Posts Made By: Greg Nowell

James Powell said:

I just received my Celestron OMNI XLT 120. Is the declination circle supposed to move? I see a screw hole but it does not have a setscrew. I tried turning the circle but it does not move.

OK, this is something I know a lot about. The dec setting circle will *usefully* be in a fixed position IF you are EXACTLY polar aligned. With exact polar alignment 90 degrees DEC is exactly due N.

In the real world, every time you set the scope up, you are a bit off true N. The best way to handle this is to adjust the DEC setting circle to the correct coordinates on a known star (Vega, Capella, whatever) once you have set up and polar aligned as best you can.

One can go further. With analog setting circles your best result will be if you re-set coordinates as you move to different parts of the sky. So if you're hunting around Sagittarius you can "locally tune" by moving to Antares in Scorpius and setting your circles for the coordinates of Antares. If you decide to move from there to Perseus, you might want to "locally tune" on one of the stars in Cassiopeia. To do this exactly you will need, in the field, the coordinates of a bunch of bright stars. By keeping your setting circles "locally tuned" you will get maximum accuracy--if you "locally tune" on Antares in Scorpius and then move to Cassiopeia, any error in your setup will be magnified. But by re-tuning in Cassipeia you will again minimize errors. When yo re-tune, you need to re-tune in both RA and Dec.

So, to sum up: The Dec setting circle should have a set screw (if you want to lock it down in an observatory context) but you will rarely use it (in most applications, where you set up and break down the scope) because movable observers are best advised to move the Dec setting circle and adjust it often.

So, if you see no way to make the Dec setting circle move, you have an issue to raise with Celestron. Shine a strong light down there and make sure there is no screw.

NOW THAT WE'VE SAID ALL THAT, there is a very easy way to use setting circles in which locally tuning and the position of the Dec circle, or for that matter, the RA circle, simply doesn't matter.

If you have a reasonable star chart like Sky Atlas 2000, flip it open to Lyra. Look at Vega. Now look at where M57 is. You will notice that to get to M57 from Vega you need to move from about 18:37 in RA to 18:53. The difference, then, is 15 minutes in RA. All you have to do is move 15 minutes E from Vega and then head due South from Dec +38:49 to +33:00. That's about 5 degrees and 3/4 of one degree more. Looking at your dec markings, you can do that. You can count off in Dec or, as you get close, just look in the eyepiece and continue to sweep.

In other words, you don't need EXACT coordinates and EXACT positioning of your setting circles to find stuff, because with your star chart you can get the ANGLE you need to move and ANY of the measurements on the analog setting circles will help you get the ANGLE provided that you remember you are looking to count off the difference--5 3/4 degrees--not move from the exactly designated position of Vega (18:37 +38:48) to the exactly designated postion of M57 (18:53 +33). When you move South 5 3/4 degrees it doesn't matter if your dec circle says you are starting from 40 or -22 +38:48 or whatever. The important thing is simply to look at the markings (never mind the numbers) and move 5 3/4 degrees.

There is some inaccuracy in this estimation technique and it is "friendliest" in wide field refractors but I have used it, with some care, in a c14, which is only 3/4 of a degree field of view. In my refractors I have 2.7 degrees in one and 4.25 degrees in the other, which is huge, and provides for a lot of slop. You have two potential sources of error, a bit of estimate error on the star chart, and a bit on the actual setting circles.

So you can blow off having your setting circles accurately set if you use them instead as angle counters to get from one known point to your destination. For this to work you need a good star chart.

It is more complicated to DESCRIBE than it is to DO, but you can move around pretty quickly once you get the hang of it, and it is a technique that occupies the gray zone between star hopping and using a computer (you are using an analog computer to measure angles).

The "Dec sweep technique" eliminates the need for measuring in Dec altogether. You go to the approximate RA and sweep till you hit the object. My impression is that the Dec sweep technique is best for brighter objects but with refinement you should be able to do just about anything.

So long as you are "on the same page" of your sky atlas 2000 (as in moving from Vega to M57) you will automatically have "local compensation" for distortions caused by slight polar misalignment.

It is helpful to know the field of view your telescope provides with its widest field eyepiece, which is basically your error tolerance with these techniques.

So, to conclude:

--In some contexts the Dec circle should not move (permanent observatory set ups).
--In most cases it should. That's the best way to get the most accuracy.
--If you learn the angle measurement technique you won't care.

Nonetheless, my preference would be to have a Dec setting circle I know I can adjust, as sometimes, it's nice to set it all up so you can use the coordinates as you read them. But on the process involves keeping a lot of numbers in your head and usually the easiest thing to do is some form of angle measurement or dec sweep.

regards
Greg N

Well Floyd I've been using a G11 for seven years and it has never seemed so nice as it does now with the FS128. That just makes for a very smooth combination. (I'm sure there are others) I used it visually with the C14 for six years but then I had a general reorientation, the Tak got the G11, the C14 moved on over to a used AP900QMD, and on the way it got a new friend to keep it company, a 102mm that was made for just too much load (for my picky tastes) when it rode on the c14 on the G11.

