different schmidt collimation inside and outside of focus

I was carefully collimating a new Celestron 9.25 with a
celestron .63 reducer last night and I noticed something odd. I was
using robofocus to move inside or outside focus the same amount
(perhaps 10 wavelengths). When I collimated inside to be concentric
rings then went outside it was perhaps 10-15% off. When I
collimated outside and went inside it was 10-15% off. I ended up
collimating in the middle to balance the two figuring whatever
effect it was would cancel at focus. I used focus max to focus and
the resulting stars were about 2.6 arcseconds FWHM in a .06 second
exposure using the focus function in maxim. The stars seemed symmetric.

My question is, what optical configuration problem leads to the
optimal collimation being different inside and outside. I could not
find anything in Harold Suiter's book to explain it, and I don't think my
secondary is 10-15% off in the center of the corrector

Thanks

Web

Could be a misaligned baffle. Also could be a thermal effect. Was the
scope thoroughly acclimated?

First thing I would do, before condemning the scope, is remove the .63
Reducer/Corrector, and the Robofocus Unit, then try collimating again.
I would assume a Robo-focus, or other Crayford Focuser attached to the
rear cell would eliminate any possibility of the influence of mirror
shift, but for the heck of it, nect time you're out with the scope, try
what I suggest, and see if you notice any differences.

I've personally never heard of anyone ever using a reducer while
Collimating an SCT.

If your Diagonal is known to be good, Collimating with this in place
shouldn't be a problem, but some will Collimate without any optional
item attached to the rear cell, other than a 1-1/4" Visual Back.

Best Collimation will always be achieved at the highest magnifications
(seeing permitting) Mark

"Mark D" wrote in message
...
First thing I would do, before condemning the scope, is remove the .63
Reducer/Corrector, and the Robofocus Unit, then try collimating again.
I would assume a Robo-focus, or other Crayford Focuser attached to the
rear cell would eliminate any possibility of the influence of mirror
shift, but for the heck of it, nect time you're out with the scope, try
what I suggest, and see if you notice any differences.


Agreed, or, at LEAST remove the r/c...

I've personally never heard of anyone ever using a reducer while
Collimating an SCT.

I have, and I was never happy with the result... So I now always
collimate without the r/c.


If your Diagonal is known to be good, Collimating with this in place
shouldn't be a problem, but some will Collimate without any optional
item attached to the rear cell, other than a 1-1/4" Visual Back.

Best Collimation will always be achieved at the highest magnifications
(seeing permitting) Mark

Agreed.

"webdove" wrote in message
...
I was carefully collimating a new Celestron 9.25 with a
celestron .63 reducer last night and I noticed something odd. I was
using robofocus to move inside or outside focus the same amount
(perhaps 10 wavelengths). When I collimated inside to be concentric
rings then went outside it was perhaps 10-15% off. When I
collimated outside and went inside it was 10-15% off. I ended up
collimating in the middle to balance the two figuring whatever
effect it was would cancel at focus. I used focus max to focus and
the resulting stars were about 2.6 arcseconds FWHM in a .06 second
exposure using the focus function in maxim. The stars seemed symmetric.

My question is, what optical configuration problem leads to the
optimal collimation being different inside and outside. I could not
find anything in Harold Suiter's book to explain it, and I don't think
my
secondary is 10-15% off in the center of the corrector

Thanks

Web
Are you _sure_ the backlash setting in RoboFocus is large enough?. What
you describe is exactly what would happen if this was slightly too low, so
the baffle is not quite positioned to the same angle when you focus in
each direction. Maxim has it's own 'backlash' setting, and if this was not
zero, the problem would then disappear (ideally get the Robo backlash
working, and disable all other backlash corrections).

Best Wishes

My question is, what optical configuration problem leads to the
optimal collimation being different inside and outside. I could not
find anything in Harold Suiter's book to explain it, and I don't think my
secondary is 10-15% off in the center of the corrector

Just a guess at this point, but are you having the mirror flop
back and forth?

Clear Skies

Chuck Taylor
*********************************************
Do you observe the moon? If so, try
http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

First remove any extraneous optics from the optical path.

Second, be aware that there is only 1 (back focal) distance at which
all the aberations are optimized. This distance corresponds to the
normal attachments (diagonal,...). Go for a high power EP and a
moderately bright star--dim enough for you to see the diffraction
pattern with out glare from the central star, bright enough to see the
diffraction pattern clearly. For a 9.25" SCT this should be about M4.

Third, be aware that colimation at 10 waves is at best rought
collimation. After you get it roughly collimated, move inward to 3-5
waves and continue fine collimating. When this is optimized, then
proceede to critical collimation which is performed at the point where
the out-of-focus disk is collapsing into the focused airy pattern
(about 1-1.5 waves OOF). Here you are looking for symetry around the
not quite collapsed ring. You shold be able to get the not quite
collapsed ring to be symetrical on both sides of focus.

Use lots of magnification--at this point a barlow and your highest
power ep helps to see the diffraction pattern. I often use upwards of
130X/inch here. Say 3mm or 6mm and a 2X barlow. With a bright star,
you can use magnifications that are simply unreasonable for general
observations. You are trying to see the diffraction pattern of focus
not the image of the telescope. A subtle difference.

Ignore the spherochromatism inherent in these scopes while looking at
the symetry of the not quite collapsed ring. Look only at the intensity
and not the color pattern. It is common that one side will have some
flare like spikes around the perifery of the not quite collapsed ring.
Don't worry about these, worry only about the symetry of the ring and
get it similar on both sides of focus.

