testing SCT mirrors OUTSIDE of scope, is it possible and what ins. would I need?

I have a 14" Celestron SCT that I suspect has some sort of optical problem
like spherical aberration. I can't realistically expect to do a good star
test due to poor seeing in my area, and I lack the necessary distance to
place an artificial source. This beast is HEAVY and setting up on a regular
basis only to be disappointed due to lack of stellar diffraction patterns is
getting annoying at the least. The other day, a friend suggested removing
the mirrors for bench testing. Although I have had the scope apart before
(after first correctly nothing the secondary, primary and corrector
positions), I have never removed the mirrors although I don't think it would
be too difficult a task. I have a couple of questions: 1) If I were to
remove each mirror for testing indoors, what equipment would I need? Does
standard "mirror making" test equipment work with SCT mirrors? 2) Would I
even have to remove the primary or could it be tested from within the tube?
3) Has anyone else done this type of testing and can you provide a link for
guidance?

Thanks in advance. BTW, this scope is 6 years old and was purchased used.
I don't think the original owner realized that there was a problem having
always done wide field work, however I don't know that with absolute
certainty.

Ian

--
To reply, remove the "z" if one appears in my address
"Ian Peterson" wrote in message
nk.net...
I have a 14" Celestron SCT that I suspect has some sort of optical
problem
like spherical aberration. I can't realistically expect to do a good
star
test due to poor seeing in my area, and I lack the necessary distance to
place an artificial source. This beast is HEAVY and setting up on a
regular
basis only to be disappointed due to lack of stellar diffraction
patterns is
getting annoying at the least. The other day, a friend suggested
removing
the mirrors for bench testing. Although I have had the scope apart
before
(after first correctly nothing the secondary, primary and corrector
positions), I have never removed the mirrors although I don't think it
would
be too difficult a task. I have a couple of questions: 1) If I were to
remove each mirror for testing indoors, what equipment would I need?
Does
standard "mirror making" test equipment work with SCT mirrors? 2) Would
I
even have to remove the primary or could it be tested from within the
tube?
3) Has anyone else done this type of testing and can you provide a link
for
guidance?

Thanks in advance. BTW, this scope is 6 years old and was purchased
used.
I don't think the original owner realized that there was a problem
having
always done wide field work, however I don't know that with absolute
certainty.

Ian


Ummm, if the seeing is so bad that you can't do a star test (which means
you can't collimate it properly either), what leads you to believe you
have problems with your optics?

Your problems may well be related entirely to your poor seeing, and quite
possibly also to lack of collimation. And you have said nothing about
what you are doing to ensure your scope is reaching thermal equilibrium,
which for a scope this size, and this configuration, will take a LONG
time, unless you have supplemental cooling equipment in use. And, if your
scope is not fully equilibrated, you are not going to have a good star
test, regardless of seeing OR collimation...

Before you can begin to evaluate your scope's optics, you MUST be looking
through a critically collimated, and fully equilibrated telescope...

Finally, if, as you state, your seeing is consistently bad, all the bench
testing in the world isn't gonna' get that scope to perform any better if
it's always seeing-limited...

Perhaps before you start taking the scope apart again, you should take it
somewhere that DOES have good seeing, and try it THERE. And better yet,
do this with someone present who's familiar with the care and feeding of
SCT's... THEN decide if the scope needs further evaluation

--
To reply, remove the "z" if one appears in my address
"Ian Peterson" wrote in message
nk.net...
I have a 14" Celestron SCT that I suspect has some sort of optical
problem
like spherical aberration. I can't realistically expect to do a good
star
test due to poor seeing in my area, and I lack the necessary distance to
place an artificial source. This beast is HEAVY and setting up on a
regular
basis only to be disappointed due to lack of stellar diffraction
patterns is
getting annoying at the least. The other day, a friend suggested
removing
the mirrors for bench testing. Although I have had the scope apart
before
(after first correctly nothing the secondary, primary and corrector
positions), I have never removed the mirrors although I don't think it
would
be too difficult a task. I have a couple of questions: 1) If I were to
remove each mirror for testing indoors, what equipment would I need?
Does
standard "mirror making" test equipment work with SCT mirrors? 2) Would
I
even have to remove the primary or could it be tested from within the
tube?
3) Has anyone else done this type of testing and can you provide a link
for
guidance?

