Dob vs Cass

The main thing is to determine what you want to use the scope for.

Last night I had 3 scopes out C102HD, custom built 6" F5 Dob, and 9.25" GPS. I
quickly abandoned the 102 it could not keep up. The optics in the newt are a
tad sharper than the SCT, but the SCT gathers more light. At approximately
200x the image in the newt was a tad sharper and the coloring of Jupiter was
more contrasty. The image in the newt starts to dim considerably over 200x
compared to the SCT. The SCT tracks, and has goto at the push of a button. I
am planning to install the newt on a GP soon. When it comes to deep sky
objects it is actually closer than I would have thought until you view
globulars, here the SCT shows its stuff.

I use the SCT the most. The newt is good for quick looks. The 6" F5 I built
is not the run of the mill newt it is made with expensive components. I did
not get such good performance from the mass newts I have owned in the past
except for one. Somehow Meade manged to let an exceptional 8" F6 EQ Starfinder
slip past QC. Keep that in mind.



Clear Skies
Richard

Hi Mark:

I found the images less sharp in the Cass. This was especially apparent in
globular clusters. Is this because of the larger obstruction?

More likely the problem was that the scope was not properly collimated.
Collimating an SCT is critical for best performance, but it also very easy
compared to collimating, say, an 8 inch dobsonian. ;-)

I found the Dob easier to drive, but I have had it for years. The Cass
required a lot of knob turning to hunt for the desired object. I also found
myself putting in an image compressor and 30mm EP to get a wide field to hut
down objects.

Applying a little friction to the dec and RA locks allows you to move an SCT
"by hand" very easily. Frankly, I find the ability to move in RA and Dec makes
hunting much more easy than with a dob. Using a reducer corrector is a plus.
You can apply the r/c for large objects/wide field and remove it for high
magnification subjects.

I am also considering digging deeper for a used 11"-14" Cass. Will this
images still be not as sharp as my f7 8"?


If you collimate it, the images will be better in an 11 - 14 inch SCT--far
better--than in your 8 inch dobsonian. "Sharp" is a subjective word.


I like to look at a bit of everything from planets to galaxies.

If these are your goals, an SCT would be the telescope of choice for you.


Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

Hi Mark:

I found the images less sharp in the Cass. This was especially apparent in
globular clusters. Is this because of the larger obstruction?

More likely the problem was that the scope was not properly collimated.
Collimating an SCT is critical for best performance, but it also very easy
compared to collimating, say, an 8 inch dobsonian. ;-)

I found the Dob easier to drive, but I have had it for years. The Cass
required a lot of knob turning to hunt for the desired object. I also found
myself putting in an image compressor and 30mm EP to get a wide field to hut
down objects.

Applying a little friction to the dec and RA locks allows you to move an SCT
"by hand" very easily. Frankly, I find the ability to move in RA and Dec makes
hunting much more easy than with a dob. Using a reducer corrector is a plus.
You can apply the r/c for large objects/wide field and remove it for high
magnification subjects.

I am also considering digging deeper for a used 11"-14" Cass. Will this
images still be not as sharp as my f7 8"?


If you collimate it, the images will be better in an 11 - 14 inch SCT--far
better--than in your 8 inch dobsonian. "Sharp" is a subjective word.


I like to look at a bit of everything from planets to galaxies.

If these are your goals, an SCT would be the telescope of choice for you.


Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

it is also possible that the Newt just does have better
optics and that the effect of the smaller central obstruction is noticeable.

Hi Jon:

From a casual inspection of a globular? I don't think so. ;-)

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

it is also possible that the Newt just does have better
optics and that the effect of the smaller central obstruction is noticeable.

Hi Jon:

From a casual inspection of a globular? I don't think so. ;-)

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

Chris L Peterson wrote in message . ..

No. Images in an SCT are every bit as sharp as a Newt.

Technically, that cannot be true; the 35% central obstruction of
a typical SCT softens the image *substantially* more than the 20%
CO of a typical F/7 Newtonian. But I agree with everyone else that
the effect is not likely to be obvious from casual inspection of
a globular cluster; where you would see it is when comparing fine
detail on Jupiter on a night of excellent seeing. And I also agree
with everyone else that collimation is the most likely culprit.

- Tony Flanders

Chris L Peterson wrote in message . ..

No. Images in an SCT are every bit as sharp as a Newt.

Technically, that cannot be true; the 35% central obstruction of
a typical SCT softens the image *substantially* more than the 20%
CO of a typical F/7 Newtonian. But I agree with everyone else that
the effect is not likely to be obvious from casual inspection of
a globular cluster; where you would see it is when comparing fine
detail on Jupiter on a night of excellent seeing. And I also agree
with everyone else that collimation is the most likely culprit.

- Tony Flanders

On 10 Apr 2004 13:07:36 -0700, (Tony Flanders) wrote:

Chris L Peterson wrote in message . ..

No. Images in an SCT are every bit as sharp as a Newt.

Technically, that cannot be true; the 35% central obstruction of
a typical SCT softens the image *substantially* more than the 20%
CO of a typical F/7 Newtonian.

Technically. But at the 80-90 power that Mark was using, there's just no way
that you are really going to see a difference in sharpness between two scopes
because of the different COs. As you note, the difference might show up in
subtle contrast details with planetary viewing, but never on DSOs.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On 10 Apr 2004 13:07:36 -0700, (Tony Flanders) wrote:

Chris L Peterson wrote in message . ..

No. Images in an SCT are every bit as sharp as a Newt.

Technically, that cannot be true; the 35% central obstruction of
a typical SCT softens the image *substantially* more than the 20%
CO of a typical F/7 Newtonian.

Technically. But at the 80-90 power that Mark was using, there's just no way
that you are really going to see a difference in sharpness between two scopes
because of the different COs. As you note, the difference might show up in
subtle contrast details with planetary viewing, but never on DSOs.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...
On 10 Apr 2004 13:07:36 -0700, (Tony Flanders)
wrote:

Chris L Peterson wrote in message
. ..

No. Images in an SCT are every bit as sharp as a Newt.

Technically, that cannot be true; the 35% central obstruction of
a typical SCT softens the image *substantially* more than the 20%
CO of a typical F/7 Newtonian.

Technically. But at the 80-90 power that Mark was using, there's just no
way
that you are really going to see a difference in sharpness between two
scopes
because of the different COs. As you note, the difference might show up in
subtle contrast details with planetary viewing, but never on DSOs.

I did do a start test while I was out and the Cass was out a bit, but not
real bad. The Newt is dead on. I looked at the scope today and I don't see
any way to collimate it. It doesn't really matter for what I am borrowing
it for it will be fine.

I was surprised at the magnitude of the difference as well. I expected a
much more subtle difference than I found.

After more thought, the scope may be even older. It is a Celestron with an
orange tube.

Mark C