Then go for a cold one, you will have about expended all the mental
energy you can muster this night. And don't be worried if it takes
several nights of semi-frustration. Once you do get it perfect, the
scope will performe so much better you won't be able to believe it.
Then, once it is perfect, go very far out of your way never to subject
the scope to any jarring and it will hold collimation for decades!

Then go for a cold one, you will have about expended all the mental
energy you can muster this night. And don't be worried if it takes
several nights of semi-frustration. Once you do get it perfect, the
scope will performe so much better you won't be able to believe it.
Then, once it is perfect, go very far out of your way never to subject
the scope to any jarring and it will hold collimation for decades!


I wonder how many SCT owners collimate to anywhere near this degree of
accuracy? In fact, I wonder how many collimate at all? And given the
mirror flop issue, just how meaningful is extremely accurate collimation
anyway? And how far out of one's way can one go to avoid "jarring", when
one of the features of SCTs is their portability? Our club's observatory is
up a bumpy dirt road!

Dennis

"Dennis Woos" wrote in message
...
Then go for a cold one, you will have about expended all the mental
energy you can muster this night. And don't be worried if it takes
several nights of semi-frustration. Once you do get it perfect, the
scope will performe so much better you won't be able to believe it.
Then, once it is perfect, go very far out of your way never to subject
the scope to any jarring and it will hold collimation for decades!


I wonder how many SCT owners collimate to anywhere near this degree of
accuracy?

Not many. And not many have even completed critical collimation WITHOUT
mechanical aids like Robofocus. And that's a real shame, because the
latter is not difficult at all, though finding a night when you can dial
up the magnification enough to get all the way there can be elusive.

There are very few folks who have ever looked through a critically
collimated and fully equilibrated SCT, where both conditions were as they
should be at the same time...

But because this is the case doesn't mean it is especially difficult to
achieve. Most folks simply don't know how, and don't understand how much
image quality is impacted when you DON'T have collimation AND
equilibration where they should be.

In fact, I wonder how many collimate at all?

I think there are quite a few that have never gone beyond getting that big
black out of focus blob into the center of the field... And there are a
smaller number who have gone to the next level and gotten the Fresnel
rings concentric on an out of focus centered star. But, of course, that
is not critical collimation, either... And yet the last step is just
pretty much a repeat of the previous one, but at much higher
magnification, and IN focus, getting a couple diffraction rings concentric
with the Airy disk of a centered star, versus the previous step getting
out of focus Fresnel rings concentric at lower power ...

And given the mirror flop issue, just how meaningful is extremely
accurate collimation
anyway?

Given all these distractions, do YOU ever observe?

Mirror flop is certainly an issue for imaging, but it's really no big deal
visually (and if you want to recheck your collimation in the event of
mirror flop going over the top, go ahead and DO it).

Folks with Dobsonians/Newtonians have to collimate regularly for most of
the same reasons, other than mirror flop, and there is a lot more that can
come out of collimation with them; yet I don't see too many of THEIR
owners rending their hair, or screaming and running from the field...

So, it depends on how the scope is being used (which was not mentioned in
the original post), how significant mirror flop is. For visual use it is
of little significance. For imaging, there are ways to work around mirror
flop, though I don't know of a simple fix (but, then, I'm not an imager,
either)... So, should everyone with an SCT simply throw up their hands
and run screaming from the field in fear of having to collimate??? I
don't THINK so...

And how far out of one's way can one go to avoid "jarring", when
one of the features of SCTs is their portability?

Right. So folks should be prepared to recollimate after a bumpy trip...
Probably after ANY trip. Collimation is easy. The degree you can achieve
critical collimation is a function of the seeing, and THAT will determine
how close you can get to critical collimation on any given night. But it
is certainly easy enough to check your collimation every time you set up,
once the scope has equilibrated, even if you can't run the scope up to 50
or 60X/inch to do the last step all the way...

Our club's observatory is up a bumpy dirt road!


Some of life's better things are worth a little extra work! So drive up
that bumpy road and then collimate...

--
Jan Owen

To reach me directly, remove the Z, if one appears in my e-mail address...
Latitude: 33.6
Longitude: -112.3


Dennis

Good thought Roger,

I will try removing the reducer, and try increasing the backlash to see if
either helps.

Web

"Roger Hamlett" wrote in message
...

"webdove" wrote in message
...
I was carefully collimating a new Celestron 9.25 with a
celestron .63 reducer last night and I noticed something odd. I was
using robofocus to move inside or outside focus the same amount
(perhaps 10 wavelengths). When I collimated inside to be concentric
rings then went outside it was perhaps 10-15% off. When I
collimated outside and went inside it was 10-15% off. I ended up
collimating in the middle to balance the two figuring whatever
effect it was would cancel at focus. I used focus max to focus and
the resulting stars were about 2.6 arcseconds FWHM in a .06 second
exposure using the focus function in maxim. The stars seemed symmetric.

My question is, what optical configuration problem leads to the
optimal collimation being different inside and outside. I could not
find anything in Harold Suiter's book to explain it, and I don't think my
secondary is 10-15% off in the center of the corrector

Thanks

Web
Are you _sure_ the backlash setting in RoboFocus is large enough?. What
you describe is exactly what would happen if this was slightly too low, so
the baffle is not quite positioned to the same angle when you focus in
each direction. Maxim has it's own 'backlash' setting, and if this was not
zero, the problem would then disappear (ideally get the Robo backlash
working, and disable all other backlash corrections).

Best Wishes