Deep Sky Objects in light-pollution with 4" Dob - Given up for now

[flord.lord]

Hi,

Until recently, I have been using my 4" Dobsonian (f/4 A=100, D=400) for lunar and planetary viewing. I've been pretty impressed!

So I decided to take a shot at some bright DSOs, apparently what my scope “specialises” in. For the past three nights I have been trying to locate M3 through the wide 25 mm eyepiece (x16). Now, I live in a light-polluted city suburb. The limiting naked-eye magnitude is about 3.2 and through the 'scope it's about 7 (I think).

I'm finding it almost impossible to star-hop through the scope as the nearest star visible to Beta Comae Berenices (where I am supposed to start) is Cor Caroli, and traversing quite a large distance through the scope is difficult - basically I don't know what I'm seeing.

I'm sure M3 has already passed through my FOV - though anything above mag 5 in the scope just looks like a dim fleck, so I don't know how to distinguish it from other stars.

Is there a technique I'm missing out on? Or - as I think - I will either have to wait for dark winter skies or go to the countryside.

Thanks!

[helicon]

It should be visible in your scope since it can be seen in binos from my light-polluted backyard. At 18 arc minutes in size and magnitude 6.2 it should resemble a puffball at the eyepiece. One of the techniques you can use when the LP is bad is to aim by dead reckoning. Basically this is sighting up the scope to the location shown on your chart (or phone ap). Then you can sweep to the right and left, then back to the center point, and after that either raising or lowering your scope and repeat the motion. Eventually you'll get it into your eyepiece's field of view. Also note that it is midway between the bright star Arcturus and Cor Caroli... as shown in this chart:

[flord.lord]

It should be visible in your scope since it can be seen in binos from my light-polluted backyard. At 18 arc minutes in size and magnitude 6.2 it should resemble a puffball at the eyepiece. One of the techniques you can use when the LP is bad is to aim by dead reckoning. Basically this is sighting up the scope to the location shown on your chart (or phone ap). Then you can sweep to the right and left, then back to the center point, and after that either raising or lowering your scope and repeat the motion. Eventually you'll get it into your eyepiece's field of view. Also note that it is midway between the bright star Arcturus and Cor Caroli... as shown in this chart:

How much light pollution do you have? As it's summer, the sky never really gets black, more like dark grey (at least where I am). But the key thing I don't understand is how I'm supposed to distinguish a 6 mag star and a 6 mag anything else - In my 25 mm they just look super dim.

The only stars visible to the naked eye to me on that chart are Arcturus, Cor Caroli and Muphrid - it would be easier if I could see β Comae Berenices, but it's just not visible.

[helicon]

Bortle 6-7 with a limiting magnitude of about 4 on most nights. The globular would appear as a faintish cotton ball, not looking anything like a 6th magnitude star, as the individual stars of the cluster are too faint to be resolved. Still it's bright enough as deep sky objects go to be seen even from the city through a scope.

[flord.lord]

Bortle 6-7 with a limiting magnitude of about 4 on most nights. The globular would appear as a faintish cotton ball, not looking anything like a 6th magnitude star, as the individual stars of the cluster are too faint to be resolved. Still it's bright enough as deep sky objects go to be seen even from the city through a scope.

Thanks, I guess I'll try again when it clears up. As a matter of interest, is using 16x through my small 4" the problem? Do I need a higher magnification?

[helicon]

16x is fine. I can see M3 in my 10x50 binoculars (10x). Once you find it at 16x you can switch to a higher power eyepiece to zero in on it.

[ARock]

Globular clusters will look like a puff of smoke, so are sensitive to sky contrast. It should be visible in your scope. Not sure about your sky darkness though.

Things to try, assuming you are using Stellarium, a red dot finder, and the 25mm EP.

1. In the Red Dot finder , find Arcturus and Cor Corioli and place the scope close to midway between them towards Arcturus.
Then try the EP.

2. In stellarium, set up the Oculars plugin with your scope and EP settings so that you have the FOV circle for the EP. Start from Arcturus and move short distances towards M3 by way of the brightest stars. At every step match the stars you see in the 25mm with what you see in Stellarium. It helps if you set the LP level in stellarium to match what you see in the scope.
Long hops need patience and lots of tries. A 10x50 bino can help you scout the area.

