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Posts Made By: John Biretta

June 15, 2002 01:57 PM Forum: Equipment Talk

Observatory aperture does not win?

Posted By John Biretta
Hubble can do better than Palomar 200" for small targets
(stars, etc., anything that looks stellar) due to
the atmosphere. On Earth, light from a faint
stellar target gets mixed with about one square arcsecond
of sky background light, which makes it hard to see.
For Hubble, stellar targets get mixed with only about
0.003 square arcsecond of a fainter sky background --
thats advantage gained by taking away the atmosphere.

The 20th mag number for Palomar 200" isn't for
CCD; you can get down to 24 - 25 magnitude range
with 200" and CCD if you try hard enough.

It is sobering to think that the *resolution*
of Palomar 200" is about the same as your TV102
lightcup, due to the atmosphere. Both have resolution
around 1 arcsecond. Though, of course,
Palomar collects alot more photons.

June 24, 2002 11:22 PM Forum: Equipment Talk

Cleaning EP's and ever popular lens cleaning pen

Posted By John Biretta
I use Lens Pens alot. As already noted, one should be
careful to keep the cleaning pad itself clean and
free from grit, etc. For this reason, I only use them
as last cleaning step (after compressed gas, brush,
etc.). Even then, I try to use each one only a
couple times before discarding it. They are best way
I've found for removing last traces of grease from
lenses.

WalMart used to carry them for $5 or $6 but not any more.
Anyone know other places to get them cheap?


June 28, 2002 06:34 PM Forum: Equipment Talk

How good is TV85's color correction?

Posted By John Biretta
I've got a TV85. I do not see any color or
purple halo, etc., at focus at any power on
bright stars or planets. You can see color
in out-of-focus star images -- you'll see red
halo on one side of focus, and green halo on the
other side.

Color correction is not as good as Vixen Fluorites
or modern AP Triplets, but is much, much better
than achromats or the TV Pronto / Ranger.

I think (I'm guessing) that they use same glasses
in the TV76, TV85, and TV102 since color correction
of these scopes are similar, but the 76 and 85
have slightly better correction than 102.

Its a great scope; highly recommended.

July 3, 2002 08:46 PM Forum: Equipment Talk

Attack of the filters - 1st light & question

Posted By John Biretta
I think the problem you mention is the filter. Here
is possible explanation and test:

When you screw the filter into the Everbright, the
optical beam is about 15 - 20mm in diameter when it
hits the filter -- thats how much of the filter is
being used. But if you just hold the filter in
front of the eyepiece, you are only using about 1 or
2 mm of the filter (size of exit pupil). So my
guess is that the filter glass has some bad
astigmatism on scales like 15 - 20 mm on the glass.

Here's a simple test. Screw the filter into
the Everbrite, put star just outside of focus
(run focusser out) and note which direction the
image is elongated. Now unscrew the filter
exactly 1/4 turn, and repeat the test. If the
problem is the filter, the elongation
direction of the star image should rotate by
90 degrees.

Good luck.

July 5, 2002 09:36 PM Forum: Equipment Talk

Need some advice on a visual back C9.25"

Posted By John Biretta
You'll probably want to rotate the diagonal to
get a comfortable viewing position, and I
*think* this will be easier with AP/Lumicon
style 2" back -- its just one or two thumbscrews.
I think the dedicated SCT diagonals have a big
ring you'll need to loosen/tighten if you want to
rotate the diagonal.

I use a refractor-style 2" mirror diagonal
and am very happy with it.

July 18, 2002 09:50 PM Forum: Equipment Talk

Binoviewer Vignetting

Posted By John Biretta
There are two types of vignetting to
consider -- vignetting at the center of
the field, and vignetting at the edge of
the field. For planetary work you only
care about the field center, but for other
stuff you care about both.

Vignetting at the field center does not
depend on the eyepiece focal length / field
size. It only depends on the scopes F ratio
and the size of the prisms. Most of the binoviewers
add about 5 inches to the optical path. So a
unit with say one inch prisms should be OK
with focal ratio 5" / 1" = F5 or slower.
For one of the binoviewers with small
prisms its 5" / (1/2") = F10 or slower.

Vignetting at the field edge is more complex.
It depends on the scope light cone, the size of
prisms in the binoviewer, and the size of the
eyepiece field stop. If the binoviewer is one of the
microscope derived units with 12mm prisms, and
you use an eyepiece with a 27mm field stop,
the field edges are going to be completely
vignetted (dark) no matter what the
scopes focal ratio is. Fast scopes will probably
have some vignetting at the field edge, even
with large prisms -- maybe 50% light loss at the
edge -- detectable but not terrible.

July 23, 2002 01:28 AM Forum: Equipment Talk

telepod head question

Posted By John Biretta
Never heard of anyone puting slow-motions on a Telepod.
There are electronically driven alt-az mounts though,
and old Unitron Alt-Az mounts with manual slow motions
(if you can find one used). - John B.

July 25, 2002 12:10 AM Forum: Equipment Talk

Who is using 2" accessories on a C9.25?

Posted By John Biretta
I've got similar setup, too. I also use the UO
orthos; the 9mm and 12.5mm are great for planetary.
No focus problems with 2" diagonal. I've been using
an AP Maxbright, but any modern 2" mirror diagonal
should be fine. - John B.

August 17, 2002 10:03 PM Forum: Equipment Talk

Deep sky filters-do they work?

Posted By John Biretta
My favorite filter is the Lumicon O-III; it gives very
dramatic results on emission line nebula. Their
"Deep Sky" filter gives some improvement on all types
of objects, but is less dramatic than O-III.
I've not tried other brands of filters. - John B.

August 22, 2002 02:36 AM Forum: Equipment Talk

ParaCorr

Posted By John Biretta
Weight: It is about same as your 9 Nagler or 22
Panoptic. I forget exact number.

Focus: I have the older style non-tunable top.
If you focus with an eyepiece, and then insert
Paracor, and put eyepiece in Paracor, you will need
to move the focusser *in* about 1/2 inch to reach
the new focus with Paracor added.

Camera: yes you can use Paracor with camera.
If you have a standard 2" camera adapter (with
T thread) on your camera, you just unscrew the
2" tube and replace it with bottom section of
the Paracor (has all the glass in it). If you can
focus camera OK without Paracor, you should be fine.

- John B.

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