Focal reducer for RC - how?

[kalon]

Embarrassing question time: How do you fit your focal reducer to a Ritchey-Cretien telescope?

I currently have my CSO 8" RC setup as follows:

OTA->(Extender rings)->Focuser->Focal reducer->Off-Axis Guider->Filter wheel->Camera

I can get perfect focus without the FR (same imaging train otherwise) using the requirement to be 159mm of back focus from the focuser. The FR requires 80-85mm of back focus from the back flange of the FR, but in my cursory searching I haven't found what I need to do to achieve focus.

I tried to get BOTH of these exact - back of focuser @159mm and back of the focal reducer @80-85mm but got at best an out-of-focus donut like I would use for collimating. I tried varying the distance from the FR to the camera as well as testing the full range of draw on the focuser, but couldn't improve it (but could make it worse). I even pulled off the extender rings one at a time to try for large-scale focuser changes.

I think it is a back focus issue, but can't find anything to put me on the right path. Resources will be greatly appreciated!

[Martin_S]

Kalon, I have an RC6. This is my setup at f/4.7 .It's not the most orthodox setup but it works good with my ASI385 camera.

Martin

[kens]

Which FR is it? I fitted my CCD47 to the end of a 2" nosepiece and it slides inside the focuser. With 78.5mm spacing from the rear thread to sensor I need to focus in by 36mm My spreadsheet tells me that with 83.5mm spacing I would need to focus in by about 41mm

[kalon]

It is a cheapie GSO 0.75x FR from Bintel (https://www.bintel.com.au/product/bintel-0-75x-rc-reducer/) I just want to know that I'm putting it in the correct position in my imaging train to start with, and what impact it has to my backfocus calculations...

Googling has shown nobody posting a "This is how I'm set up" images ANYWHERE... this implies it is so easy it is considered basic knowledge, but given I've never managed to make it work, I'm asking the embarrassing question

[kens]

So you can remove the 2" tube to leave a female M48 thread on the telescope side and a male M48 thread on the camera side.
Lets assume without the FR you have a 2" nosepiece that fits into the focuser.
You can now screw the FR onto the end of the nosepiece. Measure from the back flange to the camera to see if you can get the 80-85mm spacing needed. If it is shorter add some extension tubes. If it is longer you would need to replace the nosepeice with a shorter one or use an extension tube in its place with an appropriate stop.
Once you've done this you need to reduce the overall backspacing from the scope to camera by around 40mm.
I achieve this by replacing the 50mm M90 extension with a 25mm one and also remove a total of 15mm of spacers further back in the optical train.
Do you have the measurements of all components from the back of the scope to the camera?

[Karlz]

I find looking at a distant hill or something over 1km away to help figure what will work as at times I use no reducer to reducer directly mounted to camera to adding spacer in between reducer and camera. I found doing this in daylight hours saves the frustration at night. All need different spacing from OTA to focuser. I have a RC8 and use CCDt67 reducer.

[Karlz]

Hi Kevin,

Here is my image train with the reducer at 86mm from senor. hope this helps.