6.5 OA beats 16" Meade Starfinder Dob...

After both scopes cooled down and stabalized and at similar magnifications,
the 6.5 OA shows more detail on Mars than the Starfinder. The Starfinder
was collimated to about 250x (I was too excited on receiving the dob to
collimate the scope further than that). The 6.5 OA I had collimated dead
on. Comparing around 200x or so on both the 6.5 inch just gave a much more
detailed image. My guess is the planet is so bright that the diffraction
from both the spikes and the fact that the mirror cell holds onto the edges
of the mirror causes some difficulty resolving the detail. Splitting
doubles was the same the OA at the same mags just did a better job on
splitting doubles.

Although the title of this message was just to grab your attention on other
things the 16 inch just kicked the OAs butt. Looking at M15 and M2, they
look like little clouds in the OA but in the 16 inch dob that were
magnificent globular clusters similar to what you see in pictures. Just
incredible. Galaxys actually show their elliptical nature clearly where in
the OA they just look like boring grey blobs.

All in all, I found a perfect mate for my 6.5 inch OA, a nice 16 inch Dob.

Now if the Meade mechanics for the starfinder were any good I'd be all set.
This scope has the finder mounted on the top part of the dob instead of
along the side. The focuser is angled up instead of to the side. The
focuser wobbles unbelievably, very difficult to get focus (especially at
f/5) when the image shifts almost a degree! The dampening times were
horrendous and the base is "sticky" when sliding in the azimuth. All of
these things are fixable, but it's a shame that for less than $100 more all
these problems could have been solved for me

Mike.

Oh boy, I'm getting some popcorn for this one.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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The Starfinder
was collimated to about 250x (I was too excited on receiving the dob to
collimate the scope further than that).

It seems a bit odd to make a comparison between a scope that is perfectly
collimated and one that is not.

Jon

Mike,

The Meade 16" Dob is a work in progress as they come from the
factory. With a few hundred in accessories, they become nice scopes.
My has a JMI focuser upgrade, heavier springs in the main mirror
cell, along with ventilation hiles drilled in the rear of the cell. Also,
I replaced the spider assembly with one from ProtoStar with their
3.1" secondary (it still gives 100% illumination where it is mounted).
Also, the teflon pads have been upgraded one the mount, and I
used a lazy susan and teflon pad arrangement on the base board for
smoothness and stability. One little hidden gotchais the Meade uses
nylon screws all along the outer edge of the primary to provide extra
lateral support for the primary mirror. They should only be tight
enough to contact the edge of the mirror, with no pressure. If they
are tight, they can create astigmatism. I had this problem with mine,
which went away when I loosened the screws. The mirror performs
well now.

After the mods, the scope holds collimation well, and can give
knockout planetary images at well in excess of 500X on good
nights. The views it gave me of Mars were spectacular, and the
best I've ever seen in any scope. That last bit of collimation, as
well as tube ventilation, makes all the difference. With my 16"
Saturn's moon Titan resolves to a disk, and double stars resolve
to limits well beyond what a 6.5" can provide. Seeing needs to
cooperate, though. On a good night, double stars look refractor-
like in appearance.

The Meade 16" Dobs are a great bargain, but need a few mods
to become great performers. The problem is usually not a poor
primary mirror, but the mechanics supporting the mirrors, and
the focuser. Adding a ParaCorr doesn't hurt either.

That said, the 6.5" Off-Axis and the Meade 16" sound like a nice
combination.

Thanks, Tom Davis

"Mike Fitterman" wrote in message
...
After both scopes cooled down and stabalized and at similar
magnifications,
the 6.5 OA shows more detail on Mars than the Starfinder. The Starfinder
was collimated to about 250x (I was too excited on receiving the dob to
collimate the scope further than that). The 6.5 OA I had collimated dead
on. Comparing around 200x or so on both the 6.5 inch just gave a much
more
detailed image. My guess is the planet is so bright that the diffraction
from both the spikes and the fact that the mirror cell holds onto the
edges
of the mirror causes some difficulty resolving the detail. Splitting
doubles was the same the OA at the same mags just did a better job on
splitting doubles.

Mike.

Tom,

Thanks for the input and advice. I think this will really help.

See inline comments:


"Tom Davis" wrote in message
...
Mike,

The Meade 16" Dob is a work in progress as they come from the
factory. With a few hundred in accessories, they become nice scopes.
My has a JMI focuser upgrade,

My focuser seems to be placed wrong. It's right along an axis of the spider
and is mounted too high on the tube rather on the side like a typical dob.
Did you have this problem?

heavier springs in the main mirror
cell, along with ventilation hiles drilled in the rear of the cell.

Did you just use the same mirror cell? I noticed that the mirror has clips
all around the edges. Does yours have these? Did you replace them with
something else?

Also,
I replaced the spider assembly with one from ProtoStar with their
3.1" secondary (it still gives 100% illumination where it is mounted).

