Re: Flexure Measurement - How Much is Too Much ?

Roland Christen

Looking good is one measurement. All the stars are round is comforting, and that's all that some want in their images. I have seen round stars in posted images on CN where people have imaged with mounts that have huge periodic errors, imaged unguided with those mounts, imaged with small short guide scopes etc., and had roundish stars. Broke all the rules and still had round stars. But if you look closely at full size you can see they lack sharpness and resolution, and most of them are out of focus because they also did not refocus critically each frame as the temp dropped. I compared one of my images of M57 with one I saw posted on CN. The stars were round on the CN image but they were 3 times as big and the resolution in the nebula was also really poor. Almost the same focal length, image scale and aperture!

To get really good images takes a lot of effort, very good tracking of the main optics, and a steady and hyper-accurate mount.


-----Original Message-----
From: Mike Dodd <mike@...>
Sent: Mon, Jun 21, 2021 3:15 pm
Subject: Re: [ap-gto] Flexure Measurement - How Much is Too Much ?

On 6/21/2021 3:29 PM, M Hambrick wrote:
> Roland has stated on multiple occasions that using a separate guide
> scope is not the ideal way to do guided imaging because of flexure. How
> much flexure is considered excessive ?

ANY flexure is too much! :-) You can read about my experience here:

> ...after a single 600 second exposure I measured the
> shift in the position of the guide star centroid to be 0.053 pixels in
> the X-direction, and 0.304 pixels in the Y-direction. This doesn't seem
> to be too bad in the X-direction, but I am not so sure about the
> Y-direction. Does anyone have any thoughts on this ?

The proof of the pudding is in the eating. If your images look good,
then your flexure is so small you shouldn't have to worry about it.

--- Mike

Roland Christen

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