was searching how focal reducers work and this explanation has done this for me. Great work 👏👏
After 45+ years in the field of amateur astronomy, this is the BEST explanation of this issue I've ever seen, read or heard! Please keep up the great work!
It's been a really good summer. Lots of clear nights. Last winter, I was lucky to get two good nights in a month. Usually, winters are much better, though.
Mmmh, the spreading of butter analogy is great, the overall conclusion is great, and tradeoff explanation as well! But unless I'm going insane, the light rays crossing is wrong: when in focus, regardless of the focal ratio, the light rays cross at the level of the sensor, because our targets are effectively at infinity. So you are picturing out of focus cameras in your diagram... Your light rays crossing explanation is good at showing why in focus images have more brightness per pixel than out of focus images :) And why faster focal ratio is more sensitive to critical focus! And it also shows that the maximum incidence angle of light rays is higher for low focal ratios - but it doesn't relate back to the "butter spreading" effect of focal length for a given aperture...
Salut, Cuiv! There is an actual physical crossover of the light rays between optics. Where it happens depends on the optical design, of course. In the case of a concave mirror, if the subject is beyond C (center of the mirror's curvature), the real image forms somewhere between F (focal point) and C. This is what allows for there to be an image circle of some diameter or other. As I understand the optical design of a SCT, the secondary hyperbolic mirror appears at about 350 mm from the primary (-50 from the primary's 400 mm focal length), so the light from the primary has not yet reached F, thus the image cannot yet have inverted. Ergo, inversion does not happen between the primary and secondary as is sometimes suggested. The light is then reflected from the hyperbolic secondary into the baffle and down through the image train. At some point the light from the secondary reaches focal point and the real image forms a bit behind F. This causes inversion of the image and allows for the real image to spread out into the 2 dimensions of the image circle. (It also explains how reducers work. A reducer simply and literally would reduce this image circle, concentrating light energy into a smaller image circle, giving a shallower focal length perspective and lower F ratio). The key thing is the real image forms a bit after the focal point. How much after depends on the optical design. If that spread didn't happen, the real image would be a mere dot and useless. You can see this effect when you look at a reflection in a concave mirror. Prior to the focal point, the reflection is magnified and not inverted. At the focal point (F), the object will look like it takes up the whole mirror. Beyond the focal point, the object inverts. At C, the object is to scale, and beyond C it is de-magnified. The light reflected from the subject comes to a dot at the focal point, but that's not where the real image forms. I know it's not intuitive. This is one of the best diagrams I know to illustrate it. As you can see, the real image forms between C and F. drive.google.com/file/d/1CC_RKtJQZmAJOfr51dbrF_Luvet8nmlE/view?usp=sharing In the following link, you can see what happens to an image in a concave mirror before, at and after the focal point in live motion. ua-cam.com/video/3e-LZPHBA2M/v-deo.htmlsi=04FBgBZDtCyVqsaB Portraying this is very complicated and those of us with scientific backgrounds tend to get caught up in rabbit holes of complexity that don't help students. I decided to stay focused on the goal of keeping it simple and focused on focal length in order to avoid the technical rabbit holes that always haunt these topics. Thus, the diagram of the SCT is not to scale, lacks pieces and the focal point is shown long before where it would actually happen in the image train as it would simply be very difficult to illustrate and needlessly complicate the diagram. I am not an optical engineer and it's been a long time since I took a physics course, so if my understanding is off, let me know. A bientot!
@@SKYST0RY Hello hello! Ah, I see, this is a common trap - the diagram you linked to is indeed correct, but the target is close to the mirror. Try redrawing that diagram as the target is getting further and further away from the mirror! You'll see that the image formed becomes closer and closer to f. For a target at infinity (which for all intents and purposes is our case), it is at f, with no crossing.
