Checking Optics of 10" Dob in Day Time?

In article , Dan Chaffee
wrote:


Actually, Carl does measure astigmatism with the simple foucault test
and the couder mask. My experience is that large and medium scale
roughness is VERY easy to recognize and I had no problem seeing this
in my ealiest mirror making days. Micro-ripple is tougher to confirm
and requiring some experience, but not as damaging as large and medium
scale roughness.

Dan,

Astigmatism is very hard to measure in a Foucault test unless it is
really bad. I participated in a mirror testing program with the ATM
list. One of the mirror was astigmatic by about 1/4 wave. Of the 15
people who tested the mirror by Foucault, Ronchi, and other simple
bench tests, NOT ONE OF THE TESTERS IDENTIFIED ASTIGMATISM! However,
there were about four interferometry tests, and all four tests easily
identified the astigmatism.

I know that Carl tests for astigmatism. It is easier to identify
BEFORE PARABOLIZING with a "ball bearing" bench test. I suspect that
that is how Carl does it, but I don't know.

Back to Wayne. You can do a star test either at night or in the day
using a remote star or artificial star (see Suiter's book.) Any more
sophisticated bench test will require you to remove the mirror from the
telescope. You could track down a mirror maker to help you set up a
bench test like the Foucualt test. If you wanted to I could help you
do it, but we are not very close in space.

Scott

it is well collimated. I did it about 4 months ago

I try to check mine weekly, if not nightly.
jc

--
http://mysite.wanadoo-members.co.uk/jc_atm/

"W. Wat son" wrote:

I'd like to check the optics of my Dob during the day and before this
evening. Are there some simple tests I can run? I can at least, I think,
get it into a dark area. We have many windows. I could take to an office
where it is very dark.

Look at a relection of the sun from a *distant spherical* object like a
Christmas tree ball.

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Scott wrote:

Astigmatism is very hard to measure in a Foucault test unless it is
really bad. I participated in a mirror testing program with the ATM
list. One of the mirror was astigmatic by about 1/4 wave. Of the 15
people who tested the mirror by Foucault, Ronchi, and other simple
bench tests, NOT ONE OF THE TESTERS IDENTIFIED ASTIGMATISM!
However,
there were about four interferometry tests, and all four tests easily
identified the astigmatism.

I know that Carl tests for astigmatism. It is easier to identify
BEFORE PARABOLIZING with a "ball bearing" bench test. I suspect that
that is how Carl does it, but I don't know.

In meassage # 4245 on the Zambuto Yahoo group, Carl goes into detail
in how he uses zonal measurements to calculate the astigmatism of a
full
fledged paraboloid. Since it's a private group, I'm not at liberty
to cut and past on a public forum, but anyone serious about mirror
making should log on that group and read what a very magnanimous
master has to reveal about the many fine points of the craft. We are
talking here about classic astigmatism, not smaller higher order
assymetrical errors that will indeed most likely elude skillful
measurements.

I don't try to measure astigmatism because it is simply easier to check
for in the ball bearing test. If I detect it there,it's back to
grinding. If I can't see it in the ball bearing test, I have no reason
to bother to measure it.

Dan

In meassage # 4245 on the Zambuto Yahoo group, Carl goes into detail
in how he uses zonal measurements to calculate the astigmatism of a
full
fledged paraboloid. Since it's a private group, I'm not at liberty
to cut and past on a public forum, but anyone serious about mirror
making should log on that group and read what a very magnanimous
master has to reveal about the many fine points of the craft. We are
talking here about classic astigmatism, not smaller higher order
assymetrical errors that will indeed most likely elude skillful
measurements.

Good to know. Thanks Dan, I'll have a look.

-tom