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Telescope Limiting Mag.


Author Message

Jeff Medke

#31 / 43


said the following, though I may be wrong:

Quote:

>Hi,

>'14.6 in a 4,5" reflector'???? Where does that magnitude come

>from: GSC maybe? I don't think one can see that faint even with

>an 8" under high power and a 6.5 {*filter*} eye limit!

>I just wonder where some people get their info from....

Since you apparently have not been paying attention to the

work I have been doing in faint photometric sequences, or to

the preaching I have been doing about the inaccuracy of GSC

magnitudes, here is the story for about the sixth time:

The photometric sequence used surrounds the active galaxy

AKN 120, in Orion. The photometry was done by Hamuy and

Maza; the citation to their paper is Hamuy & Maza 1989,

Astronomical Journal, 97, 720. The paper is available on the

web from the very large URL at the bottom of this posting.

The photometric observations were made using .6, .9, and 1

meter telescopes at Cerro Tololo Inter-American Observatory

on CTIO's UBV(RI)kc photometric system. These magnitudes

have not so far been challenged by the professional

community. Brian Skiff, an astronomer at the Lowell

Observatory, was my collaborator in gathering this paper,

interpreting its contents, generating a sequence based on

it, and inspiring the observations.

My observation of the AKN 120 sequence was conducted at an

altitude of 6530 feet near Coronado Peak, Arizona, in the

late summer of 1997, when the {*filter*}-eye limit (for me) was

6.8. The seeing, which is equally important when making

astro amateur, Telescope Limiting Mag. - Page 3 http://www.science-bbs.com/17-astro-amateu

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6.5

: {*filter*} eye limit!

: I just wonder where some people get their info from....

: >

: > >I can get 14.6 in a 4.5" reflector from my site. An 8" SCT

: > >that can't do 15.5 magnitude under my skies is a broken one,

: >

These magnitude limits aren't really exceptional - its what a

skilled observer can accomplish under dark, steady conditions. Under

fairly typical dark skies (Lm ~ 6 -6.2), I have observed stars

8-nch - upper 15's

10-inch ~ 16.0 (Note both 8 and 10-inch are Cave's)

17.5 inch ~ 16.8 to 17.0

20-inch 17.2 to 17.4

These estimates were made under dark, _steady_ skies, and at

magnifications of 20 to +25x/inch of aperture.

Observers like Tom Polakis and Barbara Wilson who are blessed

with great skies can often exceed these 'limits' by 0.5 to

1.0 magnitudes. (Plus they are great observers..)

- Rich Jakiel

Mon, 14 Aug 2000 03:00:00 GMT

Jeff Medke

#34 / 43


the following, though I may be wrong:

Quote:

>Jeff:

>I am interested in knowing the details on the 4.5 inch>reflector which you use for these observations. Is

>it commercially available or is something custom??

It is homebuilt. It was originally a telescope of uncertain

origin picked up at the Stellafane swap tables in, IIRC,

1980 or so. It is now thought the primary was ground by a

Stellafane founder, but this cannot be proven; what can be

proven beyond any reasonable doubt is that the mirror is old

and was home-spun. It has been recoated at least twice. The

mount for the original scope was not so hot, and I disliked

the secondary which was not very flat, optically speaking.

In 1988 I dismembered the telescope and put the optics into

storage. Some time later, I built an OTA around the primary

and set it on a dobsonian mount. The current incarnation of

the telescope is quite ugly, but very functional. You may

see a picture of it (and me, sorry) at:

http://shutter.vet.ohio-state.edu/astronomy/herschel400/

The OTA is an oversized and overlong square, with internal


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and epsilon Equulei (1.1 sec). I was able to see clearly

elongated the stars 48 Cassiopeiae (.8 sec) and 32 Orionis

(.9 sec). All those separations are from a 1984 source, of

course - I believe that whole observing project was done out

of a "Backyard Astronomy" article in S&T at that t ime. I

conducted the observations over several months from sites

near Akron, Ohio, USA.

