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Posts Made By: John Biretta

June 20, 2006 04:56 AM Forum: Refractors

Re: Single-Element "Apo" Lens For Moon?

Posted By John Biretta

I gave it a try. What I came up with was a 6" F15 with BK7 singlet, one aspheric surface (control spherical). Its diffraction limited across a 20 Angstrom bandwidth at 5500 Angstroms.

July 15, 2006 03:31 AM Forum: CCD Imaging and Processing/Solar System

Jupiter last night

Posted By John Biretta

Jupiter with a C14 + ToUcam in good seeing
(well, good for 27 deg. elevation) and thin
clouds. Probably too much aperture for seeing.

Anyone down under get some recent pics?

Exposure info: K3CCD, 10 FPS, 1/25 sec,
brightness 50%, saturation 50%, gamma 0%,
gain 90%, auto color balance, format 640x480.
Camera has internal sharpening disabled.
4 deg wedge prism used to reduce dispersion.

Processing: Registax 3, 1200 of 1500 frames,
reduced to 75%.

September 22, 2006 10:18 AM Forum: Refractors

Re: Case for 3 inch F15???

Posted By John Biretta

Some of the scope bags at telescope.com (Orion)
would fit. Like the XT-8 case. Might be a
little "baggy" since its made for an 8" Newt OTA.
or the "Observer 60 Altaz". They list the approx
dimensions. There are a lot of sizes.

October 2, 2006 12:11 PM Forum: Meade

This can't be good....

Posted By John Biretta

Shareholder class action lawsuit vs. Meade....
http://biz.yahoo.com/iw/061001/0168225.html

October 2, 2006 11:23 PM Forum: CCD Imaging and Processing/Solar System

Re: Low Jupiter

Posted By John Biretta

Nice stuff! When you use IR R G B, what do you do
with the IR channel? Do you just add it to R and
display as red? Or is something more interesting
going on?

October 16, 2006 04:50 PM Forum: Refractors

Re: 5" Achro Optical Test

Posted By John Biretta

Try it again with a deep green filter.

October 31, 2006 03:50 PM Forum: Digital SLR AstroPhotography

Nikon lens -> Canon EOS adapters?

Posted By John Biretta

I'm planning join the Canon 20Da craze. I was wondering if
my trusty Nikon manual lenses could be used on it. Has
anyone tried the Novoflex or Camera Quest adapter for
Nikon lens to Canon EOS body? I realize none of the
fancy lens electronic stuff will work, but probably won't
miss it too much for astro. Supposedly infinity focus works,
but would be nice to confirm.


November 2, 2006 02:53 PM Forum: Eyepieces

Burgess/TMB 4mm Planetary -- superb in refractors

Posted By John Biretta

I just wanted to register an interesting impression
of the BO/TMB 4mm Planetary. In particular, the
performance at the field edge. It is remarkably good in
refractors. I tried it in a TMB 100mm F8 and an Orion
ED80 (80mm F7.5). Both displayed nice Airy disks all the
way to the field stop -- very, very impressive. Even at the
field edge there was no astigmatism and lateral color was
almost non-existent. Swapping with some others --
Nagler 3.5mm T6, Nagler 3-6 Zoom @4mm, Tak LE 5mm,
UO 4mm Ortho, UO 5mm Ortho, and Orion 5.2mm Lanthanum
showed the BO/TMB to have the best edge performance of the
bunch. (From memory I expect it would also be
superior to Radians, where I did not like the edge
performance due to lateral color.) It seems perhaps the
BO/TMB has been optimized for F8-ish refractors. Perhaps
not surprising after all, considering the designer and
manufacturer.

An interesting test would be in the TeleVue NP-101 (F5.4)
but I have not tried it. Can anyone comment?

