Determining Focal Reduction

[niteman1946]

Hi Group,

I use an Atik 383L+mono camera mounted to a  12"LX200 Classic scope.  With the stackup of camera, filter wheel, OAG, and focuser, I suspect my focal reduction is somewhat greater than the native F10.

Is there a way in PI to solve for the reduction (possibly using plate solve?).  And if so, how do I get there?

Thanks for your help,

Mark

[Geoff]

The ImageSolver script will give you the focal length. Just look in the process console after applying the script or check the fits header file.
Geoff

[niteman1946]

Hi Geoff,
Thanks for the response.

I tried a couple of images of IC5146, Cocoon Nebula and got curious results. 
a) IC5146_F10 returned a focal length of 4391mm (see attached image)
b) IC5146_F6.3 returned a focal length of 2766mm

The F10 version should be the native focal reduction of the 12"LX200 (i.e. F10).  The F6.3 version was shot with the Meade f/6.3 reducer/corrector in the light path.  Both fit files are in the attached DropBox link.
So I do not understand why either of them are so high.  More than likely I am not using the tool correctly.

It is interesting that the ratio of the shorter to longer focal lengths is 0.63.

If you are so inclined, I wouldn't mind if you gave these two images a go.  And see what you come up with.
My goal is to be able to state accurately what actual focal reduction both of these images were shot at.

Thanks,

Mark


https://www.dropbox.com/sh/lypvmqu6x0dttrn/AE13mZANNQ

[oldwexi]

Hi Mark!
When you change the PixelSize to the correct value of 5.4 for the Atik383L
you get for the F10 Image 3203 mm focal length which could be correct
depending how far away you have positioned the camera.

The result will be more precise if you use a single uncropped Image for the measurement.

Gerald

[niteman1946]

Hi Mark!
When you change the PixelSize to the correct value of 5.4 for the Atik383L
you get for the F10 Image 3203 mm focal length which could be correct
depending how far away you have positioned the camera.

The result will be more precise if you use a single uncropped Image for the measurement.

Gerald

Gerald,

Thanks for the input.  Almost too easy!

Here are the focal lengths and reductions using the correct 5.4 pixel size:
F10 Image:  3203mm, and therefore f10.51
F6.3 Image:  2009mm, and therefore f6.59

BTW, the DropBox images are individual uncropped subs.

Thanks again,

Mark

[oldwexi]

Hi Mark!
i am happy that i could help.

Loading your dropbox F10-file and checking it in PI with   File/FITS Header says:
Image is stacked (integrated) with PI 1.8 RC7 using,
average, additive + scaling, linearFit, 7 Images etc etc...

Second the Atik383L mono raw Image size is
Horizontal Resolution: 3362 pixels
Vertical Resolution: 2504 pixels
Your Image has:
3354 by 2529(!) pixel

Also, when you stretch your image with STF you can see the black borders on the  left and above
which where created after Registration by stacking.

So from my point of view measurement on this stacked Image does not
really result in precise data, you have to use a single raw image.

Gerald

[niteman1946]

Gerald,

I should have never doubted you.  I stand corrected.  A senior moment.

FWIW, using a sub instead of the integrated F10 image produces very close to the same results.

Thanks,

Mark