[thegableguy (Chris)]

I just took delivery of a new GSO 8" f/5 from Andrews Comms and had a shot at Helix last night. For some reason it was far, far darker than it should have been, ie several stops. Trying to work out why.

I've collimated it this morning and it was significantly off (and there are smudges on the primary - WTH Andrews???), but it's perfect now. I've just compared it to my ED80 with the same camera & settings, and have been dismayed by the results. Using the same crop-sensor DSLR at the same exposure setting, the same target is actually slightly BRIGHTER through the ED80 than through the Newt. Surely this shouldn't be..?

Worth noting:
- the Newt is using the GSO coma corrector which adds about 1.1X to the focal length
- the ED80 uses a 0.85 flattener / reducer

Even so, the focal ratios are now f/6.375 for the ED80 and f/5.5 for the Newt. Any image through the vastly bigger Newt should require a shorter exposure, surely...? My much older Skywatcher 200mm f/5 certainly did. I'M CONFUSED.

Any help appreciated!