SingAstro

Thank you everyone for the kind advice.

Can anyone tell me the difference between metal back distance and back focus distance?
At first I thought they mean the same, but Takahashi quotes different figures for both, so I am not sure what to make of it.

It doesn't matter. All of them should be able to reach focus. The only thing you should care about is the distance for the reducer/flattener to the camera, which should be correct if you are using the tak camera adapters.

MooEy wrote:It doesn't matter. All of them should be able to reach focus. The only thing you should care about is the distance for the reducer/flattener to the camera, which should be correct if you are using the tak camera adapters.

In the case of the FSQ 85ED when reducer is used, the back focus listed is 82mm. The metal back is 72.2mm. Do I have 82mm to squeeze in my camera and filter wheel, or just 72.2mm?

One measures the rear of the glass to the sensor, the other measures the mechanical distance.

72.2mm should be correct. You will need an adapter from the back of the reducer to CA-35, then the CA-35 Wide T-Mount. Total distance for those 2 adapters should come close to 28mm, and the camera body will take roughly another 44mm, giving a total of 72.2mm.

Why are you going this far?

~MooEy~

I am considering the Sky 90, FSQ 85 or the William Optics GTF81.
The Sky 90 and WO GTF81 are priced way lower than the FSQ 85. Is the extra cost for the FSQ 85 worth it?

Thank you everyone. Finally got my kit together.
Managed to purchase a second hand William Optics Megrez 80 and a Vixen Sphinx SXW for $1.2k
Did a test shot and am very happy to get lots of galaxies at first try.

Let me guess, M81+ M82 in Ursa Major region?

By the way, which camera or ccd are u using? I myself have a Skywatcher 80ED f6.25...may think of imaging when I get older...

Regards,
Junwei

It's M106 in Ursa Major.
Was using the Orion G3 monochrome camera.

Nice, thanks!

Regards,
Junwei