No subject
Thu Feb 4 12:37:29 PST 2010
g from the following.
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1. Air turbulence
2. Tube current
3. Spherical aberration
4. Rough surfaces
5. Zonal aberrations
6. Turned edges
7. Astigmatism=20
8. Pinched optics
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Hope you enjoyed=2C Roger
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The moon and stars to govern the night..... Psalm 136:9
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All=2C<BR>
 =3B<BR>
Hey guys=2C just some good information. I am sure many are aware of the fol=
lowing information=2C but it is still some good reading.  =3B<BR>
 =3B<BR>
Do you know how to star test your telescope? Have you ever collimated your =
scope in the daytime using an artificial star? =3B<BR>
 =3B<BR>
1. <STRONG>Artificial star:</STRONG> Any very shiny well polished sphere. I=
use a =3Bround brass cabinet knob that I sit on the top of a =3Bho=
llow metal tube. =3BSome people use an insulator on a utility pole at a=
distance=2C or the corner of a chromed auto bumper. There is a formula for=
the distance=2C but hey don't worry about this. =3BJust make sure that=
the =3Bsphere is at least a few hundred feet away and you can focus on=
it. =3BA larger scope requires =3Ba greater distance.  =3B<BR>
 =3B<BR>
During the early mornings when the wind is calm and the sun is shinning ver=
y brightly. =3BSit your =3Bscope is in the shade=2C to avoid heat o=
n the OT. Observe =3Bthe disc at very high magnification. Look for the =
<STRONG>airy disc</STRONG>. Are they of the same brightness all around the =
star? Now de-focus your scope. =3BAre the rings on the inside of focus =
exactly the same as the outside? Very few scopes will pass this test. <STRO=
NG>Probably the most sensitive =3Btest of all. </STRONG>I have seen "te=
xtbook" images =3Bin only one scope that I =3Bhave ever tested=2C&n=
bsp=3Ba Takahashi 4-inch refractor=2C and I think it was a 5-inch Astro-Phy=
sics refractor. =3BOf course we all know that these are high resolution=
=2C very high quality =3Bscopes=2C and cost as much as a very good used=
car. I attempted to do a test on a Questar one night=2C but the seeing was=
not good enough.  =3B =3B =3B =3B =3B =3B<BR>
 =3B<BR>
Are you familiar with the <STRONG>"airy disc"</STRONG> named after 19th cen=
tury scientist Sir George Airy. This is a critical test of your scope. Have=
you ever observed the airy disc =3Bin your scope?<BR>
No=2C this is not the out of focus diffraction rings=2C <STRONG>but the ver=
y faint ring or rings surrounding the perfect in focus star at very high ma=
gnification. =3BThis can be almost impossible for a larger scope as&nbs=
p=3Bit =3Brequires at least 30x per inch of aperture. =3BIf this te=
st is performed at night "almost" perfect seeing is required.</STRONG><BR>
<STRONG></STRONG> =3B<BR>
<STRONG>I had an excellent quality 80 mm f/15 Japanese refractor quite a fe=
w years ago. I noticed when working on double stars =3Bthat the airy di=
sc was brighter on one side as compared to the =3Bother (180=B0). Howev=
er=2C it was =3Bvery slight and I could not see the miscollimation whil=
e observing or defocusing on a star. =3BA precise adjustment of the&nbs=
p=3Baperture resolved this problem. I am somewhat afraid to mess with the a=
djustments on a refractor. I would never=2C let me repeat=2C never =3Bt=
ouch =3Ban Astro-Physics=2C Tak=2C or any other $5=2C000 (+) OTA. This =
is not a job for the amateur=2C wanting to learn. =3B =3B</STRONG><=
BR>
<STRONG></STRONG> =3B<BR>
<STRONG>I also had an ETX 90 with superb optics. When comparing to a $5=2C0=
00 90 mm Questar the images were not much different. The ETX was a =3B1=
996 model=2C the first year that they were introduced. I am not sure how we=
ll the mirrors are in the newer=2C made in China ETX's. The same problem as=
the 80 mm. At very high magnification the airy disc was brighter on one si=
de=2C again=2C exactly 180=B0.</STRONG><BR>
<STRONG></STRONG> =3B<BR>
<STRONG> =3BI could move the test star very slightly from the center an=
d could find perfect collimation within the scope. Being a =3BMaksutov-=
Cassegrain scope without those primary adjustments screws (Laura =3Bwas=
talking about this =3Bin an e-mail a couple of days ago=2C regarding a=
certain =3B100 mm Newtonian) I could not put the scope in perfect coll=
imation. It was so slight that the performance of the scope was not really =
compromised. A great little scope indeed=2C that would split double stars f=
ar beyond its limits=2C so this would indicate the miscollimation was not t=
hat great. </STRONG><BR>
 =3B<BR>
<STRONG>A good book that I would like to recommend that will explain everyt=
hing above and much more.</STRONG><BR>
<STRONG></STRONG> =3B<BR>
<STRONG><FONT size=3D3><EM>Star Testing Astronomical Telescopes </EM>by Har=
old Richard Suiter. This guy is really a genius at optics. He is an experim=
ental physicist.</FONT></STRONG><BR>
<STRONG><FONT size=3D3></FONT></STRONG> =3B<BR>
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