sp900nc magnification?

[Bazzaar]

The term "magnification" is irrelevant in imaging.

Of course its relevant, how absurd!

The field of view or image scale changes with the size of the sensor,

The field of view or image scale changes with the focal length of an eyepiece, therefore they are the same. You have stated no distinction that disallows the use of the word magnification yet.

you don't start seeing smaller details in the image because a webcam gives a greater "magnification" than a DSLR at the same focal length.

You might do if the webcam pixel size was less than the DSLRs.[

If I image with my Skymax150 at F24 (focal length 3600mm) this would give me a "magnification" of 600x with a webcam, which is of course rubbish because the practical maximum magnification for visual use is about x300.

So what happens at magnifications above this "practical maximum magnification"? does it stay at x300? does it disappear? no you get a poor image at x600!! still magnifying.

If I then use a DSLR at the same focal length the "magnification" will then be less.

No, if you use ANY camera with a larger sensor the magnification will be less. Because the sensor size is analogous of ep focal length and magnification equals objective f/l divided by sensor size or ep focal length

It is the field of view that changes.

And all the objects therein and so are magnified

Using the term "equivalent field of view" is OK when comparing eyepieces with cameras but not "magnification" as this can lead to confusion.

I still dont see anything to be confused about

The formula shows where the idea focal ratio or planetary/lunar imaging of F20 to F32 comes from. Ideally you need at least a block of 2x2 pixels per 1 arcsec of resolution, but not more than 3x3 to get the best out of a telescope/camera combination. For planetary/lunar imaging this tends to be somewhere between F20 and F32.

Peter

I think this last paragraph is where you actually get to the crux of your argument. That there are optimum scope/camera combinations that need to be calculated to insure maximum information is gathered in as short a time as possible. And these are dependant on more aspects of a sensor than the overall dimension.

Nothing you have said discredits the simple truth that a camera sensor of size X produces the same magnification in a system as an eyepiece of focal length X.

Regards

Barry

[RogerTheDodger]

Cornelius (Peter) is right, but the term he was looking for is "apparent" magnification. It is relative to the focal length divided by the diagonal of the the sensor in mm - BUT only when the comparison is printed out at the same size - i.e. if the printout of the image obtained from an SPC900 through an 8" newt is printed out on 10x8" paper is compared to the same object imaged with a canon 1000d printed on the same size paper, the SPC image will "appear" to be at a higher magnification... it isn't, but it appears to be.

[Cornelius Varley]

Cornelius (Peter) is right, but the term he was looking for is "apparent" magnification. It is relative to the focal length divided by the diagonal of the the sensor in mm - BUT only when the comparison is printed out at the same size - i.e. if the printout of the image obtained from an SPC900 through an 8" newt is printed out on 10x8" paper is compared to the same object imaged with a canon 1000d printed on the same size paper, the SPC image will "appear" to be at a higher magnification... it isn't, but it appears to be.

Exactly. Image scale and magnification are not the same thing. If a webcam and a DSLR have the same pixel size then the field of view of the webcam is smaller, it is not more magnified than the image taken with a DSLR, just a smaller area.

If I image planets and the moon at F24 (which I always do because this gives the best arcsec/pixel ratio for my telescope/camera) I am not actually imaging at 600x. If I was to then image with a DSLR, that has a bigger sensor than a webcam, would I then be imaging at a lesser magnification ?. No I wouldn't. The arcsec/pixel ratio would stay nearly the same due to a slightly diiferent pixel size but I would would see any less detail on the final image, I would be image a greater area than the webcam with the same level of detail.

Visual magnifiication and imaging scale are not the same. When imaging it is quite possible to image at focal ratios and "magnifications", as you like to call it, beyond what would be capable visually because the normal rules on practical maximum magnification don't apply to imaging.

Peter

[Bazzaar]

Sorry Roger but you are actually confirming my argument, the definition of magnification is to make something appear to be a different size. And then you go on to state that this size difference is only true if you use the same ruler each time! you dont say!

Peter, I believe you are arguing from a point of view that pixel size of an object determines magnification, ie an object 20x20 pixels is magnified compared to one that is 10x10 pixels.

I'm arguing that this is not what this discussion is about at all.

If you displayed two pictures on a PC screen from two different cameras and one covered 10% of the screen and the other 20% of the screen, the 20% image is magnified more. Pixel dimensions, image scale arc seconds/pixel not relevant. You use the whole of the information from each camera and view them on the same screen. Magnification is the apparent size of an object. The OP asked why his webcam image of Saturn appeared bigger than when he viewed with a 25mm ep. The webcam sensor is 6mm and gives an equivalent image of a 6mm ep. The webcam gives apparent magnification.

Barry

[Cornelius Varley]

So what happens at magnifications above this "practical maximum magnification"? does it stay at x300? does it disappear? no you get a poor image at x600!! still magnifying.

Could you expand a bit more on this statement?

Peter

[Bazzaar]

Certainly Peter,

That was in response to this statement of yours;

If I image with my Skymax150 at F24 (focal length 3600mm) this would give me a "magnification" of 600x with a webcam, which is of course rubbish because the practical maximum magnification for visual use is about x300.

Ok, so we are above this rule of thumb figure of x300, what of it?

Barry

[Cornelius Varley]

Certainly Peter,

That was in response to this statement of yours;

If I image with my Skymax150 at F24 (focal length 3600mm) this would give me a "magnification" of 600x with a webcam, which is of course rubbish because the practical maximum magnification for visual use is about x300.

Ok, so we are above this rule of thumb figure of x300, what of it?

Barry

does it stay at x300? does it disappear? no you get a poor image at x600!! still magnifying.

The practical maximum magnification rule doesn't apply to imaging. Because there is no magnification when imaging, only image scale which defined as the area of sky or moon measured in arcseconds, it doesn't matter what the apparent "magnification" is.

The term "magnification" is only relevent to an optical system, ie one with a lens/mirror and an eyepiece. The area of the sky/moon may appear to be the same field of view as that given by a 6mm eyepiece at the same focal length, but you don't get a "poorer" image just an equivalent field of view. The image is not degraded by shooting at an equivalent field of view or magnification given by the 6mm eyepiece.

If I then use the telescope at the same F number and a camera that has an equivalent field of view of a 12mm eyepiece, would this give a better image than a camera with an equivalent field of view a 6mm eyepiece? The only thing that has changed is the image scale, ie the field of view measured in arcseconds. This larger image could then be cropped down to 640x480 and would look identical to the original webcam image.

In imaging it is field of view or image scale that is important, not magnification, because in imaging there is no true magnification.

I am quite happy to image at an apparent "magnification" of 600x because I know this is not a problem for the telescope and I know that at "600x" (F24) I will get a better image than at "300x" (F12). The arcsecond/pixel ratio is closer to the optimum value for my telescope at F24 than at F12.

Peter

[Bazzaar]

You are still stuck on the point that an image size is only defined by pixel hight and width, and that you can resize any image in post processing and hence have lost any dimensional references to determine resultant magnification. I understand that, I understand that all you can then refer to is the size of the object in angular dimensions.

It is still not the point of the answer to the OPs question.

The OPs question has been answered simply and accurately.

Regards

Barry