Did you have to move the secondary (and focuser forward) to do this? How
much did you have to do this if you did? I know the Meade secondary is 4"
and an inch difference seems like an awful lot.

Also, the teflon pads have been upgraded one the mount, and I
used a lazy susan and teflon pad arrangement on the base board for
smoothness and stability. One little hidden gotchais the Meade uses
nylon screws all along the outer edge of the primary to provide extra
lateral support for the primary mirror. They should only be tight
enough to contact the edge of the mirror, with no pressure. If they
are tight, they can create astigmatism. I had this problem with mine,
which went away when I loosened the screws. The mirror performs
well now.

I have a feeling mine has this problem based on what I was seeing at higher
powers. I'll have to give this a shot.


After the mods, the scope holds collimation well, and can give
knockout planetary images at well in excess of 500X on good
nights. The views it gave me of Mars were spectacular, and the
best I've ever seen in any scope. That last bit of collimation, as
well as tube ventilation, makes all the difference. With my 16"
Saturn's moon Titan resolves to a disk, and double stars resolve
to limits well beyond what a 6.5" can provide. Seeing needs to
cooperate, though. On a good night, double stars look refractor-
like in appearance.

I figure mine will too, but it's going to take a lot of tweaking...at least
I'm hoping....


The Meade 16" Dobs are a great bargain, but need a few mods
to become great performers. The problem is usually not a poor
primary mirror, but the mechanics supporting the mirrors, and
the focuser.

You can say that again!!!!!! The mechanics are just plain awful.

Adding a ParaCorr doesn't hurt either.

That said, the 6.5" Off-Axis and the Meade 16" sound like a nice
combination.

The 6.5 on the equatorial makes a nice planetary scope. I'm hoping to get
some wheels on the 16" so I can move it around myself. Right now it takes 2
people to handle it. Maybe I need to build that roll off observatory I've
always wanted :-)

Thanks again for your help Tom.

Mike.

"Mike Fitterman" wrote in message ...

After both scopes cooled down and stabalized and at similar magnifications,
the 6.5 OA shows more detail on Mars than the [16-inch] Starfinder.

Sure, that's exactly what I would expect, unless you are observing from
the southern hemisphere. In my part of the world (New England) it is
fairly rare for the seeing to be so good that even a 6-inch scope will
perform to its limit for objects at the zenith. That is especially
true at this time of year; the best seeing is usually in the summer.
And with Mars as low as it is for observers north of the tropics,
it would require truly extraordinary seeing for a 6-inch scope to
perform to its potential.

As for why the 16-inch is worse than the 6.5-inch, and not just no
better, my first guess would be thermal effects. Don't understimate
how long it takes for a 16-inch mirror to reach thermal equilibrium
even in the best of circumstances. Particularly true for Meade Dobs,
which are famous for lousy cooldown characteristics.

- Tony Flanders

"Tony Flanders" wrote in message
...
"Mike Fitterman" wrote in message
...

After both scopes cooled down and stabalized and at similar
magnifications,
the 6.5 OA shows more detail on Mars than the [16-inch] Starfinder.

Sure, that's exactly what I would expect, unless you are observing from
the southern hemisphere. In my part of the world (New England) it is
fairly rare for the seeing to be so good that even a 6-inch scope will
perform to its limit for objects at the zenith. That is especially
true at this time of year; the best seeing is usually in the summer.

Interesting, we don't live far from each other and I find that this time of
year produces the best seeing, especially if your willing to get up early in
the morning. Last night the seeing was spectacular early. I don't know
what it was like after 9pm but it was great up until then, getting 300+x
views with both scopes. That's as good as it gets here (as it sounds like
you know :-)

And with Mars as low as it is for observers north of the tropics,
it would require truly extraordinary seeing for a 6-inch scope to
perform to its potential.


Not true right now, over the summer yes, but it's much higher in the sky
than it was even a month ago (it's only at -6 declination where 2 months ago
it was at -16. That's a big difference here and puts it just over the line
in terms of good viewing. Contrasting last night with the *best* night I
had over the summer (and I went out on most clear nights) where I could only
get 245x max before the image starting breaking down.

As for why the 16-inch is worse than the 6.5-inch, and not just no
better, my first guess would be thermal effects. Don't understimate
how long it takes for a 16-inch mirror to reach thermal equilibrium
even in the best of circumstances. Particularly true for Meade Dobs,
which are famous for lousy cooldown characteristics.

I would expect thermal expects to be visible in the form of wavyness of the
image which is what I've seen in the past with optics that haven't cooled
down. Especially when you take a star and put it slightly out of focus.
Not being able to see the concentric rings is an easy way to tell. When I
was comparing the scopes, both were showing signs of thermal equilibrium.
As the night moved on (and the temperture started dropping faster) I lost
the ability to get the rings as the mirror was not cooling down as fast as
the air.

