OTA with same focal ratio

[Glider]

Can anyone answer this?

If I have an 80mm refractor, focal length 320mm that's a focal ratio of 5. So that's a fast scope.

If I have a 200mm reflector, focal length 1000mm that's also a focal ratio of 5.

If I image the same object for the same time will individual subs from each have the same brightness?

Or will the refractor loose out by a factor (80/200)^2?

[shaunster]

Not sure on your exact question but imaging through the refleector will produce a much smaller FOV than using the refractor at these focal lenghts

[Whippy]

This is one of those questions that can cause arguments!!

IMO, given that both scopes are taking the same image with the same kit at the same time under perfect conditions, then yes both of the images would have the same brightness as they have the same focal ratio (ie: speed). Some people would argue that the reflector's images would be brighter due to the larger aperture. As Shaunster says, the 8" reflector image would show a much smaller FOV due to the increased focal length.

The conditions do affect scopes with longer focal lengths and it's said, scopes with central obstructions. I can't say that for sure as I only use refractors for imaging.

BTW, an 80mm scope @f5 would have a f/l of 400mm .

Tony..

[andrew s]

Glider - The image intensity for extended objects will be the same bur the larger scope will give a higher intensity for point objects (e.g. stars) The image scale will be larger for the larger scope and as this spreads the light from extended objects out ,that is why you get the same intensity as the smaller scope. Andrew

[Glider]

So, if I'm imaging say M1 I might just as well use the refractor as the reflector given I have enough camera pixels to be able to crop the smaller refractor image?

[James4]

Hi Glider, your refractor is F4 (320/80) while the reflector is F5 (1000/200). The image from the refractor will appear brighter (photographically only). However, the reflector image will appear almost 3 times as large. The typical applications here are:

Use the small fast refractor for imaging wide field shots like The Orion Nebula and the Running Man in one image. Use the larger (still fast) reflector for imaging galaxies like M81 and the like (gives a larger image scale). There's one more thing. For a given aperture the resolution is fixed (regardless of F/ratio). The 200mm reflector has a much higher resolution (.56 arc seconds) than the 80mm refractor (1.45 arc seconds). Resolution is the ability to show fine detail. Hope this helps

[ollypenrice]

I agree with James and Andrew. If the f ratio is the same then get the object as large as you can on the chip. This means choosing the right focal length. Unless of course you want a lot of sky around your object, which you might. But M1 is pretty small even at a metre of FL so use the reflector. It would be miniscule at 320mm. But are you sure it is 80/320? What is it? Sounds awful fast to me. My FSQ can be 85/328 using the reducer but that gets it down to f3.9 which is pretty unusual, hence the price of the darned thing!

Olly

[ncjunk]

For a given aperture the resolution is fixed (regardless of F/ratio). The 200mm reflector has a much higher resolution (.56 arc seconds) than the 80mm refractor (1.45 arc seconds). Resolution is the ability to show fine detail. Hope this helps

i am confused with this point as i thought the resolution was linked to focal length. In the calculations i have it is the focal length and pixel size that determine resolution and not aperture...i am quite happy to stand in the corner facing the wall in silence if i am wrong though!

[ollypenrice]

I think resolution is a confusing term because there are several 'resolutions' to consider here, as it seems to me!

If a given subject is imaged on a chip of size 'x' then the smaller the pixels on the chip the higher the resolution the chip can offer.

If the same subject is imaged on the same chip but at a longer focal length the image will be at a higher resolution but less of the object will fit on the chip.

If the same object is imaged on the same chip and at the same focal length but with different apertures then the larger aperture will increase the resolution.

And finally the medium of presentation has an effect on resolution as well. More dots per inch or a better screen will add to resolution. Likewise a bigger presentation of the image.

Any one of these can in principle, place an upper limit on all of the others. A small printed version of an image in a newspaper will be limited entirely by the print resolution, so the difference between the Hubble M31 and M31 done in an ED80 would probably not show. Blowing up a webcam image of Saturn to a four square metre poster will simply exceed any meaningful resolution in the image. Coarse pixels can make finer optical resolution pointless. Smaller pixels will be lost on inadequate optical resolution. (This is what sampling rate is all about. If you are feeling brave, see http://www.cloudynights.com/item.php?item_id=2394) Seeing and guiding impose their own limits on resolution.

Resolution is a confusing term because it applies to so many parts of the image making process.

Olly

EDIT PS: my Saturn example would probably not apply to laws-of-physics-defying Mr Damian Peach!!

[Whippy]

I think James is mistaking the visual and imaging terms of resolution. A larger aperture visually can resolve more detail at the eyepiece as opposed to a camera where pixel size and focal length are the numbers to consider.

Tony..

[ncjunk]

Olly i dont entirely agree with you but i'll either start another thread or pm you for a discussion as its quite interesting but a bit off topic.