Floyd I hope you will join the Yahoo! Losmandy group. There's a lot of technical knowhow here but over there we've got many years of archives, photos, tear downs, and all kinds of levels of expertise for whatever issue may arise as you break in to your new mount.

If you need virtual directions send me a note via the 'mart mail facility.

regards
Greg N

Hmmm. Well, there are some wires. The thing is, most of the wires I have I would have on any mount. Most of what you see dangling are the power supply, running from the battery up to the side of the mount, and dew control technology for the tube; and the Argo Navis computer. A different type of mount would deal with some of these wires but by no means all of them. For example the dew control runs a wire from the power supply to the control box, and from there I have two wires going up to the tube; a third wire is going down into the eyepiece box where I also run a heater.

A CGE would relieve me of the Argo Navis (and its wires) but so would the Gemini unit for the G11. It would remove two wires from the G11 control panel, if memory serves, the ones you plug into the RA and Dec motors. That's about it. It would be nice not to have wires but the main thing is most of them are not due to the mount. I'll take the wires and push-to over go-to because even though I have wires I have less system complexity and higher gearing in the steppers. For me that works out better.

I also have an AP900QMD era. You don't hear anyone accusing AP900s of having too many wires but they have the same as a G11. You didn't say if you want to take pictures or not. But at any rate for a 5 inch refractor you should do fine with a G11 even if you DO photography.

Incidentally after I bought the AP900 I kept the G11. I have it on a wood tripod (berlebach) and use my 5 inch refractor on it. So I think the wire thing with G11s is overblown. A long thread on Yahoo! Losmandy has been unfolding on just this topic so you're welcome to join and read the archives.

Greg N

Let me add that I don't know anyone who disputes that the Tak mounts are a step above the Losmandy. I'm not sure that they cure the wire problems but I'll let someone else speak to that. Speaking for myself, I would still need a power supply cord and all the wires that go with the various active heating systems.

Tak uses sand cast aluminum, Losmandy uses CNC. I can't speak to the innards of the Tak, but its the use of steel on the main axes of the G11 makes it very heavy, in proportion to mounts such as the AP900.

You haven't even said why you want a German equatorial. You could get a very nice alt-azimuth mount and a 4" refractor and get your feet wet. If you want tracking, well, then, A german equatorial is nice (I wouldn't use anything else). If you do your own machining or have a friend who can help you make an adapter, a wood tripod is a wonderful combo with the G11.

The conundrum you pose is an interesting one because I always say people underestimate the importance of the mount. And certainly a Tak mount (or an AP) is saying you refuse to underestimate the need for a good mount. But the thing is I HAVE an AP900 but it doesn't change that the G11 is a GREAT mount for my five inch f/8 refractor.

In fact that's what I would tell you to do. Get a Tak FS128 on the used market for around $3.2k and get the Tak mount. I'm not sure why you want a refractor anyhow but I like mine (in addition to the SCT). But being who I am I would probably get as a first combination a Celestron 9.25 with a side mounted 90 or 102mm refractor on the G11 mount.

There are many very nice combinations out there, and the G11 can be part of them; but you seem to be well informed so I don't think you will make a wrong choice. In the kind of equipment choices you're looking at there is no wrong choice, but I think the 5" refractor is better. The TEC140 is a good choice too, but as I say, a used FS128 is nothing to sneeze and and you can save significantly.

I've got a picture of the FS128 on the G11 with wood tripod here (in my cluttered garage) and it doesn't have the wires, but then, I already posted wires. I was just setting it up to see how it balanced.

Alex: Allow me to clarify a bit what makes for good pointing. There are half a dozen sources of flexure, polar misalignment, orthagonality, and other error that will make systems have pointing error. This is also true of professional observatories (except they usually nail the polar alignment). There are a number of error terms that are standardized in the industry, they include MA, ME, NP, IH, DF, and a few others. Type that series letters into google and you'll get some astronomical engineering hits. Software Bisque sells a laptop system which will also do this analysis.

Gemini compensates for these sources of pointing errors by taking the alignment stars and building a statistical model of the mount. The first few alignment stars get you "most of what you need," and you can build a super-refined model using a hundred stars (or forty, or sixty, you can choose) all over the sky for an observatory set-up. But most of the time when we say "build a model" we are talking 3, 5, maybe 10 stars being fed into the computer, depends on the person and the application.

