Field flattener, OSC camera and backfocus.

[LaurenceT]

I'm just finalising my first imaging train since migrating from a star tracker and mirrorless camera/lenses but I'm not really getting to grips with backfocus. My scope, an Evoguide 50ED has a backfocus of 20mm. I'm connecting a Skywatcher field flattener for this scope which has a back focus of 17.5mm and I'm connecting a ZWO ASI533 mc Pro. Because I have added the FF the back focus of the scope+FF is now 17.5mm, is this correct?

When adding my ASI533 mc Pro camera in order to achieve focus I can simply screw in the camera with the 11mm T2 extender in place, is that correct? Could I, should I so wish, add "anything" between the camera sensor and the FF as long as doesn't exceed 17.5mm and still reach focus?

Thanks for looking. 

[AstroNebulee]

6 minutes ago, LaurenceT said:

I'm just finalising my first imaging train since migrating from a star tracker and mirrorless camera/lenses but I'm not really getting to grips with backfocus. My scope, an Evoguide 50ED has a backfocus of 20mm. I'm connecting a Skywatcher field flattener for this scope which has a back focus of 17.5mm and I'm connecting a ZWO ASI533 mc Pro. Because I have added the FF the back focus of the scope+FF is now 17.5mm, is this correct?

When adding my ASI533 mc Pro camera in order to achieve focus I can simply screw in the camera with the 11mm T2 extender in place, is that correct? Could I, should I so wish, add "anything" between the camera sensor and the FF as long as doesn't exceed 17.5mm and still reach focus?

Thanks for looking. 

Is there supposed to be 55mm of back spacing between the FF and zwo 553 mc pro? I could be and probably wrong 

[LaurenceT]

4 minutes ago, AstroNebulee said:

Is there supposed to be 55mm of back spacing between the FF and zwo 553 mc pro? I could be and probably wrong 

You could well be right Lee, the image train diagrams I have seen all mention 55mm but I was unclear whether that was the maximum hence my complete confusion. How does this tie in with the back focus of the Skyguider FF with it's available 17.5mm?

Edited March 13, 2022 by LaurenceT

[AstroNebulee]

55mm is the general rule of thumb for most zwo cameras, though mine is around 62mm for my set up. So hopefully 55mm will be the one for you and will sometimes be plus or minus 1 I've read before. 

[LaurenceT]

2 minutes ago, AstroNebulee said:

55mm is the general rule of thumb for most zwo cameras, though mine is around 62mm for my set up. So hopefully 55mm will be the one for you and will sometimes be plus or minus 1 I've read before. 

So the back focus of the field flattener of 17.5mm is not a factor in adding a camera that requires 55mm? You can see the depth of my confusion😅

[Elp]

Evoguide as I understand doesn't do 55mm backfocus as you can't use a DSLR with it. You calculate based on what you're directly connecting to. If it is 17.5mm BF for the field flattener, find out your sensor to end of thread measurement where your adaptor ring terminates against your camera (my 183mm pro is 6.5mm), take that away from the BF and you use adaptor rings to make the difference.

[Budgie1]

Looking at the documentation, the Evoguide field flattener does have a back focus of 17.5mm.

This means that the light coming through the field flattener is focused on a point 17.5mm behind the last lens in the field flattener. This is where you want your camera sensor to be. 

As you're using the ASI533MC, this has the sensor 6.5mm from the front face of the camera, so adding the 11mm spacer will bring this to the 17.5mm spacing required.

If you want to add anything between the camera sensor and the field flattener, like a UV/IR cut filter, then it has to go into the 11mm spacer . You should have got a filter holder with the camera that looks like THIS, which screws inside the 11mm spacer and will accept a 1.25" filter.

[LaurenceT]

Thanks @Budgie1, I was able to achieve focus on distant trees this morning.