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[curare]

Hello All,

I just registered a few days ago and have been enjoying the site. I'm an extreme novice astronomer and am presently borrowing a Questar 3.5 (f/14) from the school where I teach. I would like to buy my own scope to use this summer. The two scopes I'm considering are:

A.) Zhumell 10" reflector (f/4.92)

www.telescopes.com/products/zhumell-10-inch-dobsonian-reflector-telescope-38399.html#

B.) Zhumell 8" reflector (f/8.7)

www.telescopes.com/products/zhumell-10-inch-dobsonian-reflector-telescope-38399.html#

So my main point of consideration is whether to go deep sky or medium range. I've been reading books on astronomy and telescopes, but book knowledge is no substitute for the experience that I lack. So I am hoping to pick your brains Here are some questions that are in my head:

1. The resolving power of the 8" is roughly twice that of the 10". By my reckoning, this seems to be a function of the aperture difference because both scopes have roughly the same focal length. Could applying a dimming filter (like a moon filter) to the eyepiece of the 10" increase its resolving power by decreasing the amount of light filtering through from the aperture? My concern here is that even though I am very interested in observing deep sky objects, I would hate to lose my ability to see the planetary detail I've been enjoying through the Questar.

2. My one complaint about the Questar is that even though the images are beautiful, they're small and it strains my eyes to look at an object like Saturn. The Questar has a maximum useful magnification of about 270x, whereas the 10" Zhumell has a MUM of about 600x. But what will the planets look like through the scope? Will they be blurry discs or will I still be able to see surface detail?

3. How will deep sky objects look different on the 8" and 10"? Does the higher resolving power of the 8" mean that I will be able to see more detail in distant nebulae? Or will the greater light collecting ability of the 10" give me that greater detail (as well as greater image size)?

That's it for now. After writing this, I'm figuring that since I have access to a high quality f/14 scope, I might as well go for the other end of the spectrum with the 10" f/4.92. I just want to make sure I'll see things with clarity and detail.

Thanks very much for any advice you can give,
Tim

[katodog]

To me, that is no question. I would go for the bigger apeture. It always depends on light gather, and the bigger apeture gives you that.

Also, as is my experience, nothing beats an excellent camera. My scopes are not that good, but the camera makes up for it.

[Chicago Astronomer - Astro Joe]

Tim,

It's a savy astronomer who chooses his equipment carefully and with advance thought. Your Dilemma is quite a good one and you are about to choose between magnification resolution or light gathering ability.

First just a basic run on terms:

Aperture & Light-Gathering Power

Aperture is the size of the telescope's light gathering element; whether it is an objective lens, as in a refractor, or its main mirror, as in a reflector. The bigger the lens or mirror, the more light the telescope will gather, and the brighter the image will be.


Resolution
This is a measure of how sharp and well-defined an image the telescope can produce. A telescope's ability to resolve fine detail depends on a combination of both its aperture and the quality of the telescope's optics. In most cases, good resolution of an image is more important than magnification.


Focal Length

This is the length of the telescope's light path, from the main mirror or objective lens, to the focal point (the location of the eyepiece). The longer the focal length, the higher the magnification the telescope will produce for any given eyepiece. The telescope's focal length is usually marked on the telescope tube.

Focal Ratio

Focal ratio is an indication of the "speed" of the telescope's optical system, and is given by the focal length divided by the aperture. Longer focal lengths, over f/8, are "slower," while shorter, under f/6 are referred to as being "faster."

Generally, the slower and simpler an optical system is, the gentler the curvatures of its optics and the freer from optical aberrations it will be. For example, slow Newtonians and Refractors have very gentle curvatures and complete freedom from coma and field curvature. A telescope is considered slow if its focal ratio is f/10 or greater. Medium speeds occur at focal ratios of f/7 to f/9. Focal ratios f/6 or less are considered fast.

Beginners are often under the false impression that the surface brightness of an extended object image is determined by a telescope's focal ratio. In reality, surface brightness is determined by the telescope's total light grasp and the magnification. Stars differ from extended objects in this respect by actually becoming easier to see at higher magnifications, again, completely regardless of focal ratio.