As it happens, you can get Gemini type accuracy with an Argo Navis computer, which allows for a multi-star alignment that will increase your pointing accuracy. You can talk to Gary Kopf at Wildcard Innovations. (send him an email) There is also an Argo Navis Yahoo! group. Software Bisque, Gemini, and Argo Navis do not use the same program to achieve their results, but they are all conceptually similar and are addressing more or less the same variables. The gemini system is a bit more "opaque" to the user, you just zap stars and it figures out what to do; Argo Navis requires a few button clicks to put is compensation model, which they call TPAS, to work. There are other systems in use including CGE and rumor has it that AP will introduce a similar system one of these days.

If you want bullet proof reliability on a mount you can use and abuse, that's the G11 with steppers. but 95% of imagers, or 98%, prefer Gemini. I use Argo Navis but I don't image.

With Argo Navis you can get about 5 arc minute pointing accuracy on a G11 equipped with 8192 tic encoders. You could get down to below 2 arc minutes with 10000 tic encoders. (you would disucss these options with Gary Kopf)

Gemini and Argo Navis are your logical options to improve pointing accuracy. Gemini will slew automatically, which is nice, and the servo motors have a smoother motion. The steppers are more rugged for throwing into the car and for "set it and forget it" characteristics in extreme temperature changes. If you got to a real cold site, for example, Gemini may stall unless you adjust the worm gear. Argo Navis has a better user interface than Gemini.

But all things considered, most Losmandy sky photographers are using Gemini. Since I switched to Argo Navis pointing accuracy is no longer an issue in my observing life. There is one functionality Gemini has that Argo Navis does not. Gemini can be set to compensate the *tracking* (not just the pointing) for polar alignment error. This would be useful for longer exposures, but you would still get slight field rotation (say you were ten arc minutes off true N in your polar alignment). You have to perfect your polar alignment to eliminate field rotation. Argo Navis can't fix that for you because it is not connected to your drive (which can be a good thing from other points of view). But field rotation for someone who is 10 arc minutes off true N isn't going to be a huge issue.

Both gemini and Argo Navis will correct pointing for polar misalignment, and both have systems to help you improve your polar alignment.

I would suggest that you review your setup procedures too. The following things can make you lose accuracy as the night progresses:

a. failure to torque down those azimuth lock screws on the deck of the mount head--if the mount moves, your pointing will go off
b. failure to tighten the altitude second altitude screw so that mount is tightened down in altitude after you have done polar alignment--it will shift very slightly as you use it, if you don't
c. some people don't have the dsc box set for the right number of tics....often off by a little....you'll be either 4096 or 8192 on most configurations
d. observing on some surfaces such as concrete can allow for twisting motions of the scope to turn the tripod. Not a problem on grass or rough surfaces.

In addition to polar alignment, the single biggest source of pointing error on systems that don't "model" is the accuracy of the right angle of the optical axis to the actual mechanical axis of the mount; the optical axis should be at right angles to the dec axis. (This is "NP" in professional parlance, non-perpendicularity) If you're using a Newtonian and rotating it in its saddle this situation is pretty hopeless; I don't know of any system that will give you good accuracy with a Newt unless the optical axis is real spot on. You could use a Newt and stay accurate, you'd just have to be sure not to rotate it.

Refractor and cat owners on the other hand used to shim their mounts to improve the orthagonality. I have done this procedure more than once but I now consider it a stone age technique. All the good systems will compensate for this mechanical error mathematically (except for Newts that are rotated in the rings).


regards
Greg N

Well there are a lot of very favorable comments on the web about the pointing accuracy of the ASGT or SGT or whatever it is called these days, the CG5 go-to. On the other hand, you are getting chinese quality control, and if they're beating their prison labor that day your unit may not come out too well, or have lead paint, or whatever. I personally own and use a Losmandy and am fond of it, and recommend it; and the people that have G8s are also doing well.

HOWEVER, I have to say, that I have thought that in general Vixen offers the most consistent quality control in the astronomy biz, on all dimensions. If you had a Great Polaris mount you probably wouldn't hate it. I know I like my Super Polaris.

Basically I own three known commercial GEMs (ap900qmd, super polaris, G11) and am enthusiastic about all of them, in their respective applications, but I'm not a go-to user. I give an edge to Losmandy because of the durability of the construction and the higher carrying capacity, and the fact that you don't have those %#!%~!! clutch levers. But Vixen is also an excellent choice, and has the edge over the Losmandy alternatives on certain aspects, including the likelihood that you won't need to adjust the worm gear when you get it, and a fairly high level of qc/qa.

If you want 30 year dependability I'd go with Losmandy, one of the annoying things about Vixen is that their electronics really aren't on a par in terms of taking abuse and the like, and the company has made some bad choices about power supplies (9 v for the one motor drive, 12 v for the two motor drive, weird stuff like that) that has put some users into the unfortunate position of burning out the very pricey electronics. (OTOH some people burn out their Gemini motors, if you get the go-to; the steppers appear to be built to last until kingdom come) The Vixen electronics are typically in the paddle and if you drop 'em forget about it. Losmandy electronics are bolted on to the mount in a box that could stop a .45 slug. I was very annoyed to have to replace motors AND controller on an SP unit (with GP stuff) because the original electronics box fizzed out; everything OK except for that controller box.