Scratch that- your correct but it was the newspaper one that threw me as i dont think its relavent...(only as a description of what resolution is)

Like a lot of technical docs the problem is you have to be specific and explain all the assumptions and conditions before making a statement .

I tend to view resolution in terms of arc sec per pixel and then length of subs as providing depth, which is also resolution in a way(as you state)

...i am waffling...sorry...

[ollypenrice]

I think James is mistaking the visual and imaging terms of resolution. A larger aperture visually can resolve more detail at the eyepiece as opposed to a camera where pixel size and focal length are the numbers to consider.

Tony..

Hmmm... A larger aperture telescope of a given focal length can produce smaller spot sizes than a smaller aperture instrument of the same focal length so would this not improve resolution, assuming the pixel array could distinguish between the two? Genuine question, I don't know - but I can't on the face of it see why not.

Olly

[ncjunk]

What is the definition of resolution, that is the problem. I could argue that the two telescopes of differing aperture have the same resolution from the point of view of arc sec per pixel. The only defining factor then is depth of image which to equal out would require a longer sub in the smaller aperture than the larger. I am stating depth of image as being another term different to resolution and i suppose this is where the F number comes into it. I can see why you would also define it as resolution but for me resolution has alway been a quantifiable number based on physical dimensions, as per the newspaper print 300dpi.

Thats my thoughts hope its not too much waffle..

[ollypenrice]

What is the definition of resolution, that is the problem. I could argue that the two telescopes of differing aperture have the same resolution from the point of view of arc sec per pixel. The only defining factor then is depth of image which to equal out would require a longer sub in the smaller aperture than the larger. I am stating depth of image as being another term different to resolution and i suppose this is where the F number comes into it. I can see why you would also define it as resolution but for me resolution has alway been a quantifiable number based on physical dimensions, as per the newspaper print 300dpi.

Thats my thoughts hope its not too much waffle..

Resolution is fairly easy to define outside the context of imaging, or even within it in global terms. How about, 'The ability to distinguish detail on small scales.'

But all sorts of things determine this ability. Seeing, guiding, optical resolution, pixel resolution, print and screen resolution. One definition doesn't really fit all as far as I can see. I have often thought this but never dscussed it before. Interesting.

Olly

[RobH]

All other things being equal, a larger telescope will have the ability to show finer detail, in other words, have greater resolution.

Here's an interesting article about the subject.....

the astroscopic labs - article: The Resolution of a Telescope - Dawes, Rayleigh and Sparrow

[ncjunk]

So we are defining resolution in terms of image definition and quality. Perfectly resonable. I tend to think of the calculated resolution and then worry about the actual image resolution, as in quality or depth, at a later date as that depends on atmospheric conditions.

I have read that 0.5 to 1.5 arc seconds per pixel is the optimum resolution.

I was reading this earlier as well-

CCD pixel size

I still dont quite like the use of resolution for both calculated equipement resolution and image resolution (as in final image quality).

For me one contains additional parameters that affect the end result and the other is a calculated value of resolution (like the cars mpg...if you drive like a nutter or overload the car the mpg will fall but the theoretical ideal doesnt change)

Btw good link Rob, very interesting

[Whippy]

Hmmm... A larger aperture telescope of a given focal length can produce smaller spot sizes than a smaller aperture instrument of the same focal length so would this not improve resolution, assuming the pixel array could distinguish between the two? Genuine question, I don't know - but I can't on the face of it see why not.

See, I don't think that's the case Olly. Would a larger scope with the same focal length as a smaller scope resolve more of a cluster given both have the same camera? IMO, if anything I think the larger aperure scope would struggle more if the conditions are less than perfect, which they quite often are! And how do reducers come into this equation? Are they redundant? I've also read that scopes with central obstructions are more affected by conditions too but that's only what I've read, not from own experience. I've certainly noticed that even my ED120, which isn't a large scope by most standards doesn't show it's best in less than good conditions.

Tony..

[ollypenrice]

See, I don't think that's the case Olly. Would a larger scope with the same focal length as a smaller scope resolve more of a cluster given both have the same camera? IMO, if anything I think the larger aperure scope would struggle more if the conditions are less than perfect, which they quite often are! And how do reducers come into this equation? Are they redundant? I've also read that scopes with central obstructions are more affected by conditions too but that's only what I've read, not from own experience. I've certainly noticed that even my ED120, which isn't a large scope by most standards doesn't show it's best in less than good conditions.

Tony..

Hi Tony, I think it would. Take an extreme example of two scopes of 1 metre FL, one of 10mm aperture (!!) and one of 500 mm aperture. They will not produce images of equivalent resolution even if they both resolve to their Dawes Limit and share a focal length. Why not? Because the optical resolution of the small one (which can be expressed in arcsecs per mm on the chip) does not meet the potential resolution of chip measured in pixels per mm. The pixels are, if you like, smaller than the resolution of the optics and could record finer detail if the optics delivered it.