I believe this to be about 'typical' performance for

double-star splitting, but I am not sure.

Quote:

>Thanks for any information you can provide.

Anytime! Was there anything I missed that you wanted to know

about?

--

Jeff Medkeff | Check out the s.a.a. photos page at

Rockland Observatory | http://shutter.vet.ohio-state.edu/saa.htm

Sierra Vista, AZ |

Tue, 15 Aug 2000 03:00:00 GMT

David W. Knisel

#35 / 43


Hi there. A number of small star fields or star clusters have had their

individual star magnitudes measured photometrically, so there is

considerable data to judge how faint a scope (and an individual) can

go. There are several of these fields shown in Chris Luginbuhl and

Brian Skiff's book, OBSERVING HANDBOOK AND CATALOGUE OF

DEEP-SKYOBJECTS, so you might check it out. I know a good 2.4 inch refractor

will (under good conditions) go to around 12.5 or so. I have seen 14.6

magnitude stars in an 8 inch, and my ten inch has taken me to past 15,

but going to 14th magnitude with a 4.5 inch might be a little much to

expect (but again, some people have better eyes than I do).

--

Prairie Astronomy Club, Inc. http://www.4w.com/pac

Hyde Memorial Observatory: http://www.blackstarpress.com/arin/hyde

***********************************************

* Attend the 5th Annual NEBRASKA STAR PARTY *

* July 18-25, 1998 http://www.4w.com/nsp ************************************************

Mon, 21 Aug 2000 03:00:00 GMT


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Jeff Medke

#36 / 43



Quote:

>Hi there. A number of small star fields or star clusters have had their

>individual star magnitudes measured photometrically, so there is>considerable data to judge how faint a scope (and an individual) can

>go. There are several of these fields shown in Chris Luginbuhl and

>Brian Skiff's book, OBSERVING HANDBOOK AND CATALOGUE OF

DEEP-SKY

>OBJECTS, so you might check it out.

Brian Skiff and I are working to bring some of these

sequences to the WWW. Its a little slow going, but we have

some usable sequences up at:

http://shutter.vet.ohio-state.edu/astronomy/mags/

--Jeff Medkeff | Check out the s.a.a. photos page at


Sierra Vista, AZ |

Mon, 21 Aug 2000 03:00:00 GMT

Berto Monar

#37 / 43


Thanks Bill for the good advice. But most of the points you mention are

applied by

most observers at least to certain extents.

Clear skies are a must and a steady breeze helps much in this regard, darkadaptation does wonders. But....

My best performance under good (nearly near-ideal!?!) skies close to zenith

was

magnitude 14.4 (comparison star near T Pyx) with the 10"/ f4.5. I just wonder

how

much I could improve on that? Perhaps the optics are not that good, or tube

currents

keep on stirring, my eyes go bad....

Following my previous posting I had some discussions (e-mail exchanges)

with B Skiff

in which he actually supports the claims of Jeff. Surely dark and dry high

altitudesites will boost the limiting magnitude somewhat (1,5 magn OK?) when using

high

magnifications.. As a result I tend to accept now Clark's limits, although they

are

hard to achieve.

Still I remain sceptic about the claimed 14.6 with a 4,5". Come-on Jeff, admit

it


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was 14,0!

Tell me Bill, do you need to cook the carrots first?

Regards,

Berto

/////

Quote:

> >'14.6 in a 4,5" reflector'???? Where does that magnitude come from:

GSC maybe?

> >I don't think one can see that faint even with an 8" under high power

and a 6.5

> >{*filter*} eye limit!

> I haven't checked my 'absolute' limits, but based upon some of my

> observations with a 13cm (5.1") scope, I can achieve the limits

> predicted by Clark in "Visual Astronomy of the Deep Sky". (14.2 or

> 14.3). Jeff's figure is about 0.6 magnitude fainter than Clark's

> formula gives for a 4.5" scope.

