Calculating arc-seconds per pixel ....

[daz]

I thought it about time I put my brain in gear and worked some sums through! So, I want to calculate the arc-seconds per pixel a given camera is going to see.

So, how do I calculate the angular size of an object?

and then how do I convert that into pixel size?

Ta!

[Kaptain Klevtsov]

Quite simply you draw a right angled triangle with a long flat base which is the focal length of the 'scope, the height is a pixel, then the thin pointy angle is arc seconds per pixel.

1000mm FL and 7 micron pixel example

ATAN (7E-3 / 1000) = 0.00040107 degrees

degrees X 3600 = arcseconds = 1.444 arcseconds per pixel

HTH

Captain Chaos

[daz]

It most certainly does Cap'n!!

So, if my f/l is 1200, and pixel size is 5.6 microns, I get:

0.962569096 arc-seconds per pixel

Which, on an array of 640*480 pixels give me 616.04 * 462.03 arc-seconds FOV.

Saturn is approx 20.13 seconds of arc, so I could fit approx 30 vertically in my FOV.

Increasing the focal length (2x barlow) gives me approx 308*231 arc-seconds FOV, and I get 15 Saturns.

Have I understood correctly??

[Kaptain Klevtsov]

Think so!

Captain Chaos

[daz]

[MartinB]

Sounds complicated!! Try this: -

Sampling in arcseconds = (206.265 / (focal length in mm) ) x (pixel size in microns)

[daz]

Thanks Martin

[lunator]

Daz

You do realise that knowing this detail will allow you to measure faint double stars?

Something you should consider

[daz]

I did actually.

I must admit, I never did work through the equations you posted some time ago. Time just seems to slip by!

[lunator]

Daz

with a CCD you dont need to work much out. There is software avaialable that will do all the measurements for you.

Most double star observers/measurers work this way now.

I am old fashioned using visual methods.

Although the eye beats technology when it comes to tracking down the neglected doubles.

Cheers

Ian