I'm still expecting the 16 inch to beat the 6.5 inch on a good night of
seeing (such as last night). I actually believe, as Tom suggested, that the
mechanics of the scope were getting in the way of really great views at high
power. I'm also with you that on most nights around here the 6.5 inch will
do just as good if not better than the 16 inch because of the lousing seeing
and the way the temperture changes here. That's why I'm happy to have both
scopes now, because there is plenty of things the 16 inch can do that the
6.5 can't do regardless of collimation, seeing, etc... Now if I only could
move it as easily as the 6.5 inch :-)

Mike.



- Tony Flanders

That's all fine and good, but don't assume that the 16" Meade represents the
performance of a fine large Dobsonian.

Del Johnson


"Mike Fitterman" wrote in message
...
After both scopes cooled down and stabalized and at similar
magnifications,
the 6.5 OA shows more detail on Mars than the Starfinder. The Starfinder
was collimated to about 250x (I was too excited on receiving the dob to
collimate the scope further than that). The 6.5 OA I had collimated dead
on. Comparing around 200x or so on both the 6.5 inch just gave a much
more
detailed image. My guess is the planet is so bright that the diffraction
from both the spikes and the fact that the mirror cell holds onto the
edges
of the mirror causes some difficulty resolving the detail. Splitting
doubles was the same the OA at the same mags just did a better job on
splitting doubles.

Although the title of this message was just to grab your attention on
other
things the 16 inch just kicked the OAs butt. Looking at M15 and M2, they
look like little clouds in the OA but in the 16 inch dob that were
magnificent globular clusters similar to what you see in pictures. Just
incredible. Galaxys actually show their elliptical nature clearly where
in
the OA they just look like boring grey blobs.

All in all, I found a perfect mate for my 6.5 inch OA, a nice 16 inch Dob.

Now if the Meade mechanics for the starfinder were any good I'd be all
set.
This scope has the finder mounted on the top part of the dob instead of
along the side. The focuser is angled up instead of to the side. The
focuser wobbles unbelievably, very difficult to get focus (especially at
f/5) when the image shifts almost a degree! The dampening times were
horrendous and the base is "sticky" when sliding in the azimuth. All of
these things are fixable, but it's a shame that for less than $100 more
all
these problems could have been solved for me

Mike.

It would be interesting to try your 16" Meade with an off-axis aperture mask
(about 6" of clear aperture) and compare those views with the OA6.5 I
imagine they will be very close. It's a lot cheaper than another scope if
you have a big dob!

"Del Johnson" delastro@{right star in Orion's belt}.sdsu.edu wrote in
message ...
That's all fine and good, but don't assume that the 16" Meade represents
the
performance of a fine large Dobsonian.

Del Johnson


"Mike Fitterman" wrote in message
...
After both scopes cooled down and stabalized and at similar
magnifications,
the 6.5 OA shows more detail on Mars than the Starfinder. The
Starfinder
was collimated to about 250x (I was too excited on receiving the dob to
collimate the scope further than that). The 6.5 OA I had collimated
dead
on. Comparing around 200x or so on both the 6.5 inch just gave a much
more
detailed image. My guess is the planet is so bright that the
diffraction
from both the spikes and the fact that the mirror cell holds onto the
edges
of the mirror causes some difficulty resolving the detail. Splitting
doubles was the same the OA at the same mags just did a better job on
splitting doubles.

Although the title of this message was just to grab your attention on
other
things the 16 inch just kicked the OAs butt. Looking at M15 and M2,
they
look like little clouds in the OA but in the 16 inch dob that were
magnificent globular clusters similar to what you see in pictures. Just
incredible. Galaxys actually show their elliptical nature clearly where
in
the OA they just look like boring grey blobs.

All in all, I found a perfect mate for my 6.5 inch OA, a nice 16 inch
Dob.

Now if the Meade mechanics for the starfinder were any good I'd be all
set.
This scope has the finder mounted on the top part of the dob instead of
along the side. The focuser is angled up instead of to the side. The
focuser wobbles unbelievably, very difficult to get focus (especially at
f/5) when the image shifts almost a degree! The dampening times were
horrendous and the base is "sticky" when sliding in the azimuth. All of
these things are fixable, but it's a shame that for less than $100 more
all
these problems could have been solved for me

Mike.

The smaller off-axis telescope will probably prevail for a couple reasons.
First, it is probably made to a higher standard as it is more expensive per
aperture than the Meade Dobsonian. Second, the smaller telescope will
probably cool down a lot faster.

Finally, any telecope worth its weight works best at full aperture, even on
planets.

Del Johnson



"Chuck" wrote in message
t...
It would be interesting to try your 16" Meade with an off-axis aperture
mask
(about 6" of clear aperture) and compare those views with the OA6.5 I
imagine they will be very close. It's a lot cheaper than another scope if
you have a big dob!