Could you explain telescope magnification to a camera lens guy?

I'm trying to understand what I'd actually see with a telescope, given a certain magnification.

I know that, when using a camera and a camera lens, the moon will be projected onto your camera sensor at about 1mm in diameter for every ~110mm of lens focal length. I.e. with a full frame DSLR (sensor 24mm tall) you'd need a lens with a 24*110 = 2640mm focal length to completely fill the frame vertically.

As far as I understand the specs of (reflector) telescopes (e.g. the Sky-Watcher Skyliner 200P Dobsonian), you have a main mirror diameter - e.g. 203mm (8"), a focal length - e.g. 1200mm, and then you use eye pieces to obtain a desired magnification.

The maximum practical magnification usually seems to be rated as twice the mirror diameter (e.g. x406 for the scope described above), and the actual magnification is the focal length divided by the eyepiece. E.g. with a 1200mm focal length, and 25mm and 10mm supplied eyepieces, you can get 1200/10=120x, and 1200/25=48x magnification.

But... what do those numbers actually mean in terms of what you'd see through the eye piece? I.e. at what magnification would you effectively fill your view with the moon?

If attaching a camera to the telescope, how do you calculate the effective size in mm of the projected image on the sensor? I.e. what would I use to get a 24mm projection of the moon on a full frame DSLR sensor?

I assume greater magnification ratios are working like an extender/teleconvertor with a camera lens - that is, they're just filling your view with a smaller (and greater magnified) region of the main mirror?

Just think of a scope as a big prime telephoto lens . Pretty much works the same way . Just depends on the scope and what kind it is . A refractor only has a glass lens at the front. but a camera lens has a front and rear lens . All scopes have an Eye Piece you view through at the exit. A cadioptric scope uses mirrors and glass to bounce light inside the tube until it reaches the exit where the eye piece is . A dobsonian works same as a cadioptric only the tube is longer to the exit . The eye piece determines the magnification power of the scope . Smaller the # of the eye piece the stronger the magnification is . The larger # of eye piece means lower power . Reason us cause some space objects do not require alot of power to see as to where objects that are smaller or farther away need more power to see farther distances . Camera lens are the smaller the wider FOV where as a scope eye piece is backwards , higher the mm the lower power it is and the higher or smaller the mm the more power (magnification) it is . Hopefully I’ve helped some in explaining .

sploo wrote in post #19001330
you have a main mirror diameter - e.g. 203mm (8"), a focal length - e.g. 1200mm, and then you use eye pieces to obtain a desired magnification.

The maximum practical magnification usually seems to be rated as twice the mirror diameter (e.g. x406 for the scope described above), and the actual magnification is the focal length divided by the eyepiece. E.g. with a 1200mm focal length, and 25mm and 10mm supplied eyepieces, you can get 1200/10=120x, and 1200/25=48x magnification.

So you put a camera mount adapter where the eyepiece would go...and it has no optics of its own, presumably. So you have 1200mm mounted on your camera.

Wilt wrote in post #19001527
So you put a camera mount adapter where the eyepiece would go...and it has no optics of its own, presumably. So you have 1200mm mounted on your camera.

That makes some sense, but...

Celestron wrote in post #19001439
Just think of a scope as a big prime telephoto lens . Pretty much works the same way . Just depends on the scope and what kind it is . A refractor only has a glass lens at the front. but a camera lens has a front and rear lens . All scopes have an Eye Piece you view through at the exit. A cadioptric scope uses mirrors and glass to bounce light inside the tube until it reaches the exit where the eye piece is . A dobsonian works same as a cadioptric only the tube is longer to the exit . The eye piece determines the magnification power of the scope . Smaller the # of the eye piece the stronger the magnification is . The larger # of eye piece means lower power . Reason us cause some space objects do not require alot of power to see as to where objects that are smaller or farther away need more power to see farther distances . Camera lens are the smaller the wider FOV where as a scope eye piece is backwards , higher the mm the lower power it is and the higher or smaller the mm the more power (magnification) it is . Hopefully I’ve helped some in explaining .

...I still don't understand what a particular magnification number actually means in practical terms. I.e. is "magnification" a consistent term for scopes - i.e. does x50 mean the same thing on an 8" scope with a 1200mm focal length as it does on a 6" scope with a 900mm focal length? What magnification would give me a view of the whole moon (i.e. filling my view, but not cropping any part of the moon)? Is the eyepiece irrelevant when using a camera - i.e. a 1200mm focal length scope is just that when used with a camera, as an eyepiece isn't used?

A bit of google research leads me to believe that "magnification" is about the size of objects in degrees; e.g. the moon has a size (from Earth) of around 0.5 degrees, so a 50x magnification would make it appear as 25 degrees.