> I'm willing to give Jeff the extra 0.6 magnitude -- based upon his

> skies, his experience, his youth, and the various articles I've read

> over the years pertaining to this topic. The bottom line: There are

> no 'absolutes' when it comes to telescopic limiting magnitude.

Mon, 21 Aug 2000 03:00:00 GMT

Berto Monar

#38 / 43


Thanks Bill for the good advice. But most of the points you mention areapplied by

most observers at least to certain extents.

Clear skies are a must and a steady breeze helps much in this regard, dark

adaptation does wonders. But....

My best performance under good (nearly near-ideal!?!) skies close to zenith

was

magnitude 14.4 (comparison star near T Pyx) with the 10"/ f4.5. I just wonder

how

much I could improve on that? Perhaps the optics are not that good, or

annoying tube

currents.

Following my previous posting I had some discussions (e-mail exchanges)with B Skiff

in which he actually supports the claims of Jeff. Surely dark and dry high

altitude

sites will boost the limiting magnitude somewhat (1,5 magn OK?) when using

high

magnifications.. As a result I tend to accept now Clark's limits, although they

are


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hard to achieve.

Still I remain sceptic about the claimed 14.6 with a 4,5". Come-on Jeff, admit

it

was 14,0!

Tell me Bill, do you need to cook the carrots first?

Regards,

Berto

/////

>'14.6 in a 4,5" reflector'???? Where does that magnitude come from: GSC

maybe?

>I don't think one can see that faint even with an 8" under high power and a

6.5

>{*filter*} eye limit!

I haven't checked my 'absolute' limits, but based upon some of my

observations with a 13cm (5.1") scope, I can achieve the limits

predicted by Clark in "Visual Astronomy of the Deep Sky". (14.2 or

14.3). Jeff's figure is about 0.6 magnitude fainter than Clark's

formula gives for a 4.5" scope.

I'm willing to give Jeff the extra 0.6 magnitude -- based upon his

skies, his experience, his youth, and the various articles I've read

over the years pertaining to this topic. The bottom line: There are

no 'absolutes' when it comes to telescopic limiting magnitude.

Mon, 21 Aug 2000 03:00:00 GMT

Jeff Medke

#39 / 43



Quote:

>Following my previous posting I had some discussions

>(e-mail exchanges) with B Skiff in which he actually

>supports the claims of Jeff. Surely dark and dry high

>altitude sites will boost the limiting magnitude somewhat

>(1,5 magn OK?) when using high magnifications.. As a

>result I tend to accept now Clark's limits, although they

>are hard to achieve.

Such an observation is *never* easy. The night in question

was one of several I spent on Coronado Peak with another

observer, during the period that I was setting up the Akn

120 sequence. This particular observation occurred over the

time period comprising the American "Thanksgiving" holiday,

that is, late November. Tom Polakis, a Phoenix observer, has

also remarked here on s.a.a. how unusually good the nights

were around this time. The moon was a thin crescent, 63

degrees *below* the horizon at the time we started


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observing.

My methodology was designed to grope for the dimmest star I

possibly could. "Normal" observing was simply not in the

program. Observing commenced about an hour before the

sequence reached the meridian (about 1:00 am local), which

was about an hour and a half after dark adaptation

commenced. I used a magnification of 110x for most of the

observation time; this was enough to render the field stop

invisible and, in my opinion, more magnification would not

have helped. A description of the telescope is posted

elsewhere; for this particular observation I also utilized a

black shroud that fit over the eyepiece and covered my head,

neck, and shoulders. The eyepiece used was a narrow-angle

one. Think about this: there is a good reason for using such

a thing.

Earlier in the observing season I had been contending with

some 'noise' that my eyes were generating; I discovered that

this was caused by mild dehydration (I was new to Arizona at

the time), and I had the problem{*filter*}ed by this time. I made

it a point to eat a balanced meal at about 8:30pm that

night. I didn't do any caffeine, and I was not on any

prescriptions at the time.

The first observations I made while dark adapted clearly

showed sequence star H of magnitude 13.84. At this time the

{*filter*}-eye limit was 6.8. These observations were fairly

casual, and remained so for about 20 minutes. I was

eventually able to get sequence star J at mag 13.92 without

undue difficulty.

