Observing report, 10" dob and TV76, 3/13/2004

Date: Saturday evening, 3/13/2004
Location: Cottonwood Spring, Joshua Tree National Park, Calif.
Equipment: GS 10" f/5 dob and Tele Vue 76 (3" f/6.3) APO refractor

Stargazing from Cottonwood with both 10" f/5 dob and Tele Vue 76. Was
more a night to test out the TV76 and compare a bit to the 10" than a
night of "serious" observing.

Trapezium - Could only resolve the 4 main stars with either scope. With
the 10" things got really murky with higher powers. With the 3" stars
stayed a lot sharper but still no E or F components. Maybe i'm expecting
a bit too much from the little TV76. I'm also starting to think the
mirror in my 10" isn't that great.

1977, DN, Orion - Running Man neb - Visible in both scopes.

M78, DN, Orion - Visible in both scopes. Nearby NGC 2071 easily seen in
the 10".

2169, OC, Orion - The 37 Cluster.

2392, PN, Gemini - Eskimo neb - Spent some time using both scopes and
switching around eyepieces. Looks quite good at higher powers in the
10".

M35, OC, Gemini - Pretty in both scopes. Nearby open cluster NGC 2158
also easily seen.

Horsehead neb - UTL.

2362, OC, Canis Major - Tau Canis Majoris - Probably my favorite open
cluster. It sparkles and shines no matter what the scope or power.

2359, DN, Canis Major - This is a really interesting object. At first it
didn't really look like much in the 3" or the 10". Then i tried the UHC
filter in the 10". Wow! It looks like a bunny rabbit's head with big
ears. Very interesting object.

M66/M65/NGC 3628, EG, Leo - The Leo Trio - Easy with the 3". All 3 fit
in same field with both scopes. M66 is even fairly easy to spot in 10x42
binoculars.

M95/M96/M105, EG, Leo - All three plus NGC 3384 in the 3". With the 10"
can also pick up M105's second companion NGC 3389. Panning around the
area with the 10" can see 7 or 8 galaxies.

Castor - I just realized a week or so ago that Castor was a double star.
Using the Nagler 3-6mm zoom in the TV76 the star splits very nicely. Two
white headlights coming at me in the sky.

2419, GN, Lynx - The most distant globular cluster. 300,000 light years
from Earth! Took a while to find it with the 3". It's dimmer than i
remembered. Once located with the 10" i found i could see it with the 3"
but with difficulty.

Realm of the Galaxies - I packed up the 10" about 10pm. While i still
had the TV76 setup i just panned around the Realm without trying to
identify anything. I probably saw close to a dozen galaxies this way in
the 3". Pretty neat. I think my next time out will be to concentrate and
identify these galaxies.

Three closing comments... 1. I'm very happy with the Tele Vue 76. Very
sharp. Sharper than the 10" dob at higher powers. 2. I need a better
alt/az head for the TV76. At low powers the Bogen photo head works okay
but not at higher powers. 3. It's best to concentrate on using one
telescope (plus binoculars) than switching back and forth between two
scopes. Distracting and time consuming.

-Florian at Stargazing dot com
Palm Springs, Calif.

snip
3. It's best to concentrate on using one
telescope (plus binoculars) than switching back and forth between two
scopes. Distracting and time consuming.
unsnip

Agreed. Occasionally I will set up multiple scopes and invariably only one
really ends up really getting used. Swearing to never do it again, it's
only a matter of time ;o)


---
Martin
Remove "ilikestars" from email address

"Florian" wrote in message .. .

Trapezium - Could only resolve the 4 main stars with either scope. With
the 10" things got really murky with higher powers. With the 3" stars
stayed a lot sharper but still no E or F components. Maybe i'm expecting
a bit too much from the little TV76. I'm also starting to think the
mirror in my 10" isn't that great...

Three closing comments... 1. I'm very happy with the Tele Vue 76. Very
sharp. Sharper than the 10" dob at higher powers...

Yes, I'm sure the TV-76 is sharp; a small, high-quality refractor is
a wonderful thing.

But don't be too quick to condemn the mirror on the Dob. Most mirrors
are pretty good these days; the problems you are describing sound much
worse than could be ascribed to a mediocre mirror. Are you talking less
sharp at identical powers, or less sharp at identical exit pupils, i.e.
3X higher for the Dob than the refractor?

My best guess is that the problems are thermal. Deserts are famous
for cooling off quickly at night; a Dob would be unlikely to keep up
unless carefully designed for good thermal characteristics and/or
equipped with a fan. See Bryan Greer's article in the May S&T
for how to diagnose this.

Other things to think about are that at identical magnifications,
the exit pupil of the Dob (and the amount of light collected)
are likely to show up aberrations in your own eye that are
invisible in the smaller scope. Also, whereas the seeing is
often good enough to allow a 3-inch scope to perform at or
near its theoretical limit, that happens much more rarely with
a 10-inch scope. That is one of the biggest mental adjustments
required when moving from a small scope to a big one.

