I've had both a 180 and now a 210 Mewlon...both tack sharp, but both have had different airy disk patterns on inside and outside of focus. Phil Harrington calls this spherical aberation...but is that what it really is on a Mewlon? Any other explanations...?
Mewlon's airy disk discrepancy
Started by falsetto, 08/15/2006 01:56AM
Posted 08/15/2006 01:56AM
Opening Post
Posted 08/15/2006 08:12PM
#1
Scott Richter said:
I've had both a 180 and now a 210 Mewlon...both tack sharp, but both have had different airy disk patterns on inside and outside of focus. Phil Harrington calls this spherical aberation...but is that what it really is on a Mewlon? Any other explanations...?
If they're really tack sharp, I wouldn't worry too much about it. We had a thread about this in another Astromart forum, I forget which one. The bottom line is that you can't compare SA on one design to SA on another. They can look very different in the star test, even if the quantified level of SA is equal. If they focus easily ("snapping" into focus), you likely don't have a problem.
Posted 08/15/2006 10:38PM
#2
What do you mean by "airy disc patterns"? If you are referring to the fresnel rings that you see inside and outside of focus, it is my opinion that these will never be exactly the same in a compound telescope or practically any other for that matter, except on the most steady of nites. On one side of focus you see the atmosphere or jet stream or something whipping by.
What you look for is that the airy disc is the same size, and you see the same size shadow of the secondary at the same point on both sides of focus.
Kevin
What you look for is that the airy disc is the same size, and you see the same size shadow of the secondary at the same point on both sides of focus.
Kevin
Posted 08/15/2006 11:36PM
| Edited 08/15/2006 11:37PM
#3
The difference inside / outside focus could be any
number of things: atmospheric turbulence, warm/cool
air pooling inside the tube, temperature difference
across the optic, spherical aberration, astigmatism,
turn-edge, zones, etc.
If the seeing is very steady, and you are looking at
the zenith, and scope has had an hour or two to
temperature equilibrate, and the outdoor temperature
is not dropping (at least not very fast), you can
begin to rule-out the temporary effects and think
more about problems in the optics.
Another issue with these compound scopes is that the
spherical aberration depends on the separation between
the primary and secondary mirrors. So while you are
focusing you are also changing the spherical. You may
need to figure out the backfocus (distance from rear of
scope to eyepiece) that gives the least spherical.
The designer might have included backfocus for say a
1.25" diagonal in their calculations, but maybe you've
got a 2" diagonal + JMI focuser + extension tube +
Barlow + binoviewer, etc. on there.
number of things: atmospheric turbulence, warm/cool
air pooling inside the tube, temperature difference
across the optic, spherical aberration, astigmatism,
turn-edge, zones, etc.
If the seeing is very steady, and you are looking at
the zenith, and scope has had an hour or two to
temperature equilibrate, and the outdoor temperature
is not dropping (at least not very fast), you can
begin to rule-out the temporary effects and think
more about problems in the optics.
Another issue with these compound scopes is that the
spherical aberration depends on the separation between
the primary and secondary mirrors. So while you are
focusing you are also changing the spherical. You may
need to figure out the backfocus (distance from rear of
scope to eyepiece) that gives the least spherical.
The designer might have included backfocus for say a
1.25" diagonal in their calculations, but maybe you've
got a 2" diagonal + JMI focuser + extension tube +
Barlow + binoviewer, etc. on there.
Funding Member