3. Make a paper setting circle for the Az axis and an digitial inclinometer for the Alt axis and use it as a pushto. Google Manual setting circles for a Dob.

M22 is another bright globular and an easier hop. You can use it to figure out if your sky permits seeing globs at all now. If it doesn't you can look for open clusters, which are easier to see.

Do you have the book "Turn Left at Orion" ? It is very useful for beginners.

Good luck

[Buckethead 2.0]

16x is fine. I can see M3 in my 10x50 binoculars (10x). Once you find it at 16x you can switch to a higher power eyepiece to zero in on it.

This is my latest target as well, so thank you Michael for your helpful responses. It should help me too.

[MrShorty]

I don't think you are missing out on any specific technique, it is mostly a matter of practice.

Rather than starting Cor Caroli, my starhop for M3 tends to start at Arcturus. I move perpendicular to the line between Arcturus and Mufrid through a couple of "groupings" of stars until I find the parallelogram of 4 stars indicated in this picture. The top 2 stars in this grouping point almost directly to M3, so I can then follow the line through those two stars to M3.

I would agree, too, that it could be worth trying a few other bright globulars. M22 has already been mentioned. M13 in Hercules is a pretty easy one to find and should be high in the sky right now.

By all means, don't give up. Starhopping is a bit of a skill that requires some practice and you will get better at it.

[pakarinen]

Before I got push-to, I’d hop from Arcturus. M22 is a monster and might be easier for you.

[John Baars]

M3 is not the easiest to find, without a finder. You picked up quite a difficult one, since there are no real "bright" reference stars nearby, once you have passed the star near Arcturus.
You can use Stellarium to duplicate the view of your 25mm eyepiece. Starhopping is not easy the first time you do it.
Good chance that it passed without even noticing it in the bright backgroundglow. It seems rather bright, but that's because its brightness is calculated as if it is a single star...
But.. all the light is spread out. So M3 is far far more dimmer than you'd expect.
If I may suggest another object: M57. The road from Wega to there has rather a lot moderate bright stars. And still...it will be a hard time getting there, even if you know that the field of view you have with your 25mm eyepiece is some 3 degrees, and you only have to cross 6 degrees. That will be more than enough challenge while learning to starhop.
M57 may seem les bright. But is is smaller than M13 and sharper defined, so it is easier.

[KingClinton]

Star hopping is your best friend when using your dob.
It may seem daunting at first but with practice it becomes second nature, like riding a bike.

It's all about following a trail of crumbs or stars. Check in stellarium first and plan a "route" to follow. Remember that your view in the eyepiece is flipped and upside down with a Newtonian scope.
If you do use Stellarium then remember to use the oculars plugin configuration tool and check the horizontal and vertical flip boxes under the telescopes tab.
Now it should match the view in your eyepiece and make star hopping that much easier.

I enjoy a good star hop in my dob and it gets you bang on target every time!
I use Skysafari at the scope on a tablet in night mode.

Perseverance is key, it will pay off.

[flord.lord]

Globular clusters will look like a puff of smoke, so are sensitive to sky contrast. It should be visible in your scope. Not sure about your sky darkness though.

Things to try, assuming you are using Stellarium, a red dot finder, and the 25mm EP.

1. In the Red Dot finder , find Arcturus and Cor Corioli and place the scope close to midway between them towards Arcturus.
Then try the EP.

2. In stellarium, set up the Oculars plugin with your scope and EP settings so that you have the FOV circle for the EP. Start from Arcturus and move short distances towards M3 by way of the brightest stars. At every step match the stars you see in the 25mm with what you see in Stellarium. It helps if you set the LP level in stellarium to match what you see in the scope.
Long hops need patience and lots of tries. A 10x50 bino can help you scout the area.

3. Make a paper setting circle for the Az axis and an digitial inclinometer for the Alt axis and use it as a pushto. Google Manual setting circles for a Dob.

M22 is another bright globular and an easier hop. You can use it to figure out if your sky permits seeing globs at all now. If it doesn't you can look for open clusters, which are easier to see.

