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M13 Propeller--How Low Can You Go?

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#1 Phillip Creed

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Posted 15 June 2013 - 10:57 AM

One of the things that makes M13 one of my favorite globular clusters is the "spider" chains of stars that branch out from the cluster and its "3-bladed propellor" of dark lanes (star-poor regions).

One thing I was wondering, though, is what power was needed. I had seen the Prop before many a time in my 12" scope and my old 16" scope under high powers. So I decided to test this out from my driveway here in Canton, OH.

Now I know my spot's not the best. I can see all the stars in the Little Dipper on a good night. Call it a Bortle Class 8 sky, if you will. But globs are nice in that they're DSO's that are suited for observation under high power, which helps darken the sky background and enhances contrast. I also flocked opposite the focuser on both my 8" f/4.9 and 12" f/4.9 reflectors, as well as in front of the primary mirror on the 8".

First, the 8". I had no trouble seeing the propeller with the 10mm Delos + 2X barlow (200X). So I tried it again with a (HEAVY!) pairing of my 30ES-82 + 4X Powermate (133X), and I could still see it, though it was much harder. A 10mm Delos by itself (100X) didn't show or hint at anything.

I tried this again last night using the newly-flocked 12" scope. The 10mm Delos (150X) showed it easily. The 14mm Delos (107X) was juuuuuuuuust enough power to eek out the dark lanes. It was tough, but they were clearly there. Aperture definitely helps.

I tried again using the 22mm Vixen LVW (68X), but nothing was hinted at.

So my best guess is about 100X is the bare minimum, at least based on my scopes, my eyes and my observing spot. I've got a good set of eyes, but I'm not in Stephen O' Meara's league.

Question--what say you? Many of you are blessed with far, far darker skies than mine in your backyard, with sharper vision to boot. How low can you go and still see the propeller in M13?

Clear Skies,
Phil

#2 youngamateur42

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Posted 15 June 2013 - 11:20 AM

Caught it in my six inch from light polluted skies at 180x, not easy, but there

#3 Astrodj

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Posted 16 June 2013 - 12:28 AM

I have seen the propeller with my XT10 at 86x from my backyard. For comparison purposes, I can make out all the stars in the little dipper on only the best nights at this time of year, when it is higher in the sky. My absolute best ZLM ever is about 5.4.

I was using a Meade series 4000 13.8 SWA in the 10".

#4 bigdob24

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Posted 16 June 2013 - 03:03 PM

I have looked for this many times with no luck.
I have. 24" dob, have looked through a 30" and a 48" and always the same
I have even looked at some sketches that have been drawn.
I will continue the search , maybe it will shine through for me some night
Also have looked for it with my 132mm refractor too.
Any advise on how to pick it out?

#5 Phillip Creed

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Posted 16 June 2013 - 04:14 PM

Dan,

Here's how I learned how to find it.

In any finderscope under dark skies, you can see M13 is flanked by two 7th-magnitude stars nearby. M13 forms a broad triangle with these two stars, which are spaced about 30' apart. You should be able to get both stars and M13 in the same FOV with a decent low-power 2" eyepiece.

Once you got both stars and M13 in the same FOV, imagine drawing a line between the two 7th magnitude stars. You want to be looking for the 3-lane propeller in the part of the cluster that lies closest to the midpoint of that line.

Clear Skies,
Phil

#6 star drop

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Posted 16 June 2013 - 06:29 PM

I have looked for this many times with no luck.
I have. 24" dob, have looked through a 30" and a 48" and always the same
I have even looked at some sketches that have been drawn.
I will continue the search , maybe it will shine through for me some night
Also have looked for it with my 132mm refractor too.
Any advise on how to pick it out?

In a 25" telescope on a good night the propeller lanes are not dark as in smaller telescopes. They are dimmer than the surrounding areas but not that much so, they give me the impression of hollowed out star filled grooves in the cluster. I get a similar grooved impression when I observe the rays of Tycho on Earth's moon. Keep your magnification low, 100x is plenty.

#7 star drop

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Posted 16 June 2013 - 11:53 PM

"How low can you go and still see the propeller in M13?"

Fifty seven magnification using a ten inch Orion Dobsonian (f/4.7 with a 21mm Ethos) at Cherry Springs, PA. The propeller blade pointing away from the cluster center appeared shorter than the other two blades. That was the smallest telescope in which anyone had asked me to find it. Both of the telescope owners were able to see it as well once they knew what to expect.