On a full frame camera (if my dodgy math is right), a 2750mm focal length lens would have a vertical angle of view of about 0.5 degrees; so that fits with the 115 rule for the moon; 2750mm/115=24mm about the height of a full frame sensor.

I've found some references to high magnifications resulting in a darker image, so I guess it is as simple as a 1200mm focal length telescope being a 1200mm "lens", and the eyepiece is just magnifying a smaller region of the field of view. I see also that a larger (wider) eyepiece will obviously show a larger field of view, even though the magnification is still the ratio of the scope focal length to the eyepiece focal length. Makes sense... I think.

Maybe this will help explain what you are wanting to know as far as magnification of a scope . Not really sure if your wanting lens comparison to a scope size or just trying to figure out what size scope will produce a full moon size image on your FF camera ?

https://www.astronomic​s.com …ow-big-a-scope-do-i-need/

https://www.astronomic​s.com …ical-terms/magnification/

https://www.astronomic​s.com/info-library/

https://www.astronomic​s.com …ook-through-my-telescope/

Celestron wrote in post #19001938
Maybe this will help explain what you are wanting to know as far as magnification of a scope . Not really sure if your wanting lens comparison to a scope size or just trying to figure out what size scope will produce a full moon size image on your FF camera ?

https://www.astronomic​s.com …ow-big-a-scope-do-i-need/

https://www.astronomic​s.com …ical-terms/magnification/

https://www.astronomic​s.com/info-library/

https://www.astronomic​s.com …ook-through-my-telescope/

Very useful - thanks.

I'm essentially trying to answer the question "is it worth buying a telescope, and if so, what?". I have the Canon 100-400II, plus 2x and 1.4x extenders, so I can effectively make a 400mm f/5.6, a 560mm f/8, an 800mm f/11, and with both extenders stacked, an 1120mm f/16.

I'm more interested in astrophotography than observing, so I get the gist that a longer focal length/smaller aperture scope is the way to go (vs a shorter focal length/wider aperture).

Assuming I were to get a 1200mm f/5.9 scope (the Skywatcher 200, with a 203mm main mirror) I assume the main differences to my 1120mm f/16 lens setup would be greater light collection (so shorter exposures for dim objects), and probably sharper results (and I'd be able to crop a digital capture further than what I'd get away with on the lens + extenders)?

Am I right thinking that for astrophotography the eyepieces (and the magnification ratio you get with them) is irrelevant, or can you use an eyepiece (essentially acting like an extender) and optically enlarge and "crop" the image before it hits the camera?

PS I'm ignoring the obvious issue that an 8" telescope is going to need a much better tracking mount than the one I have for a camera + lens.

If your trying to get a closeup of the moon you’ll need a scope , if your just trying to get a image of the moon to fill the FF on your camera you’ll need a big lens and your extenders .

Let me give an example of what i do to get an image of the moon with my XSi 450D . Adapters must be used to attach the camera to a scope . Now my scope mount has tracking motors which is a necessity to get a good image . I use what is called a T-Adapter that replaces my camera lens with a T-Ring . Then when i view the moon through the view finder of my scope the moon just tightly fills the view enough that i usually take two images , one make space between the top of the moon and the top of the frame . Then i adjust the view so that there is a space between the bottom of the moon and bottom of the frame . Two images one with the top of the moon clipped and one with the bottom of the moon clipped . Then i stack the two images and it makes a whole moon with space at the bottom and top . This is because of the scope size but viewing through the camera viewer whatever you see will be the same as the images produced . Now if i went to a smaller scope , say a 6” SCT , that gives me a wider field of view which makes the moon smaller also then i would not need two images to stack to make one .

But if i want a closeup to see craters like when i use a high power eyepiece then i need a different adapter for my camera that holds an eyepiece in place that allows the camera to see the same view in my camera viewer for a close up image .

But since your not into astronomy if i were you just use a big lens on a tracking unit and get the best image you can then enlarge it to a point before pixelation makes your image distorted .

Astronomy.Tools

Everything you need in the FOV Calc. Click imaging mode. Just enter the metrics of the gear (focal length & pick a sensor or enter dimensions of your sensor). Pick a subject from the messier list or the solar system list to see the resulting FOV.

