View Full Version : Celestron Omni 127 XLT

2008-May-03, 09:33 PM
I bought a Celestron Astromaster 130EQ first. I was excited when I bought it but was disappointed when I received it. The mount and overall craftsmanship/workmanship of the scope is very poor. I ended up selling it and got my money back. It was a Christmas telescope.

I decided that with the respect Celestron commands in the astronomical community I can't expect much from a $178 telescope. So I'll try their pricier Celestron Omni 127 XLT. When GW's money came yesterday, I picked up this telescope first thing when I woke up.

I have assembled the telescope. The OTA is definitely much better than the Astromaster's. The pieces, optics, and construction was definitely the feel of a real telescope. The CG4 is also a much better mount than the CG3 of the Astromaster. All around, I got what I paid for $629+tax. Because I want to align the scope very well, I also bought the polar alignment scope for the CG4.

However, the when I got the mount out of the box it was in there just kind of namby pamby. And the worst part is the declination setting circle which I can't seem to change. When it is pointing directly north with respect to the rest of the mount, the declination reads 76 degrees. The Astromaster's CG3 read 0 degrees when it pointed North directly when it should read 90 degrees. So twice in a row both Celestron products out of the box do not show outstanding quality. The declination setting circle is important to me because that is how I will locate DSOs.

Has anyone else had a similar experience? I need to find a better manufacturer. What do you think of Meade and Orion and Vixen? Would you know how I can adjust the CG4's declination setting circle? I could send this back to Celestron, but that would mean that I would be without a telescope.

2008-May-03, 10:32 PM
The setting circles on a mount the size of the CG-4/EQ-3 are practically useless. Setting circles are not a reliable method to find faint DSOs, especially with an f/10 Schmidt Cassegrain.

If you still need them aligned, setting circles are meant to be rotated to align them. But it really requires practice and I have never even tried to use setting circles before. Even on my mount, which costs what you payed for your whole telescope, the setting circles do not have the accuracy to pinpoint objects with ease. It's faster just to star hop.

2008-May-04, 07:45 AM
I agree the setting circles aren't accurate. And I do star hop, but setting circles aren't completely useless. Here's the strategy and tactic I would use.

First assumptions:
1. Mount is as level as possible.
2. Polar alignment done.
3. Finder scope aligned with OTA.

I don't expect the setting circles to single handedly get me to my DSOs, but that's where I would first point. Once, I'm there, I can star hop using combo finder scope and OTA. Sometimes the finder scope is in an awkward position an example of this is when you're looking for something at the zenith; the setting circles would help to get me into the ballpark first instead of using the finder only.

Although many people ignore the setting circles altogether, that doesn't mean Celestron can get them wrong and check it off in their quality assurance tests before it goes out the door. If they don't intend on getting it right, take the feature off completely and pass on the savings to me. On principle, any manufacturer should want to at least give best effort for their instruments' configuration even if their accuracy is to the nearest hundreds only. This thing was so off by a weird number, it doesn't look like they even tried.

The RA setting circle can be adjusted, but the declination setting circle seems to be fixed. If I forcibly rotate it, something will break.

My gear is new and while I was tempted to buy a pricier rig, I just couldn't afford it; I wanted a manual system anyway where I can put myself in the shoes of Hipparcos, Galileo, Newton and Messier, so it wasn't so bad. I'm sure that my tripod alone, cost more than the instruments they're famous for using, and yet they live on centuries later because of their astronomical and scientific contributions. One day I would like to upgrade to a better setup, but that may have to wait until I win the lottery.

2008-May-04, 04:31 PM
With an equatorially mounted scope you use Right Angle Sweep to find your object, NOT star hopping. Well you can but that's the hard way. There you need stars to hop from and in a light polluted environment they may be hard to come by.

Right angle sweep doesn't need but one star. Find a star near the object and look up the relationship to the object. Say your chart shows it 4 degrees west and 3.5 degrees north of the star. Put in an eyepiece with a known field of view. I like one of a bit over 1 degree which would be an eyepiece that gives a bit less than 50x with a 50 degree plossl or one that gives a bit under 65 power for an eyepiece with a stated 65 degree field of view.

