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Old 15-11-2021, 09:23 AM
bluesilver (Peter)
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Join Date: Jul 2018
Location: Australia, Tasmania
Posts: 317
Working out back focus?

Hi, I am in a little bit of a muddle with figuring out back focus,

I understand what it all means in general, but i kind of have 3 different numbers here and just hoping someone might be able to help me sort this simple issue out please.

My scope is a Skywatcher Esprit 150, and it has the Field flattener installed.
So on page 8 of my paper book manual that comes with the scope, it gives me two figures of back focus for this scope.

This is exactly how it is worded:
3) A length to ensure that the distance from the last lens of the field flattener to the CCD sensor is exactly 100mm

As it can be difficult to evaluate the position of the field flattener lenses inside the frame, the distance can be more conveniently measured from the lenses mount edge, as shown in the picture. ( basically the measurement is taken from the base of the field flattener to the CCD )
https://inter-static.skywatcher.com/...1416868668.pdf
From there to the sensor, the correct distance should be exactly 96mm

So i have two figures there, and i am guessing 96mm is the one to use as it is measured from the base of the field flattener ( not from the end or the thread ) to the CCD

Now after all that, the camera that i have is the ASI 2600mc, according to ZWO, that correct back focus is 55mm and the sensor is 17.5mm inside the body of the camera.

So i have all the included a photo of how it is all layed out:

On the far left is the adaptor that screw onto the scope,
Followed by the Field Flattener
Followed by the adaptor to go from the Field Flattener to the spacers ( this is 43mm)
Followed by a 21mm spacer
Followed by a 16.5mm spacer
Followed by the asi2600mc camera

So all up i have a total of 98mm from the base of the field flattener to the CCD

So how do i work out if i have the correct back focus with all those numbers?
98mm seams close to the figure in the book of 96mm, but what about this figure of 55mm back focus for the asi 2600mc that the ZWO site gives me?

As you can see, i am in a bit of a confused mind here at the moment, or will the setup i have work?

Any help would be appreciated.
Thanks.
Peter.
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