Do you have the book "Turn Left at Orion" ? It is very useful for beginners.

Good luck

Thanks! I tried setting up the oculars plugin in Stellarium. I calculated the field of view in my 25mm eyepiece on a website and it gave me "50 degrees". I'm assuming this is 0° '50? Because in Stellarium I have to enter in ° and '. Sorry if I'm being a newbie about that.

[John Baars]

50° 00'
Fifty degrees and zero (arc) minutes.
When you see this: 50°00' 00" it means: 50 degrees, zero arc-minutes and zero arc-seconds.

[Ruud]

You can set Stellarium to use decimal degrees: F2(configuration) --> tools --> use decimal degrees

The Babylonian degrees-minutes-seconds system is called the sexagesimal system and has 60 as base. 2°12' 30" = 2 + 12/60 +30/(60x60) = 2.208333 degrees.

I think I'll set Stellarium to decimal myself.

[ARock]

Thanks! I tried setting up the oculars plugin in Stellarium. I calculated the field of view in my 25mm eyepiece on a website and it gave me "50 degrees". I'm assuming this is 0° '50? Because in Stellarium I have to enter in ° and '. Sorry if I'm being a newbie about that.

You dont have enter the symbols in the Ocular Plugin on Stellarium. Entering in decimal degrees is fine.
You do not have to calculate the FOV on another site and enter it. The Ocular plugin calculates it for you.
For eyepieces you need to enter the aFOV, Focal length and Field Stop.
The aFOV depends on the type of eyepiece and can be got from the eyepiece specs. Yours are most likely Plossl and so a aFOV of 52 degrees. So enter 52 in the aFOV. In the focal length field enter 25 and leave 0 in the Field Stop unless you know it. Some EP specs include a field stop. Give it a name "Orion 25mm" and you should be set. Stellarium will calculate the True FOV and draw a circle of the appropriate size. In your case it should be around 3 degrees.

[helicon]

I took a look at M3 last night [mention]flord.lord[/mention] (it has been awhile) in the 10" Dob. Pretty well resolved at 135x with some sparklers at the edges. Found by aiming the scope at the point about 2/5 of the way between Arcturus and Cor Caroli. The sky was not completely dark (9:45) meaning the view was slightly muted. About 20 minutes later (more darkness) I peeked at M13, which was a bit more impressive, and also, the sky was darker.

Just sort of a test to see whether M3 was fully visible in a darkening sky...

[flord.lord]

:

You can set Stellarium to use decimal degrees: F2(configuration) --> tools --> use decimal degrees

The Babylonian degrees-minutes-seconds system is called the sexagesimal system and has 60 as base. 2°12' 30" = 2 + 12/60 +30/(60x60) = 2.208333 degrees.

I think I'll set Stellarium to decimal myself.

[mention]Ruud[/mention]
Unfortunately, I have mobile Stellarium+ and I don't think I can enable that feature. Here is what I have currently got

[flord.lord]

I took a look at M3 last night @flord.lord (it has been awhile) in the 10" Dob. Pretty well resolved at 135x with some sparklers at the edges. Found by aiming the scope at the point about 2/5 of the way between Arcturus and Cor Caroli. The sky was not completely dark (9:45) meaning the view was slightly muted. About 20 minutes later (more darkness) I peeked at M13, which was a bit more impressive, and also, the sky was darker.

Just sort of a test to see whether M3 was fully visible in a darkening sky...

Thanks. Annoyingly, it's been consistently cloudy here and I haven't had a chance to try again. My highest magnification now is 100x; an 8mm eyepiece with a 2x Barlow. Also for the sky to get usably dark it has to be about 22:45

[Ruud]

Unfortunately, I have mobile Stellarium+ and I don't think I can enable that feature. Here is what I have currently got

Now that's a shame. I have discovered that I prefer decimal. In the desktop version you make the change once for the entire program, including the oculars and angular distance plug-ins. (I found an exception: the location window remains in minutes and seconds mode.)

The sexagesimal system is from the 3rd millennium BCE. Maybe the makers of Stellarium mobile can be moved to update to decimal, at least as an option. Ask them!

Good luck.