Center on the star. Note a star at the northern edge of the field of view. Move the scope so it's at the southern edge and note another star near the northern edge. Make this move 3 times then on the forth move just center that star that was at the northern edge of the field of view. Now you do the same going west but for 4 full fields of view and the object is centered in your eyepiece. Once you learn this method is is far faster than the time needed to read this. At first you may have to lock one axis so you don't move on a diagonal but once you get the hang of it its fast and quick. I went through my Herschell 400 with an average seek time of less than 1.5 minutes counting the time at the charts to calculate the move. I would hit 100 objects in three hours using this method. No setting circles needed and the polar alignment need not be all that precise. Being off two or three degrees has little effect on the accuracy. I'd just haul out the 10" f/5 and plop it down. Look to see the polar axis appeared to be pointing north by standing back 10 feet and eyeballing the mount and sky.

If your scope has 6" setting circles or larger you can just use the relative position they show to do the same thing. It doesn't matter that they aren't reading correctly, just that you move accordingly. Here you use RA for east and west not degrees of course. Even though my scope does have circles with the needed accuracy I never use them. To do so means using a red LED light to read them and that just further hurts my dark adaption. Also it takes longer and is less accurate -- at least for me.


2008-May-04, 09:59 PM
Once you learn this method is is far faster than the time needed to read this. At first you may have to lock one axis so you don't move on a diagonal but once you get the hang of it its fast and quick. I went through my Herschell 400 with an average seek time of less than 1.5 minutes counting the time at the charts to calculate the move. I would hit 100 objects in three hours using this method.

Nice. I have never tried his method and it seems intuitively obvious. But I wonder if it's more difficult in alt-azi mode? Tracking north, south, east or west is not always easy. Probably won't work well for dobs.


2008-May-05, 12:32 AM
Nice. I have never tried his method and it seems intuitively obvious. But I wonder if it's more difficult in alt-azi mode? Tracking north, south, east or west is not always easy. Probably won't work well for dobs.


Note my first words, " With an equatorially mounted scope you use Right Angle Sweep... "

In the 50's when I got my start nearly all scopes were on GEM mounts but for 60mm refractors. Dobson hadn't had his brilliant idea. So everyone used right angle sweep. But then Dobson changed the face of amateur astronomy and star hopping was in vogue since RA sweep didn't work with a Dobsonian mount or any other alt-azimuth mount, as you suggest, unless you are viewing right along the meridian. In the process RA sweep was forgotten though it still applies for equatorially mounted scopes.


2008-May-05, 05:09 AM
Hey Rick,

That's what I meant by star-hopping, I didn't know how to term it, but I was going to use the properties of the telescope and eyepiece to help me arrive where I want to go.

I know I can just transpose the degrees of the declination setting circle, but the problem isn't because I can't do the math. It's the fact that I spent $629+tax on a precision instrument that for whatever reason the dec circle is 14 degrees off.

Sure, I can just say, 0 degrees is 14 degrees, but why? If you buy a car, would you be okay with the speedometer reading 14mph when the vehicle is at rest, so you constantly have to subtract 14 from your velocity? Plus, this boasts Celestron's name, a respected brand in the astronomy community. I'm not expecting perfection. In fact, I know the circles are inherently inaccurate, but they still help. So no, I refuse to be the crutch for a problem that could have been prevented at the quality assurance department of Celestron.

2008-May-05, 07:14 AM
As far as equatorial mounts go that price is FAR from the cost of a precision mount. They cost many thousands of dollars and that's without a scope.

Setting circles for your mount are worthless. They are just decoration. It's always been that way. Even the 6" circles on my Astrola mount bought in the 50's for what in today's dollars would be about $2500 had useless circles as they had no vernier scale which is needed to use them with any accuracy.

Since the right angle sweep is more accurate than 6" circles WITH a vernier scale the circles are pretty useless and really just window dressing. Even if they appeared to read right they'd be too far off to be of any use. They are also much slower to use. Consider them decoration. You can get electronic circles for most mounts. The generic ones need a bit of work to adapt to a particular mount. They will be fairly accurate, far more accurate than mechanical circles of the size on your scope would be. But mounts in your price range don't have the precision alignment of the axes and bearings to make them as accurate as you'd likely expect. Using the circles as being relative so you only are measuring a few degrees from one location to another, you can get the needed accuracy but the errors of such mounts when you are using them over 360 degrees these errors really add up making them worthless as others have said.

It turns out optical accuracy is far cheaper to achieve than is mechanical accuracy.

