Hi Alexandra,
The hard part is the D-ERF, which you have covered!
You will need these two adapters from AOK swiss (both the front & rear adapter).
http://www.aokswiss.ch/ayo/pst.html
You will need a blocking filter, such as a 10mm or 15mm Coronado blocking filter, you can get that commercially or used. Alternatively, you can use the PST's blocking filter but it requires some work and other adapters, and it's only 5mm so its limiting on larger sensors, but it is an option.
https://optcorp.com/products/coronado-b ... ing-filter
You will need a simple 2" extension of about 80~140mm in total (you can stack these) and these are commercially made as typical 2" extensions in 35mm, 50mm and 80mm lengths commonly. The focus point needs to be 200mm after the rear collimating lens in the PST etalon's housing and the diagonal of your blocking filter will absorb some of that. So it's tyipcal to have 140mm of extension after the rear PST adapter then the blocking filter, and after that is your camera and stuff. You can also get extensions that incorporate non-rotating helical focusers to give yourself fine focus in the imaging train.
Here's examples of 2" extensions commercially, they're common:
https://agenaastro.com/blue-fireball-2- ... nsion.html
https://agenaastro.com/blue-fireball-2- ... nsion.html
Example of 2" extension with 1.25" helical focuser:
https://www.amazon.com/gp/product/B07HR ... UTF8&psc=1
Since you're using an SCT it's a lot simpler and you need less stuff.
The front PST adapter will have M48 threading to allow for standard filters to thread on. This is where you thread on a UV/IR block filter, or a Red CCD imaging filter, or a Baader 35nm HA filter to eliminate the rest of the heat (knowing your DERF passes 393 & 430 so there's still some heat coming through that is not needed/wanted since this is HA with the PST). I would suggest the Baader 35nm HA filter first. After that the Baader Red Imaging CCD-IR block filter.
You insert this into your SCT visual back.
Now, harvesting the PST etalon... that's another story. I have done all of this recently, so I can share via my threads:
Here I harvest a PST:
viewtopic.php?f=9&t=25420
Here I make a PST module that works in SCT, refractor, etc:
viewtopic.php?f=9&t=26560&start=25#p239784
I still ask for help doing this all the time because I'm not the expert, but hopefully this gets you started!
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Here's my current PST module in my C8 Edge with the D-ERF:

- C8Edge_PSTMod_BF10mm_09232019.jpg (72.81 KiB) Viewed 805 times
From left to right:
2" Visual back ->
Baader Red CCD IR blocking Imaging Filter (ERF #2) ->
AOK Swiss Front PST Adapter -> PST Etalon ->
AOK Swiss Rear PST Adapter ->
2" adapter with 1.25" helical focuser ->
Coronado 10mm Blocking Filter -> ASI290MM camera
This allows me to focus the primary mirror with the SCT's focuser with the collimating lens in the appropriate spot to be at F10. Then I fine focus with the helical focuser near the camera.
Very best,