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06-01-2019, 09:28 AM   #1
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Astrotracer question

New owner of a K3-ii and am going to be using Astrotracer for some night sky shots with a 50mm and 135mm lens and was wondering how often I need to reaim the camera if taking multiple exposures of constellations or DSO's to later stack?

Thanks, Mark

06-01-2019, 10:09 AM   #2
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Hi Mark,

Welcome to the Forum.

As to re-aiming your camera, consider the field of view of your lenses and the size of your target object.

The sky rotates 1 degree in 4 minutes. You can check the lens reviews here in the Forum (Pentax F Prime Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database) for the field of view size. For a 135mm lens, the FoV is only 10 degree wide. You will probably want to re-aim every 10-20 minutes.

Your stacking software will have to be able to deal with aligning exposures that do not cover exactly the same area (and may need to be rotated as well). For Az/El mounts, like a tripod, the sky will generally appear to rotate around the center of the field as well as across the FoV during an exposure.
06-01-2019, 11:04 AM - 1 Like   #3
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Join Date: Jan 2011
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Download the free software Stellarium to help plan astrophotography sessions Stellarium Astronomy Software

It includes a field of view simulator. You select your target, sensor, focal length, and time of day. It shows where your target will be located. You can fast-forward to see how your target drifts through the field.
06-01-2019, 11:32 AM   #4
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Most important to gain accurate focus, you can't use AF. Use a bahtinov mask and try and secure the focus ring when focus is correct, it is easy to bump it out of focus when repositioning the camera.

Astrotracer is a bit hit and miss, stay away from metal objects like your car.

06-01-2019, 01:03 PM   #5
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QuoteOriginally posted by AstroDave Quote
Hi Mark,

Welcome to the Forum.

As to re-aiming your camera, consider the field of view of your lenses and the size of your target object.

The sky rotates 1 degree in 4 minutes. You can check the lens reviews here in the Forum (Pentax F Prime Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database) for the field of view size. For a 135mm lens, the FoV is only 10 degree wide. You will probably want to re-aim every 10-20 minutes.

Your stacking software will have to be able to deal with aligning exposures that do not cover exactly the same area (and may need to be rotated as well). For Az/El mounts, like a tripod, the sky will generally appear to rotate around the center of the field as well as across the FoV during an exposure.
Thanks for the info AstroDave, I'm hoping to get out tonight and play with it. I use Deep Sky Stacker and it adjusts for field rotation. Hopefully I'll be able to get some decent shots (weather permitting).

Mark
06-01-2019, 01:05 PM   #6
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Thanks everybody for the useful information, hopefully the weather will cooperate this evening and I'll be able to try the camera out.

Mark
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