Finally, it sounds as though you are still somewhat stuck in a
small-scope mentality; most of the targets that you mention are
small-scope targets rather than big-scope targets, and when you
talk about big-scope targets like galaxies, you talk about them
in small-scope terms -- detectability -- rather than in big-scope
terms. In a 10-inch, you should be looking for the spiral arms
in M51 or the dust lane in NGC 3628, not just thinking about
detectability and maybe gross morphology.

- Tony Flanders

Finally, it sounds as though you are still somewhat stuck in a
small-scope mentality; most of the targets that you mention are
small-scope targets rather than big-scope targets, and when you
talk about big-scope targets like galaxies, you talk about them
in small-scope terms -- detectability -- rather than in big-scope
terms. In a 10-inch, you should be looking for the spiral arms
in M51 or the dust lane in NGC 3628, not just thinking about
detectability and maybe gross morphology.


The night in question was more to just compare the scopes a bit (not =
that they are really comparable scopes at all) and not make detailed =
observations. When i'm observing from home it often _is_ often more a =
question of "detectability" with the 10" since the skyglow interferes =
with seeing faint details.

-Florian

"Florian" wrote in message ...

When i'm observing from home it often is often more a
question of "detectability" with the 10" since the skyglow interferes
with seeing faint details.

True enough! When it comes to galaxies, dark skies are paramount. M31 and
M33 show better to the unaided eye under dark skies than in a 10" scope
under heavy light pollution.

- Tony Flanders

Hello, Tony,
I think this paragraph below is remarkably insightful and goes to a major difference is observing
techniques and philosophy that is related to equipment and type of object: those who are seeking
detection and those who are seeking detail or structure, of you prefer that word. The larger scope
permits seeing detail on galaxies, or at least structure, and a small scope does not. Each approach
is valid but they are very different.
Unfortunately, galaxies get fainter faster than amateur scopes get bigger, so many users of 16
inch and larger scopes who look at Abell clusters of galaxies are pretty much limited to the kind
of detection one might do on the Messiers with a three inch refractor, as far as I can tell from
their observing reports. Of course the members of Abell clusters are so far away that seeing them at
all is no doubt deeply gratifying.
I myself favor objects that I can see some structure in with whatever scope is at my disposal,
but I do mix in a bit of detection as well.
Since I am interested in structure, since it gives me the " wow!" effect, I try to choose a
telescope by the following algorithm: first I define the boundaries of what is reasonably convenient
for me, in terms of set up, portability, and ease of use at the eyepiece. Then I choose the largest
apertures within those boundaries. Then I choose the highest quality scope of that aperture, that I
can afford.
This algorithm has led me successively, based on my living conditions and observing
circumstances at the time, to a 1.25 inch hand held RFT from ANRA,
then to a 6 inch f/5 Dob from Telescopics, and then to an 11 inch ELT Dob, all of which have been
wonderful instruments. Each one made possible the observation of structure, but in different classes
of objects.
I think recently you posted in SAA about a procedure for choosing a telescope, giving different
weights to various aspects, such as quality, size, optical design, and so forth, but I can't seem to
find it and would appreciate it if you would refer me to that posting, as I thought it was astute. I
can't seem to find it in a google search.
Ciao,
Bill Meyers

Tony Flanders wrote:
(snip)

Finally, it sounds as though you are still somewhat stuck in a
small-scope mentality; most of the targets that you mention are
small-scope targets rather than big-scope targets, and when you
talk about big-scope targets like galaxies, you talk about them
in small-scope terms -- detectability -- rather than in big-scope
terms. In a 10-inch, you should be looking for the spiral arms
in M51 or the dust lane in NGC 3628, not just thinking about
detectability and maybe gross morphology.

"Jan van Gastel" wrote in message ...

I don't think I agree with your statement, that optical quality is less
important for observing DSO's then it is for observing planets.

Well, high optical quality is *always* beneficial, if you can afford it.
There's also many different kinds of optical quality. Ideally, a
reflector should have a mirror with a highly accurate figure, smooth,
and well baffled against ambient light. For viewing planets, a good
figure is essential and a smooth surface is fairly important, but
baffling is quite irrelevant, because the planet will easily outshine
any other light. For viewing galaxies, baffling is very important,
a smooth surface is fairly important, and a good figure is relatively
unimportant, because most visible detail in galaxies is very large
compared to the size of the Airy disk.

But there are always exceptions, which is why it is always nice to
have an optical system that is good in every way. Even galaxies can
have bright cores that show fine detail. And M42 challenges a
telescope in every possible way. The Huyghenian section is bright
enough to show sub-arcsecond detail where the figure is paramount.
Observing the E and F stars requires both a good figure and a
smooth surface. And the outer reaches of M42 are as faint as any
the outer arms of a galaxy, requiring a smooth figure and good
baffling. Likewise with planetary nebulae; it is very common to
have both a bright section that takes the highest possible
magnification and a very faint section that challenges your
faint-fuzzy skills.