@@SKYST0RY The more I read your comment, the more I see misconceptions, let me clarify a bit for some of those in a couple of replies. Misconception 1 "if the image formed at f it would be a dot" - no, the parallel light rays entering the objective at 90 degrees from the objective represent only the dot at the center of the FOV. Another part of the target that is, say 2 arcminutes away from the center of the FOV will also have parallel light rays entering the system, but at an incident angle of 89.967 degrees (90 minus two arcminutes). The light rays being parallel, they will also form an image on the plane of f, but not at f. Actually depending on the design they will form an image very, very slightly in front of the plane of f (because it's not a plane but a focal sphere with a very large radius which is why having curved sensor would be quite nice), which is field curvature that our optics have to fight via field flattener, etc. And the distance on the focal plane/sphere between your center point of your star 2 arcminutes away from the center will be smaller for shorter focal lengths, and that is what actually explains the impact of focal length on "dimming" the image You can check the wikipedia article "infinity focus"
@@SKYST0RYMisconception 2: how the focal reducer works. On any system, we'll have those converging light rays (before crossing in this case), whose maximum incidence angle depends on the focal ratio of the main optical system. A reducer is basically just a magnifying glass/converging lens that grabs the whole light cone and makes it steeper. This makes the focal plane of the system be closer, thus diminishing the overall focal length. And of course the maximum incidence angle of the light rays also increases. This is why when using narrowband filters on a fast system, you'll get less bandpass shift if you place the filter in front of the reducer rather than after in a filter drawer. The reducer doesn't rely on having rays crossing before reaching it (in fact those rays would then be divergent)
@@CuivTheLazyGeek I think, in regard to item 2, we are talking about exactly the same thing with different language. My scientific background is psychology, and I've only audited physics in university, and that was many years ago, so I am unlikely to use the parlance of physicists. However, I do prefer to avoid technical jargon and keep things to language people can understand. Even in my field, I prefer to avoid psychobabble. In regard to focal points and where images form, when diagrams show light converging to a single point, that's just a simplified representation of reality. It's no more true to life than a circuit schematic that shows complex wiring as a couple lines. Diagrams are "shorthand" for real life. In real life, the subjects we image subtend an area of the sky and the real images our optics produce of them take up an area within an image circle. The image is made from millions of light waves with wavelengths of nanometers, and when it comes into focus, discreet light waves originating from a point on those objects converge in the real image. But the many millions of light waves converge over an area that has the two dimensions of width and breadth (in the image circle). Thus, a diagram may show rays of light converging to a point. In reality, millions of light waves converge into millions of points spread out along the real image--the image circle. But all the diagrams I've seen illustrate this poorly to anyone without a scientific background. And having spent several hours trying to find the best language to explain this, I think spoken language is also wanting. I think I will have to find a better way to illustrate what focus is. (Actually, I just reread your first reply and I think there, too, we are very much describing the same thing but conceptualizing it from very different perspectives. Which tells me it's too late, and I need to get some sleep. I may do astrophotography but I am very much an early bird, usually up before the sun rises, and I am too foggy headed to think "in focus".)
Very nice video. I’ve spent a lot of energy on Cloudy Nights trying to explain what a “fast telescope” really means in the age of low read noise CMOS cameras when nearly all telescopes are fast enough to avoid read noise and now it’s just down to what image scale and total light gathering power you want. To that end, I think one additional analogy with the “butter spread on toast” is to also show that with a larger aperture, you get to start with more butter even if it may be spread out more because if the target fits in the field of view (like your Hamburger galaxy example), a telescope with 2x the aperture will have 4x more butter. And how it’s spread out is the F-ratio, but you start with more butter and that is always good. With a low read noise CMOS camera, especially if doing broadband imaging, you’ve escaped read noise so collecting light faster with a fast scope may not be a material advantage and really it’s down to how many target photons per second you collect. One final point is if the long focal length image is “dimmer” but spread out wider (your toast analogy again), you can always make it brighter in post processing by simply downsampling it to the same pixel scale as the short focal length telescope. Yes you lose the potential resolution advantage but once you’ve escaped read noise, this is basically equivalent as shooting with a short focal length (ignoring the field of view difference here). I think a video that kind of explains both of these factors as kind of a matrix would complete the picture.
Finally! This is the first video about focal length and focal ration that makes sense and explains it truthfully. I've watched so many and my reaction has always been always been "Nah, that can't be right." ...and it wasn't! The focal length tendency to stretch the image like butter on a piece of bread, now that make sense (and reminds me of Lord of the Rings!) and explains what I'm actually seeing.
