Re: "Believe it or not"


Aug 28, 2006

 


----------------------------

#32891 Aug 28, 2006

Five of us saw this Sunday a.m. at OSP (Oregon Star Party) what we

thought was an

impossibility with the AP prototype 8" Mak. OSP (central Oregon

desert-5,200 feet)

was very dark and had excellent seeing. Howard Banish had his

excellent 28" driven

Dob set up near by and was commenting about the great seeing we were

having and

observing Uranus and 4 of its moons at 1100X, it was 2:15 a.m.



Just for fun I put the Mak on M57 and increased the power to just

over

800X using the Baracon and a 6 SPL. The image was quite good for the

high magnification, the 13th mag. Star just outside the ring needed

no

averted vision so I used it to focus on. To my astonishment I saw a

slight hazy spot near the center of the ring and moments later a star

appeared in its center, and then disappeared.



This happened 4 times over a 5 minute period. 3 other folks observed

and

confirmed the same sight. Wanting to be sure I got Mel Bartel to

confirm

if we were crazy or not. Mel is one of the best observers and ATMer

in

the country, his home page: www.bbastrodesigns.com/. He

confirmed

our observations buy focusing on the 13th mag. Star, when the seeing

made the star tight the central star would appear.



Until Sunday a.m. I had only seen the central star in a 20" dob.

Needless to say I am very impressed with Rolands Mak design and

execution.



Jeff Vickers



----------------------------

#32892 Aug 28, 2006

Whatever happened to theoretical limits :-O?



Does this mean that the central star is NOT mag 15+ ?



Chime in anytime experts ;-)



Mark



-----Original Message-----

From: jjvickers@...

To: ap-ug@yahoogroups.com

Sent: Mon, 28 Aug 2006 11:30 AM

Subject: [ap-ug] "Believe it or not"





Five of us saw this Sunday a.m. at OSP (Oregon Star Party) what we

thought was an

impossibility with the AP prototype 8" Mak. OSP (central Oregon

desert-5,200 feet)

was very dark and had excellent seeing. Howard Banish had his

excellent 28" driven

Dob set up near by and was commenting about the great seeing we were

having and

observing Uranus and 4 of its moons at 1100X, it was 2:15 a.m.



Just for fun I put the Mak on M57 and increased the power to just

over

800X using the Baracon and a 6 SPL. The image was quite good for the

high magnification, the 13th mag. Star just outside the ring needed

no

averted vision so I used it to focus on. To my astonishment I saw a

slight hazy spot near the center of the ring and moments later a star

appeared in its center, and then disappeared.



This happened 4 times over a 5 minute period. 3 other folks observed

and

confirmed the same sight. Wanting to be sure I got Mel Bartel to

confirm

if we were crazy or not. Mel is one of the best observers and ATMer

in

the country, his home page: www.bbastrodesigns.com/. He

confirmed

our observations buy focusing on the 13th mag. Star, when the seeing

made the star tight the central star would appear.



Until Sunday a.m. I had only seen the central star in a 20" dob.

Needless to say I am very impressed with Rolands Mak design and

execution.



Jeff Vickers







---------------

Check out AOL.com today. Breaking news, video search, pictures, email and IM. All on demand. Always Free.





[Non-text portions of this message have been removed]



----------------------------

#32896 Aug 28, 2006

If one optical system puts less energy into the Airy Disc than another, then

there is no way that you can say that one aperture has such and such a

limiting magnitude in a case like this. For instance, a 35% obstructed optic having

1/4 wave error will have an effective Strehl ratio of only 64% as compared to

an unobstructed aperture having no optical errors. As soon as you introduce a

central obstruction of any appreciable size on top of an optical error of 1/4

wave, the light glare surrounding the Airy disc increases in intensity and the

central peak drops in intensity. This has the effect of dropping the contrast

immediately surrounding the star, and will probably hide it if it's buried

inside a nebula. When Mike Simmons use to attend star parties, he used the

visibility of stars inside the Dumbell nebula as a test of this kind of contrast.



Roland Christen



In a message dated 8/28/2006 4:17:27 PM Central Daylight Time,

pandrolmb@... writes:



> This is a subject that I've not had much success with in the past :-(. One

> mfr. anyway, reports the limiting mag. for a 16" SCT with 32% c/o at 15.5.

> Surely an 18" dob would be expected to reach CLOSE to mag 16 anyway? Does

> anyone know if there is a general formula/s for limiting magnitudes? I would think

> that a dob, for instance, would be expected to reach slightly deeper than

> say, an RC, of equal aperture, simply because of the reduction in reflecting

> surfaces, hence "formula/s". I'm sure that baffling, and general scope design

> play significant roles as well. Also; seeing conditions are obviously a major

> factor in any such predictions. What then is assumed to be constant seeing

> condition for generally accepted "limiting magnitude"? Is there such a thing?