However, in fast Newts, the situation is quite the opposite.
Field edge performance was fairly dismal. Both F4.4 and F5
Newtonians showed huge coma at the field edge -- much more
coma than from the Newt optics alone. This is rather
consistent with some of Floyd's comments using a Newt. some weeks back
http://www.astromart.com/forums/viewpost.asp?forum_post_id=460761
(But I think the true value of this eyepiece may have been entirely
missed by that review.)

Apparently the eyepiece has significant Newtonian-like
coma at the field edge, which compounds and worsens the
normal fast Newtonian coma. (I could clearly see this by
placing the field edge of the eyepiece at the optical
axis of the F4.4 Newt. using some side-ways shifting
magic.) But in refractors, which tend
to be slower and have opposite-signed coma, the eyepiece
tends to cancel the OTA coma, leading to really superb
performance.

In other respects the performance was also excellent --
contrast, sharpness, field curvature, eye relief, etc.
all very nice. It seems this eyepiece may be a bit of
a "gift" to refractor owners.

I will try to make a more detailed review when I can.
But this was just so remarkable I wanted to give a
quick comment.

December 1, 2006 04:08 PM Forum: Telescope Making

Roddier testing

Posted By John Biretta

After some prodding by Mr. Highstone and questions from a few others, I
have written up a short cookbook on performing the Roddier test.
Essentially it is a star test performed outdoors and recorded with
a CCD camera (I use a ToUcam webcam). The resulting images are then processed
through special software to derive the aberrations including PTV, RMS, Zernike
coefficients, and Strehl ratio.

I have parked the stuff for now at http://tech.groups.yahoo.com/group/roddier/.
You will need to join the group to get the stuff (sorry, I want to keep out
spammers). All the stuff is in the files area. The cookbook is about
3 MB PDF file, and there are some sample images to play with.

If you just want to try out the software, you can skip to about page 26 in the
cookbook to the "Roddier Analysis" section, and grab the example images. Have fun.

Clear skies. - John B.

December 26, 2006 02:28 PM Forum: Refractors

Re: Testing of optics through magnification

Posted By John Biretta

James Fusco said:
Testing a lens with high magnification until the image breaks down, this limit is it a test? Does this indication a quality of optics or design.
Jim

I think this "test" can reveal bad optics. But I feel it is less useful
for distinguishing amongst a range of good to perfect optics. Once
you reach enough power to resolve the Airy disk, and the Strehl ratio is
well within the range of being diffraction limited (say >90% Strehl), I
think this test tells you much less.

This test is very closely tied to the eye's resolution.
The human eye can resolve around 1 - 2 arcminutes, and allowing for
some minimal sampling by the eye (say Nyquist type of sampling or
2 samples across the Airy disk), the Airy disk will become well-resolved
for most people around 30x to 60x per inch. Once the Airy disk is
resolved to the eye, differences between good and great optics will be
very difficult to judge. On some lunar or planetary scene, great optics
will give slightly higher contrast and visibility of fine features
than merely good optics. But even testing side-by-side it will be difficult
to judge subtle differences with seeing effects present, etc.

Note that this connection to the eye's resolution also makes it
difficult to compare results from different people, or even the
same person on different nights. People with poorer eyesight,
or having a day with blurry vision, will say the image looked
"sharp to them" at more power per inch. In effect they could
make the Airy disk bigger before it was fatter than their eye's
resolution.

As already pointed out, this test is also very sensitive to seeing
effects, making it difficult to compare scopes evaluated on
different sites, different nights, or even different moments of
variable seeing.

A better test, I think, is to look at the out-of-focus
patterns of rings inside and outside focus -- the so-called
"star test." The patterns are much larger than the eye's resolution,
and mostly removes the eye's performance from the test. A perfect optic
would give identical patterns on either side of focus. But the test is
also extremely sensitive and can reveal small optical errors of little
consequence. This test can also be made quantitative by comparing what you
see with software programs like Aberrator. Critics of this test (mainly a
few makers of high-end scopes) will point out that most people observe with
images "in focus" or that their particular type of scope can't be tested
with this method.