#### Re: Star Eccentricity Help

Hi Mike

I think you may be thinking of star flatness?

i snagged this description out of PI's subframe selector - btw in this doc they spec 0.42 as being perfectly round

Eccentricity is a measure of star profile distortion. Given an elliptical star profile with major axis diameter a and minor axis diameter b where a is greater than or equal to b, the star profile eccentricity equals (1 - b2 / a2)0.5, the star profile aspect ratio equals b / a and the star profile flatness equals a / b - 1. For all stars fit in an image, their Eccentricity is weighted by the StarResidual value to minimize the influence of bad fits. A distortion with an eccentricity less than about 0.42 is not perceptible to most people. The table below shows the relationship between these measures.

On Sun, Aug 9, 2020 at 12:54 PM Mike Shade <mshade@q.com> wrote:

The value of .5 is stated as round, I am curious if .05 is what people mean.  According the Maxim's help, a value of .1 means that the star is longer in one axis by 10%, so a value of .5 would be 50%.  That would be horribly out of round.  Want to make sure that I am following.  With my system, PW 17/AP1600 (non AE)/STL6303E camera/Optec Gemini rotating focuser/.63" pixel/5' subs I generally get roundness values from .01 to .05, depending on where the system is pointed/seeing.

In War: Resolution

In Defeat: Defiance

In Victory: Magnanimity

In Peace: Goodwill

Sir Winston Churchill

Already, in the gathering dusk, a few of the stars are turning on their lights.

Vega, the brightest one, is now dropping towards the west.  Can it be half

a year since I watched her April rising in the east?  Low in the southwest

Leslie Peltier, Starlight Nights

International Dark Sky Association: www.darksky.org

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Brian Valente
Sent: Sunday, August 09, 2020 9:23 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Star Eccentricity Help

>>>  Is the solution a Planewave?? J

either that, or I hear AP has a 17" reflector they are working on :)

>>What should I expect for good eccentricity - .35ish??

generally eccentricity of 0.5 or lower is considered 'perceptibly round'

I'm not sure i would 'expect' that, but i would certainly aim for that

On Sun, Aug 9, 2020 at 9:10 AM Tony Benjamin <tonybenjamin@...> wrote:

Thanks Brian. I’m sure mirror flop is a definite possibility, even with the Edge mirror being “locked down”. Is the solution a Planewave?? J

What should I expect for good eccentricity - .35ish??

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Brian Valente
Sent: August 9, 2020 9:04 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Star Eccentricity Help

Okay thanks

I'm poking around in your logs some, and I'm waiting to hear back from someone else I asked a Q about, but I can see your RA corrections are primarily one-sided at higher altitudes (and a corresponding increase in RA and therefore your eccentricity goes up), and as you get lower in the sky, RA corrections become more even on both sides, and your RA RMS and eccentricity both go down

My suspicion is mirror flop or something shifting in your equipment

On Sun, Aug 9, 2020 at 8:54 AM Tony Benjamin <tonybenjamin@...> wrote:

No, no seperate guide scope. I'm using an Ultrastar guide camera on a QHYOAG.

--

Brian

Brian Valente

--

Brian

Brian Valente

--
Brian

Brian Valente

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