calculating focal length

[bird]

This topic comes up a lot, especially in this forum where barlows and powermates are used.

Here's a page that describes how to accurately calculate your effective focal length:

http://www.acquerra.com.au/astro/docs/focallength.html

regards, Bird

[davidpretorius]

thanks bird, spot on!

[iceman]

Great stuff Anthony, thanks.

A suggestion or two:

1) Include an example.

For example, Have one of your images of Jupiter, and include the measurements you used - where/how you measured the # of pixels in Jupiter (across the equator or from N-S pole?), what was the size of Jupiter in arcseconds when you recorded it, and where did you get this info from? Include some pixel sizes for some common cameras (Toucam, DMK, etc).

2) Update your homepage

Links to new articles, and updated images :)

[bird]

Thanks Mike, already done #2 yesterday :-)

I'll add an example to the page, and you're welcome to link to it.

.. and done #1 thismorning.

regards, Bird

[iceman]

Excellent, great, perfect!

Although I hadn't refreshed your homepage when I looked this morning (it was cached) which was why I suggested #2 :)

Great example. Thanks.

[Dennis]

Hi Anthony

Thanks for a very useful formula and the worked example – what were previously murky waters for me, are now crystal clear!

Cheers

Dennis

[iceman]

From my recent Jupiter image (2nd April), I calculated my focal length to be 10,028m (or f/33).

349 pixels across, 5.6 microns, 40.2 arcseconds.

Thanks for the formula and example!

[iceman]

btw Anthony I'd love to see the image scale of your Jupiter images with a camera with smaller pixels!!

[davidpretorius]

thought i would be a smart ass and make the image go from 650 to 1400...........even then the bloody thing hass mnore detail than my best this season...........................

WOW:lol:

[bird]

DP, if I was using a toucam or dmk camera with 5.6u pixels then the image should be 7.4/5.6 = 1.32 times the original size, ie Jupiter would be 461 pixels across the equator. It would only *just* fit inthe 640x480 ccd :-)

cheers, Bird

[davidpretorius]

i like my 1400...........call it rough as guts maths, but it looked awesome on my 24" monitor...........ie fill the screen!!!!

much more fun than actually calculating it properly :rofl:

plus you had a great chance to rub it in to us all..........."yes, young grass hoppers, one day you can come scratch the clouds of jupiter with uncle bird, but until then..... keep practising on your baby pixels" :lol:

[davidpretorius]

yup, the old GRS would fill up my 640 x 480 screen!

[iceman]

A sure sign of quality data when it looks that good at that size!

[davidpretorius]

scary isnt it!