A barlow will complicate the situation since the magnification factor you get will depend upon the barlow to sensor spacing which is not the same thing as the 146mm backfocus recommendation for your EdgeHD. In addition to the variation you will get in the magnification some barlows are designed to work best at a given spacing between the barlow and the sensor which will likely not fall at that 146mm recommendation.
So, what you need to do (if using a barlow) is to determine what magnification factor you want from said barlow and then in keeping with any recommendation on the barlow to sensor spacing fix that barlow to sensor spacing and then position said barlow so that you achieve focus at the same mirror positions on the EdgeHD as given when the system is NOT equipped with the barlow and at the 146mm backfocus recommendation for the EdgeHD.
Basically, what you need to do is satisfy both the target barlow to sensor spacing and the EdgeHD's mirror spacing (focus) that would normally be achieved at the 146mm backfocus setting.
That said, there is a range of spacings (different from the above) that will still provide good results but it is highly unlikely that a 146mm spacing will give you the optimum results when using a barlow on the EdgeHD.
Here is an example when using a barlow with a recommended 55mm spacing between the barlow and the camera's sensor. That 55mm barlow spacing may come about either from a target magnification factor (say 2X) and/or a design requirement for the barlow.
1.) Fix that 55mm spacing between the barlow and the camera's sensor.
2.) Focus the EdgeHD so that there is 146mm between the rear baffle on the EdgeHD and the camera's sensor (when NOT using the barlow). This will place the mirrors in the EdgeHD at the system's optimum location (there is some leeway here, so don't obsess over a few millimeters).
3.) Determine the spacing that is needed between the rear baffle on the EdgeHD and the barlow so that the system remains in focus at the same mirror spacings as given in step 2.
Step 3 is the "hard" part and it may require some experimentation, although you can use the magnification factor of the barlow to help determine what part of the 146mm backfocus spacing will be needed between the rear baffle on the EdgeHD and the barlow itself (given that in the above example you know that you'll have 55mm of spacing between the barlow and the camera's sensor).
In any case, note that the spacing that is determined in step 3 is very likely NOT simply the difference between 146mm and 55mm. As a first approximation and assuming that the 55mm barlow spacing produces a magnification factor of 2X you can calculate that the barlow will consume 55mm / (barlow magnification) which in this case is 55mm / 2 or 27.5mm of the target 146mm spacing on the EdgeHD. So, that spacing between the rear baffle on the EdgeHD and the barlow would be 146mm - 27.5 = 118.5mm and the total spacing between the rear baffle on the EdgeHD and the camera's sensor would be 118.5mm + 55mm = 173.5mm (which is NOT 146mm).
Note also that barlows differ and as one example note that the barlow to camera sensor spacing for the Astro-Physics Advanced Convertible barlow at a 2X magnification is 90mm (this would be the spacing shown in step #1 above, substitute for the 55mm).
Finally, there is leeway here so don't get hung up on a few millimeters. Plus, you'll probably get okay results with just about any spacing (within reason, don't be "off" by 50mm).
Here is another post that I made concerning barlow use on a EdgeHD. This includes a diagram which may help.
Edited by james7ca, 22 September 2020 - 04:55 AM.