Back focus Distance for ASI 178MC

cherron3

I am new to the field of astrophotography, and I have a couple of questions about achieving the correct back focus. I'm thinking I need to get this right so I can rule this out. Mainly interested in capturing the Moon, Jupiter, and Saturn. It's 12.5 mm to the sensor; the nosepiece( 28 mm) and a ZWO duo band filter (5 mm) add another 33 mm bring the total to 45.5 mm. This leaves 9.5 mm. So, would I need an additional 9.5mm of spacers to achieve the correct back focus distance of 55mm? I will be connecting to an Astro Tech field flattener for an AT 115 EDL.

Also have Celestron 9.25 XLT with a 0.63 reducer installed. Back focus is 105mm. Do I add some additional spacers to my image train to achieve the correct distance? Looks like I would need an additional 60.5 mm to reach the correct back focus here.
Hopefully, someone can let me know if I am on the right path here. Thanks in advance

w7ay

cherron3 ZWO duo band filter (5 mm)

How thick is your filter glass??!!!

Glass (approx refractive index of 1.5) will add one third of the glass thickness (i.e. 0.5 out of 1.5) to the optical path. If the filter glass is 2mm thick, add 2/3 mm, if the filter glass is 3mm thick, add 1mm to the optical path.

Where did you get the 5mm from? That would correspond to a filter that is 15mm thick.

Chen

cherron3

I measured the filter body not the actual filter.

cherron3

it's a 1.25 inch zwo duo filter; filter body is 5 mm; thought the filter body would be what was actually adding length to my image train. it adds to the length of the nosepiece