Great video! I will never be able to settle between fast scopes and insane focal lenghts. I now shoot with a 10" Newtonian 1200mm f/4.7 and it's in my opinion a very sweet spot between zoom and speed. Some 20 hours of exposures on a target with this scope outputs a shit ton of details. Would even go as far as upgrading the tube to a 12" Fast Newt 1200mm f/4.0 which would exponentially increase the light gathered. However I also crave a RC telescope that goes beyond 2000mm, just to have the power of imaging small galaxies. Do you think it would be worth the effort? Seems like not only do I need a new camera with large sensor and pixels (just as you explained in this awesome video), but at that focal if I'm not mistaken you kind of depend on good to excellent seeing, which I don't always have. What do you think? Also, you got yourself a new subscriber :D
Hello, and thank you! I think a RC for distant objects is definitely worth it. You'll have to spend more time on target, and success will depend more on the quality of your sky, for sure. But even OK seeing will be workable. Basically, the better the seeing, the fewer the subs you'll have to cull. Shoot shorter subs so you can be more aggressive with your culling. Good subs are the foundation of good images.
Good video. We DO use things like Barlows and flateners to compensate and/or augment the telescope's focal length. I do use a relatively longer focal length than others and I DO use a more expensive larger sensor, cooled camera for my pics as well.
Great Video! I was scared to pick a long focal length telescope with a high f-ration for several years. For a year now I own an 8" RC with 1624mm focal length with f/8 ratio and I love it since I have a good focuser on it. Last night I shot the first set of exposures on the core of M 16 with it. The first 3.5h of integration time are already amazing. Sure the dimmer details will be revealed only tomorrow or the night after once the second and third night have been successful. I also own an f/7 Apo that also provides good results with just 4 hours of integration if the target fills the full sensor area. Those close-up shots have their own magical theme. They show what all of us dream about whereas low focal length wide field shots reveal this mindblowing size of our galaxy. A dimmer result is not worse than a bright result from my point of view. The dimmer photos transfer this mystical appearance of nebulae and what is happening inside of them. After having seen your video I think I am also going to use my RC telescope on targets that do not appear to be a perfect fit for it but the result will be an amazing pay back of the invested time. 👍
I don't think I could ever go back to a small aperture telescope. And, like Dylan O'Donnell, I like deep field imaging. I want to get in close. I love big FL telescopes for the way they open up the cosmos.
@SKYST0RY I loke big bu77s... I mean telescopes.😂😂😂 For few targets I will still use my 420mm Apo refractor. but the majority will now be done with the larger ones. I aga8n had big fun with my RC on the Eagle Nebula last night. Just my 2600MC Pro has a too wide field of view.
I am too elderly and feeble to use an 8" SCT. I do have a 6" SCT, a x0.63 reducer and a Hyperstar 6 v4. So, I can use the SCT plus a x3 Barlow for planets and bright, tiny deep space objects, the SCT plus the x0.63 reducer for about 90% of the Messier catalogue and the SCT plus the Hyperstar for really big targets. So, I find the SCT to be a very versatile optical tube.
Can you do a video on over and under sampling? And how does hyperstar and rasa work to make a large aperture telescope faster? Is it that the light path is shortened? If thats the case, can i replace the secondary mirror in my 17.5" dobsonian (i call it the Godsonian) with a camera? Assuming of course i get the back focus right. And lastly, what if you out the camera sensor BEFORE the point where the image crosses instead of AFTER?
I've covered a little on over sampling already. One of the days, I'll try to tackle it more in depth. But I can tell you the RASA and Hyperstar work by removing the secondary mirror hyperbolic mirror from an SCT, so the telescope is then working with only the primary mirror's native 400 mm focal length and f/2 ratio. I don't think you can just remove the secondary from a SCT and replace it with only a camera because you need a lens system to correct aberrations in the light that will result.
Great video and information, so would you be able to use a 2 x Barlow on an 8 inch f5 Newtonian and get the same image as the 8 inch sct or would the Barlow give poor quality image , thanks for taking the time to make these videos
In principle, I would think so. I don't have a Newt but I understand they can be subject to image deformation and require good correctors. I don't know how a Barlow lens may interact with that.
