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Do you flip?


Joel Shepherd
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There are two terms to distinguish in imaging, 'flip' and 'rotate.'  A flipped image is a mirror image so this will never be natural. (It's the equivalent of turning the page over and looking from the other side.) A rotated image is just turned clockwise or anticlockwise. A Newt won't flip an image of itself but some stacking software does do this - rather irritatiingly!

Olly

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Hmm. Okay, my terminology is probably wrong. Let me try again.

If I look through a Newt at a terrestrial object (top of tree), it appears upside down in the eyepiece. I assume that if I replace the eyepiece with a camera, the image recorded will be upside down as well. And with left and right reversed?

Assuming that is true for an astrophoto, do folks usually correct for it during processing?

(After I posted the original message and there were no responses I decided that they must ... but now I'm not so sure.)

Thanks -- Joel.

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I never rotate/flip neither the telescope or camera. As doing to can upset a few things like flats, balance and framing. This is espectially important when it comes to mosaics, where you need to figure out your overlaps and ensure that what you planned on your computer is exactly what your telescope sees.

I've set my camera up so it matches whatever rotation stellarium says it should see (using the oculars plugin):

post-5513-0-85373600-1422693394_thumb.jp

Its worth setting up beacuse it makes planning your session so much easier.

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Personally I go for 'north is up' in final presentations most of the time. Sometimes, especially with large mosaics in portrait orientation (long side running N/S) I'll rotate them to fit on the screen and occasionally images just look better this way on or that, so I go with what seems nicest.

I agree with Rob that being orientated along RA and Dec where possible has an awful lot going for it long term.

BTW, I wasn't nit picking on the terminology. Some software does generate a true flip. I have two scopes parallel on one mount with cameras orientated the same way. At capture they look identical on the capture screen. Once viewed on other software, though, one camera's image needs a double operation to make it match the other (and the sky.) It needs 'flip image horizontal' and then 'rotate 180.'

Olly

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Hmm. So I'm realizing this is more involved than I first thought. Olly, I didn't interpret what you were saying as nit-picking :-) ... or, maybe more accurately, I'm a nit-picker about language and your thoughts made me realize I was being imprecise.

Anyway ... I've completely ignored the issue of camera rotation. If I take two sets of images of the same target with the telescope and the same position, but the camera is rotated 30-degrees between sets, the resulting images will differ in rotation by 30-degrees ... even before taking the telescope's inherent light paths into account. Which means that I'm probably not going to be able to "normalize" images I already have (i.e., rotate so top is north) because I have no idea how the camera was positioned relative to the telescope. (For that matter, I have no idea which way the camera considers "up".)

But if I do figure out the camera rotation and correct for that (maybe with the help of Stellarium), then 'flip image horizontal' and then 'rotate 180' should correct for the telescope's (standard Newtonian reflector) light paths.

Please let me know if I'm wildly off target.

Thanks! -- Joel.

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