| |
Main
Date: 23 Oct 2006 10:00:44
From: dan&lisa
Subject: MN66 owners
|
Hello,
I had my MN66 pointed at Vega a couple of nights ago checking for
collimation. I noticed that the diffraction pattern was fine on one side of
focus while on the other, two of the inner diffraction rings were thicker
and nearly merged. Is this a sign of a defect or inherent in the design?
Dan
|
|
| |
Date: 23 Oct 2006 08:43:01
From: Uncle Bob
Subject: Re: MN66 owners
|
On Mon, 23 2006 10:00:44 -0400, dan&lisa wrote:
> Hello,
>
> I had my MN66 pointed at Vega a couple of nights ago checking for
> collimation. I noticed that the diffraction pattern was fine on one side of
> focus while on the other, two of the inner diffraction rings were thicker
> and nearly merged. Is this a sign of a defect or inherent in the design?
>
> Dan
By thicker, do you mean brighter? I think you want to look for
differences in brightness in the rings inside and outside of focus.
What you're trying to determine is the focal length of different parts of
your mirror. Ideally, all parts should come into focus at the same point,
if there is a zone (region) of the mirror that has a different focal
length than the remainder of the mirror, its reflected energy will be more
diffuse than the other zones, and hence the rings will be of different
brightness.
In the case of a Mak-Newt, which is a compound scope, there
are two elements that are responsible for final figure--the meniscus plate
(corrector plate) and the mirror itself. Before you get all freaked out
over a couple of rings (which might be tube currents, after all) take a
ronchi (150 lines per inch or so) screen and look at a bright star
thorough it. (Don't use an eyepiece). A parabolic figure will show
straight lines in the image, a spherical figure will show curved lines.
You're talking a parabolic figure with your scope (since the corrector
plate changes the figure of the spherical mirror to a parabola).
The lines should be straight. If there are zigs or zags in them, then it
may indicate a zone. But I doubt you'll find a zone in your mirror.
If you don't have a ronchi screen, you can download one and print it out
on transparency stock with a laser printer (like at kinkos). I think the
Stellafane ATM website has some images you can download to make ronchi
screens.
If you find anything strange, check to make sure your corrector plate is
nice and secure, not pinched (it's held to the cell with screws on the
inside of the scope). Don't spin it in the cell. There should be a notch
in the cell that corresponds to a tab on the tube to maintain orientation
of the corrector plate. It's important that you not rotate the corrector
plate to a different position.
Now after typing all that,
you better let us know what you find. ;-)
Uncle Bob
Fairfax, CA
|
| |
Date: 23 Oct 2006 12:01:11
From: atasselli@hotmail.com
Subject: Re: MN66 owners
|
Uncle Bob wrote:
> On Mon, 23 2006 10:00:44 -0400, dan&lisa wrote:
>
> > Hello,
> >
> > I had my MN66 pointed at Vega a couple of nights ago checking for
> > collimation. I noticed that the diffraction pattern was fine on one side of
> > focus while on the other, two of the inner diffraction rings were thicker
> > and nearly merged. Is this a sign of a defect or inherent in the design?
> >
Probably slightly tilted focuser and/or secondary.
...
> If you find anything strange, check to make sure your corrector plate is
> nice and secure, not pinched (it's held to the cell with screws on the
> inside of the scope). Don't spin it in the cell. There should be a notch
> in the cell that corresponds to a tab on the tube to maintain orientation
> of the corrector plate. It's important that you not rotate the corrector
> plate to a different position.
Absolutely irrelevant, as the corrector is symmetric and the field
itself is coma-corrected.
Andrea T.
|
| | |
Date: 23 Oct 2006 15:15:43
From: dan&lisa
Subject: Re: MN66 owners
|
Andrea,
Thanks for your reply. The rings were round, in and out of focus. Wouldn't a
tilted focuser show some type of off-centeredness?
Regards,
Dan
<atasselli@hotmail.com > wrote in message
news:1161630071.512183.326630@e3g2000cwe.googlegroups.com...
>
> Uncle Bob wrote:
>> On Mon, 23 2006 10:00:44 -0400, dan&lisa wrote:
>>
>> > Hello,
>> >
>> > I had my MN66 pointed at Vega a couple of nights ago checking for
>> > collimation. I noticed that the diffraction pattern was fine on one
>> > side of
>> > focus while on the other, two of the inner diffraction rings were
>> > thicker
>> > and nearly merged. Is this a sign of a defect or inherent in the
>> > design?
>> >
>
> Probably slightly tilted focuser and/or secondary.
>
> ...