@@SKYST0RY yes would be an intresting experiment, I think another problem would be getting a mount that would be able to track with a large newt and long FL
Yes it would be horrible, I own a really high quality astro-physics barlow for planetary work. Tried it on my 8" newt and 130mm refractor on M101 and it was as blurry as anything. I also own a 9.25 edgehd and there is no comparison.
guiding and seeing! There is a crossover point for those too. An 8" sct with its long focal length may not perform better than an 8" f4 with the same camera because of seeing, or a mount cant guide well enough to not stray light across the sensor.
It's true that lower focal length will require better seeing and more exacting guiding, but I haven't found it to be a huge impediment. Modern tech has made guiding more precise and margin for error more forgiving.
Many thanks for your fascinating video. In simple terms, I have a ZWO585MC camera with a couple of different scopes. One has a FL of 336mm and the other 660mm. Would I get sharper images if I had my camera set to bin 1 for the shorter FL and perhaps bin 2 for the longer one? I look forward to your thoughts. Many thanks
That partly depends on the optics of the telescope. I haven't covered issues such as airy disks yet and the complications they bring to matching sensors. But I have the Player One Uranus-C camera which also has the Sony IMX585 sensor. I can use it on my 81 mm refractor which has a similar focal lengths to yours and the outcomes are fine. Being a little over sampled isn't as much of an issue these days as it used to be.
@@SKYST0RY thanks very much for your reply. I may look into comparing bin 1 and bin 2 with my larger scope and see which is best. Keep the great videos coming 👍👌
@@dougiesmart1623 I totally forgot to note: I only ever use 585 at bin 1 when imaging. Sometimes bin 2 for plate solving if there is ambient moonlight or poor seeing. But I always image at bin 1.
Great video. In the end, it's all tools. Comes down to using the right tools for the project. Not going to build a house with a tack hammer or do upholstery with a 4lb. sledge hammer. All forms of photography involves robbing Peter to pay Paul. No such thing as one size fits all.
Very true. My bad. I did not see the telescope model as important as the principle what happens with FL vs F ratio applies to any telescope, so I was not concerned with the telescope model's details and, in fact, left out a great deal of it.
I get confused very fast when comparing F Ratios of scopes with different Aperture and Focal length, but when I compare a 4” F5 530 mm FL to a 11” F1.8 530mm FL , things make more sense to me.
F ratio is deceptive. It is not a good tool for comparing OTAs of different aperture, but only to compare the performance of an OTA at one focal length to itself at another focal length. I'll try to cover this some time.
This is true, but the main limitation of focal length will be due to seeing. At some point extra focal length won’t resolve additional details due to atmospheric blur.
You failed to mention the atmospheres impact on images thus you might trick some people to get larger and larger aperture. With long time exposures you quickly find your self not needing all that large aperture, only benefiting from it under specific imaging conditions.
A rule of thumb is that a telescope's useful magnification is twice its aperture in millimeters. It's more that the bigger the aperture, the more useful magnification you can have. However, this is an old limit. Techniques such as speckle imaging and other developments have extended this a bit. But it's probably unnecessary. The C8, for example, has 480x without a reducer and it's well within the aperture to magnification guideline.
Perfect example and clarity of describing focal lengths. New to the game and was searching how focal reducers work and this explanation has done this for me. Great work 👏👏
After 45+ years in the field of amateur astronomy, this is the BEST explanation of this issue I've ever seen, read or heard! Please keep up the great work!
Great and easy to understand explanation!
Great explanation!
Subscribed! Thank you for explaining those hard to understand and confused information in a simple way.
Great illustration and explanation! Thank you.
Very concise explanation of focal length. Great channel all round. Really envious of all the clear nights you seem to get🙂
It's been a really good summer. Lots of clear nights. Last winter, I was lucky to get two good nights in a month. Usually, winters are much better, though.
One word, WOW!!!