Thanks in advance. BTW, this scope is 6 years old and was purchased
used.
I don't think the original owner realized that there was a problem
having
always done wide field work, however I don't know that with absolute
certainty.

Ian


Ummm, if the seeing is so bad that you can't do a star test (which means
you can't collimate it properly either), what leads you to believe you
have problems with your optics?

Your problems may well be related entirely to your poor seeing, and quite
possibly also to lack of collimation. And you have said nothing about
what you are doing to ensure your scope is reaching thermal equilibrium,
which for a scope this size, and this configuration, will take a LONG
time, unless you have supplemental cooling equipment in use. And, if your
scope is not fully equilibrated, you are not going to have a good star
test, regardless of seeing OR collimation...

Before you can begin to evaluate your scope's optics, you MUST be looking
through a critically collimated, and fully equilibrated telescope...

Finally, if, as you state, your seeing is consistently bad, all the bench
testing in the world isn't gonna' get that scope to perform any better if
it's always seeing-limited...

Perhaps before you start taking the scope apart again, you should take it
somewhere that DOES have good seeing, and try it THERE. And better yet,
do this with someone present who's familiar with the care and feeding of
SCT's... THEN decide if the scope needs further evaluation

"Ian Peterson" wrote in message
nk.net...
I have a 14" Celestron SCT that I suspect has some sort of optical problem
like spherical aberration. I can't realistically expect to do a good star
test due to poor seeing in my area, and I lack the necessary distance to
place an artificial source. This beast is HEAVY and setting up on a
regular
basis only to be disappointed due to lack of stellar diffraction patterns
is
getting annoying at the least. The other day, a friend suggested removing
the mirrors for bench testing. Although I have had the scope apart before
(after first correctly nothing the secondary, primary and corrector
positions), I have never removed the mirrors although I don't think it
would
be too difficult a task. I have a couple of questions: 1) If I were to
remove each mirror for testing indoors, what equipment would I need? Does
standard "mirror making" test equipment work with SCT mirrors? 2) Would I
even have to remove the primary or could it be tested from within the
tube?
3) Has anyone else done this type of testing and can you provide a link
for
guidance?

Ian,

There is really no point in taking the scope apart - and some risk. It was
designed to work as a unit and should be tested that way. The easiest way
is to wait for the proper night to do star testing, or to test with a
distant "point source." Folks often use the reflection off an insulator or
ball bearing for the latter. Perhaps there are some folks in your area who
are experienced at star testing and would be willing to look at it for you.

One way to test would be a null test under autocollimation. To do it right
you need a flat somewhat larger than the clear aperture - obviously not a
commonly available item in 14.5" size. You could do it with an oil flat if
you set up the scope pointing downward into the flat. This will show you
how the optics "look," but you'd need to do some work to put any numbers on
what you see. If you do this you are certainly going to see a bit of S.A.
and probably some roughness - perfect scopes exist more often in the mind
than in reality.

I suggest making sure the scope is collimated and has enough time to come to
equilibrium, and then simply see how it works. Do you like the views it
provides? It should certainly provide nice views of deep sky objects under
dark skies, and the aperture should allow nice lunar and planetary views.
If you are looking for a nice textbook Airy disk and diffraction rings I
think you will be disappointed. A 14" scope is often seeing limited,
sometimes thermally limited, and rarely shows textbooks star images. If you
are looking to enjoy the benefits of a large aperture under dark skies I
think you will not be disappointed.

Avoid the dark side of the hobby - don't be the type of amateur astronomer
who sees only aberrations in telescopes. There are a lot of great things to
see, especially when you have a reasonable amount of aperture.