>

>







[Non-text portions of this message have been removed]







----------------------------

#32899 Aug 28, 2006

--- In ap-ug@yahoogroups.com, pandrolmb@... wrote:

>

> Does anyone know if there is a general formula/s for limiting

magnitudes? >



Mark,



Please see

adsabs.harvard.edu//full/seri/PASP./0102//0000212.000.html

It is the best paper I have read on the subject.



Seeing mag 15 with 8" is not the problem - any well made scope should

do this in dark skies. The other night in mag 6+ NELM skies I

glimpsed the mag 14.1 and 14.7 stars around M57 with my 160FL binoviewed!



The problem with the M57 central star is that it is surrounded by that

very bright ring of nebulosity. As Peter said the seeing has to be

good enough to throw high magnification at it and the scope needs

superb contrast.



Glimpsing the M57 central star in 8" is a remarkable achievement and a

testament to the quality of the optics, the seeing and the skill of

the observer. I have never seen it in less than 20".



Clear skies,

Milt



----------------------------

#32900 Aug 28, 2006

There are a couple of thoughts I would like to share. The focuser

played an important roll in observing the central star at 800+ X. The

firm yet finely adjustable focusing with absolutely no image shift

allowed us to accurately focus on the 13 mag star just outside of the

ring. The accuracy of focus on such an object was very necessary.



The modeling and tracking of my G11 was spot on allowing us to

concentrate on observing without worry about drifting at 800X.



I have owned an used for planetary viewing Zeiss Ortho's, Pentax

SMC's, Radians, and SPL's. Over the years of reading many of the

debates over which was the best for light through put, resolution,

coatings, number of lenses, etc. I really could not see enough of a

difference between any of them that set them apart at my usual

observing power which is normally 150X-300X.



One of the observers was not used to the narrow field of view with

the 6mm SPL and the close eye relief. Of course he observes with a

large dob (28"er) and swears by his 7mm Radian of which we all know

is an excellent eyepiece.



I was surprised when trying the 7mm radian pushed to nearly 800X that

it was clearly a lot dimmer and lost a lot of resolution, It would

not show the central star. I guess because of the number of lenses

used. The 6mm SPL was necessary to observe the central star at 800X.

I sold my Ortho's when I got my SPL's so I had none of them to try

observing with but less glass is more at that high of power IMHO.



Roland's excellent Mak along with being in the right place at the

right time made all of the difference. Definitely a highlight of

observing for all of us that night. FWIW



Jeff



----------------------------

#32903 Aug 29, 2006

Here is what Mike Simmons wrote about using M27 to evaluate telescopes...



"In M27, the Dumbbell Nebula, I look for the extent of the nebulosity and

the number of stars in the nebulosity. In a really good telescope, under

dark sky conditions, M27 is quite a bit longer in the direction

perpendicular to the bright dumbbell shape. In a SUPER telescope, up to a

dozen or more stars can be seen imbedded in the nebulosity, an excellent

test for field contrast."



Clear skies, Alan

----- Original Message -----

From: chris1011@...>

To: ap-ug@yahoogroups.com>

Sent: Monday, August 28, 2006 5:35 PM

Subject: Re: [ap-ug] "Believe it or not"





> If one optical system puts less energy into the Airy Disc than another,

then

> there is no way that you can say that one aperture has such and such a

> limiting magnitude in a case like this. For instance, a 35% obstructed

optic having

> 1/4 wave error will have an effective Strehl ratio of only 64% as compared

to

> an unobstructed aperture having no optical errors. As soon as you

introduce a

> central obstruction of any appreciable size on top of an optical error of

1/4

> wave, the light glare surrounding the Airy disc increases in intensity and

the

> central peak drops in intensity. This has the effect of dropping the

contrast

> immediately surrounding the star, and will probably hide it if it's buried

> inside a nebula. When Mike Simmons use to attend star parties, he used the

> visibility of stars inside the Dumbell nebula as a test of this kind of

contrast.

>

> Roland Christen

>

> In a message dated 8/28/2006 4:17:27 PM Central Daylight Time,

> pandrolmb@... writes:

>

>

> > This is a subject that I've not had much success with in the past :-(.

One

> > mfr. anyway, reports the limiting mag. for a 16" SCT with 32% c/o at

15.5.

> > Surely an 18" dob would be expected to reach CLOSE to mag 16 anyway?

Does

> > anyone know if there is a general formula/s for limiting magnitudes? I

would think

> > that a dob, for instance, would be expected to reach slightly deeper

than

> > say, an RC, of equal aperture, simply because of the reduction in

reflecting

> > surfaces, hence "formula/s". I'm sure that baffling, and general scope

design

> > play significant roles as well. Also; seeing conditions are obviously a

major

> > factor in any such predictions. What then is assumed to be constant

seeing

> > condition for generally accepted "limiting magnitude"? Is there such a

thing?

