Quote:
Originally Posted by tempestwizz
I think there are multiple factors involved. A major one is the effective difference in applied light per pixel on your sensor, (assuming it’s the same sensor) given the difference in focal length. Tracking errors will exacerbate this effect.
Cheers
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Not sure I follow.
If I take a photo of a featureless blue sky with any f/5 lens or OTA, their focal lengths will have no impact on the brightness of the image. That's quite literally what the f ratio means. If the aperture stays the same and the focal length increases, the f ratio changes and thus the brightness. If the ratio stays the same, that means both are receiving the same amount of light per pixel. Whether that light is from a larger or smaller patch of the sky doesn't change the net amount received.
Also not sure what difference the tracking can make. As I said, both OTAs were used with the same mount, camera and settings, within about 90 seconds of one another. Any impact from tracking would be identical between them, regardless of how accurate.
The only conclusion I can draw is that the newt is somehow less "efficient" and that its f/5 ratio is more in theory than practice. About 20% of the light is being lost somewhere, just curious as to where. Is some of the light from the primary going around the edges of the secondary? Is there something funky going on with the focuser assembly that 20% of the image isn't making it to the camera sensor? I'd really like to know!
My best guess is it's a combination of things:
- some light is blocked by the secondary (about 6% apparently);
- some light is escaping back out into space, ie going around the secondary
- some light is landing outside the camera sensor (so as to avoid vignetting around any images)
Bit of a shame that it all adds up to the thing being nearly a stop less efficient than advertised, though. It's a big strike against reflectors for me personally, especially when considering how inconveniently cumbersome and fiddly they are. Their main benefit is supposed to be their light-gathering capabilities, but they're considerably less efficient than they're supposed to be. (At least, the three I've owned are.)