Mmmh, the spreading of butter analogy is great, the overall conclusion is great, and tradeoff explanation as well! But unless I'm going insane, the light rays crossing is wrong: when in focus, regardless of the focal ratio, the light rays cross at the level of the sensor, because our targets are effectively at infinity. So you are picturing out of focus cameras in your diagram... Your light rays crossing explanation is good at showing why in focus images have more brightness per pixel than out of focus images :) And why faster focal ratio is more sensitive to critical focus! And it also shows that the maximum incidence angle of light rays is higher for low focal ratios - but it doesn't relate back to the "butter spreading" effect of focal length for a given aperture...
Salut, Cuiv! There is an actual physical crossover of the light rays between optics. Where it happens depends on the optical design, of course. In the case of a concave mirror, if the subject is beyond C (center of the mirror's curvature), the real image forms somewhere between F (focal point) and C. This is what allows for there to be an image circle of some diameter or other. As I understand the optical design of a SCT, the secondary hyperbolic mirror appears at about 350 mm from the primary (-50 from the primary's 400 mm focal length), so the light from the primary has not yet reached F, thus the image cannot yet have inverted. Ergo, inversion does not happen between the primary and secondary as is sometimes suggested. The light is then reflected from the hyperbolic secondary into the baffle and down through the image train. At some point the light from the secondary reaches focal point and the real image forms a bit behind F. This causes inversion of the image and allows for the real image to spread out into the 2 dimensions of the image circle. (It also explains how reducers work. A reducer simply and literally would reduce this image circle, concentrating light energy into a smaller image circle, giving a shallower focal length perspective and lower F ratio). The key thing is the real image forms a bit after the focal point. How much after depends on the optical design. If that spread didn't happen, the real image would be a mere dot and useless. You can see this effect when you look at a reflection in a concave mirror. Prior to the focal point, the reflection is magnified and not inverted. At the focal point (F), the object will look like it takes up the whole mirror. Beyond the focal point, the object inverts. At C, the object is to scale, and beyond C it is de-magnified. The light reflected from the subject comes to a dot at the focal point, but that's not where the real image forms. I know it's not intuitive. This is one of the best diagrams I know to illustrate it. As you can see, the real image forms between C and F. drive.google.com/file/d/1CC_RKtJQZmAJOfr51dbrF_Luvet8nmlE/view?usp=sharing
In the following link, you can see what happens to an image in a concave mirror before, at and after the focal point
in live motion. ua-cam.com/video/3e-LZPHBA2M/v-deo.htmlsi=04FBgBZDtCyVqsaB
Portraying this is very complicated and those of us with scientific backgrounds tend to get caught up in rabbit holes of complexity that don't help students. I decided to stay focused on the goal of keeping it simple and focused on focal length in order to avoid the technical rabbit holes that always haunt these topics. Thus, the diagram of the SCT is not to scale, lacks pieces and the focal point is shown long before where it would actually happen in the image train as it would simply be very difficult to illustrate and needlessly complicate the diagram.
I am not an optical engineer and it's been a long time since I took a physics course, so if my understanding is off, let me know. A bientot!
@@SKYST0RY Hello hello! Ah, I see, this is a common trap - the diagram you linked to is indeed correct, but the target is close to the mirror. Try redrawing that diagram as the target is getting further and further away from the mirror! You'll see that the image formed becomes closer and closer to f. For a target at infinity (which for all intents and purposes is our case), it is at f, with no crossing.
@@SKYST0RY The more I read your comment, the more I see misconceptions, let me clarify a bit for some of those in a couple of replies.
