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Tips for imaging very north objects? PHD / polar alignment / under vs over polaris etc


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Hi! I have plans to image things like the iris nebula and the integrated flux nebula near polaris, but I have noticed that despite my best attempts to polar align (using Ekos' polar alignment routine with my astrocam and getting things to a matter of seconds away from true) I still see field rotation across my images, such as a recent M81/82 daat set I collected and put through pixinsight's blink feature. I have made sure my guidecam is roughly aligned with the telescope (M81 and M82 were both in the FOV during the beforementioned session, so guide stars were in the same place as the imaging. Perhaps cone error or maybe untrue bearings are to blame in my HEQ5? Alternatively, my scope (now an 8" TS photon) does seem easy to wobble in the vixen style dovetail and puck, perhaps the scope is rotating in its seat due to flexture?

I also noticed when I first got PHD working (only got guiding recently) that it failed to perform the calibration on RA, and in the end I realised I was pointed at polaris, and as such the small RA movements PHD was making would have almost no effect. Can PHD setting be tuned to work up there or am I out of luck for guiding up in that region?

I also am curious about objects that are under the pole. Can they be reaonably imaged or are there restrictions with our EQ mounts when it comes to the observable area under the celestial pole? My mind goes a bit numb thinking about how the mount would need to position itself to do something like follow polaris around if it were below the pole vs above it, while keeping polaris in the center of the field and without rotation.

Advice appreciated! Forecast is poor tonight but I see an absence of clouds so I shall chance it!

Many thanks

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Does this field rotation matter if it is not visible within one sub? All it means, assuming you stack using an alignment which does 'right-left-up-down and rotation' is a slight loss of keepable real estate around the edges. What you gain is a bit of inherent dither.

As for imaging 'under the pole,' we are doing this whenever we begin imaging a target which is rising in the east and lying at an altitude below that of our latitude. I'm at a lower latitude than you yet I do this all the time. Since I have dark horizons I certainly don't have to wait for my targets to rise above  an altitude of 44.19 to get started. To be honest, I've never given this a thought.

Olly

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I suspect that cone error and polar misalignment are the main sources of field rotation in combination with a meridian flip, when imaging near the polar region. During frame alignment/plate solving after a meridian flip, you compensate for a rotation in one axis (axis of cone error, or alt/az) by a rotation along two other axes (ra and dec). The best you can do is to check how much cone error and polar misalignment you have, try to minimise it, and crop the stacked image to get rid of what's left of field rotation. It's annoying if you have a small sensor and/or a long focal length, to have to cut away a large portion of an image. But there only so much you can do about it.

As for PHD calibration, unless you use st4 guiding, you can and should calibrate away from the polar region. Do this near the meridian and near the celestial equator. If you use eqmod, PHD will know where the scope is pointing, and adjust the guiding accordingly.

The ts 8" isn't exactly a small scope, and you should definitely consider mounting it with a Losmandy plate, rather than a vixen. The heq5 should be able to carry the weight, but your setup is sensitive to any mechanical instabilities. If you don't have the means to upgrade the mount, consider upgrading the dovetail and puck.

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I had, and still somewhat have, significant cone error in my 8" newt and field rotation on a target at 74 degrees in DEC was definitely an issue. Best advice i can think of is to look into fixing cone error if you dont have the sensor area to spare to crop the corners out.

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