- Tony Flanders

"Jan van Gastel" wrote in message ...

I don't think I agree with your statement, that optical quality is less
important for observing DSO's then it is for observing planets.

Well, high optical quality is *always* beneficial, if you can afford it.
There's also many different kinds of optical quality. Ideally, a
reflector should have a mirror with a highly accurate figure, smooth,
and well baffled against ambient light. For viewing planets, a good
figure is essential and a smooth surface is fairly important, but
baffling is quite irrelevant, because the planet will easily outshine
any other light. For viewing galaxies, baffling is very important,
a smooth surface is fairly important, and a good figure is relatively
unimportant, because most visible detail in galaxies is very large
compared to the size of the Airy disk.

But there are always exceptions, which is why it is always nice to
have an optical system that is good in every way. Even galaxies can
have bright cores that show fine detail. And M42 challenges a
telescope in every possible way. The Huyghenian section is bright
enough to show sub-arcsecond detail where the figure is paramount.
Observing the E and F stars requires both a good figure and a
smooth surface. And the outer reaches of M42 are as faint as any
the outer arms of a galaxy, requiring a smooth figure and good
baffling. Likewise with planetary nebulae; it is very common to
have both a bright section that takes the highest possible
magnification and a very faint section that challenges your
faint-fuzzy skills.

- Tony Flanders

(Tony Flanders) wrote in message m...

There's also many different kinds of optical quality... For viewing galaxies, baffling is very important, a smooth surface is fairly important, and a good figure is relatively unimportant, because most visible detail in galaxies is very large compared to the size of the Airy disk.

I'd rather say there is many different factors influencing optical
quality.
Optical quality itself boils down to the efficiency of contrast
transfer. And all of the factors mentioned (and some that wern't)
influence this efficiency in more or less particular way. Depending on
the circumstances, baffling can be anything from very important to
relatively unimportant. Surface smoothnes does matter, but so does
figure error. In fact, at similar RMS error levels, these two also
cause similar contrast drop. When this contrast drop is plotted
against minumum contrast required for resolution - which is higher for
dim than for bright low-contrast objects - it turns out that dim
low-contrast objects lose more in limiting resolution all the way down
to ~1/2.5 wave s.a. level, starting from perfect aperture. At 1/4 wave
level, bright low-contrast object resolution loss is little more than
10%, while that for dim low-contrast object is about twice as much,
despite the samllest resolvable details for dim objects being roughly
four times larger. The reason is that even moderate figure errors - or
surface roughness - cause significant energy increase not only in the
proximity of the Airy disc, but also far beyond it.

With the effective optical quality dropping bellow ~1/2.5 wave level,
contrast/resolution loss becomes greater for bright than for dim
low-contrast objects. Knowing that "good" amateur instruments operate
at ~1/3 wave s.a. optical quality level, and those not so good at ~1/2
wave, popular notion that deep-sky requires less in terms of optical
quality seems to be having some theoretical background, although the
notion likely is not generally corrrect.

(Tony Flanders) wrote in message m...

There's also many different kinds of optical quality... For viewing galaxies, baffling is very important, a smooth surface is fairly important, and a good figure is relatively unimportant, because most visible detail in galaxies is very large compared to the size of the Airy disk.

I'd rather say there is many different factors influencing optical
quality.
Optical quality itself boils down to the efficiency of contrast
transfer. And all of the factors mentioned (and some that wern't)
influence this efficiency in more or less particular way. Depending on
the circumstances, baffling can be anything from very important to
relatively unimportant. Surface smoothnes does matter, but so does
figure error. In fact, at similar RMS error levels, these two also
cause similar contrast drop. When this contrast drop is plotted
against minumum contrast required for resolution - which is higher for
dim than for bright low-contrast objects - it turns out that dim
low-contrast objects lose more in limiting resolution all the way down
to ~1/2.5 wave s.a. level, starting from perfect aperture. At 1/4 wave
level, bright low-contrast object resolution loss is little more than
10%, while that for dim low-contrast object is about twice as much,
despite the samllest resolvable details for dim objects being roughly
four times larger. The reason is that even moderate figure errors - or
surface roughness - cause significant energy increase not only in the
proximity of the Airy disc, but also far beyond it.

With the effective optical quality dropping bellow ~1/2.5 wave level,
contrast/resolution loss becomes greater for bright than for dim
low-contrast objects. Knowing that "good" amateur instruments operate
at ~1/3 wave s.a. optical quality level, and those not so good at ~1/2
wave, popular notion that deep-sky requires less in terms of optical
quality seems to be having some theoretical background, although the
notion likely is not generally corrrect.