MN66 Collimation - BINGO!

wrote:
Dan wrote:

I tweaked a little more as you (Andrea) suggested and now have what appears
to me as perfect or near perfect collimation. A little difficult to tell
with the seeing but I feel confident that the scope is quite well
collimated.

Only issue remaining is that at low power I see slight oval patterns to out
of focus stars; horizontally when outside focus and vertically inside focus;
i.e., foci line up horizontally /vertically. Adjusting the focuser doesn't
seem to change things. Do I need to make very fine adjustments? While the
adjustments I made were not coarse they were not on the level of the fine
tweaking I did with the secondary mirror screws.

Thanks for the help, it's greatly appreciated.



Dan,

Glad you sorted out the collimation issues. As for the the oval
pattern, ignore that. It is NOT astigmatism (!!). It is due to the
severe illumination fall-off of the MN66 ad the edge of the field. Some
paraxial rays make it and some don't. Don't worry about it and start
observing.

Andrea T.


Andrea - could this effect also be caused by using a very small
secondary? I notice the same problem in my 8" f/8 and had thought that
it must be due to minor astigmatism. But the secondary is as small as I
could make it (ma 1.55" - right at the limit). Could the loss of light
at the edge, if the secondary weren't in precisely the right spot, cause
this problem? The in-focus image with this scope is awfully sharp.

Thanks - Chris

Andrea,

The oval patterns I mention are also present at the center of field but not
as pronounced. The problem completely disappears at moderate to high power.

I am not getting all the messages from this group so apologize if I
seemingly ignore posts. I would not have known that anyone replied had I not
seen starburst's message.

Dan

Dan wrote:
The oval patterns I mention are also present at the center of field but not
as pronounced. The problem completely disappears at moderate to high power.

Hunh. Interesting. Just for a lark, try observing the problem at low
power, and then rotating the eyepiece. See if the pattern rotates with
the eyepiece, or stays in place.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

I did try that as well as used other EPs but nothing changed.

Dan

"Brian Tung" wrote in message
...
Dan wrote:
The oval patterns I mention are also present at the center of field but
not
as pronounced. The problem completely disappears at moderate to high
power.

Hunh. Interesting. Just for a lark, try observing the problem at low
power, and then rotating the eyepiece. See if the pattern rotates with
the eyepiece, or stays in place.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Dan wrote:
I did try that as well as used other EPs but nothing changed.

I assume you mean that the pattern stayed in place, rather than rotating
with the eyepiece, right?

Maybe I'm not reading Andrea right, but I don't see why that should
happen when the star is in the center of the field, and I particularly
don't see why the magnification should matter, if the eyepieces aren't
at fault. Is it possible that the secondary isn't *centered*?

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Brian Tung wrote:
Dan wrote:
I did try that as well as used other EPs but nothing changed.

I assume you mean that the pattern stayed in place, rather than rotating
with the eyepiece, right?

Maybe I'm not reading Andrea right, but I don't see why that should
happen when the star is in the center of the field, and I particularly
don't see why the magnification should matter, if the eyepieces aren't
at fault.


Because most likely than not at low power (and possibly large FOV) the
center of the ray bundle isn't exactly at the center of the EP (which
is difficult to estimate anyway) . However it is is the high power that
matters and if the secondary were off-center than this would affect
also the high power views, IMO.

Andrea T.

Brian, Andrea,

When I rotate the eyepiece the distortion's orientation does not change. I
suspect also that the focuser needs some fine adjustment. I'll give it a
last try, however, at magnifications above 100X the distortion is gone so I
won't fuss with it too much. I'll have some clear skies tomorrow so will
post again late tomorrow night.

Thanks for the replies.

Dan

wrote in message
oups.com...

Brian Tung wrote:
Dan wrote:
I did try that as well as used other EPs but nothing changed.

I assume you mean that the pattern stayed in place, rather than rotating
with the eyepiece, right?

Maybe I'm not reading Andrea right, but I don't see why that should
happen when the star is in the center of the field, and I particularly
don't see why the magnification should matter, if the eyepieces aren't
at fault.


Because most likely than not at low power (and possibly large FOV) the
center of the ray bundle isn't exactly at the center of the EP (which
is difficult to estimate anyway) . However it is is the high power that
matters and if the secondary were off-center than this would affect
also the high power views, IMO.

Andrea T.

starburst wrote:
wrote:
Dan wrote:

I tweaked a little more as you (Andrea) suggested and now have what appears
to me as perfect or near perfect collimation. A little difficult to tell
with the seeing but I feel confident that the scope is quite well
collimated.

Only issue remaining is that at low power I see slight oval patterns to out
of focus stars; horizontally when outside focus and vertically inside focus;
i.e., foci line up horizontally /vertically. Adjusting the focuser doesn't
seem to change things. Do I need to make very fine adjustments? While the
adjustments I made were not coarse they were not on the level of the fine
tweaking I did with the secondary mirror screws.

Thanks for the help, it's greatly appreciated.



Dan,

Glad you sorted out the collimation issues. As for the the oval
pattern, ignore that. It is NOT astigmatism (!!). It is due to the
severe illumination fall-off of the MN66 ad the edge of the field. Some
paraxial rays make it and some don't. Don't worry about it and start
observing.

Andrea T.


Andrea - could this effect also be caused by using a very small
secondary? I notice the same problem in my 8" f/8 and had thought that
it must be due to minor astigmatism. But the secondary is as small as I
could make it (ma 1.55" - right at the limit). Could the loss of light
at the edge, if the secondary weren't in precisely the right spot, cause
this problem? The in-focus image with this scope is awfully sharp.

Thanks - Chris

It isn't *also*. It is *only* due to the small secondary. Get a (much)
bigger one and see how it will disappear.

Andrea T.

hell jujst buy a whole new telescope and see if the second one fails too - then a
third - then a sixth! Right?



" wrote:

starburst wrote:
wrote:
Dan wrote:

I tweaked a little more as you (Andrea) suggested and now have what appears
to me as perfect or near perfect collimation. A little difficult to tell
with the seeing but I feel confident that the scope is quite well
collimated.

Only issue remaining is that at low power I see slight oval patterns to out
of focus stars; horizontally when outside focus and vertically inside focus;
i.e., foci line up horizontally /vertically. Adjusting the focuser doesn't
seem to change things. Do I need to make very fine adjustments? While the
adjustments I made were not coarse they were not on the level of the fine
tweaking I did with the secondary mirror screws.

Thanks for the help, it's greatly appreciated.



Dan,

Glad you sorted out the collimation issues. As for the the oval
pattern, ignore that. It is NOT astigmatism (!!). It is due to the
severe illumination fall-off of the MN66 ad the edge of the field. Some
paraxial rays make it and some don't. Don't worry about it and start
observing.

Andrea T.


Andrea - could this effect also be caused by using a very small
secondary? I notice the same problem in my 8" f/8 and had thought that
it must be due to minor astigmatism. But the secondary is as small as I
could make it (ma 1.55" - right at the limit). Could the loss of light
at the edge, if the secondary weren't in precisely the right spot, cause
this problem? The in-focus image with this scope is awfully sharp.

Thanks - Chris

It isn't *also*. It is *only* due to the small secondary. Get a (much)
bigger one and see how it will disappear.

Andrea T.