Hi Paul...I've got that one but what I'm trying to do is extrapolate that to the 4x6"photo print of my star trails.
If I had used the st-4 or st7e I know this would be a lot easier than photo analysis.
jimmy
From: Paul Sterngold <psterngold@...>
Reply-To: Losmandy_users@...
To: Losmandy_users@...
Subject: RE: [Losmandy_users] G11 Fix (III)
Date: Wed, 23 Aug 2000 06:50:25 -0700 (PDT)
A simpler formula for #1 below is:
Arc-secs per pixel = (pixel width / FL) * 206
You can nearly do this one in your head. Certainly with a simple
calculator.
It provides almost exactly the same results. This is the formula
recommended in several articles including the S&T website (Sky Pub).
Paul Sterngold
--- Neal Barry <nbarry@...> wrote:
Jimmy,
1. You can calculate arcseconds per pixel with the following formula:
ArcSec = 3600 * ARCTAN( (PixelWidth * 10^-6) / FL )
Where:
ArcSec = Width of Pixel in Arcseconds
PixelWidth = Width of CCD pixel in microns
FL = Objective focal length in meters
For Example, the TV 101 with ST8 would be:
ArcSec = 3600 * ARCTAN ( ( 9 * 10^-6) / .540 )
ArcSec = 3.44
2. If you want to find the dimensions of a projected image (ie, a large
format camera) the formula above works as well. Just pretend that a
pixel
is
the size of the negative. There is no direct way to state the angular
dimensions of a photographic print unless you know the FL of the
objective
lens, size of the negative, AND the magnification used during printing.
3. You can estimate the arcsecond tracking error by visually measuring
the
peak to peak pixel deviation of the star trail from an ideal mean trail.
Then multiply this deviation by the arcseconds per pixel resolution of
your
CCD camera. I wrote a small Visual C program a while back to calculate
it
more precisely directly from the image file. I'd send you a copy, but I
can't seem to find it right now.
4. Yes, doubling the FL roughly halves the arcseconds per pixel unless
you
are dealing with very short focal lengths.
Neal
-----Original Message-----
From: James A. Thibert [mailto:thibertj@...]
Sent: Tuesday, August 22, 2000 7:14 PM
To: Losmandy_users@...
Subject: Re: [Losmandy_users] G11 Fix (III)
Hi Neal,it's looking good.I put a third 1"long bearing in each of
my axies
and it improved nicely but not like yours.
By coincidence,I offset my mount and took a few before and after
startrails.
The after star trailsARE VERY REULAR AND i TOO USED A 101 POINTED
AT THE M31
REGION.I used a 2x big barlow in front of my camera.
My star trails can be described as 2 different appearing trails:
the bright stars look loke a straight linr composed of light balls
(stars
)connecetd together like a string of pearls.
The dimmer stars leave wavy trails but are much thinner of course
than the
bright stars.
How would I measure the arcsec error?
I had tried to calculate my arcsec resolution with a 12.5mm ortho
eyepiece
and on the 60mm scale it took 300 seconds for a star to go end to
end.I
calculate dthe that give an st-4 pixel is 13.5 x 16 microns that
on the ra
side,1 pixel at prime focus would be 5.5 arcsecs.The 2x barlow
would halve
that would it not?
If correct,how then do you extrapolate that to a 4 by 6"photo?
I shot 30 minutes at the pleades and have round stars even under
an 8 power
magnifier.
Can you shed any light on this for me?
thanks
Jimmy
From: "Neal Barry" <nbarry@...>
Reply-To: Losmandy_users@...
To: <Losmandy_users@...>
Subject: [Losmandy_users] G11 Fix (III)
Date: Tue, 22 Aug 2000 12:55:59 -0700
Hi all,
Well, I'm reasonably pleased with the improvement that I have
been able to
achieve in my G11's tracking and periodic error. Take a look at
to see the
results.
I am curious how the PE shown on the web page above compares to other
G11
mounts out there. I still am not sure if the problems with the mount
I
received are typical or a fluke.
I have made the fix permanent since my original post back on 8/13,
but
I
did
risk damaging the mount in the process. Specifically, I pressed in a
new
needle bearing with .002" stainless steel shim stock wrapped around
the
OUTSIDE of the bearing (I know, I was against it in an earlier
post ;-) I
do
not recommend this! You can easily damage your mount unless you are
VERY
careful. Five things can go wrong.
1. The needles in the bearing can seize.
2. The bearing can easily distort due to the amount of force require
to
press it in.
3. The fit can bee too good, and the shaft will bind due to the
relative
misalignment of all three needle bearings.
4. You can permanently knock the RA wheel gear bearing sleeve off
axis
or
bend it.
5. You can expand the OD of the RA wheel gear bearing sleeve so that
the
wheel gear will no longer fit.
However, this strategy did cause the ID of the needle bearing to
shrink
(exactly the amount needed in my particular case) and I now have a
very
nice
on-axis fit with no measurable slop. The OD of the RA wheel gear
sleeve
bearing did expand slightly (approx .0002") and I needed to lap
it a bit to
ensure that it would not bind over an appreciable operating
temperature
range.
Anyway, now that the RA shaft was not flopping around, I was
finally able
to
see how well the mount would track. Now I had a fairly regular
PE of about
11-12 arc seconds. After lapping the RA worm and wheel gear it
dropped
to
under 10 and became even more regular. After training the
SkyWalker II PEC,
it tracks within 2 arc seconds. Not too shabby given where this all
started.
Neal
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