When agonizing between TAK Epsilon and Tak FSQ, which have similar focal lengths but very different aperture (the Epsilon almost double the Baby Q) I was aware that the Epsilon would produce smaller spot sizes than the refractor. It visibly does, too, if you compare images. On the other hand it has diffraction spikes which personally I don't like and which, in effect, increase the size of small stars, at least in terms of their 'visual impact.'

But that is another story.

In terms of beating the seeing, I agree with you based on experience, though I have no knowledge of the theory. Small aperture seems less affected than large. I did read something about it but can't remember where.

Olly

[Whippy]

Fair enough Olly, and I'd agree with you in your analogy but I think this is where the physics might be right but in the real world it's not likely to happen finding two scopes like that is it? I'm not also certain how many times when you'd actually find the conditions good enough (certainly in this country anyway!) where the resolution of your setup would beat the seeing and where does oversampling comes into the equation?

FWIW, I read somehwere that imagine the atmosphere is like a collection of cells, all of differing sizes. A smaller scope wouldn't cross one or more of these cells while a larger scope would, therefore losing it's advantage. That's how I understand it anyway!

Tony..

[ollypenrice]

I know what you mean, Tony, but I will admit that an Epsilon at about 500mm FL can out resolve my FSQ at about the same FL. But in the real world I went for the FSQ. I prefer the pic from the refractor myself, but it is personal. Your description of seeing and aperture is pretty much how I remember reading about it myself. I bet there is something it.

I think the limitations of seeing impose themselves at rather longer FLs. I would guess at about 1.5 metres or so? Don't really know, and it depends on how bad it is, of course. Active optics systems seem to me (looking at the results of others) to make a huge difference to images from mid to large SCTs. Don't buy a C14 without an AO system, especially since you can obviously afford it!!

Glider, we are hijacking your thread but not intentionally! Sorry.

Olly

[philj]

Interesting question Noel and a good thread. This is a subject that I have been trying to figure out for some time. FL and apeture is the thing, I cant speak for reflectors much but I have 2 imaging refractors both F7. If I want wideish fov I use my 102mm but if I want to go for detail in M1 I use the 127 at F7 fl. This gives a much nicer image scale and more detail BUT it takes longer to get it. Swings and roundabobuts Im afraid:D

philj

[Glider]

So the larger 127mm takes longer to collect the image than the smaller 102mm. This is what I'm wondering. If you don't want the 'detail' then the 102mm does it quicker. It's difficult because I'm giving points for quick rather than good if you see what I mean.

If you like this thread have a crack at the one below called 'ISO'.

[ollypenrice]

Interesting question Noel and a good thread. This is a subject that I have been trying to figure out for some time. FL and apeture is the thing, I cant speak for reflectors much but I have 2 imaging refractors both F7. If I want wideish fov I use my 102mm but if I want to go for detail in M1 I use the 127 at F7 fl. This gives a much nicer image scale and more detail BUT it takes longer to get it. Swings and roundabobuts Im afraid:D

philj

If they are both f7 I don't see why one should really be faster than the other. What I suspect is that an image quality which looks acceptable when the feature of interest is small does not pass muster when seen in the greater detail of longer FL. Look what happens when you zoom out from 100 percent on your screen. The 'quality' seems to improve. I think this arises from two things;

- the eye can't see the defects when they are smaller.

- the defects (noise) fall below the resolution of the screen.

Very little in your picture is smaller in scale than noise, which is why gentle smoothing can be got away with.

If you want your image to stand up at 100 percent you seem to need about twice the data required to enable it to look noise free at 50 percent. I have plucked those numbers out of the air really based on gut instinct. (I did two M42s for my colour v mono article in Astronomy Now. Then I combined them just for myself. The effect was mainly visible at 100 percent. It was worth having but made little difference when viewed at a smaller size.)

So I think f7 is f7 but object scale is the active ingredient here.

Olly

[Macavity]

So I think f7 is f7 but object scale is the active ingredient here.

I get that impression too. And it depends on the nature of the object. Is it a "point" source (stellar), or an extended source (nebula, planetary) etc. I guess some (most?) astro-objects fall between these extremes? Limiting magnitude seems relatively easy to understand, beyond that, it may be something of an experimental (subjective) science?

If I draw hope, it is that smaller aperture and slower scopes ("odd" chip and pixel size) etc. need not EXCLUDE one from "having a go" at astro-photography. And most things can be "tweaked"? But then, I'm mostly waiting on the arrival of my run-off observatory, and a few(!) clear nights, to test this (or any!) hypothesis...

[ollypenrice]

Well, you are not going to be worrying about noise on stars so the critical stuff is the faint stuff at the bottom of the brightness range. That is what needs all the time if it is to look good at 100 percent. Brown dust is the worst of all, as in M78, around the Coccoon and Iris and in so many VdB objects. You get the main image in no time and then need to treble it to get a bit of murky brown! Grrr....

Olly