Today I had the chance to test the PST Etalon/BF on the Celestron Omni XLT 120.

It’s not the perfect setup as the Etalon is not placed perfectly at 200mm from the prime focus point. I still need to take it 1/1,5cm forward but I was curious to see if I was able to reach the focus and try the different D-ERF positions.

Differently from the Vixen A80M setup, I had to place the D-ERF inside the Omni 120 tube as the diameter of the D-ERF is only 75mm (70mm considering the mm taken by the cell).

So I had to find a place that didn’t sacrifice too much the Omni 120 aperture but also didn’t cause any internal air turbulence. I tried 2 settings based on two of the 3 internal diaphragms: the ones that can be easily reached from the tube extremes. The first is the one behind the front lens (less turbulence but also less surface) and the one just in front of the focuser (more turbulence but also more surface).

Later in the evening I’ve found a way to put the D-ERF with the cell in the middle of the tube so that I solved the dilemma and have the maximum aperture available.

To solve the focus/etalon distance problem I substituted the original Omni 120 focuser with a new one from TS that is shorter by 3 cm. That is enough to have the etalon “almost” in the right place and reach the focus with the second focuser. So the first focuser is just used as a spacer/adaptor for the real “ex-PST” block (2″ nose + Etalon + SC Crayford Focuser + BF5). Just a note about the BF: the BF5 now it’s a BF6 as I enlarged the holes 😀

Here’s a picture of the setup:

Despite of the very long Omni 120 backfocus I wasn’t able to reach the focus for very very little (with the DMK41). No problem with the Hyperion Zoom in visual. So I replaced the original nose of the DMK41 with the shorter one I bought months ago and also removed the Baader Barlow adapter. With this setup I have a lower magnification but I have room for precise focusing.

I made a few tests and here’s a comparison of the 3 setups I tested (Vixen A80M mod, Omni 120 with Deep ERF and Omni 120 with Front ERF):

Well, there’re a lot more details and you have to consider that with the 100mm setup I have double the surface than with the Vixen (70mm in reality).

The gallery shows some of the images I took today. Don’t put much attention to the dirt etc as I simply wanted to test as man setup as possible and I was not in perfect conditions. Anyway I’m already happy with the results.

Tomorrow I’ll do more test… with a clean CCD 😀