Your experience is one reason I always suggest our new club members hold off buying a scope until they've attended several star parties and seen and handled many different scopes. Then you start to realize the compromises involved when you shoe horn a scope into a price range that's affordable. Actually today you get one heck of a lot more for your buck than you did when I started back in the early 50's. Then the only way to afford a scope over 60mm was to make it yourself and even then it could cost $100. I bought gas back then for 19 cents a gallon! In gas money that very simple scope cost me $1800 today! And it had no setting circles, was as unsteady as a Don Knotts character with a 30 degree field of view (most scopes were more like 20 degrees back then). My current imaging telescope and mount couldn't be bought at any price under 6 figures and that was in those 1950's dollars.

So to most of us that setting circle "problem" is a so what type issue. It's been that way for the more than 50 years I've been in the hobby when dealing with mounts in that price range. Remember too that the RA circle works in sidereal time and is not driven. Thus you need to reset it to the correct sidereal time each time you move to another object trying to use the circles. A major pain at best. Another reason few ever tried to use them. Of the hundreds of members in our club over the last 48 years I only know of one who ever even succeeded in using the circles that came with his 8" scope. It cost in 1960 dollars about $2000 and did have a vernier dial. He had a watch modified to sidereal time and did use them. Of course he found 3 objects a night while the rest of us using right angle sweep found dozens but he made them work. I only fault Celestron and all other manufacturers of mounts and scopes in that price range for even putting setting circles on the mount knowing they are worthless window dressing.


2008-May-05, 02:29 PM
Hey RickJ,

Thanks for that reply. I learned that these compromises have been largely accepted by the astronomical community at these price ranges.

The setting circles are important to me because I would like to not have any electronics on the mount. I want to learn things about the sky that electronics will cheat me out of. Aside from the fact that I cannot afford the more expensive GoTo systems, I would like to be able to put myself in the shoes of the fathers of modern astronomy. Electronics back then didn't exist, and their setting circles are less accurate than ours today, but I'll allow myself that cheat. However, I'm discovering that the electronic GoTo mounts are more affordable than no-electronics precision mounts.

So you recommend Losmandy? What is your opinion on the Vixen GPD2? I don't want a lot of gear. In fact, one set will be fine for me, but I expect it to work well. So if it will cost a lot of money at first, so be it because that will mean that I won't have to spend much in the future and it has the capacity for growth into astrophotography. Thanks again.

2008-May-05, 06:16 PM
The dobsonian with electronic setting circles would teach you plenty about the sky and be far more accurate. Early astronomers didn't have setting circles, they are a rather new invention. They always referenced their discoveries from other objects which were in turn referenced from other objects. The chain was fragile. This is why many objects in the NGC catalog and even some M objects are either unknown today or there's a big debate over what they were seeing. Take a look through the NGC project where an astronomer is trying to pin down each NGC object to what was really seen. It isn't easy. Early earthly surveyors using a rag tied to a wagon wheel were more accurate at surveying the earth than astronomers were trying to survey astronomical objects.

For a beginner an equatorial mount can be very confusing as some threads by beginners here will show. In some parts of the sky east is west and north is south, somes both are reversed at once. It can make it seem like you can't point the scope at the object you want to point it at. This is especially true with GEM mounts like you are considering. You have to flip the scope around doing what is called a "meridian flip" when you go from one side of the meridian to the other. This is fast and easy IF you know what you are doing. I've watched beginners in our club pick up their scope and turn it around totally blowing any polar alignment because they couldn't figure it out and wouldn't wait for one of us to help them.

Again even the equatorial mount is a rather new invention. Early astronomers had very crappy mounts, most being very difficult to use. How they did anything still amazes me. They'd have been in heaven with a dobsonian mount. Simple and elegant that can easily handle huge telescopes and move with just a light touch of the finger with perfect smoothness. But don't try to look a the zenith! They don't go there without extreme difficulty.

You really do need to find a local club and attend a few star parties. It will help you far more than anything else you can do.


2008-May-05, 07:15 PM
Star hopping with a Dobsonian mounted reflector is how I learned the sky.

It's a technique I would recommend to anyone.

No electronics needed (although using some planetarium software on a laptop is a nice option to paper charts).

2008-May-06, 06:11 AM
Thanks, Rick. I've joined the LAAS, and I've got a Star Party booked this weekend.

However, I'm not giving hope on my declination setting circle. Celestron and the retailer from which I bought this telescope agreed they'll replace the mount in time for this weekend's festivities. Part of the fun of hunting DSOs is using know-how from class. I'm looking forward to the challenge.