So, in overall it is a very close draw between Losmandy and Vixen IMHO. The SGT series has drawn some faithful adherents but it isn't what I would get.

regards
Greg N

Rob: Well I'm not sure what the issue is. Judging from your equipment you know full well that to get to a decent magnification in your fast refractor you need 1.25" format for the range 10mm and under, and for your larger aperture you'll use longer fls and hence two inch.

Some of us BTW have standardized on Pentax and therefore two inch territory starts at 30mm. One need not make excuses for being addicted to Pentax.

Among people with smaller investments, 1.25" eyepieces are a good way to break into the hobby, and on the humble but popular c8, "the coffee can scope with an aperture punch", one gets perhaps 1.07 degrees fully illuminated fov, one might want to stick with 1.25 inch format eyepieces (but I've enjoyed two inch wide fields).

There are a few small Newts out there--I have one--that have 1.25 focusers.

Since 1.25 inch covers everything from the 24 Pan to Edmund RKEs to Chinese plossls to the TMB super monos sold by Teton telescopes, there is something to be said for the tremendous range and variety available in this format.

In my C14 planetary viewing is typically at 14mm to 10mm, and I don't need an Ethos for that thanks; I do fine with the XW14 and the XW10, and Mark Rieck's 10mm TMB super mono, which I've had for months, and finally got to use for about five minutes the other night (pretty nice Mark).

What I CAN sympathize with is the HASSLE of switching eyepieces with adapters and without adapters etc. I solved this problem by just buying an adapter for every 1.25" eyepiece, the eyepiece lives in the adapter and so EVERYTHING is treated "as if" it was a 2".

regards
Greg N

Well Floyd you're sure spending a lot of money for no reason. One night I forgot my 2nd counterweight for the G11/C14. I had one 21 lb but left the other one at home.

Fortunately my observing buddies contributed their soda bottles which I filled with water. About four 20 ouncers as I recall. Maybe five, but I don't think so. I used a velcro strap. With the one counterweight I had all the way at the bottom, I used it as a table to arrange the water-filled soda bottles around the CW shaft. Then I cinched the velcro belt around them and they were securely in place from one side to the other.

With this system I was good for the night. I was a bit out of balance but by tightening the clutches I could do what I wanted to do. So for economy's sake I suggest you just go to the recycling bin and get some soda bottles....and on a more serious note, something like that filled with lead shot would do fine.

regards
Greg N

The problem is that in the refractor forums some of the various refractor sellers have annihilated each other's reputations. Among the least-annihilated are TEC, AP, Tak, Pentax, Vixen. I have a list of the most-annihilated but won't get into it.

Damned if I know what to make of it except that refractors are a religion.

regards Greg N

It all depends on how much money you want to throw into the mount. Also, Jon is not entirely right about the barlow solving all magnification problems. In the special instance of photographing planets, if you have an f/4 instrument you need double barlows (like a 5x and a 3x) to get up to the f/40 and f/50 ranges imagers sometimes use. It's much easier to work with a long fr scope like an f/10 with one 5x barlow (for example).

If you're shooting deep sky it goes the other way, it is easiest to break into astrophotography with a fast scope (such as a small fast refractor) because it is most tolerant of tracking errors.

If the f/10 is an sct there are reducer correctors which will cut the focal ratio down to f/6.3 and at the same time flatten the field which is helpful.

So, in any case, as you move to a faster scope you won't have to spend as much on the mount to get good results on deep sky; tracking periodic error correction doesn't need to be as good for using a web-cam on planets, but if you're at f/50 it certainly is helpful to have precise hand paddle control, and precision typically means higher cost.

Now, as for visual applications, the faster scope can have a decisive advantage in wide field.

Mechanical issues need however to be considered. An f/4.5 dob can shake quite a bit in the wind, I often find them difficult to use. An f/10 SCT is often steady where the similar aperture f/4.5 is shaking (most of my experience is with an Obsession 15"). In a newtonian or refractor longer fr means longer length but in the bizarre world of SCTs you get shorter, and more squat, evne though it's f/10. In conditions of a breeze or wind the shorter scope will also have an advantage for imaging applications.

In terms of view, my friend's f/4.5 15" Obsession offers 1.6 degrees which is pretty good compared to my C14's puny 0.73 degrees. Nonetheless for wide field applications my side-mounted f/6.5 102mm refractor is a very viable alternative even with the smaller aperture.

What to make of all this?

Not much, except that no one scope meets all engineering criteria.

regards
greg n