But before you get too hung up on focal length, you really need to undo your mind on the idea of "aperture" (terrestrial camera speak, this is commonly used, but really the f-number is the focal-ratio, not the aperture). The 200mm of your intended scope you're looking at is the aperture, 8", which defines how much surface area, light collection and potential actual resolution (based on angular resolution) can be achieved. The focal length relative to that aperture gives you y our focal-ratio, or f-number, such as F4 to F6 on such an instrument. If your goal is to image high resolution solar system subject matter, like the moon, a larger aperture (the size opening into the instrument) is where that resolution comes from, not the focal length. If you want to image deep space, focal-ratio is much more important for photography speed. You have to retrain your mind to start thinking differently than terrestrial photography where the idea of aperture is a misnomer of focal-ratio and that resolution is the sensor's pixel array, when it's not, in this subject matter. Really you'll find lots of misnomers and "slang" on a terrestrial camera based place, like this, and you'll find out its not what you think it is (this is fine, just preparing you to relearn the vocabulary and what things really are as it matters a lot and determines what you can, cannot do. Also consider, to do some things, you need good seeing (minimal atmospheric turbulence) and to image something at a fine image scale with a long exposure you need an incredibly robust mount to handle it all and good weather conditions, for hours at a time. It's one thing to use little camera lenses to image wide field, and completely different to image astrophotography with finer image scales. And the most important thing to have is a robust tracking mount. It starts there. That determines what you can possibly mount and image with. Prepare for a rather beasty budget too. Depends on your goal and expectations and how dedicated you want to get into this. You also need to consider light pollution and if you can even do what you want to do.

The more info about what you want to do, goals, overall, your sky quality, budget, etc, will help greatly in guiding you there.

There's a lot more to this, but it's all calculable. I highly recommend you come to an astrophotography based forum like CloudyNights or StarGazersLounge to get a lot more information before buying anything.

Very best,

Don't forget the telescope mount; arguably, for long exposure astrophotography, the mount is more important than the telescope.

The mount should be able to hold the telescope/camera rigidly.
The mount should also be able to auto-guide accurately, so that stars are nice and tight and round, not trailed like dashes.

Most Equatorial telescope mounts can track reasonably well; that is they have a motor(s) that compensated for the Earth's rotation, allowing the telescope to follow the object across the skies.

However, if you want to take photos at long focal lengths and with durations of several seconds to minutes, you will also need to Autoguide.

Autoguiding is where a separate Guide Camera is calibrated to monitor a guide star. If the guide star drifts off a pixel on the autoguider camera, the camera send a signal to the mount to nudge it back into the calibrated location.

Cheers

Dennis

Short answer to "is a scope worth it?"...Absolutely, especially depending on what kind of scope and what kind of mount you have. Is it necessary...not really. It depends on what you want to photograph. When I sold my astro gear I had around $50,000 worth of gear. Sadly I didn't have enough time with it all to get any amount of photography done with it, but such is life. That's a tale for another day.

Three images, first two done with a Sigma 150-600mm, first at 600mm and second with a 2xTC for 1200mm (both handheld), and third done with a Meade 12" LX200R (schmidt cassegrain telescope) with a 3048mm focal length...

IMAGE LINK: https://flic.kr/p/SBHz​tQ

IMAGE LINK: https://flic.kr/p/2gcb​ERP

IMAGE LINK: https://flic.kr/p/bBSY​d9

IMAGE LINK: https://flic.kr/p/n8Gq​Hk

Celestron wrote in post #19002045
But if i want a closeup to see craters like when i use a high power eyepiece then i need a different adapter for my camera that holds an eyepiece in place that allows the camera to see the same view in my camera viewer for a close up image .

That's a useful thing to understand - thanks.

MalVeauX wrote in post #19002054
Astronomy.Tools

Everything you need in the FOV Calc. Click imaging mode. Just enter the metrics of the gear (focal length & pick a sensor or enter dimensions of your sensor). Pick a subject from the messier list or the solar system list to see the resulting FOV.

But before you get too hung up on focal length, you really need to undo your mind on the idea of "aperture" (terrestrial camera speak, this is commonly used, but really the f-number is the focal-ratio, not the aperture). The 200mm of your intended scope you're looking at is the aperture, 8", which defines how much surface area, light collection and potential actual resolution (based on angular resolution) can be achieved. The focal length relative to that aperture gives you y our focal-ratio, or f-number, such as F4 to F6 on such an instrument. If your goal is to image high resolution solar system subject matter, like the moon, a larger aperture (the size opening into the instrument) is where that resolution comes from, not the focal length. If you want to image deep space, focal-ratio is much more important for photography speed. You have to retrain your mind to start thinking differently than terrestrial photography where the idea of aperture is a misnomer of focal-ratio and that resolution is the sensor's pixel array, when it's not, in this subject matter. Really you'll find lots of misnomers and "slang" on a terrestrial camera based place, like this, and you'll find out its not what you think it is (this is fine, just preparing you to relearn the vocabulary and what things really are as it matters a lot and determines what you can, cannot do. Also consider, to do some things, you need good seeing (minimal atmospheric turbulence) and to image something at a fine image scale with a long exposure you need an incredibly robust mount to handle it all and good weather conditions, for hours at a time. It's one thing to use little camera lenses to image wide field, and completely different to image astrophotography with finer image scales. And the most important thing to have is a robust tracking mount. It starts there. That determines what you can possibly mount and image with. Prepare for a rather beasty budget too. Depends on your goal and expectations and how dedicated you want to get into this. You also need to consider light pollution and if you can even do what you want to do.

