Skywatcher 100ED focus with reducer

[matthasboldlygone]

I like to have my telescope roughly focussed before going outside, otherwise I spend hours winding the focus in and out all the way trying to find the right position.

Without the reducer I know the camera sensor needs to be 900mm from the front lens, I can measure this and then tweak it when outside in the dark.

If I add the 0.85 reducer I know the camera sensor needs to be 55mm behind the reducer, but not sure how far out or in the focus tube should be. Will it be 900mm to the reducer or 900mm to the sensor or 900mm x 0.85 ?

👍

[Budgie1]

The focal length of the Evostar 100ED with the Skywatcher field flattener attached is 900 x 0.85 = 765mm.

What I've done is; I set the focus outside without the field flattener on, so 900mm, and when I came in after imaging I drew a line with a Sharpie on the focus tube where it meets the housing. Then I did the same with the field flattener attached. So I now have two lines on the tube and I know roughly where focus will be without having to measure each time.  

[matthasboldlygone]

12 hours ago, Budgie1 said:

The focal length of the Evostar 100ED with the Skywatcher field flattener attached is 900 x 0.85 = 765mm.

What I've done is; I set the focus outside without the field flattener on, so 900mm, and when I came in after imaging I drew a line with a Sharpie on the focus tube where it meets the housing. Then I did the same with the field flattener attached. So I now have two lines on the tube and I know roughly where focus will be without having to measure each time.  

I've got various scribbles on the focus tube now  

So when you put the reducer on did you have to wind the focus tube inwards by 135mm? 

765mm from front lens to the reducer, then 55mm to the sensor?

I tried some daytime focusing, but only had a tree about 50 meters away as a target, and this didn't seem to match focus on stars.

[Budgie1]

I have slightly different setups for both.

The field flattener screws directly onto the focus tube with a T-mount between the field flattener and the camera.

When I don't use the field flattener, the double thumbscrew adaptor screws onto the focus tube, then the 2" reducer and the T-mount is screwed to the reducer.

The result is that there's only a 19mm difference between the focus lines on the tube.

When I set the focus with these setups I didn't measure the physical distance from the front lens to the camera sensor. I setup outside, pointed to a bright star, centred with the finder scope. Then got a rough focus using live view on the camera (DSLR) and fine tuned using the focus tool in APT. Once set, I put a line on the tube with a Sharpie.

Now I know where the focus will be with each setup but I'll still use the APT focus tool to tune it.

I hope that helps.