#8 JasonBurry

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Posted 17 June 2013 - 01:37 PM

I see the propeller regularly with a 12.5mm in my 8" dob, for right about 100x. I don't recall spotting it at lower power.

I usually observe M13 with the 12.5 in a 2x barlow, and at 200x, the propeller is fairly easy to see.

The biggest problem is to know that the propeller is NOT centered in the glob.

J

#9 Starry eyes

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Posted 18 June 2013 - 01:06 AM

The best view I saw of the propeller was with the Lick 36" refractor. Probably around 500X. It was quite prominent. It floats in and out with my 18" between 160 and 400X. At 100X not so much.

#10 azure1961p

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Posted 18 June 2013 - 10:19 PM

I never knew it was there. The chains are there but no propeller - I never interpreted it as such. Ill say with the 8" 150x is optimum - maybe 135x. By contrast M15 needs at least 200x and 240x is better. I've fouls m92 has a more impressive central core than 13. Jus sayin.

Pete

#11 Patricko

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Posted 19 June 2013 - 11:37 AM

This is an excerpt from what I posted over at a different website:

Hi everyone,

I got a second night out with the 60/700mm refractor under a New Moon
sky. From my intown location I was seeing stars of 5.4 mag near the
zenith with only very light high passing clouds. Seeing was a 2/5;
transparency a 8.5/10. A short time ago I had wondered if the propeller
feature in M13 could be observed visually in a small 60mm refractor. I
got my answer.

M13 was climbing high in the east and I started off with my 35mm Ultima.
Of course the propeller was not to be seen at such a low power and
neither was the GX NGC 6207. Using an 18mm Meade 5000 UWA still showed
no hint of either object. M13 was hinting at some resolution around the
outer edges at this power. Next was the 14mm UWA Meade 4000 eyepiece,
this gives a power of ~50x. I studied hard looking for any sign of
either object, no dice.

Really the 14mm UWA is not the eyepiece to be using for extremely faint
objects because it has a lot of elements and the coatings transmit
approximately 74% of most light waves. Stars of 12.2 mag were glimpsed.
Thinking perhaps less glass would be helpful I tried a 13.8mm Meade SWA
4000. The FOV decreased in size and the outer FOV correction distorted
in comparison to the 14mm UWA but the contrast and light-throughput
noticeably increased. A 13.25mag field star was now being held with
averted vision near NGC 6207 but the GX remained invisible.

I want to pause and call attention to what I just wrote. The 14mm UWA
allowed me to see stars with averted vision to 12.2 magnitude. The
13.8mm SWA allowed me to see stars to 13.25 magnitude!!! This is a HUGE
difference. I next tried my 11mm UWA, no improvement in magnitude depth
and still no hint to NGC 6207 or the propeller. Next up was a 9mm SWA,
this allowed for a 13.3 mag star to be detected, now I was seeing deeper
and with extreme effort caught glimpses of NGC 6207 fuzzy core. For
several minutes I studied M13 at this power, the cluster was really
beginning to resolve and not just the chains stars, some across the core
were visible now as well. I got the feeling that 2 of the propellers
were being detected but couldn't hold them long enough without being
sure. Damn! I need a mount that tracks.

Finally, I decided to use a 6.4mm Meade 4000 SP, this gives ~109x and
~.55mm exit pupil. This eyepiece has a much smaller FOV than all the
other eyepieces I was using. I had to study one object at a time. First
the field of NGC 6207..... is that it??? Yes, yes I got it! I got the GX
with slight averted vision with a 60mm telescope!!!! Not only that but I
was catching repeatedly the nearby 13.9 magnitude star just to the east
of NGC 6207! Folks, this is the FAINTEST star I have ever managed to see
with a 60mm refractor! The excitement was almost overwhelming and I
manually tracked this GX for about 20 more minutes before turning my
sights on M13. At this point M13 was BLINDINGLY bright and was actually
ruining my dark adaptation. However, using slight averted vision brought
out the "Y" shaped feature. Several times I would manually move the
mount, watch M13 and the "Y" off to the SE float across the FOV and then
do it again and again.

My conclusion is this.... eyepieces REALLY have a major impact upon how
deep you can see with a small scope. Just look at the great magnitude
difference between my 14mm UWA and the 13.8mm SWA. There is NO WAY that
the .2mm difference is what accounted for the 1.05mag increase in
faintness. The 14mm UWA has the best corrected FOV I have ever seen in
an UWA bar none. The 13.8mm isn't near as corrected but it blows its big
brother out of the water where faintness detection is concerned.

