I have a 16” dobsonian with f/D 4.5 (hand crafted by Dobson Factory in France) and a former microscope Zeiss Binoviewer modified by Denis Levatic, with an optical path of 123mm it seems, and prisms open at 25mm. And with a T2 connection...
I am still debating on what I need: I want to use it with my 16” dobsonian, with lowest magnification possible, with as little vignetting as possible, and 1.25” eyepieces.
I tried last weekend the Siebert OCA that has 3 configurations possible on my dobsonian.
- in 1.25x configuration : I can not reach focus.
- in 2x configuration : I can not reach focus.
- in 3.5x configuration : I can reach focus and I have at least 20mm of back focus left.
Now, with other solutions :
- with a 2x barlow, I can reach focus but as said previously, I get crazy high magnification (like 4.5x) and a horrible eye relief with TV Plössl 32mm or E-Pl 10x/25.
- with a Baader GPC (glass path corrector) 2.6x, I can not reach focus, it is supposed to compensate 85mm of optical path...
- with a Explore Scientific Focal Extender 3x, I can reach focus and have maximum 4mm of back focus left.
So, my question is : what solution exists with only one configuration of magnification, that would be the smallest possible magnification, without too much vignetting ? I know it can be done with a 3.5x Siebert OCA and a 3x ES Extender but is it possible to have less magnification and still reach focus ? I have not tried a Powermate 2x with T connection but that would be ideal, but I can’t find any info on what light path I could compensate... massive headache...
Many thanks and clear skies,