Back focus question

[brisen (Brian)]

I am trying to work out the spacers I need for the correct focus of the 12" RC.

The total back focus of these is 288mm from what I have managed to find (they don't supply a manual). The focal point is apparently 127 mm from the 2" connection on the focuser which has 50 mm maximum travel.

In trying to work out what I need and where I thing I am confusing myself with this, is based on the following.

The focus point from the 2" connection on the focuser to the chip on the CCD needs to be 127 mm.

The Atik One 6.0 CCD has a back focus of 27 mm and I have the OAG on the front which takes up 24 mm of back focus. I have filters in the mix and assume the depth of these is 3 mm, therefore the 1/3 rule suggests adding another 1 mm. This gives me 52 mm of back focus used out of the 127 mm available to get focus from the focuser. Based on this, there is a need to fill 75 mm with spacers to achieve the back focus from the focuser to the CCD chip.

The OTA comes with 100 mm worth of M117 extension rings which the focuser screws onto and these are placed between rear of the OTA where the M117 thread starts and the focuser. The focuser comes in at 90 mm in length. Taking this into account I have used 190 mm of the available 288 mm, leaving 98 mm of back focus but the focal point is 127 mm from the edge of the 2" adapter on the focuser. So essentially I need to lose 29mm of spacer between the back of the OTA and the focuser. The M117 rings are 2 x 25 mm and 1 x 50 mm.

If I have calculated this correctly, I need to have 75 mm of extension tube between the focuser and the OAG to give me the 127 mm from the connection to the chip.

On top of this I think I need to take out the 2 x M117 extensions and just keep the 1 x 50 mm M117 in place and attach the focuser to this. Given I think I need to lose 29 mm off the extension to stay within the 288 mm back focus, if I take out the 2 x 25 mm M117 extensions I am losing an extra 21 mm. The focuser has 50 mm travel so I should essentially have 29 mm of focuser travel spare to achieve focus.

Does this look correct? I will be replacing the 2" adapter with the M68 threaded adapter and dropping this to M48 and M42 into the OAG and that gives me 25 mm to work with to effect that change. Having typed this up I think I have clarified what I need to do, but if someone can confirm I haven't lost the plot that would be greatly appreciated.

Brian

[Joshua Bunn (Joshua)]

Hi Brian.

After reading your post, I would recommend placing some card behind the scope at night And pointing at the moon. Where the moon comes to focus, is your backfocus. You want to get this right, and this will do that. HTH.
Regards Joshua

[Imme (Jon)]

Originally Posted by Joshua Bunn

Hi Brian.

After reading your post, I would recommend placing some card behind the scope at night And pointing at the moon. Where the moon comes to focus, is your backfocus. You want to get this right, and this will do that. HTH.
Regards Joshua

Very sensible advice

[brisen (Brian)]

Originally Posted by Joshua Bunn

Hi Brian.

After reading your post, I would recommend placing some card behind the scope at night And pointing at the moon. Where the moon comes to focus, is your backfocus. You want to get this right, and this will do that. HTH.
Regards Joshua

Hi Josh

Thanks, I hadn't thought of that approach. Once I get caught up with work I will get it outside and try this.

Brian

[JA]

Hi Brian,
I would suggest using some camera bellows and some Heath Robinson (or not) connections to enable a precise control of the focal plane position, which you could then remove and measure.

Best
JA