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
Hotmail: Trusted email with Microsoft=92s powerful SPAM protection. Sign up=
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All=2C<BR>
 =3B<BR>
I've used everything from an =3Boatmeal box with a light inside and a p=
inhole on =3Bthe side=2C to the old reliable insulator on a telephone p=
ole=2C to Polaris with a Ronchi grating. I prefer the Ronchi grating on a b=
right star=2C and not usually Polaris as it's kind of dim. In some ways=2C =
maybe you don't want to star test your optics either. If you suspect someth=
ing major wrong=2C sure. However=2C if you are happy and just curious=2C it=
may not be wise to dash your hopes and dreams by finding your optics are a=
stigmatic=2C over/under corrected=2C have a turned down edge. Sometimes it'=
s better to let sleeping dogs =3Blay/lie=2C whatever...<BR>
 =3B<BR>
Fred<BR>
<BR> =3B<BR>
<HR id=3DstopSpelling>
From: drivester at hotmail.com<BR>To: lvas at lvlug.org<BR>Date: Wed=2C 17 Feb 20=
10 13:45:09 +0000<BR>Subject: [LVAS] Star Testing a Telescope<BR><BR>
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All=2C<BR> =3B<BR>Hey guys=2C just some good information. I am sure man=
y are aware of the following information=2C but it is still some good readi=
ng.  =3B<BR> =3B<BR>Do you know how to star test your telescope? Ha=
ve you ever collimated your scope in the daytime using an artificial star?&=
nbsp=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 =3Bhollow 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.&nbs=
p=3BJust make sure that the =3Bsphere is at least a few hundred feet aw=
ay 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 very brightly. =3BSit your =3Bscope=
is in the shade=2C to avoid heat on the OT. Observe =3Bthe disc at ver=
y high magnification. Look for the <STRONG>airy disc</STRONG>. Are they of =
the same brightness all around the star? Now de-focus your scope. =3BAr=
e the rings on the inside of focus exactly the same as the outside? Very fe=
w scopes will pass this test. <STRONG>Probably the most sensitive =3Bte=
st of all. </STRONG>I have seen "textbook" images =3Bin only one scope =
that I =3Bhave ever tested=2C =3Ba Takahashi 4-inch refractor=2C an=
d I think it was a 5-inch Astro-Physics refractor. =3BOf course we all =
know that these are high resolution=2C very high quality =3Bscopes=2C a=
nd cost as much as a very good used car. I attempted to do a test on a Ques=
tar one night=2C but the seeing was not good enough.  =3B =3B =
=3B =3B =3B =3B<BR> =3B<BR>Are you familiar with the <STRON=
G>"airy disc"</STRONG> named after 19th century 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 ri=
ngs=2C <STRONG>but the very faint ring or rings surrounding the perfect in =
focus star at very high magnification. =3BThis can be almost impossible=
for a larger scope as =3Bit =3Brequires at least 30x per inch of a=
perture. =3BIf this test is performed at night "almost" perfect seeing =
is required.</STRONG><BR><STRONG></STRONG> =3B<BR><STRONG>I had an exce=
llent quality 80 mm f/15 Japanese refractor quite a few years ago. I notice=
d when working on double stars =3Bthat the airy disc was brighter on on=
e side as compared to the =3Bother (180=B0). However=2C it was =3Bv=
ery slight and I could not see the miscollimation while observing or defocu=
sing on a star. =3BA precise adjustment of the =3Baperture resolved=
this problem. I am somewhat afraid to mess with the adjustments on a refra=
ctor. I would never=2C let me repeat=2C never =3Btouch =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>&=
nbsp=3B<BR><STRONG>I also had an ETX 90 with superb optics. When comparing =
to a $5=2C000 90 mm Questar the images were not much different. The ETX was=
a =3B1996 model=2C the first year that they were introduced. I am not =
sure how well 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 brighte=
r on one side=2C again=2C exactly 180=B0.</STRONG><BR><STRONG></STRONG>&nbs=
p=3B<BR><STRONG> =3BI could move the test star very slightly from the c=
enter and could find perfect collimation within the scope. Being a =3BM=
aksutov-Cassegrain scope without those primary adjustments screws (Laura&nb=
sp=3Bwas talking about this =3Bin an e-mail a couple of days ago=2C reg=
arding a certain =3B100 mm Newtonian) I could not put the scope in perf=
ect collimation. 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 far beyond its limits=2C so this would indicate the miscollimation w=
as not that great. </STRONG><BR> =3B<BR><STRONG>A good book that I woul=
d like to recommend that will explain everything above and much more.</STRO=
NG><BR><STRONG></STRONG> =3B<BR><STRONG><FONT size=3D3><EM>Star Testing=
Astronomical Telescopes </EM>by Harold Richard Suiter. This guy is really =
a genius at optics. He is an experimental physicist.</FONT></STRONG><BR><ST=
RONG><FONT size=3D3></FONT></STRONG> =3B<BR>From pictures in this book =
you can determine if your star image is suffering from the following.<BR>&n=
bsp=3B<BR>1. Air turbulence<BR>2. Tube current<BR>3. Spherical aberration<B=
R>4. Rough surfaces<BR>5. Zonal aberrations<BR>6. Turned edges<BR>7. Astigm=
atism <BR>8. Pinched optics<BR> =3B<BR>Hope you enjoyed=2C Roger<BR>&nb=
sp=3B<BR> =3B<BR> =3B<BR><BR><FONT face=3DArial> =3B<BR><FONT s=
ize=3D4></FONT> =3B</FONT><BR><FONT face=3DArial></FONT> =3B<BR><FO=
NT face=3DArial></FONT> =3B<BR><FONT face=3DArial></FONT> =3B<BR><F=
ONT face=3DArial></FONT> =3B<BR><FONT face=3DArial></FONT> =3B<BR><=
FONT face=3DArial></FONT> =3B<BR><FONT face=3DArial> =3B<BR><BR>&nb=
sp=3B<BR></FONT><EM><FONT face=3DArial><STRONG> =3B =3B =3B </S=
TRONG></FONT>The moon and stars to govern the night..... =3B =3B Ps=
alm 136:9</EM><BR><BR><BR><BR><BR>
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