Misconception 1 "if the image formed at f it would be a dot" - no, the parallel light rays entering the objective at 90 degrees from the objective represent only the dot at the center of the FOV. Another part of the target that is, say 2 arcminutes away from the center of the FOV will also have parallel light rays entering the system, but at an incident angle of 89.967 degrees (90 minus two arcminutes). The light rays being parallel, they will also form an image on the plane of f, but not at f. Actually depending on the design they will form an image very, very slightly in front of the plane of f (because it's not a plane but a focal sphere with a very large radius which is why having curved sensor would be quite nice), which is field curvature that our optics have to fight via field flattener, etc. And the distance on the focal plane/sphere between your center point of your star 2 arcminutes away from the center will be smaller for shorter focal lengths, and that is what actually explains the impact of focal length on "dimming" the image
You can check the wikipedia article "infinity focus"
@@SKYST0RYMisconception 2: how the focal reducer works. On any system, we'll have those converging light rays (before crossing in this case), whose maximum incidence angle depends on the focal ratio of the main optical system. A reducer is basically just a magnifying glass/converging lens that grabs the whole light cone and makes it steeper. This makes the focal plane of the system be closer, thus diminishing the overall focal length. And of course the maximum incidence angle of the light rays also increases. This is why when using narrowband filters on a fast system, you'll get less bandpass shift if you place the filter in front of the reducer rather than after in a filter drawer. The reducer doesn't rely on having rays crossing before reaching it (in fact those rays would then be divergent)
@@CuivTheLazyGeek I think, in regard to item 2, we are talking about exactly the same thing with different language. My scientific background is psychology, and I've only audited physics in university, and that was many years ago, so I am unlikely to use the parlance of physicists. However, I do prefer to avoid technical jargon and keep things to language people can understand. Even in my field, I prefer to avoid psychobabble. In regard to focal points and where images form, when diagrams show light converging to a single point, that's just a simplified representation of reality. It's no more true to life than a circuit schematic that shows complex wiring as a couple lines. Diagrams are "shorthand" for real life. In real life, the subjects we image subtend an area of the sky and the real images our optics produce of them take up an area within an image circle. The image is made from millions of light waves with wavelengths of nanometers, and when it comes into focus, discreet light waves originating from a point on those objects converge in the real image. But the many millions of light waves converge over an area that has the two dimensions of width and breadth (in the image circle). Thus, a diagram may show rays of light converging to a point. In reality, millions of light waves converge into millions of points spread out along the real image--the image circle. But all the diagrams I've seen illustrate this poorly to anyone without a scientific background. And having spent several hours trying to find the best language to explain this, I think spoken language is also wanting. I think I will have to find a better way to illustrate what focus is.
(Actually, I just reread your first reply and I think there, too, we are very much describing the same thing but conceptualizing it from very different perspectives. Which tells me it's too late, and I need to get some sleep. I may do astrophotography but I am very much an early bird, usually up before the sun rises, and I am too foggy headed to think "in focus".)
Very nice video. I’ve spent a lot of energy on Cloudy Nights trying to explain what a “fast telescope” really means in the age of low read noise CMOS cameras when nearly all telescopes are fast enough to avoid read noise and now it’s just down to what image scale and total light gathering power you want. To that end, I think one additional analogy with the “butter spread on toast” is to also show that with a larger aperture, you get to start with more butter even if it may be spread out more because if the target fits in the field of view (like your Hamburger galaxy example), a telescope with 2x the aperture will have 4x more butter. And how it’s spread out is the F-ratio, but you start with more butter and that is always good. With a low read noise CMOS camera, especially if doing broadband imaging, you’ve escaped read noise so collecting light faster with a fast scope may not be a material advantage and really it’s down to how many target photons per second you collect.
One final point is if the long focal length image is “dimmer” but spread out wider (your toast analogy again), you can always make it brighter in post processing by simply downsampling it to the same pixel scale as the short focal length telescope. Yes you lose the potential resolution advantage but once you’ve escaped read noise, this is basically equivalent as shooting with a short focal length (ignoring the field of view difference here).
I think a video that kind of explains both of these factors as kind of a matrix would complete the picture.
That idea of aperture = butter is excellent! The more aperture, the more butter you start with.
Totally agree!! Perfect explanation!
Finally! This is the first video about focal length and focal ration that makes sense and explains it truthfully. I've watched so many and my reaction has always been always been "Nah, that can't be right." ...and it wasn't! The focal length tendency to stretch the image like butter on a piece of bread, now that make sense (and reminds me of Lord of the Rings!) and explains what I'm actually seeing.
I am glad you appreciate it. Engineers occasionally have conniptions over how I try to make these topics intuitively understandable.
Perfectly explained, great content as always.
