Can I collimate using flatfields

[blueskies123_89]

Hi,

Just wondering, am I right in saying that for a collimated scope, the brightest part of the image should be directly in the centre of the image.

The idea is that I am thinking of writing a program to check the location of the illumination of the field. To collimate, I would then try and centre the brightest part of the field. This might allow accurate collimation during the day, and lower the workload at night.

I really don't know if my thinking is correct, so if someone could enlighten me that would be great! Any other ideas also welcome.

Thanks
George

[Barrykgerdes]

Hi George

The flat field used in astro photography is a check on the sensitivity of the ccd over its entire surface and is used to correct the luminance of the exposure. It normally has nothing to do with collimation.

Correcting the collimation of the OTA is done to make sure that the focal plane of the light path is at the position where all parts of the image are in the best focus.

With a refractor telescope it is not normally done as most telescopes do not have facilities for adjustment although it is necessary for best results.

On a Newtonian or SCT It is very important and it is done in two stages. The first is to get the off focus image of the secondary mirror centred in the eyepiece. The second is to then do a very fine adjustment using diffraction rings caused by the secondary "obstruction" on a highly magnified star off focus to make them concentric.

This is a very brief description perhaps over simplified. Others may give you more detailed information.

Thierry Legault used to have has a very informative article on collimation on his web site. I think it is still there.

http://perso.club-internet.fr/legault

Barry

[blueskies123_89]

Does that mean that there is no correlation between the area of luminance and the collimation? My reasoning was that:
* the areas away from the centre are going to be of softer focus
* softer focus may lead to spread of of light?
* hence darker?

[Barrykgerdes]

If you want to collimate a telescope this reasoning will have no practical bearing.

I have corrected the link to Thierry's page. Have a look at what he says about collimation.

Barry

[blueskies123_89]

ok, thanks for the help and the link.

George