Finally, the 6.4mm Meade SP has very little eye-relief, is hard to use
ergonomically, and has a small FOV. It needs to be used with a scope
that tracks because objects float thru the small FOV quickly. With this
said the 6.4mm Meade SP showed me the faintest star I have ever seen
with averted vision in a 60mm telescope. It shows NGC 6207 from my
intown backyard, and the Y shaped propeller feature can be seen and held
steady in M13. This eyepiece and my new little 60mm have motivated me to
invest in a mount that tracks because I want to spend all my time glued
to they eyepiece and like Thomas has mentioned it is a necessity when
viewing at high powers. I can tell you after using my Alt/Az mount
tracking at high powers manually is not fun when you're trying to eek
out every last photon.

Clear skies,
Patrick

#12 Edwin Quiroga

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Posted 19 June 2013 - 07:51 PM

Really, really remarkable, Patrick! Thanks for sharing.

#13 Mike4242

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Posted 23 June 2013 - 11:36 AM

I find I can see it best at a lower power. I saw it in my 6" refractor the other night with a 14mm Meade 5000 UWA at 70x. It was actually easier for me to see it at this magnification than at higher ones. At higher mags it seems to get lost in the darkness between the stars.

#14 bremms

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Posted 24 June 2013 - 10:48 PM

Magnitude difference? Between similar EP's? The difference transmission between those EP's is a couple of percent at best. Some other factor was contributing.
13.9 in a 60mm? mag 5.4 sky? Not saying it didn't happen.. But, 60mm only gives you 5 magnitudes of gain assuming no losses.

#15 Scanning4Comets

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Posted 26 June 2013 - 08:00 AM

You can see the propeller in the top right of M-13 in this photo below:

http://yorkastro.org...2012/08/M13.jpg

I haven't seen it in many years.....Looks like it is hunting time on M-13 again! Thanks for this post!

Cheers,

#16 Sasa

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Posted 26 June 2013 - 10:37 AM

13.9 in a 60mm? mag 5.4 sky? Not saying it didn't happen.. But, 60mm only gives you 5 magnitudes of gain assuming no losses.

Using Pogson's equation in this way is too rough. There are several important effects that are not included. The most important is the influence of high magnification. I can also reach in 100mm refractor stars below mag 14 from my backyard with 4.5-5 mag skies. Far from naive estimation based on Pogson.

#17 Fuzzyguy

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Posted 26 June 2013 - 10:54 AM

I've looked for the propeller several times over the last year and a half, but never saw it. I didn't know what I was looking for though. Now with this post, I've got an idea of what and where to look for it! Thanks for the post and all the images!

#18 Charles Funk

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Posted 26 June 2013 - 11:51 AM

Ok, I am looking at the photo, I just do not see this propeller. Anyone have a overlay, showing the exact location of this feature? I have never heard of this before, and am curious to see what the heck you guys are talking about...

#19 JasonBurry

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Posted 26 June 2013 - 02:20 PM

I haven't an overlay, but view that image in the link full size. The propeller feature is offset from the core, about 1/3rd the radius from the centre in the 1:00 or 2:00 direction.

There are 3 dark lanes radiating from that point. One points directly away from the core. The other 2 "frame" the core.

The propeller feature is about half the diameter of the whole glob's main body. Just a very low contrast trio of intersecting apparent dark lanes or star-poor regions.

J

#20 Sasa

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Posted 26 June 2013 - 04:29 PM

I see something like this on my picture of M13 that I took some time ago. But it just right from the core (i.e. at 3:00). Is it the one you are talking about or is it something else?

http://www-hep2.fzu....bj/orig/m13.jpg

#21 star drop

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Posted 26 June 2013 - 05:01 PM

Nice image, Sasa! The propeller in your image has its hub to the lower left of the core about one quarter of the way out from the center with one blade pointing to the eight o'clock position, one blade pointing to the twelve thirty position and the third blade pointing to the four o'clock position.

#22 Jim Nelson

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Posted 26 June 2013 - 05:12 PM

I just do not see this propeller. Anyone have a overlay, showing the exact location of this feature?


Quick 'n' messy:
http://i205.photobuc...bo/Untitled.png

#23 Sasa

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Posted 28 June 2013 - 02:17 PM

Thanks Ted,

I can see it now. Did not notice it before, just the more obvious one in the halo right from the core.

Alexander






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