Great video! I will never be able to settle between fast scopes and insane focal lenghts. I now shoot with a 10" Newtonian 1200mm f/4.7 and it's in my opinion a very sweet spot between zoom and speed. Some 20 hours of exposures on a target with this scope outputs a shit ton of details. Would even go as far as upgrading the tube to a 12" Fast Newt 1200mm f/4.0 which would exponentially increase the light gathered. However I also crave a RC telescope that goes beyond 2000mm, just to have the power of imaging small galaxies. Do you think it would be worth the effort? Seems like not only do I need a new camera with large sensor and pixels (just as you explained in this awesome video), but at that focal if I'm not mistaken you kind of depend on good to excellent seeing, which I don't always have. What do you think?
Also, you got yourself a new subscriber :D
Hello, and thank you! I think a RC for distant objects is definitely worth it. You'll have to spend more time on target, and success will depend more on the quality of your sky, for sure. But even OK seeing will be workable. Basically, the better the seeing, the fewer the subs you'll have to cull. Shoot shorter subs so you can be more aggressive with your culling. Good subs are the foundation of good images.
Good video. We DO use things like Barlows and flateners to compensate and/or augment the telescope's focal length. I do use a relatively longer focal length than others and I DO use a more expensive larger sensor, cooled camera for my pics as well.
Thanks for a very informative video :)
Great Video! I was scared to pick a long focal length telescope with a high f-ration for several years. For a year now I own an 8" RC with 1624mm focal length with f/8 ratio and I love it since I have a good focuser on it.
Last night I shot the first set of exposures on the core of M 16 with it. The first 3.5h of integration time are already amazing. Sure the dimmer details will be revealed only tomorrow or the night after once the second and third night have been successful. I also own an f/7 Apo that also provides good results with just 4 hours of integration if the target fills the full sensor area.
Those close-up shots have their own magical theme. They show what all of us dream about whereas low focal length wide field shots reveal this mindblowing size of our galaxy.
A dimmer result is not worse than a bright result from my point of view. The dimmer photos transfer this mystical appearance of nebulae and what is happening inside of them.
After having seen your video I think I am also going to use my RC telescope on targets that do not appear to be a perfect fit for it but the result will be an amazing pay back of the invested time. 👍
I don't think I could ever go back to a small aperture telescope. And, like Dylan O'Donnell, I like deep field imaging. I want to get in close. I love big FL telescopes for the way they open up the cosmos.
@SKYST0RY I loke big bu77s... I mean telescopes.😂😂😂
For few targets I will still use my 420mm Apo refractor. but the majority will now be done with the larger ones.
I aga8n had big fun with my RC on the Eagle Nebula last night. Just my 2600MC Pro has a too wide field of view.
@@astrofromhome Do you find the larger sensor of the 2600 is difficult to align on the 420 mm refractor?
I am too elderly and feeble to use an 8" SCT. I do have a 6" SCT, a x0.63 reducer and a Hyperstar 6 v4. So, I can use the SCT plus a x3 Barlow for planets and bright, tiny deep space objects, the SCT plus the x0.63 reducer for about 90% of the Messier catalogue and the SCT plus the Hyperstar for really big targets. So, I find the SCT to be a very versatile optical tube.
It's definitely my favorite OTA. Short or long focal length, that wide aperture makes them amazing.
Is the 3x the best barlow for the 6? I have 2 c6 with the hyperstar an starizona reducer and don’t know much about planetary.
Fantastic as usual👍
Can you do a video on over and under sampling?
And how does hyperstar and rasa work to make a large aperture telescope faster?
Is it that the light path is shortened?
If thats the case, can i replace the secondary mirror in my 17.5" dobsonian (i call it the Godsonian) with a camera?
Assuming of course i get the back focus right.
And lastly, what if you out the camera sensor BEFORE the point where the image crosses instead of AFTER?
I've covered a little on over sampling already. One of the days, I'll try to tackle it more in depth. But I can tell you the RASA and Hyperstar work by removing the secondary mirror hyperbolic mirror from an SCT, so the telescope is then working with only the primary mirror's native 400 mm focal length and f/2 ratio.