Between this star and the next in the sequence is a variable

star, which could not be used for my purpose. I felt that

the next 'good' star, sequence star L, should be in reach

considering the steady seeing and the ultra-clarity of theair. Accordingly, I sat on a low brick wall with my eyes

wrapped in the black shroud for about a half hour. There is

some disagreement in the logs about how long I did this; my

log (written after the fact) says 30 minutes, my partners'

log says about 20 minutes. Either way, I was in total

darkness for a mind numbingly long time.

At the end of the period my partner centered the field for

me and guided me back to the scope. He put the shroud hole

over the eyepiece, and put my hand on the focuser. After I

knocked the scope off the field, he did it again. I was able

to place my eye at the eyepiece without exposing it to any

light whatever beforehand.Star L was immediately apparent. It blinked on and off as I

went from direct to averted vision. With averted vision, the

star appeared rather well above the threshold of vision, and

I thought that I had a good shot at sequence star M, which

is 14.83 magnitudes - only a .15 magnitude difference from

sequence star L. So I looked for it, but I never could see

it, never could even talk myself into the idea that I could


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just barely possibly see it. I was stuck with star L at

14.68.

Quote:

>Still I remain sceptic about the claimed 14.6 with

>a 4,5". Come-on Jeff, admit it was 14,0!

Sorry, I won't recant what I know I achieved. The nearest

stars to 14.0 are sequence stars I and J, at 13.91 and

13.92, respectively. These stars were quite apparent even

before doing the Herschel trick of covering up the eyes.

Star L was seen, and seen quite clearly. Actually, L is

14.68, so we should probably round up, but that would be

like fibbing.

Quote:

>My best performance under good (nearly near-ideal!?!)

>skies close to zenith was magnitude 14.4 (comparison

>star near T Pyx) with the 10"/ f4.5. I just wonder how

>much I could improve on that?

Probably quite a bit. I don't know what kind of 10" f/4.5

you have, but my own scope of that aperture and length is a

real photometric sequence dog. The scope is just not up to

the task; the best I have done with it so far is 14.83 (Akn

120 sequence star M - however star N at 14.95 was

invisible). I have not had nights *as good* as I had the

night with the 4.5", but I have had some close ones. Still,

I would have expected to do better with the larger aperture.

My conclusion is that the construction of my OTAs accountsfor some of the difference. My 4.5" is a very souped-up

scope, while my particular 10" is practically a straight

commercial product from Meade. This may or may not be a

factor with your instrument, of course.

The other thing that I believe is that the 10" is affected

much more by seeing than the 4.5", to the point that even on

good nights much more degradation in star images is

noticeable than with the 4.5". On a night of truly perfect

seeing, I think I might make up some of that difference. (We

had a recent night of perfect seeing, but transparency was

not so good.) My older experiences (with a 10" Coulter dob)

bear this supposition out.As to how to improve on it: you have to do everything right.

You must be in excellent shape physically (the medical

sciences have shown that small health problems can

non-trivially affect scotopic vision, even just being 'out

of shape' can), you have to be well-hydrated and

well-oxygenated (I live at 4500 feet, so the 6000 some-odd I

observed at was no big deal for me), and you must not be


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hungry. You must be perfectly comfortable - I sat in a

padded office chair for the observations - and warm.

I should stress that being in 'good shape' physically is, in

my opinion, an important factor. In summer I swim daily. In

winter I walk long distances (about 2 miles per day

minimum), usually up and down mountains. I bicycle in all

weather. I work at this, but I do it mainly because I am

asthmatic and if I don't work at it I lead a slightly less

pleasant life. But I have noticed a pattern among other

observers that I consider to be especially talented or

skillful, and that is that most of them are in good physical

shape, and most of them exercise and eat right

intentionally, to stay that way.

