This post is for discussing optimal back focus distance (metal spacing) for RASA8 when filters with different thicknesses are used.
The quality of images made by Celestron RASA8 is very sensitive to the back focus distance (metal spacing). I still don't have clear understanding how it works and what's the right back focus distance in different scenarios.
Let's start from the built-in glass window, by my measurement:
- 46mm in diameter
- 2.28mm ~ 2.33mm in thickness
But I truly don't know if it's counted in the 29mm back focus distance described in the user manual (the reason I don't want to use 25mm on top of the camera adapter - the padding at the bottom of the camera adapter is soft, could be suppressed when tightening the clamping ring, not accurate at all. 29mm is from the corrector module metal surface to camera sensor).
Still, the 29mm everybody uses is different from Celestron published "Recommendation for Customized Adapter (RASA8")" guide. (https://celestron-si...dapter_Rev1.pdf). In this guide, from the corrector module metal surface to camera sensor should be 28.73mm. This is something that I'd like to use as more accurate measure.
In the following views we can see the 28.73mm is based on the scenario that the clear glass window is installed.
Not sure about the optical back focus distance added by this clear window. Assume it's still the simple 1/3 rule, it adds about 0.8mm.
Now, with 3mm filter installed and clear window removed, the back focus distance increases 0.2mm comparing to the scenario only the glass window is installed.
I'd like to hear from more people about how to design the spacing. Let me share my own first test images in my first-light session (ABE and stretch in PixInsight only): https://www.astrobin.com/9h9seb/
The stars looks okay at the corner. But HFR values are around 1.9 - 2.0. (Perfect V curves in SGP with Celestron AF Motor).
This was done by the following setup:
- Glass window removed.
- Camera adapter 3.5mm (padding ripped)
- Starizona Filter Slider (18.70mm)
- Astronomik 12nm Ha 2" (1mm thick)
- ASI1600mm-Pro (6.5mm)
I measured the total thickness of the Starizona filter slider installed together with M42 camera adapter (again, padding ripped), it's about 22.20mm. On the scope side of the Starizona filter slider, there is a 1mm thick M42-M48 ring, but the 1mm was consumed by the "sunk" circle of M42 camera adapter.
Putting the numbers of the metal pieces together, WITHOUT the glass window:
3.5 + 18.7 + 6.5 = 28.7mm
Let's assume the clear glass is removed, and use my "lucky number" here that 28.7mm of metal spacing worked with 1mm thick filter, for 3mm thick filter the metal spacing should be increased 0.67mm to 29.37mm (Astronomik 12nm Ha is only 1mm thick). ;-)