Magnification

Magnification can be changed by simply switching eyepieces in the telescope. To find out how much magnifying power an eyepiece gives, divide the focal length of the eyepiece into the focal length of the telescope.

-----------

Your choice of a Dob as your first scope is a wise one. Stay away from the Equitorial Mounts, go-to's and SCT's for now. You will find the simplicity of the Dob design easy to work with and it "forces" you to learn the night sky by star hopping to find your targets. If you will use it for your students, then it will work out well.

Your questions:

Could applying a dimming filter (like a moon filter) to the eyepiece of the 10" increase its resolving power by decreasing the amount of light filtering through from the aperture?

No. What a filter would do is to decrease the overall brightness of the entire object without "sharpening" the image. Different color filters impart different characteristics to an object, but not so much for resolving. Some have stopped down the aperture of the objective to achieve what you are asking...similar to squinting your eyes..things become a bit sharper. But I wouldn't sacrifice the aperture for the minimal gain.

But what will the planets look like through the scope? Will they be blurry discs or will I still be able to see surface detail?

Many are quite dissapointed in their views through a telescope, thinking the images will look like the outside of the boxes that the telescopes arrive in. Astronomy, (for the most part), is shades of grey and small. You can see the bands and whirls of Jupiters atmosphere, The Casini/Enke divisions of Saturns rings and even terrestial markings/ice caps on Mars. But you will never be zooming in through nebulas, or looking at the surface of Pluto. You can with good magnification get great views of Lunar mountain ranges and really small craters tho.

How will deep sky objects look different on the 8" and 10"? Does the higher resolving power of the 8" mean that I will be able to see more detail in distant nebulae? Or will the greater light collecting ability of the 10" give me that greater detail (as well as greater image size)?

Objects will appear a bit brighter in the 10" than in the 8". Resolving power is cool, but if the image is too dim to see it, what's the point? Brighter is better.

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

Lets compare the specs on both the 8" and 10" scopes you are interested in:

8" Dob

$349.00

10" Dob

$499.00

The differences are not all that drastic.

* There is only a 50mm difference in magnification

* The limiting magnitude is too damn close to worry about
(14.52 on the 10" & 14.05 on the 8")

* In astronomical observation, we rarely go to the extreme maximum on magnification, Usually staying within the low to medium magnifications. The 8" 479x and the 10" is 600x.

* Same finder scope (8x50), but I would get a right angle finder scope for much more comfort and ease.

* The 10" has the ability to use 2" eyepieces and comes with a 2" 32mm. 2" eyepieces are quite cool and a pleasure to peer through. Good for wide angle views and sweeps of the sky.

* Both have good focusers

* 6 pound difference. You are a strong and robust young man - not an issue.

* A $150.00 difference between the two.

Summary:

The 10" has a two inch larger aperture, and therfore images would be brighter and slightly more ability to "push" magnitude. But here in the city where our skies are a perpetual Orange muck, (Which I call The Great Chicago Nebula.. ;D ), it's not all that vital. Yes..I hear all the time..."I'm taking it out to dark skies to view Deep Sky Objects", but in all reality...how often is that? There is a formula that demonstates the exponential increase from a 8" to a 10", but I don't know it. But I have observed objects in both apertures and the increase is marginal.

Final opinion...

If $150.00 is no object for you, then by all means go with the 10" Dob. The 32mm 2" eypiece and extra 2" inches of mirror will be worth it.

#Thumbsup#

[Kalvis]

Some suggestions:

I'm not very familiar with Zhumell optics, but I think they are low-to-mid grade. If you're going with a scope that large, I would stick with brands that have a higher reputation for quality. Orion, Discovery, Cave, Criterion Dynascope and Coulter Odyssey come to mind, but I'm sure there are others as well. Of course, with higher quality comes higher price for equivalent sized optics.

I guess if I had a budget of $570 and wanted a nice new dobsonian, then I would go with something like this from Stargazer Steve: stargazer.isys.ca/6inch.html I have read nothing but great things about Stargazer Steve's products and I love the quality and features he offers.