Your optics must be clean. They must be *perfectly*

collimated. You must not have a lot of glass in your

eyepiece - IIRC I used a Kellner for my personal best, but I

have used Ramsdens too for this kind of observation.

It is pretty rare for all of these conditions to coincide

with perfect atmosphere. I worked pretty hard to achieve all

these things at the same time, in a dedicated three month

long pursuit of the faintest star I could see in that

instrument. I am still kind of pursuing fainter stars, but

not as fanatically. Others who have worse climates than

Arizona's might have to work on this for much more than 3

months to get as far as I did, of course.

Quote:

>Bill said:

>> I haven't checked my 'absolute' limits, but based upon some of my

>> observations with a 13cm (5.1") scope, I can achieve the limits

>> predicted by Clark in "Visual Astronomy of the Deep Sky". (14.2 or

>> 14.3).

A fair amount of Clark's material is flawed by an erroneous

interpretation of Blackwell's data. There are also some

mathematical errors. For a primer on this, see:

http://www.***.com/ ~mbartels/visual/visual.html

http://www.***.com/ ~mbartels/visual/nils/blackwel.html

Clark's stuff is nice as far as it goes, but the *real*

cat's pyjamas in this field are by Bradley Schaefer. He has

been studying the question of visual limits for much longer,

and has conducted several tests. His complete theory of

telescopic limiting magnitudes is published in:Schaefer, B. E. 1990, Pub. Astron. Soc. of the Pacific, vol

102, pp. 212-229

A "popular" version of the same thing appears in:

Sky & Telescope, November 1989, vol. 78, p. 522

There is also scattered work on UBVRI sky brightness

measurements, extinction, and so on, that he has done. All

of this work is a long bit superior to Clark's material.


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I have not yet achieved *quite* what Schaefer's formulae

define as the absolute limit, but I have been tantilizingly

close.

Quote:

>> Jeff's figure is about 0.6 magnitude fainter than Clark's

>> formula gives for a 4.5" scope.

>> I'm willing to give Jeff the extra 0.6 magnitude -- based upon his

>> skies, his experience, his youth,

Someone should point out that I was 29 years old when the

observation was made (as opposed to, say, 10), and have been

observing with optical aid since 1978 or 1979. I star ted

observing photometric sequences when Edgar Everhart

published some of Chou's sequences in S&T in, IIRC, 1982.

Quote:

>> and the various articles I've read

>> over the years pertaining to this topic. The

>> bottom line: There are no 'absolutes' when it

>> comes to telescopic limiting magnitude.

The other thing is that human vision varies greatly. I hate

to say it this way, but there is no-one here (myself

included) that has any reason to think that their vision

represents the absolute limits of all human vision. The fact

that something is unobservable to one individual or group

does not mean that there is any credible basis to believe it

will be unobservable to another person or group - especially

when all of the variables, including physiological ones, are

considered.

Calling a reported observation impossible is in consequence

calling the observer deluded, or a liar. Either way, it

isn't too pleasant. Those who have ...read more

Tue, 22 Aug 2000 03:00:00 GMT

Derek Won

#40 / 43


Quote:

> Such an observation is *never* easy...

Jeff:

This posting was awesome--it was so informative, I saved it to disk.


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Thanks for suggesting ways in which we can reach our maximum observing

potential, whatever that may be.

It was very interesting that your 10" observations were only 0.15 mags

different from your 4.5", instead of the predicted 1.74 magnitudes.

Were you observing at the same time period and with the same

magnification with the 10"? If so, removing .4 mags for seeing and .3

for other differences (ie. in optical coating, slightly better

collimation) leaves .9 magnitude of unexplained difference, which you

attribute to the OTA. This makes sense because all of the extra stray

light would produce a brighter background, wiping out very dim objects.

The implication is that telescope tweaking can make a tremendous

difference, especially at the limit of visual perception. I wonder what

would happen if you added a tube extension or other modifications onto

your 10" during one of your observing sessions.

Derek Wong

Thu, 24 Aug 2000 03:00:00 GMT

Derek Won

#41 / 43


Quote:

> >Thanks for suggesting ways in which we can reach our

> >maximum observing potential, whatever that may be.