>> If you find anything strange, check to make sure your corrector plate
>> is
>> nice and secure, not pinched (it's held to the cell with screws on the
>> inside of the scope). Don't spin it in the cell. There should be a notch
>> in the cell that corresponds to a tab on the tube to maintain orientation
>> of the corrector plate. It's important that you not rotate the corrector
>> plate to a different position.
>
> Absolutely irrelevant, as the corrector is symmetric and the field
> itself is coma-corrected.
>
> Andrea T.
>
|
| |
Date: 24 Oct 2006 06:16:20
From: atasselli@hotmail.com
Subject: Re: MN66 owners
|
dan&lisa wrote:
> Andrea,
>
> Thanks for your reply. The rings were round, in and out of focus. Wouldn't a
> tilted focuser show some type of off-centeredness?
>
> Regards,
> Dan
>
Hello Dan,
It wasn't clear from the OP whether it was a change in symmetry or a
change in brightness (or both) hence my replay. Thanks for having clear
my misunderstanding. Beware that most MNs might show a mlid degree of
5th order spherical aberration outside of focus, hence the variation in
brightness of the external rings. The real damage, in focus, is quite
small however (well below anything that might affect performance under
real sky).
Andrea T.
|
| | |
Date: 24 Oct 2006 11:27:26
From: dan&lisa
Subject: Re: MN66 owners
|
Andrea,
I wrote to Mike at ITE and he said that the mak-newt design does not produce
identical images on both sides of focus. He didn't seem concerned about what
I saw. If it was inherent in the design would not all MNs show the same
"defect"?
While I've been an observer for a long time I have never been very
interested in the technical side of scopes so have little experience to go
on in this department.
Dan
<atasselli@hotmail.com > wrote in message
news:1161695780.193027.104790@f16g2000cwb.googlegroups.com...
>
> dan&lisa wrote:
>> Andrea,
>>
>> Thanks for your reply. The rings were round, in and out of focus.
>> Wouldn't a
>> tilted focuser show some type of off-centeredness?
>>
>> Regards,
>> Dan
>>
>
> Hello Dan,
>
> It wasn't clear from the OP whether it was a change in symmetry or a
> change in brightness (or both) hence my replay. Thanks for having clear
> my misunderstanding. Beware that most MNs might show a mlid degree of
> 5th order spherical aberration outside of focus, hence the variation in
> brightness of the external rings. The real damage, in focus, is quite
> small however (well below anything that might affect performance under
> real sky).
>
> Andrea T.
>
|
| | |
Date: 24 Oct 2006 09:42:23
From: dan&lisa
Subject: Re: MN66 owners
|
Hello Andrea,
When I have another clear night I'll try to give you more details of what I
see. The problem is not in the external rings, however, it's the third and
fourth (could be second and third, I'll make a note next time) rings that
are brighter and almost merged. I'm counting the rings starting from the
center.
I noted this with various eyepieces but will try a different barlow next
time.
I purchased the scope from ITE new a few months ago and it went through
their inspection so am giving it the benefit of the doubt for now.
Regards,
Dan
<atasselli@hotmail.com > wrote in message
news:1161695780.193027.104790@f16g2000cwb.googlegroups.com...
>
> dan&lisa wrote:
>> Andrea,
>>
>> Thanks for your reply. The rings were round, in and out of focus.
>> Wouldn't a
>> tilted focuser show some type of off-centeredness?
>>
>> Regards,
>> Dan
>>
>
> Hello Dan,
>
> It wasn't clear from the OP whether it was a change in symmetry or a
> change in brightness (or both) hence my replay. Thanks for having clear
> my misunderstanding. Beware that most MNs might show a mlid degree of
> 5th order spherical aberration outside of focus, hence the variation in
> brightness of the external rings. The real damage, in focus, is quite
> small however (well below anything that might affect performance under
> real sky).
>
> Andrea T.
>
|
| |
Date: 24 Oct 2006 17:00:21
From: atasselli@hotmail.com
Subject: Re: MN66 owners
|
Dennis Woos wrote:
...
> >
>
> From the Company Seven website, describing the Orion Argonaut 6" Mak-Newt:
>
> Those who like to play "telescope testing" are likely to be pleasantly
> surprised at the Airy disk and diffraction ring patterns appearing distinct
> and nearly symmetrical in and out of focus, with no signs of zones or
> roughness - tell tale signs of patient and professional mirror making.
>
> I don't think the original poster is talking about very small asymmetries.
>
They are not "very small". Quite noticeable in fact.
Andrea T.
|
| | |
Date: 24 Oct 2006 21:51:21
From: dan&lisa
Subject: Re: MN66 owners
|
Andrea,
Thanks for putting my mind at ease. I do find the in focus images to be very
good.