Clear skies, Alan

"Ian Peterson" wrote in message
nk.net...
I have a 14" Celestron SCT that I suspect has some sort of optical problem
like spherical aberration. I can't realistically expect to do a good star
test due to poor seeing in my area, and I lack the necessary distance to
place an artificial source. This beast is HEAVY and setting up on a
regular
basis only to be disappointed due to lack of stellar diffraction patterns
is
getting annoying at the least. The other day, a friend suggested removing
the mirrors for bench testing. Although I have had the scope apart before
(after first correctly nothing the secondary, primary and corrector
positions), I have never removed the mirrors although I don't think it
would
be too difficult a task. I have a couple of questions: 1) If I were to
remove each mirror for testing indoors, what equipment would I need? Does
standard "mirror making" test equipment work with SCT mirrors? 2) Would I
even have to remove the primary or could it be tested from within the
tube?
3) Has anyone else done this type of testing and can you provide a link
for
guidance?

Ian,

There is really no point in taking the scope apart - and some risk. It was
designed to work as a unit and should be tested that way. The easiest way
is to wait for the proper night to do star testing, or to test with a
distant "point source." Folks often use the reflection off an insulator or
ball bearing for the latter. Perhaps there are some folks in your area who
are experienced at star testing and would be willing to look at it for you.

One way to test would be a null test under autocollimation. To do it right
you need a flat somewhat larger than the clear aperture - obviously not a
commonly available item in 14.5" size. You could do it with an oil flat if
you set up the scope pointing downward into the flat. This will show you
how the optics "look," but you'd need to do some work to put any numbers on
what you see. If you do this you are certainly going to see a bit of S.A.
and probably some roughness - perfect scopes exist more often in the mind
than in reality.

I suggest making sure the scope is collimated and has enough time to come to
equilibrium, and then simply see how it works. Do you like the views it
provides? It should certainly provide nice views of deep sky objects under
dark skies, and the aperture should allow nice lunar and planetary views.
If you are looking for a nice textbook Airy disk and diffraction rings I
think you will be disappointed. A 14" scope is often seeing limited,
sometimes thermally limited, and rarely shows textbooks star images. If you
are looking to enjoy the benefits of a large aperture under dark skies I
think you will not be disappointed.

Avoid the dark side of the hobby - don't be the type of amateur astronomer
who sees only aberrations in telescopes. There are a lot of great things to
see, especially when you have a reasonable amount of aperture.

Clear skies, Alan

Alan is right when he suggest "Avoid the dark side of the hobby - ",
but then you should stop reading SAA so much and observe more

Old Dust

"Alan French" wrote in message




--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

Alan is right when he suggest "Avoid the dark side of the hobby - ",
but then you should stop reading SAA so much and observe more

Old Dust

"Alan French" wrote in message




--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

"Ian Peterson" wrote in message news:
I can't realistically expect to do a good star
test due to poor seeing in my area, and I lack the necessary distance to
place an artificial source.
You might be surprised to find that a neighbor would cooperate with you
on setting up an artificial source for testing. All you need is a place to
hang a shiny Xmas tree ornament where it is in the bright sun and you
can see it from your yard. My neighbor was very cooperative and let me
hang a Xmas ball on a shed in her back yard about 500 ft away. Worked
fine for me.
Clif

"Ian Peterson" wrote in message news:
I can't realistically expect to do a good star
test due to poor seeing in my area, and I lack the necessary distance to
place an artificial source.
You might be surprised to find that a neighbor would cooperate with you
on setting up an artificial source for testing. All you need is a place to
hang a shiny Xmas tree ornament where it is in the bright sun and you
can see it from your yard. My neighbor was very cooperative and let me
hang a Xmas ball on a shed in her back yard about 500 ft away. Worked
fine for me.
Clif

Actually, you need the distance for the artificial star to allow the scope
to properly focus on the image at something relatively far away from the
optics. You can use a collimated light source but that is something
alltogether different.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!