Don't rule out used equipment. Check out www.astromart.com/classifieds and click on the Telescopes-Reflectors section. If you can't see important details like price, it's because you're not logged in. It's free to establish a username and password on Astromart, and you'll get used to and will learn to love buying and selling used astro equipment via this popular website.

My couple of years of enjoying amateur astronomy has led me to realize that the best scope is the one you use the most often. A corollary to this is the scope you use most often is the easiest one to set-up. Even in you have only one scope these same rules apply. Therefore, if you can't easily set it up in your backyard in less than three trips and less than 5 minutes you will think twice about going outside and having a quick look. For most of us with rather busy schedules, that is most of the time.

Good luck!

Kalvis

[atomant]

Hello Tim,

I personally prefer the 10in Dob. I owned an Orion Skyquest.
Big enough to give the "WOW" effect when observing planetary nebulae, globular clusters, the gas giants and the lunar surface from the city.
Small enough to break into 3 manageable parts: base, scope and accessories.
Joe can tell you more about the time we observed the moon at ridiculously high powers with the dob at CA Urban Site #2.

Antares makes a nice 10" Dob under $560 with great accessories: laser collimator, mirror cooling fan, right angle correct image finder and a smooth crayford focuser with compression rings to protect your eyepieces. See link below:
www.agenaastro.com/Pages/Products_Telescopes/Antares/Antares_525D.htm

Good luck scope hunting!

[Chicago Astronomer - Astro Joe]

It was quite an insane night at high magnifications from a "hidden" CA site right off downtown. The link to the observational session is here with pics:

astronomer.proboards23.com/index.cgi?board=Observations&action=display&thread=1122629561

On the matter of the scope:

That Antares Dob is sweet and nice right angled finder...As it should be!". Straight thru finder scopes should be banned...

A good point is what they say about the 8" to the 10" -

The 10" model gulps in 56% more starlight than 8" optics

Yet another reason to go with the 10".

The Antares is a superior scope and will certainly provide great views, (I'd like one), but don't let that dis way you into the scopes you saw. I say if you can afford it, get the best you can afford. But, like a car, as long as it gets you to your destination...

Bill is the scopemaster and has great experience with many different types. His opinions and critiques are sound.

I like Kalvis's truss scope link and what he says about quick set-up is very true. There are some of us here at the Chicago Astronomer who would rather grab their smaller aperture scopes and start viewing then spend half an hour setting up and taking down their large heavy scopes. It's what you will happily contend with and accept.

Most of us have access to two or more scopes and depending on viewing conditions, opportunities and mood, it's good to have a choice.

[stargazer]

Hey Guys

Ok I'll jump in here. As the owner of a couple different size scopes ranging from a 120mm to a 10" I will take the 10" ANYDAY. Yes two of my scopes are handmade and one was donated to me. Joe you remember the night at Triton I stuck the 5x Powermate on the cannon and showed you the moon. What a sight that was. So yes I guess I can say I am a little biased when it comes to my scopes. For the ultimate WOW factor I use the 10" but when I am showing the girl scouts the Universe I use the 6" cause I can move it myself. My 120mm so far has sat in my living room used for a spotting scope for wildlife. Yes Joe the Coyote's. So if I had to start over in picking a scope it would be the 10" hands down. No second guessing. I wouldnt change any of the ep I have though very happy there. But this is just my 2 cents worth. So far I havent steered anybody wrong. Now with 10" I went really deep sky so I knew I could see all I wanted to. So in my mind thats one thing I would highly think about.

I guess what Im saying is I would go with choice "A" the 10" F/4.92 thats not far from my scope. Mines a 4.85.

Then all of ya'll have to get your collectd butts down here and do some real viewing under dark skies. On a csc light poution they would be rated green. Were Chicago does not rate a color ( sorry Joe). so come down and see what those scopes can really see and see what ya'll are missing. I hope ya'll know Im just picking on ya.
Joe you know me. I've been trying to get you out of that city since I moved down here to enjoy some quiet and nice views.

Well time to get the geese out and let the horses out and go do some work in the garden. A farmers job is never done.

