Ah, OK, not a big user of PI
but the chart helps. Thanks...
Mike J. Shade
Mike J. Shade Photography:
mshadephotography.com
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
Antares blinks a sad farwell to fall...
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 1:07 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] 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.
toggle quoted messageShow quoted text
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.
Mike
J. Shade
Mike
J. Shade Photography:
mshadephotography.com
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
Antares
blinks a sad farwell to fall...
Leslie
Peltier, Starlight Nights
International
Dark Sky Association: www.darksky.org
>>>
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
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
No, no seperate guide scope. I'm using an Ultrastar guide camera on
a QHYOAG.
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