The more info about what you want to do, goals, overall, your sky quality, budget, etc, will help greatly in guiding you there.

There's a lot more to this, but it's all calculable. I highly recommend you come to an astrophotography based forum like CloudyNights or StarGazersLounge to get a lot more information before buying anything.

Very best,

The concept of aperture (the mirror diameter) driving resolution was somewhat new, but became fairly clear when I saw that the maximum theoretical magnification ratio of a scope is usually stated as a multiple of the mirror diameter.

I assume that the use of the various focal length eye pieces are effectively magnifying a region of the main mirror - so effectively acting like a teleconverter on a camera lens?

So, a question - if I had the following two scopes...

1000mm focal length, 200mm mirror (f/5)
2000mm focal length, 200mm mirror (f/10)

...and used a 10mm eyepiece with the 1000mm scope, I'd get the same field of view (same magnification) as using a 20mm eyepiece with the 2000mm scope. But, what would be the benefits/down sides of choosing one of those scopes over the other (ignoring size and weight)?

I assume that, if the goal were to use a scope with a camera adaptor (no eyepieces) for deep space objects then the 2000mm scope would be preferable, but if used with eyepieces then both scopes would have the same theoretical max magnification as they have the same mirror diameter, right?

nardes wrote in post #19002088
Don't forget the telescope mount; arguably, for long exposure astrophotography, the mount is more important than the telescope.

katodog wrote in post #19002119
The problem is that the Earth rotates, and for longer exposures of the night sky you can't get pinpoint stars without compensating for that rotation. Instead of asking if a scope is needed, you should spend your time researching mounts. The mount is more important because you can mount anything to it, a scope, a camera, etc.. The better the mount, the longer your exposures can be, which translates to better night-sky photos. After you decide on a good quality mount, adding a scope(s) or camera will fall into place a lot easier.

Absolutely. I currently have one of the Skywatcher Star Adventurer mounts; which just about does ok with a DSLR and a moderately large lens. If I went to a scope I understand I'd need something much more substantial (especially for astrophotograpy).

Part of me is wondering about getting a dobsonian of some form, as I understand they're a good choice for general observation, then making a mount similar to those on http://www.equatorialp​latforms.com/ (I have motors, CNC, "bits" etc.)

sploo wrote in post #19003071
Absolutely. I currently have one of the Skywatcher Star Adventurer mounts; which just about does ok with a DSLR and a moderately large lens. If I went to a scope I understand I'd need something much more substantial (especially for astrophotograpy).

Part of me is wondering about getting a dobsonian of some form, as I understand they're a good choice for general observation, then making a mount similar to those on http://www.equatorialp​latforms.com/ (I have motors, CNC, "bits" etc.)

Dobsonians are great for viewing and cheaper way of viewing ! You can get a 10” or 12” dob under $1000.00 . A 10” SCT would cost upwards $3k plus you’ll need a heavy duty mount so could run you way over $3k by final payout . But dobs work on altizumuth mounts which go up and down and left to right . They don’t track for astro photography work because they don’t keep up with rotation of the stars . That’s where you need a Equatorial mount that specifically rotates with star rotation .

Celestron wrote in post #19003194
Dobsonians are great for viewing and cheaper way of viewing ! You can get a 10” or 12” dob under $1000.00 . A 10” SCT would cost upwards $3k plus you’ll need a heavy duty mount so could run you way over $3k by final payout . But dobs work on altizumuth mounts which go up and down and left to right . They don’t track for astro photography work because they don’t keep up with rotation of the stars . That’s where you need a Equatorial mount that specifically rotates with star rotation .

I was even considering just getting an OTA and making my own Dobsonian mount, but if you look at the http://www.equatorialp​latforms.com/ products it looks as though they have a tilting platform on which you can put a Dobsonian mounted scope (so you get tracking). The substantial Equatorial mounts (e.g. HEQ5 and upwards) are pricey, so my thinking was to make my own heavy duty tracking base, on which I could put a Dobsonian scope.

In theory (in theory meaning "I don't know, but I'm assuming) I could have the tracking mount located somewhere permanently; which should mean it could be polar aligned once and then always ready for a scope on top?

EDIT: Found https://en.wikipedia.o​rg/wiki/Poncet_Platfor​m