How close to "perfectly aligned" do you need to be?

Started 2 months ago | Questions thread
hha Contributing Member • Posts: 725
Re: How close to "perfectly aligned" do you need to be?

pedz wrote:

In a set up with a rotating "star tracker" it needs to be aligned with the earth's rotation (roughly aligned with Polaris in the northern hemisphere). I'm just curious if anyone has figured out how close to perfect does this need to be?

It has a lot of factors involved but lets just assume the worse case of objects (stars) 90º to the axis of rotation. Lets assume 100mm lens which has a field of view of 23.4º on a full frame camera. Lets assume a modern 50 megapixel camera that has roughly 9800 pixels on the diagonal. And lets assume, say, a 1 minute exposure.

What is the largest angle that the star tracker could be off and still result in the error being within one pixel of the sensor?

I've been puzzling over an approach to figure this out but I started wondering if anyone has already figured such things out already. So I thought I would ask.

Has anyone figured out a formula for this or perhaps a table?

Obviously, if your polar alignment is not perfect, you will get trails.

How long a trail will you accept in your 1 minute exposure?

How are you going to process the data afterwards?

Here is my experience with a 180mm lens on a Nikon Z6 (24 Mp and 6 micron pixels) and 6.9 arcsec/pixel. If the pole axis is misaligned by 0.2 degree, you get a 15*sin(0.2)=0.052 arcsec/sec trail, i.e. 3 arcsec in 60 seconds.  This produces about a 0.5  pixel trail in each one minute exposure. Acceptable. Obviously, in a 4 hour exposure (240 shots) the frame shifts a fraction of a degree, which is OK, since the FOV on the FF Z6  is about 8x11 degrees, I then register the data in Sequator, which has no problem with dealing with the shift.

That is all there is. You can work out your own case with your own assumptions.

hha

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