Orion from Riberac

[MarkS]

You already know I had problems with the CCD misting up:
http://forum.orpington-astronomy.org.uk/index.php?topic=5467.0

I got 7 reasonable exposures of 10minutes each and I've finally processed them.  
Using a peltier-cooled and Ha modified Canon 350D using 50mm lens at F8 with CLS and IR filters.

So here it is, warts and all.
1) I used F8 to reduce the lens distortions.  This was far too slow an F-ratio to use.
2) The blurry stars in the corners were caused by the CCD being slightly out of alignment
3)  Each bright star has a multicoloured "spaceship" to its right - caused by internal reflections.

The stunning diffraction spikes are caused by the 7-bladed iris.



2/3 scaled version here:
http://gallery.orpington-astronomy.org.uk/albums/userpics/10046/orion19112009.jpg

Mark

[RobertM]

Considering all the problems you had I think that's come out rather well.  The star colours are really good, it's a lot cleaner than my uncooled version and far less 'red'.  I see you've got the outline of the witch head too, not bad for the equivalent of a 6mm f8 scope!

Robert

[Mac]

Very nice.

The high res one is fantastic.

[Fay]

So many star colours, amazing the field of view!!!!!

[Mac]

not bad for the equivalent of a 6mm f8 scope!

?

I thought it would be equivalent to a 50mm scope?

The 50mm Lens has a focal length of 50mm (it doesn't change)

Its a finsihed lens, changing the f stop wont change the focal length, as all you are doing
is stopping down the amount of light, and has been stopped down to f8 from probably f1.8

The focal length on the scope is different. The size of the object lens multiplied by the f stop gives us the focal length.
My meade is 255mm * f10 = 2550mm focal length. (the f stop on the scope is not fixed unlike the lens).

If i then stick a focal reducer on, f3.3, it then becomes 255 * 3.3 = 841mm f3 lens. If i then stop this down to f10 (restricting the light to increase the depth of field  it doesn't  go back up to a focal length of 2550 again
it stays as the 841mm f3.3 but ive restricted the amount of light)

Field of view on a 50mm lens with a correction of 1.6 for canon DSLR
25 deg H* 17 Deg V. which looking at the image is about right.

A 6mm lens would have a field of view 123 * 102 degs. (assuming not fish eye)

and the meade on the 350DSLR is about 0.5 * 0.3 which again is about right.
as the full moon is just over a full frame. 29.3′ – 34.1′

Sorry for the Hijack.

Mac.

[RobertM]

Hi Mac,

I think it's all about nomenclature.  Telescopes are refered to as objective diameter plus focal ratio (6mm f8) whereas camera lenses are refered to as focal length plus focal ratio (50mm f8).  Could be I wasn't clear or you misinterpreted what I wrote but in any case this is my explanation.

With camera lenses the iris stops down the aperture.  The more it's stopped down the less the lens diameter is used. i.e. for a 50mm f2 lens the used lens diameter will be 50/2 = 25mm and at f8 50/8 = 6.25mm.  So a 6.25mm f8 objective (telescope nomenclature) will have the same focal length of 50mm.  Because the focal length is unchanged so so is the field of view.

The field of view (for a rectilinear lens) is calculated by the following formula :

    FOV  =  2 * arctan (sensor size/(focal length * 2))

A 6mm (objective diameter) f8 scope has a focal length of 50mm so is exactly the same FOV as a 50mm (focal length) camera lens stopped down to f8.  Which, I think is what we both meant.

Robert