Starlight xpress filterwheel filter distance

[meade expert]

Hi All

Can anyone please tell me the filter distance a filter in a starlight xpress 7 position filter wheel would be from the ccd end of the filterwheel?

I keep coming up with approx. 30mm provided the camera (17.5mm back focus) is mounted flush to filter wheel body and not through T48mm threads)

I'm trying to work out if a 1.25mm filter would vignette on a aps-c size sensor using it on a F4 scope.

Does anyone know of an online calculator that can work this out?

Cheers

Joe:)

[billdan]

Hi Joe,

The general consensus is that 1.25" filters will not provide full illumination for an APS-C chip, so you will get some vignetting.

I found this thread from Stargazers lounge that may be helpful :-

You can calculate the minimum filter clear aperture required for full illumination using the formula:
minimum filter size = CCDdiag*(FL-DC)/FL + (DC/f), where FL is focal length, CCDdiag is the size of the chip diagonal, DC is the distance between the chip and the filter, and f is the focal ratio of your scope

Be aware that the minimum size here is the clear aperture not the overall filter size - a typical 1.25" filter has a clear aperture of around 28mm depending on manufacturer whereas the 36mm unmounted Baaders have a clear aperture of nearly 36mm



Cheers
Bill