The scope also cools down better than I thought it would with temps dipping
below 0°C.
Dan
<atasselli@hotmail.com > wrote in message
news:1161734421.729339.32390@i3g2000cwc.googlegroups.com...
>
> Dennis Woos wrote:
> ...
>> >
>>
>> From the Company Seven website, describing the Orion Argonaut 6"
>> Mak-Newt:
>>
>> Those who like to play "telescope testing" are likely to be pleasantly
>> surprised at the Airy disk and diffraction ring patterns appearing
>> distinct
>> and nearly symmetrical in and out of focus, with no signs of zones or
>> roughness - tell tale signs of patient and professional mirror making.
>>
>> I don't think the original poster is talking about very small
>> asymmetries.
>>
>
> They are not "very small". Quite noticeable in fact.
>
> Andrea T.
>
|
| |
Date: 24 Oct 2006 13:36:11
From: atasselli@hotmail.com
Subject: Re: MN66 owners
|
dan&lisa wrote:
> Andrea,
>
> I wrote to Mike at ITE and he said that the mak-newt design does not produce
> identical images on both sides of focus. He didn't seem concerned about what
> I saw. If it was inherent in the design would not all MNs show the same
> "defect"?
>
> While I've been an observer for a long time I have never been very
> interested in the technical side of scopes so have little experience to go
> on in this department.
>
> Dan
Hello Dan,
Non symmetrical pattern is rather normal in any MCT or MN. In fact even
my 1/40 wave rms MN has some. That is what I meant for 5th order
spherical aberration. Some MCT mfgs might do a slight aspherization of
the secondary to balance this so to have a perfectly symmetrical
fresnel pattern. In a MN this cannot be done.
Andrea T.
|
| | |
Date: 24 Oct 2006 16:50:42
From: Dennis Woos
Subject: Re: MN66 owners
|
<atasselli@hotmail.com > wrote in message
news:1161722171.393868.307270@i3g2000cwc.googlegroups.com...
>
> dan&lisa wrote:
>> Andrea,
>>
>> I wrote to Mike at ITE and he said that the mak-newt design does not
>> produce
>> identical images on both sides of focus. He didn't seem concerned about
>> what
>> I saw. If it was inherent in the design would not all MNs show the same
>> "defect"?
>>
>> While I've been an observer for a long time I have never been very
>> interested in the technical side of scopes so have little experience to
>> go
>> on in this department.
>>
>> Dan
>
> Hello Dan,
>
> Non symmetrical pattern is rather normal in any MCT or MN. In fact even
> my 1/40 wave rms MN has some. That is what I meant for 5th order
> spherical aberration. Some MCT mfgs might do a slight aspherization of
> the secondary to balance this so to have a perfectly symmetrical
> fresnel pattern. In a MN this cannot be done.
>
> Andrea T.
>
From the Company Seven website, describing the Orion Argonaut 6" Mak-Newt:
Those who like to play "telescope testing" are likely to be pleasantly
surprised at the Airy disk and diffraction ring patterns appearing distinct
and nearly symmetrical in and out of focus, with no signs of zones or
roughness - tell tale signs of patient and professional mirror making.
I don't think the original poster is talking about very small asymmetries.
Dennis
|
| |
Date: 25 Oct 2006 11:43:49
From: Uncle Bob
Subject: Re: MN66 owners
|
atasselli@hotmail.com wrote:
> Uncle Bob wrote:
> > On Mon, 23 2006 10:00:44 -0400, dan&lisa wrote:
> >
> > > Hello,
> > >
> > > I had my MN66 pointed at Vega a couple of nights ago checking for
> > > collimation. I noticed that the diffraction pattern was fine on one side of
> > > focus while on the other, two of the inner diffraction rings were thicker
> > > and nearly merged. Is this a sign of a defect or inherent in the design?
> > >
>
> Probably slightly tilted focuser and/or secondary.
>
> ...
> > If you find anything strange, check to make sure your corrector plate is
> > nice and secure, not pinched (it's held to the cell with screws on the
> > inside of the scope). Don't spin it in the cell. There should be a notch
> > in the cell that corresponds to a tab on the tube to maintain orientation
> > of the corrector plate. It's important that you not rotate the corrector
> > plate to a different position.
>
> Absolutely irrelevant, as the corrector is symmetric and the field
> itself is coma-corrected.
>
> Andrea T.
Of course, Andrea is absolutely correct, as always. I apologise for
any damage done by my misleading advice.
Clear Skies,
Uncle Bob
|
|