2008-May-07, 04:06 AM
My name is haloflightleader, and I am not smarter than a 5th grader. There is a small hole in the declination bezel that could be turned with an allen wrench to loosen the declination bezel. I was getting angry for me being ignorant, but I the Celestron manual doesn't mention anywhere that that hole even though it has grooves for a small screw, has a small allen screw inside. I wouldn't have thought of it because it was tiny, and nothing indicated that an allen wrench would turn it. My declination circle is now set.

Next problem is setting the RA circle. Before I get mad, I'm going to do some research first.

2009-Jan-09, 09:19 AM
Hello everybody,

I have just joined the forum.

I'm a complete beginner and have just bought my first telescope - a Celestron Omni XLT 150.

I joined the forum because I had problems with the DEC setting on my CG-4 mount (it seemed to be misaligned by 10 degrees all the time). However, when reading postings to the forum I discovered that this was a common problem but that the DEC setting could actually be calibrated even though now documentation of this came with the telescope ("a small hole in the declination bezel could be turned with an allen wrench to loosen the declination bezel" - see posting by haloflightleader).

I now intend to do some astrophotography. I'm therefore thinking of buying the Celestron nexImage CCD and a motor drive. Does anybody have experiences using that with the Celestron Omni XLT150/CG-4 mount (or similar)?

Best regards,

2009-Jan-10, 05:50 PM
Yes this would be a minimum set-up for deep sky astrophotography with the dual axis corrector. You probably would find the polar alignment scope useful as well though I never have used one. They didn't exist when I got started. I'd prefer the guiding corrections to be made at 1/2 sidereal rate rather than 2x as the specs indicate for this system. This may make setting the guider parameters a bit "interesting" to prevent overshoot. Maybe those with experience with this mount can help here.

But I'd spend my first year learning the ins and outs of basic astronomy before even trying this. The learning curve is steep for both. Trying to climb both at once is likely too much. Learning basic astronomy first will greatly improve your chances for finding and selecting the targets best suited for that system and your light pollution levels as well as the CCD you choose. All set-ups for deep sky work are a compromise of many factors. Right now I doubt you are even aware of but a small fraction of them you'd be making. Another reason to go slow and first learn your scope the sky as well as your sky conditions that you will be dealing with. Working with a local astronomy club usually pays big dividends here!

Planetary photography however is within reach of the beginner but for that I'd recommend a web cam not CCD. Also if you've never worked with a telescope an f/5 scope is rather demanding on collimation accuracy. Once learned it is easy to do but you might want to search out a local club for help the few first times. Photography is very demanding on exact collimation at that f ratio. I used a 10" f/5 for years and know this from experience. Even a 1/20th turn of an adjustment screw can make or break a planetary image. Deep sky photography is not quite as intollerant of accurate alignment but the difference is small. This is one reason I tend to recommend small refractors like an ED80 for beginning deep sky photography. It takes away the collimation issue. Still I started with a 6" f/4 but I had several years experience in amateur astronomy before doing so. I don't know if I'd have succeeded otherwise.

The reason for the web cam for planetary imaging is that you are fighting the atmosphere. Only for fleeting instants in time does the image come through without distortion. A USB2 web cam (640x480) can take 1000 images or so a minute capturing those fleeting clear images which software sorts out and uses (Registax is one example and is free) for the final image. You can't begin to do this with a CCD or DSLR. See the planetary images of Iceman and JAICOA in the astrophotography section for examples. Iceman has good website with a lot of useful information for beginners.

For deep sky photography, while taking many quick 30 second shots will give you an image without guiding you'll get far better ones with fewer long exposures of at least 5 minutes or longer. This requires a guiding camera as well as the imaging one. This just adds to the complications a beginner faces. Another reason to go slow especially with deep sky work.

The scope you have is usable for what you want so you are past the first hurdle. Finding a local club with one or two astrophotographers would be the next step. Another would be getting a good book on the subject. I don't know any in Swedish (my grandfather who came from there and spoke it died before I was born so I never learned the language) but a good one in English is Covington's Astrophotography for the Amateur. www.covingtoninnovations.com/astro/
It's no substitute for an astronomy club but will help you learn the questions to ask.

Good luck. I started down your road in 1954 when a scope like yours was just a dream scope compared to what was available for the amateur back then and it cost far more. You have a much better start than I did.