Quick note on eShel equipment and telescope focal

Following a question I received, here is an image of the fiber injection unit mounted on a C11 telescope with a 0.64x focal reducer and a Meade flip-flop mirror (used when I want to do some visual with the telescope & some friends!):

Distance from the back of the focal reducer to the front plate of the fiber injection unit is around 80mm. Adding 41mm for the distance from that front plate to the slit/mirror, the distance from the focal reducer to the slit is around 120mm (85mm is the nominal distance for that focal reducer). I'm then working around F/5...



Let's check this by using a long (20s) exposure of the field around CW Cep, with an Atik Titan camera (pixel size: 7.4µm):

Here is the field of view in Guide 9 starchart and the measured distance between the two marked stars: 365"

dX & dY for those two stars on the guiding image are dX=408 pixels & dY=388 pixels, giving a distance on the sensor of 4.17mm.

For a 280mm diameter telescope (C11), taking into account that the guiding port of the eShel fiber injection unit has an internal 1.5x barlow, this gives a true telescope focal at F=1570mm, so a F/ ratio of F/5.6.


Another way is to use Astrometry.Net: http://nova.astrometry.net/

I uploaded the image and obtained this result:

Then remember the formula:

telescope focal (mm) = 206 x pixel size (µm) / pixel scale("/pixel)

So in this case F = 206 * 7.4 / 0.664 = 2296mm, to divide by 1.5x to take into account the barlow inside the eShel fiber injection guiding port: F=1530mm, so F/5.5.