I don't think you can just remove the secondary from a SCT and replace it with only a camera because you need a lens system to correct aberrations in the light that will result.
Great video and information, so would you be able to use a 2 x Barlow on an 8 inch f5 Newtonian and get the same image as the 8 inch sct or would the Barlow give poor quality image , thanks for taking the time to make these videos
In principle, I would think so. I don't have a Newt but I understand they can be subject to image deformation and require good correctors. I don't know how a Barlow lens may interact with that.
@@SKYST0RY yes would be an intresting experiment, I think another problem would be getting a mount that would be able to track with a large newt and long FL
Yes it would be horrible, I own a really high quality astro-physics barlow for planetary work. Tried it on my 8" newt and 130mm refractor on M101 and it was as blurry as anything. I also own a 9.25 edgehd and there is no comparison.
guiding and seeing!
There is a crossover point for those too.
An 8" sct with its long focal length may not perform better than an 8" f4 with the same camera because of seeing, or a mount cant guide well enough to not stray light across the sensor.
It's true that lower focal length will require better seeing and more exacting guiding, but I haven't found it to be a huge impediment. Modern tech has made guiding more precise and margin for error more forgiving.
Many thanks for your fascinating video. In simple terms, I have a ZWO585MC camera with a couple of different scopes. One has a FL of 336mm and the other 660mm. Would I get sharper images if I had my camera set to bin 1 for the shorter FL and perhaps bin 2 for the longer one? I look forward to your thoughts. Many thanks
That partly depends on the optics of the telescope. I haven't covered issues such as airy disks yet and the complications they bring to matching sensors. But I have the Player One Uranus-C camera which also has the Sony IMX585 sensor. I can use it on my 81 mm refractor which has a similar focal lengths to yours and the outcomes are fine. Being a little over sampled isn't as much of an issue these days as it used to be.
@@SKYST0RY thanks very much for your reply. I may look into comparing bin 1 and bin 2 with my larger scope and see which is best. Keep the great videos coming 👍👌
@@dougiesmart1623 I totally forgot to note: I only ever use 585 at bin 1 when imaging. Sometimes bin 2 for plate solving if there is ambient moonlight or poor seeing. But I always image at bin 1.
Valeu!
Merci beaucoup, Ismael.
Great video. In the end, it's all tools. Comes down to using the right tools for the project. Not going to build a house with a tack hammer or do upholstery with a 4lb. sledge hammer. All forms of photography involves robbing Peter to pay Paul. No such thing as one size fits all.
Precisely! For every choice in photography, there is a gain and a price.
The secondary mirror in this system has a convex optical surface, not concave, as the video shows.
Very true. My bad. I did not see the telescope model as important as the principle what happens with FL vs F ratio applies to any telescope, so I was not concerned with the telescope model's details and, in fact, left out a great deal of it.
@@SKYST0RY not important for the story, yes. Just for the sake of scientific accuracy.
I get confused very fast when comparing F Ratios of scopes with different Aperture and Focal length, but when I compare a 4” F5 530 mm FL to a 11” F1.8 530mm FL , things make more sense to me.
F ratio is deceptive. It is not a good tool for comparing OTAs of different aperture, but only to compare the performance of an OTA at one focal length to itself at another focal length. I'll try to cover this some time.
This is true, but the main limitation of focal length will be due to seeing. At some point extra focal length won’t resolve additional details due to atmospheric blur.
That's true. Modern tech has reached the atmospheric limit. Some things can allow us to exceed it at times, though.
You failed to mention the atmospheres impact on images thus you might trick some people to get larger and larger aperture. With long time exposures you quickly find your self not needing all that large aperture, only benefiting from it under specific imaging conditions.
A rule of thumb is that a telescope's useful magnification is twice its aperture in millimeters. It's more that the bigger the aperture, the more useful magnification you can have. However, this is an old limit. Techniques such as speckle imaging and other developments have extended this a bit. But it's probably unnecessary. The C8, for example, has 480x without a reducer and it's well within the aperture to magnification guideline.
What on earth is an "fratio"?
Focal ratio.
@@SKYST0RY I have always heard F-ratio, not "fratio" (until now) lol