Dob vs Cass

"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

"Bob May" wrote in message
...
As others mentioned, collimation may be a problem with the Cass.
I'll note that it is also likely that the quality of the optics are lower
with the Cass as they are a bit more difficult to make (you have 3 shaped
pieces of optics as opposed to 1 and a flat with the Newt.) so the
possibilty is that your 1/20th wave Newt. is being compared to a 1/5th
wave
Cass and that isn't going to do well at all.
Get a Ronchi grating and check and see what the results are with the Cass
and the Dob and that will tell the truth of what is.

A 1/20 wave Discovery Dob?

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
Lunar Picture of the Day http://www.lpod.org/
************************************

"Bob May" wrote in message
...
As others mentioned, collimation may be a problem with the Cass.
I'll note that it is also likely that the quality of the optics are lower
with the Cass as they are a bit more difficult to make (you have 3 shaped
pieces of optics as opposed to 1 and a flat with the Newt.) so the
possibilty is that your 1/20th wave Newt. is being compared to a 1/5th
wave
Cass and that isn't going to do well at all.
Get a Ronchi grating and check and see what the results are with the Cass
and the Dob and that will tell the truth of what is.

A 1/20 wave Discovery Dob?

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
Lunar Picture of the Day http://www.lpod.org/
************************************

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

As opposed to accepting that "a 11-14 SCT will outperform your 8 inch
newt",
or that "images in an SCT are every bit as sharp as a Newt", I strongly
suggest that you look through more scopes and make this determination for
yourself.

I've looked through several types of scopes, but this was the first time I
had several hours with another scope to really get a feel for it. Most of
my club members have Newts, no refractors, and a Cass (besides the club
scope that started this thread).

Mark C

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

As opposed to accepting that "a 11-14 SCT will outperform your 8 inch
newt",
or that "images in an SCT are every bit as sharp as a Newt", I strongly
suggest that you look through more scopes and make this determination for
yourself.

I've looked through several types of scopes, but this was the first time I
had several hours with another scope to really get a feel for it. Most of
my club members have Newts, no refractors, and a Cass (besides the club
scope that started this thread).

Mark C

As others mentioned, collimation may be a problem with the Cass.
I'll note that it is also likely that the quality of the optics are lower
with the Cass as they are a bit more difficult to make (you have 3 shaped
pieces of optics as opposed to 1 and a flat with the Newt.) so the
possibilty is that your 1/20th wave Newt. is being compared to a 1/5th wave
Cass and that isn't going to do well at all.
Get a Ronchi grating and check and see what the results are with the Cass
and the Dob and that will tell the truth of what is.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!

As others mentioned, collimation may be a problem with the Cass.
I'll note that it is also likely that the quality of the optics are lower
with the Cass as they are a bit more difficult to make (you have 3 shaped
pieces of optics as opposed to 1 and a flat with the Newt.) so the
possibilty is that your 1/20th wave Newt. is being compared to a 1/5th wave
Cass and that isn't going to do well at all.
Get a Ronchi grating and check and see what the results are with the Cass
and the Dob and that will tell the truth of what is.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!

Images in an SCT will look a little duller than a Newt because of CA too.

Mike.

"Tony Flanders" wrote in message
...
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

Images in an SCT will look a little duller than a Newt because of CA too.

Mike.

"Tony Flanders" wrote in message
...
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

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.


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

Mark C

As far as I know, all Celestron SCTs can be collimated. An Orange tube C-8 can
definitely be collimated, there are three collimation screws that adjust the
position of the secondary. I believe they are normally small allen screws and
on some models they may even be hidden by a cover that pops off.

The exact procedure is pretty basic, just adjust the secondary for the best
star test. Do a websearch starting with Uncle Rods members.aol.com/rmollise
page and you should find some instructions. It seems that many SCTs are
running around that have not been collimated for many years. For optimal
performance, it needs to checked on a regular basis. SCTs are probably more
stable than the average Newt. Imagine a Newt that had not been collimated for
20 years....

Personally I like Newtonians better than SCTs but I think when comparing scopes
it is important to make sure that both scopes are properly collimated.

Also...

Historically the consensus seems to be that the Orange Tubers are quite
variable optically. Some were quite good, others, especially during periods
of high demand centered around the appearance of various comets, were not so
great.

The consensus seems to be that the recent SCTs by both Meade and Celestron
consistently have good quality optics.

Jon Isaacs