Orion Nebula - recommended Focal Length

[Alveprinsen]

What would you guys say is the ideal focal length for viewing and photography of the Orion Nebula?

I got the choice between 500mm 1350mm and 2700mm ....

I wish there were a sort of focal length calculator out there that would give you some idea of "how much would show" with different focal lengths....

Alveprinsen.

[swag72]

Would this help? http://www.12dstring.me.uk/fov.htm

Or google CCDCalc and download the free fov calculator, with images - It's a good programme.

[Alveprinsen]

Would this help? http://www.12dstring.me.uk/fov.htm

Or google CCDCalc and download the free fov calculator, with images - It's a good programme.

Hell Yeah!

This is exactly what I've been looking for.

I might as well screw on the focal reducer right away. Thanks!

[stem1989]

this link is awesome! thanks very much... favourites

[springbok]

Thanks for the link, very useful.

[Steve Ward]

You can also feed your scope and sensor details into the Ocular Plug-in in Stellarium ( as well as eyepiece details) to show the same thing.

Steve.

[ollypenrice]

These tools are vital. I model my imaging projects in SkyMap Pro but I presume they all work along similar lines.

One thing, though; you'd need a hell of a big chip to image M42 at 2.7 metres!!

The outlying parts, not shown on the palanetaria, are also nice if you have a dark site, so a widefield approach can be rewarding. 328mm gave me this version.

http://ollypenrice.s...IDE 2FLs-X3.jpg

Olly

[JamesF]

Image scale is 206265 / focal length in arcseconds per mm. If you know the size of the target in arcseconds, dividing by the figure for image scale should give you the size it appears at the image plane. If it's too big to fit on your sensor chip, you need less focal length

It's a bit more tricky with a Mak or SCT because as you move the mirror to focus, the focal length changes slightly. If I put a DSLR on the back of my 127 Mak then the Sun will just fit on the chip. If I put a 40mm extension between the camera and the visual back then the image is way too big to fit by the time I've moved the mirror to get it in focus.

As an example though working with something I'm familiar with and happen to know the number for, at 1500mm the image scale for the 127 Mak is 206265 / 1500 = 137.5 arcseconds per mm. Jupiter is currently about 48 arcseconds wide, which means it will be about 48 / 137.5 = 0.35 mm on the image sensor of my camera. However, I use a 2.5x barlow and 40mm extension that pushes the image scale up by a factor of (my best estimate) 3.3, so I'd end up with an image about 1.1mm across. The pixel size on the camera (SPC900) is 5.6um, or 0.0056mm, so the image will be 1.1 / 0.0056 = 196 pixels across. That is indeed pretty much what it turns out to be, certainly within the margin of error of not knowing exactly what the focal length is at any specific mirror position and without being able to exactly define the increase in size given by the extension.

James

[JamesF]

I've just looked up M42 in Wikipedia, which claims it is about one degree by one degree (close enough for what I'm about to do . That's 3600 arcseconds square.

Say you want to fit all of that on an APS-C sensor 15mm high and you'd like a bit of room each side, so you actually want 5000 arcseconds across the height of the sensor chip. 5000" in 15mm is 333" per mm. Therefore you'd need a focal length of 206265 / 333 = 619mm.

You might be wondering where this strange figure 206265 comes from. It's actually the number of arcseconds in one radian. It's a relatively simple piece of trigonometry to show that plate scale is 1 / focal length in radians per mm using the approximation that tan(A) = A for very small sizes of angle A (which is pretty much always the case in astronomy) when measured in radians.

Now let's just hope I've done my maths right

James

[petercoxphoto]

I use SkySafari Pro on the mac. The field of view calculator here: http://www.skyatnightmagazine.com/field-view-calculator is very handy. Plug in scope and camera details and it'll give you the field of view of the camera. SkySafari allows this to be defined as the camera field of view and this can then be overlaid on the map, showing you exactly what will and won't fit.

Cheers,

Peter