Prime focus magnification

[saturn5]

This may be a really stupid donkey question but how do you work out what magnification you have when you are taking piccies using prime focus.

[themos]

Pete Lawrence posted a formula. I am looking for it.

[themos]

http://stargazerslounge.com/index.php/topic,18009.msg187233.html#msg187233

You can work out the f-ratio used by the simple equation

F = (206.265 * Dp * Ps) / Pd

Here, Dp = diameter of the planet in pixels as measured off a capture frame

Ps = your camera's pixel size in microns

Pd = the Planets diameter in arcseconds

[themos]

sorry, that doesn't seem to be what you asked for. But I am sure it's close. Anybody help??!

[saturn5]

What i was after was what magnification i would get if i put a camera at prime focus onto my 1200mm focal lenght scope,not focal ratio,thanks anyway themos.

I have read that if you divide your scopes focal lenght by 50, that would give you your magnification,i have also read divide by 43 and 35 so am a bit confused.

[Albireo380]

The way I worked it out was:

50mm focal length seems to be about life size (x1 magnification)

So 1200mm focal length would be about x24

You also take into account any effect re the size of your DSLR chip (a x1.6 magnification on a Canon 300D, 350D or 400D)

So with one of them your 1200mm focal length prime focus exposure would be about x24 x1.6 = x38.

I tried this approach on my 2000mm focal length LX90 (prime focus x40 x1.6 = x64) and it seemed to be about right.

Tom

[steppenwolf]

The way to calculate this is as follows:-

Work out the diagonal measurement of your sensor in mm (The Square on the Hypotenuse etc). Divide the focal length of the telescope by this figure and this will give you a good approximation of the magnification.

For example, a Canon 300d has a sensor 22.7 x 15.1 The diagonal via Pythagoras is 27.26mm 1200/ 27.26 = 44 thus the magnification is X 44

If you tell me what camera you have, I'll work it out for you!

[cygnusx1]

Thanks chaps an interesting bit of info!

[saturn5]

Thanks Tom,Steppenwolf. I see where/how the variations in magnification are occuring.

Btw SW,my camera will be Canon 350d,not yet but very soon

[Kaptain Klevtsov]

This is a really awkward question to answer for several reasons.

1) If you put M31 on your camera chip and it makes an image 15mm across, the real life thing is 250,000 light years across, so it turns out that you have managed to unmagnify it tremendously.

2) Once you have worked out the corelation to a 50mm camera lens, which is said to give 1:1 magnification (for 35mm film cameras at least), you then have to factor in the amount of enlargement that you give the image. A 1:1 view of a 6MP camera would be half as big, roughly, as a 10MP image when you see it on your screen at 1:1 resolution. Both these DSLR sizes exist and have the same size sensor.

3) How big is your screen? And its resolution? A 19" monitor set at 800x600 would show the image much bigger than a 15" monitor set at 1280x1024. My 'phone has something like 800x600 resolution and a 3" ish screen, so stuff looks small on it.

The best way to resolve this is to work on arc seconds per pixel resolution, which you work out using your pixel size and focal length of the 'scope.

Made it worse now, haven't I?

Kaptain Klevtsov

[steppenwolf]

Well thanks a bunch KK, we thought we were going just fine You make a valid (several valid) point(s) but you STILL haven't allowed for the magnification that might be imparted by a spectacle wearer ......

[Kaptain Klevtsov]

Ok, ok, so I skipped a few. I've not been fed yet is my excuse. And don't sit too close to the monitor or that will effect it as well.

Kaptain Klevtsov

[saturn5]

This is a really awkward question to answer for several reasons.

1) If you put M31 on your camera chip and it makes an image 15mm across, the real life thing is 250,000 light years across, so it turns out that you have managed to unmagnify it tremendously.

2) Once you have worked out the corelation to a 50mm camera lens, which is said to give 1:1 magnification (for 35mm film cameras at least), you then have to factor in the amount of enlargement that you give the image. A 1:1 view of a 6MP camera would be half as big, roughly, as a 10MP image when you see it on your screen at 1:1 resolution. Both these DSLR sizes exist and have the same size sensor.

3) How big is your screen? And its resolution? A 19" monitor set at 800x600 would show the image much bigger than a 15" monitor set at 1280x1024. My 'phone has something like 800x600 resolution and a 3" ish screen, so stuff looks small on it.

The best way to resolve this is to work on arc seconds per pixel resolution, which you work out using your pixel size and focal length of the 'scope.

Made it worse now, haven't I?

Kaptain Klevtsov

Yes KK, ever so slightly I've decided that the magnification will be around the 40 mark ,otherwise my brain will fry trying to work it out :shaking2:

[Albireo380]

The way to calculate this is as follows:-

Work out the diagonal measurement of your sensor in mm (The Square on the Hypotenuse etc). Divide the focal length of the telescope by this figure and this will give you a good approximation of the magnification.

For example, a Canon 300d has a sensor 22.7 x 15.1 The diagonal via Pythagoras is 27.26mm 1200/ 27.26 = 44 thus the magnification is X 44

If you tell me what camera you have, I'll work it out for you!

Interesting, SteppenWolf ... I didn't know that. At least my "cack-handed" method was reasonably close.

Using your method the prime focus magnification on my 2000mm f/l LX90 would be: 2000 / 27.26 = 73.

I must admit, I am surprised at the result, as I really felt it wasnt that big a magnification, but I bow to your superior knowledge. :insects1:

Tom

[steppenwolf]

but I bow to your superior knowledge.

I wouldn't, I could be wrong (it happened once) NO, seriously, I believe that this is the correct approximation method to use as it was a repeatable calculation for any camera and was the response that I got confirmed most when I asked the same question a couple of years ago on several forums.

To put this into some kind of perspective, a standard 'film SLR' has a frame size of 36 x 24mm thus a diagonal of 43.25mm. A 'standard' 50mm lens (this is what they used to supply with every SLR) gave an approximate lifesize image (actually about 10% larger than real life) through the reflex finder so using this method of calculation and a focal length of 50mm, we get a magnification of X 1.15 - i.e. 15% greater than real life - so we are roughly 5% out - and 5% difference in magnification is not enough to detect by eye through a telescope EP.

Never mind the bullsh1t, here comes Steppenwolf .....