I hope you think about my advice . Please let us know what you decide.
B

[Chicago Astronomer - Astro Joe]

Becks,

Good first hand advice there. My best view of The Orion Nebula was thru Beck's scope at Chicago Astronomer's Al Woodstock site...

It's quite a big scope and has motorized alt/az controls...quite cool.

Becky & Husband Bill

Tim, When you get your new scope, an open invitation is extended for you to join us in a Chicago Astronomer observational session.

[curare]

Thanks for all the advice guys and gal,

I was getting a little dizzy from all the options and stats for the different scopes and last night I purchased the 10" Zhumell as I had seen nothing but good things written about it. I've even read favorable comparisons to the more established names, like Orion.

Atomant: Where were you 24 hours ago? That Antares link you sent me is tempting me to take advantage of telescopes.com's 7 day no questions asked return policy. The right angle finder is the clincher.

Katodog: I don't think the scopes I'm looking at are particularly camera-ready. I'd like to get into astrophotography, but first I'm going to learn the ropes and figure out if it'll be a lasting interest.

Joe: I have the entire summer off. I will be sailing, kayaking, biking, hiking, camping, and, most importantly, I WILL be "taking it out to dark skies to view Deep Sky Objects" any day of the week I please And thanks for the open invite. I'll will certainly be attending an observation session soon and I'll bring along that Questar as you requested.

Stargazer: You live in Barry, IL? When's the Apple Fest this year? I need to get me some of those home-made fritters; those things are unbelievable. And you're right, the skies there are pretty nice, too.

Kalvis: Thanks for the suggestion, although at this point I think I'm sold on the larger aperture scopes. I'll start with cheaper optics and more light and go from there.

A quick note on the my resolving power comments from my original questions. A book I read on telescopes referred to the arc-second as the unit of resolving power, meaning that a number of lower magnitude would actually mean increased resolving power. I didn't realize that when I was looking at the stats for the 8" vs 10" scopes because telescopes.com did not indicate the units they used to specify resolving power. I'll now assume it is the arc-second.

[stargazer]

Im going shopping today so I will ask and let you know. Are you going to come down with the scope? if you do you can stay here at the farm. Im sure we can find a spot big enough for a tent and a scope. ;D If you need a tent Im sure Bill wouldnt mind you using ours, it's big enough for 6. The farm has an offical name. "Heavens Gate Farm" We actualy live in Beverly Township but Barry is not far from us. Maybe 6 miles. So you are more then welcome to stay here you and your famly, and we wont even charge you. LOL I keep trying to get Joe down here but he wont come so maybe you can drag him down here.

[stargazer]

Tim

Applefest is from Sept 28th through Oct 2nd. So I expect you and your family down here for the weekend camping and viewing. lol. I will provide atleast 10 acre out of 80 for scopes, tents, good times for the Chicago Astronomers of this group to come down and enjoy some good times.

So............................Formal Invite Time Guys

Heavens Gate Farm

Invites
the
Members
of
Chicago Astronomers Forum
to a weekend of Stargazing and the Applefest
Sept 28th-Oct 2nd 2006

Camping at the farm, Stars from Horizon to Horizon, Mother Nature all around you, Peace and Quiet, Dark Skies,

RSVP:Either here or by PM to STARGAZER

[Chicago Astronomer - Astro Joe]

COYOTES EAT ASTRONOMERS FROM CHICAGO...EQUIPMENT ABANDONED

[stargazer]

Im still here. Just talk and they go away. They dont like people.

[stargazer]

PS I was up the hill for the lunar eclipse and about 10 coyotes were no more then 50 feet behind me and left me alone then walked between me and the house before I came down the hill. They leave you alone. They are more afraid of you then you are of them.

[curare]

Stargazer,

Thanks so much for the invite! That's a long time away, though, and it being so close to the beginning of a new school year (an extremely busy time for me) I couldn't commit so far in advance and would have to play it by ear. If the invite is still open when the time comes and my schedule will allow me to escape for the weekend then I would very much like to join you all down there.

Tim

[stargazer]

Tim

Its an open invite. Anytime you want to come down your welcome. I have plenty of room. Dont worry about the wildlife here its very peaceful. You'll like it down here.

Becky