> I have a lot of others, too. I present them in a lecture

> called "Cassini, Bond, and Barnard - How They Did It, and

> How You Can Too!" If you want to have me over to your

> club/society/whatever, I am happy to do it.

Thanks for the offer--if I join a club I'll let you know. Meanwhile, if

you give the lecture in Southern California, let me know.

Quote:

> Disappointingly, I cannot really justify making an oversize

> tube for this scope - it would require a new primary cell,

> spider, and whatnot, and it seems like more trouble than it

> is worth. But I do believe in oversize tubes, with a bit cut

> in towards the axis to allow a focuser and small secondary.

> I did this when constructing my 4.5", and I think it has

> paid off.

I would assume that this is done to decrease tube currents.

Quote:

> But once you have a good instrument, the most important

> thing to building your skill in using it is to log a lot of


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> eyepiece time. There is no substitute whatever for

> experience.

I've been trying, but El Nino has not cooperated. It's finally sunny

here but then clouds up at night.

I'm going to eventually use your suggestions along with some other

telescope tweaking and light shields at my house to see if I can see any

detail in M51 in mag 4 skies as well as to test my limiting magnitude.

I know, what's the point, but even if its impossible it will improve my

observing technique.

Derek Wong

Thu, 24 Aug 2000 03:00:00 GMT

Jeff Medke

#42 / 43



Quote:

>This posting was awesome--it was so informative,

>I saved it to disk.

Well, I'm flattered - thanks! I'm going to frame this to

look at when the crazies next get out of hand....

Quote:

>Thanks for suggesting ways in which we can reach our>maximum observing potential, whatever that may be.

I have a lot of others, too. I present them in a lecture

called "Cassini, Bond, and Barnard - How They Did It, and

How You Can Too!" If you want to have me over to your

club/society/whatever, I am happy to do it. I do not charge

for the lecture, but usually if the distance is great I try

to find some way to recoup travel expenses. If I understand

correctly, you are in southern California (sorry if I am

completely confused on that)? That isn't too far for me -

just route ten over to CA and I am mostly there. If not, I

am eventually going to get a variant of the lecture set up

on a web page, but when depends a lot on my schedule.

Quote:

>It was very interesting that your 10" observations

>were only 0.15 mags different from your 4.5", instead

>of the predicted 1.74 magnitudes. Were you observing


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I *do* believe that. I know that some people say that so

doing makes a difference that isn't noticeable but I am not

sure I agree. If I can quantify the difference with careful,

detailed observations, then I think it is "noticeable", even

if not subjectively or casually.

Quote:

>I wonder what

>would happen if you added a tube extension or

>other modifications onto your 10" during one of

>your observing sessions.

I have a tube extension in the works, as well as pre and

post secondary tube baffles to insert. A behind the mirror

baffle too. Also, I have yet to paint the mirror chamfers

black - this is also on the list. A baffle under the

focusing tube, as Bartels recommends, is in the works, as

soon as I get off my duff and work out its dimensions. This

kind of baffle sort of obviates the tube extension, but it

won't hurt me to do both.

Also non-trivial will be the addition of rotating rings to

the mount. Allowing me to sit while observing - that alone

has gained me .1 or .2 magnitudes or so.

Disappointingly, I cannot really justify making an oversize

tube for this scope - it would require a new primary cell,

spider, and whatnot, and it seems like more trouble than it

is worth. But I do believe in oversize tubes, with a bit cut

in towards the axis to allow a focuser and small secondary.

I did this when constructing my 4.5", and I think it haspaid off.

But once you have a good instrument, the most important

thing to building your skill in using it is to log a lot of

eyepiece time. There is no substitute whatever for

experience.

--

Jeff Medkeff | Check out the s.a.a. photos page at


Sierra Vista, AZ |

Fri, 25 Aug 2000 03:00:00 GMT

Jeff Medke

#43 / 43



Quote:


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