Re: [chrony-users] Application requires frequency accuracy but not time accuracy

[Brian Morgan <oopfan@xxxxxxxxxxxxx>]

Thank you for your reply! To answer your question: "To better understand 
the requirements on accuracy, how many milliseconds is one pixel in that 
image?"

My refractor's focal length is 418mm. The CCD's pixel size is 4.65 
microns. That is 2.29 arcsec per pixel which means 1568.9 pixels per 
degree at the celestial equator. There are 86400 seconds per solar day 
and therefore 240 seconds per degree. This reduces to 152.97 
milliseconds per pixel. For a target above or below the celestial 
equator you need to multiply 152.97 by the cosine of the object's 
declination. For the Andromeda Galaxy the declination is 41 degrees 
north. All said and done, that is 115.45 milliseconds per pixel. So to 
put it another way, if I were to kill the drive the galaxy would drift 
at a rate of 8.66 pixels per second.

I had mentioned that there was a temperature component. In a typical 
imaging session ambient temperature drops about 5C. It is not a straight 
line. The temperature can hold steady for an hour and then drop 1C quickly.

Typically I power up the Pi an hour before astronomical darkness. From 
experience with another Pi that is dedicated to imaging which has its 
own GPS USB dongle, it can take quite a while before the LED on the 
dongle begins to flash. The amount of time it takes to obtain a fix 
depends where I position the telescope in the field. This is not a 
problem. I like to power everything up well before I get down to 
business. Also I should mention that I am at the mercy of the weather, 
so I might go 10 days between imaging opportunities.

Having said this my expectation is that the drift file should give 
Chrony enough information to make an initial estimate of the current 
time just as soon as the Pi boots and Chrony loads. When the GPS comes 
on line, perhaps as long as 20 minutes later, Chrony should utilize the 
timestamps to formulate a more accurate, up-to-date picture.

Thank you to everyone for your help. This is a most interesting topic!
Brian


On 9/30/2019 4:11 AM, Miroslav Lichvar wrote:
> On Sat, Sep 28, 2019 at 04:20:38PM -0400, Brian Morgan wrote:
> > I wish to thank Professor Unruh. He inspired me to take a deep dive into my
> > chrony.conf file. As a software developer and devotee to the concept of
> > "minimal completeness" I have arrived at this:
> >
> > driftfile /var/lib/chrony/drift
> > makestep 0.1 3
> > refclock SHM 0 refid GPS poll 7
> That looks good to me. You could add there corrtimeratio 1000. That
> would make the frequency more stable, but if I understand the previous
> emails correctly, that might not actually be what you need.
>
> Setting minsamples and maxsamples for the refclock could improve the
> phase or frequency accuracy. But we would need to know how much wander
> the clock has (how unstable is temperature) and how much jitter is
> there in the GPS samples.
>
> > I am a great believer in sharing knowledge. Many astrophotographers keep
> > their "secrets" to themselves. I, on the other hand, like to share. I'd like
> > to offer you my website so you can follow my progress. My latest subject is
> > the Andromeda Galaxy:
> >
> > https://astrotuna.com/andromeda-galaxy-m31-the-saga-continues-siril-app/
> Very nice. To better understand the requirements on accuracy, how many
> milliseconds is one pixel in that image?

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