> >

> >

>

>

>

> [Non-text portions of this message have been removed]

>

>

>

>

> To UNSUBSCRIBE, or for general information on the ap-ug list

> see groups.yahoo.com/group/ap-ug

> Yahoo! Groups Links

>

>

>

>

>

>







----------------------------

#32912 Aug 29, 2006

Whatever the optic, detection of limiting magnitude is a

probabilistic process that follows some kind of (perhaps normal)

distribution around a mean. Variations in eyesight, sky conditions,

scopes, observer training...they all add up.



When I go hunting after limiting magnitude, I use a star chart that

has had a photometric study of the field done. Most of them seem to

be by Brian Skiff or by someone Brian knows. (Half a dozen can be

found in the Deep Sky Observer's Catalog)



I proceed by drawing the star field in as much detail as I can with

special attention to the dimmest possible stars at the limit of my

detection. Later, these can be matched up against DSS plates and the

photometric picture. (The magnitude given by sky software is never

better than approximate)



So, in the C14, my faintest confirmed detection of a star is mag 16.4.



That is not my typical result, that is my best result. A typical

result is somewhere into the fifteens.



And that does not mean I can chase any galaxy that is labeled 16.4.

Stars are much easier than galaxies, as most folks know.



I've never bothered to count the stars in M27 but on a good night

there are more than would make the task easy. It is also a somewhat

abstract question, particularly in white light, because it's hard to

say where M27 begins and ends which has an effect on the total count.



Another way to confirm that you've seen something is when you go

after it, can't find it where the sky chart says it is, see something

very close, draw it, and then go to the DSS plates and discover that

the sky software and DSS plates are in disagreement about the

object's location relative to field stars. That's a pretty strong

confirmation of the visual! Unfortunately this gets harder as time

goes by. The NGC/IC project has cleaned up a lot of the locational

errors of the most used catalog...and I already reported Pal 14's 3

arc minute positional error, and now it's fixed, so that one's off

the list (probably better to have old sky software).



Everything I read tells me I must have selected a pretty poor optical

design but I've had a lot of fun with it.



regards

Greg N



--- In ap-ug@yahoogroups.com, "Alan French" adfrench@...> wrote:

>

> Here is what Mike Simmons wrote about using M27 to evaluate

telescopes...

>

> "In M27, the Dumbbell Nebula, I look for the extent of the

nebulosity and

> the number of stars in the nebulosity. In a really good telescope,

under

> dark sky conditions, M27 is quite a bit longer in the direction

> perpendicular to the bright dumbbell shape. In a SUPER telescope,

up to a

> dozen or more stars can be seen imbedded in the nebulosity, an

excellent

> test for field contrast."

>

> Clear skies, Alan

>

> ----- Original Message -----

> From: chris1011@...>

> To: ap-ug@yahoogroups.com>

> Sent: Monday, August 28, 2006 5:35 PM

> Subject: Re: [ap-ug] "Believe it or not"

>

>

> > If one optical system puts less energy into the Airy Disc than

another,

> then

> > there is no way that you can say that one aperture has such and

such a

> > limiting magnitude in a case like this. For instance, a 35%

obstructed

> optic having

> > 1/4 wave error will have an effective Strehl ratio of only 64% as

compared

> to

> > an unobstructed aperture having no optical errors. As soon as you

> introduce a

> > central obstruction of any appreciable size on top of an optical

error of

> 1/4

> > wave, the light glare surrounding the Airy disc increases in

intensity and

> the

> > central peak drops in intensity. This has the effect of dropping

the

> contrast

> > immediately surrounding the star, and will probably hide it if

it's buried

> > inside a nebula. When Mike Simmons use to attend star parties, he

used the

> > visibility of stars inside the Dumbell nebula as a test of this

kind of

> contrast.

> >

> > Roland Christen

> >

> > In a message dated 8/28/2006 4:17:27 PM Central Daylight Time,

> > pandrolmb@... writes:

> >

> >

> > > This is a subject that I've not had much success with in the

past :-(.

> One

> > > mfr. anyway, reports the limiting mag. for a 16" SCT with 32%

c/o at

> 15.5.

> > > Surely an 18" dob would be expected to reach CLOSE to mag 16

anyway?

> Does

> > > anyone know if there is a general formula/s for limiting

magnitudes? I

> would think

> > > that a dob, for instance, would be expected to reach slightly

deeper

> than

> > > say, an RC, of equal aperture, simply because of the reduction

in

> reflecting

> > > surfaces, hence "formula/s". I'm sure that baffling, and

general scope

> design

> > > play significant roles as well. Also; seeing conditions are

obviously a

> major

> > > factor in any such predictions. What then is assumed to be

constant

> seeing

> > > condition for generally accepted "limiting magnitude"? Is there

such a

> thing?

> > >

> > >

> >

> >

> >

> > [Non-text portions of this message have been removed]

> >

> >

> >

> >

> > To UNSUBSCRIBE, or for general information on the ap-ug list

> > see groups.yahoo.com/group/ap-ug

> > Yahoo! Groups Links

> >

> >

> >

> >

> >

> >

>






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